"1 give tkafe Books
for trie founding of a C Colony' '

I • Ymn*¥MWEi&sinrY°

Healy Memorial Library

THE BRIDGEWATER TREATISES
ON THE POWER, WISDOM, AND GOODNESS OF GOD,
AS MANIFESTED IN THE CREATION.

TREATISE V.
Animal and vegetable physiology, considered
wj.th reference to natural theology.
by peter mark roget, m. d.
SEC. II. S. ETC.

IN TWO VOLUMES.
VOL. I.
[SECOND EDITION.]

"Ask now the beasts, and they shall teach thee; and the fowls of
the air, and they shall tell thee :
" Oe speak to the eauth, and it shall teach thee ; and the fishes of the
sea shall declare tjnto thee.
" who knoweth not in all these that the hand of the lord hath
WROUGHT THIS? Job, xii. 7, 8, 9.

ANIMAL

VEGETABLE
PHYSIOLOGY,
CONSIDERED WITH REFERENCE

IVATVRAL TBJQQLOGY.
...V ¦ ¦'?"",. ^/. :S
PETER M A R K R 6 G E T; M. D.
SECRETARY TO THE ROYAL SOCIETY, FULLER).* M I'HOFESSOR OF PHlSIOLOGY IN THE
ROYAL INSTITUTION OP GREAT BRITAIN, VICE PRESIDENT OF THE SOCIETY
OF ARTS, FELLOW OF. THE ROYAL COLIBGE OF PHYSICIANS,
CONSULTING PHYSICIAN TO TS$ tt^EJN CHARLOTTE'S
LYING-IN HOSPITAL, AND TO THE NpftTHERN
DISPENSARY, ETC. ETC.

VOL. I.
SECOND AMERICAN, FROM THE LAST LONDON EDITION.

PHILADELPHIA:
LEA & BLANCHAED,
SUCCESSORS TO CAREY & CO.
1839.

GRIGGS & CO., PRINTERS.

TO HIS ROYAL HIGHNESS
PRINCE AUGUSTUS FREDERICK,
DUKE OF SUSSEX, K. G.
PRESIDENT OF THE ItOYAL SOCIETY,
&C. &C. &C. &C.
THIS TREATISE
IS, WITH PERMISSION, HUMBLY DEDICATED,
«
AS A TRIBUTE OF PROFOUND RESPECT AND GRATITUDE
FOR THE BENEFITS RESULTING TO
SCIENCE
AND ITS CULTIVATORS,
FROM HIS ILLUSTRIOUS PATRONAGE,
BY HIS DEVOTED HUMBLE SERVANT,
<P. M. ROGET.

PRE FA C E.

I probably never should have ventured to engage
in the composition and publication of a work like the
present, had not that task been assigned me by my
nomination as one of the writers of the series of
Bridgewater Treatises, and had I not deeply felt the
honour done me by that appointment, as well as the
importance of the duty which it imposed. The hope,
in which I have indulged, that my labours might even
tually be useful, has been my chief support in this
arduous undertaking; the progress of which has
throughout been seriously impeded by the various
interruptions incident to my profession, by long pro
tracted anxieties and afflictions, and by the almost
overwhelming pressure of domestic calamity.
The object of this treatise is to enforce the great
truths of Natural Theology, by adducing those evi
dences of the power, wisdom, and goodness of God,
which are manifested in the living creation. The
scientific knowledge of the phenomena of life, as they
are exhibited under the infinitely varied forms of or-

Vlll PREFACE.
ganization, constitutes what is usually termed Physi
ology, a science of vast and almost boundlessjextent,
since it comprehends within its range all the animal
and vegetable beings on the globe. This ample field
of inquiry has, of late years, been cultivated with ex
traordinary diligence and success^oy the naturalists
of every country; and from their collective labours
there has now been amassed an immense store of facts,
and a rich harvest of valuable discoveries. But in
the execution of my task this exuberance of materials
was rather a source of difficulty ; for it created the
necessity of more careful selection and of a more ex
tended plan,
In conformity with the original purpose of the
work, which I have all along endeavoured to keep
steadfastly in view, I have excluded from it all those
particulars of the natural history both of animals
and of plants, and all description of those struc
tures, of which the relation to final causes cannot
be distinctly traced ; and have admitted only such
facts as afford manifest evidences of design. These
facts I have studied to arrange in that methodized or
der, and to unite in those comprehensive generaliza
tions, which not only conduce to their more ready
acquisition and retention in the memory, but tend
also to enlarge our views of their mutual connexions,
and of their subordination to the general plan of cre
ation. My endeavours have been directed to give

PREFACE. - IX
to the subject that unity of design, and that scientific
form, which are generally wanting in books profes
sedly treating of Natural Theology, published prior
to the present series ; not excepting even the unri
valled and immortal work of Paley. By furnishing
those general principles, on which all accurate and
extensive knowledge must substantially be founded,
I am not without a hope that this compendium may
prove a useful introduction to the study of Natural
History; the pursuit of which will be found not only
to supply inexhaustible sources of intellectual grati
fication, but also to furnish, to contemplative minds,
a rich fountain of religious instruction. To render
these benefits generally accessible, I have confined
myself to such subjects as are adapted to every class
of readers; and avoiding all unnecessary extension of
the field of inquiry, have wholly abstained from en
tering into historical accounts of the progress of dis
covery; contenting myself with an exposition ofthe
present state of the science. I have, also scrupulously
refrained from treading in the paths, which have been
prescribed to the other authors of these treatises ; and
have accordingly omitted all consideration ofthe hand,
the voice, the chemical theory of digestion,- the ha
bits and instincts of animals, and the structures of an
tediluvian races ; the extent of the field which re
mained, and which, with these few exceptions, em
braces nearly the whole of the physiology of the two
VOL. i. — B

X PREFACE.
kingdoms of nature, already affording ample occupa
tion for a single labourer.
The catalogue of authors whose works have fur-
nished me with the principal facts detailed in these
volumes, is too long for insertion in this place. I
have not encumbered the pages of the work by con
tinual citations of authorities ; but have given refer
ences to them only when they appeared to be parti
cularly requisite, either as bearing testimony to facts
not generally known, or as pointing out sources of
more copious information. It may, however, be pro
per to mention, that I have more especially availed -
myself of the ample materials on Comparative Anato
my and Physiology contained in the works of Cuvier,
Blumenbach, Carus, Home, Meckel, De Blainville,
Latreille, and '¦St. Hilaire, and in the volumes of the
Philosophical Transactions, ofthe Memoires and An
nales du Museum, and of the Annales des Sciences
Naturelles. I should be ungrateful were I not also
to acknowledge the instruction I have derived from
my attendance on the lectures at the Royal College of
Surgeons, delivered successively during many years,
by the late Sir Everard Home, Sir Astley Cooper,
Mr. Lawrence, Mr. Brodie, Mr. Green, and Sir
Charles Bell; and also from those of Professor Grant,
at the University of London.
I have likewise to return my thanks for the liberal
manner in which the Board of Curators of the Hunte-

PREFACE. Xl
rian Museum gave me permission to take such draw
ings of the preparations it contains, as I might want
for the illustration of this work ; and to Mr. Clift,
the conservator, and- Mr. Owen, the assistant con
servator of the museum, for their obliging assist
ance on this occasion. Mere verbal description can
never convey distinct ideas of the form and struc
ture of parts, unless aided by figures; and these I
have accordingly introduced very extensively in the
course ofthe work.*
Being compelled, from the nature of my subject,
and in order to avoid tedious and fatiguing circum
locution, to employ many terms of science, I have
been careful to explain the meaning of each when
first introduced : but as it might frequently happen
that, on a subsequent occurrence, their signification
may have been forgotten, the reader will generally
find in the index, which I have, with this view,
made very copious, a reference to the passage where
the term is explained.
I beg, in this place, to express my deep sense of
the obligation conferred on me by Mr. Davies Gil
bert, the late president of the Royal Society, to
whose kindness I owe my being appointed to write
this treatise.
* AU the wood engravings have been executed by Mr. Byfield, and the
drawings for them were, for the most part, made by Miss Catlow, whose as
sistance on this occasion has been most valuable to me.

Xll PREFACE.

I also take this opportunity of conveying my best
thanks to my friend and colleague, Mr. Children, of
the British Museum, for his kind assistance, in re
vising the sheets while the work was printing, and
for his many valuable suggestions during its pro
gress through the press.
A catalogue of the wood engravings has been sub
joined; ahd also a tabular view of the classification
of animals adopted by Cuvier in his " Regne Ani
mal," with familiar examples of animals included
under each division; both of which I conceived
might prove useful for purposes of reference. The
latter table is reprinted from that which I have given
in my "Introductory Lecture on Human and Com
parative Physiology," published in 1826, wrth only
such alterations as were required to make it corre
spond with the second and improyed edition of Cu-
yier's work,

NOTICE.
The series of Treatises, of which the present is one, is pub
lished under the following circumstances:
The Right Honourable and Reverend Francis Henry,
Earl of Bridgewater, died in the month of February, 1829;
and by his last Will and Testament, bearing date the 25th of
February, 1825, he directed certain Trustees therein named to in
vest in the public funds the sum of Eight thousand pounds ster
ling; this sum, with the accruing dividends thereon, to be held at
the disposal of the President, for the time being, of the Royal So
ciety of London, to be paid to the person or persons nominated by
him. The Testator farther directed, that the person or persons
selected by the said President should be appointed to write, print,
and publish one thousand copies of a work On the Power, Wisdom,
and Goodness of God, as manifested in the Creation; illustrating such
work by aU reasonable arguments, as for instance the variety and
formation of God's creatures in the animal, vegetable, and mineral
kingdoms; the effect of digestion, and thereby of conversion ; the
construction of the hand of man, and an infinite variety of other
arguments ; as also bp discoveries ancient and modern, in arts,
sciences, and the whole extent of literature. He desired, more
over, that the profits arising from the sale of the works so published
should be paid to the authors of the works.
The late President of the Royal Society, Davies Gilbert, Esq.
requested the assistance of his Grace the Archbishop of Canter-

bury, and of the bishop of London, in determining upon the best
mode of carrying into effect the intentions of the Testator. 'Act
ing with their advice, and with the concurrence of a nobleman im
mediately connected with the deceased, Mr. Davies Gilbert ap
pointed the- following eight gentlemen to write separate Treatises
on the different branches ofthe subject as here stated:
THE REV. THOMAS CHALMERS, D. D.
PROFESSOR OF DIVINITY IN THE UNIVEESTY OF EDINBURGH. ^8'
ON THE POWER, WISDOM, AND GOODNESS OF GOD AS MANIFESTED
IN THE ADAPTATION OF EXTERNAL NATURE TO THE MORAL
AND INTELLECTUAL CONSTITUTION OF MAN. '

JOHN KIDD, M. D. F. R. S.
REGIUS PROFESSOR OF MEDICINE IN THE UNIVERSITY OF OXFORD.
ON THE ADAPTATION OF EXTERNAL NATURE TO THE PHYSICAL
CONDITION OF MAN.

THE REV. WILLIAM WHEWELL, M. A. F. R. S.
FELLOW OF TRINITY COLLEGE, CAMBRIDGE.
ASTRONOMY AND GENERAL PHYSICS CONSIDERED WITH
REFERENCE TO NATURAL THEOEOGY.

SIR CHARLES BELL, K. G. H. F. R. S. L. & E.
THE HAND: ITS MECHANISM AND VITAL ENDOWMENTS
AS EVINCING DESIGN.*

PETER MARK ROGET, M. D.
FELLOW OF AND SECRETARY TO THE ROYAL SOCIETY.
ON ANIMAL AND VEGETABLE PHYSIOLOGY.

THE REV. WILLIAM BUCKLAND, D. D. F. R. S.
CANON OF CHRIST CHURCH, AND PROFESSOR OF GEOLOGY IN THE
UNIVERSITY OF OXFORD.
ON GEOLOGY AND MINERALOGY.

THE REV. WILLIAM KIRBY, M. A. F. R. S.
ON THE HISTORY, HABITS, AND INSTINCTS OF ANIMALS.

WILLIAM PROUT, M. D. F. R. S.
CHEMISTRY, METEOROLOGY, AND THE FUNCTION OF DIGESTION,
CONSIDERED WITH REFERENCE TO NATURAL THEOLOGY.

His Royal Highness the Duke of Sussex, President of the
Royal Society, having desired that no unnecessary delay should
take place in the publication of the above mentioned treatises, they
will appear at short intervals, as they are ready for publication.

CONTENTS
OF THE FIRST VOLUME,
IiNTTRODUCTION.
Chapter I. — Final Causes - ... If
II. — The Functions of Life - - - 39
PART I.— THE MECHANICAL FCNCTIONS.
Chapter I. — Ohganic Mechanism - - - - 56
§ 1. Organization in general - - - 56
2. Vegetable Organization - - - - 60
3. Development of Vegetables ... 71
4. Animal Organization - - 79
5. Muscular Power - 97
Chapter II. — The Mechanicai Functions in Zoophytes - 110
§ 1. General Observations ----- 110
2. Porifera, or Sponges ... 113
3. Polypifera - ... 122
4. Infusoria ----- . 136
5. Acalepna .--•-- 142
6. Echinodermata ..... 147
Chapter in. — Mollusca - - - 1ST
§ 1. Mollusca in general - - - 157
2. Acephala • - - 159
3. Gasteropoda .... 165
4. Structure and formation of the Shells of Mollusca - 168
5. Pteropoda ----- 185
6. Cephalopoda ... - 186
Chapter IV. — Articuxata - - 193
§ 1. Articulated animals in general - - - 193
2. Annelida ------ 194
3. Arachnida ... - 202
4. Crustacea ..-.-- 204
Vol. I. C

XVJIJ CONTENTS,
.Chapter V. — Insects ...... 212
\ 1. Aptera t - - . 212
2. Insecta alata r 214
3. Development of Insects .... 215
4. Aquatic Larva? - 220
5. Terrestrial Larvae ----- 222
6. Imago, or perfect Insect .... 225
7. Aquatic Insects . . - - . 237
8. Progressive motion of Insects on land - 239
9. Flight of insects - ' - - - - 242
Chapter VI. — Vertebrata - . - - 254
| 1. Vertebvated Animals in general ... 254
2. Structure and composition of the Osseous Fabric - 256
3. Formation and development of Bone - - 263
4. Skeleton of the Vertebrata - - 269
Chapter VII.— Fishes - ... 284
v
Chapter VIII.— Reptilia - - - 302
§ 1. Terrestrial Vertebrata in general - 302
2. Batrachia 303
3. Ophidia ... 310
4. Sauria .... 317
5. Chelonia - - - . - 321
Chapter IX — Mammalia - - - 330
§ 1. Mammalia in general . - . . . 330
2. Cetacea ..... 333
3. Amphibia - - - 336
4. Mammiferous Quadrupeds in general ' . 337
5. Kuminantia - 345
6. Solipeda - - - 356
7. Pachydermata - . - 357
8. P.odentia ...... 350
9. Insectivora - - . 362
\0. Carnivora ...... 354
11. Quadrumana - 367
}2. Man • - - - - 369
Chapter X. — Vertebrata capable of Flying ... 373
5 1. Vertebrata without feathers, formed for flying - 376
§. Ejirds ---,-., 382

LIST OF ENGRAVINGS:

VOLUME I.
Fi& Page
1 Rolifef redivivus, (from "Muller) . . . .58
2 Vibrio trilici, (Bauer) .... 58
3 Simple vegetable cells, (Slack) . . .61
4 Fucus vesiculosus, transverse section, (De Candolle) .¦ 61
5 Ditto, longitudinal section, (id.) . . . .61
6 Compressed cells of vegetables, (Slack) . . .61
7 Hexagonal and elongated cells, (id.) . j . .61
8 Elongated cells, (id.) . . . . . .61
9 Fibrous cells, (id.) ....... 61
10 Reticulated* cells, (id.) ..... 61
12 Junction of cells to form a tube .... 65
13 Beaded vessels ...... 65
14 Spiral vessels, or Trachea; . .¦ . .• .. 65
15 Annular vessels •-.... 65
16 Punctuated vessels . . . . .¦ .65
17 Transitions of vessels from one class to another . . 65
18 Woody fibres . . . . .¦ . .65
19 Nervures of a leaf ... J .. 65
20 Cells composing the cuticle, (De Candolle) . . .69
21 Stomata magnified, (Amici) . . . .69
22 Arrangement of stomata in cuticle, (De Candolle) . . 69
23 Roots terminated by spongioles, (id.) ... 69
24 Cells composing a spongiole, (id.) .• . .69
25 Animal cellular substance ..... 82
26 Blood vessel . . . . . . .84
27 Section of blood vessel, with the valves open . . 84
28 Ditto, with the valves closed . . . . .84
29 Striated surface ofthe scale ofthe Cyprinus alburnus, (Hei
singer) ...... 92
30 Ditto of the Perea Jluvialilis, (Carus) . . , . .92
31 Imbricated arrangement ofthe scales of fishes, (Heisinger) 92
82 Section of the bulbs of hair, magnified , . . .93
33 Quill of Porcupine, (F. Cuvier) .. . . .96
34 Transverse section of the same, (id.) . . . . " 9&

XX

LIST OF ENGRAVINGS..

Fig.
35 Longitudinal section ofthe root of ditto, (id.)
36 Capsule of bulb of ditto laid open, (id.)
37 Muscle in a state of relaxation
38 The same muscle contracted
39 Diagram illustrating the action of oblique muscles
• 40 Semi-penniform muscle ....
41 Penniform muscle 
42 Complex muscle 
43 Tendon of muscle 
44 Trapezius muscle ...
45 Muscular Structure of the Ear-drum, (Home)
46 Orbicular muscle ofthe Eye-lids, (Albinus)
47 Muscular structure ofthe Iris, (Home)
48 Muscular fibres of a sucking disk
49 Longitudinal muscular fibres of a bloodvessel
50 Transverse muscular fibres of ditto .
51 Muscular fibres ofthe human stomach, (Cooper)
52 Muscular fibres ofthe heart, (id.)
53 Magnified view of a Sponge, (Grant)
54 Spicula in the texture of a Sponge, (id
55 Gemmule of a Sponge, (id.) .
56 Lobularia. Alcyonium pelasgica, (Deterville)
57 Detached polype of ditto, (id.) . '
58 Zoanthus, (Actinia sociala,) (Ellis)
59 Hydra viridis, (Trembley) .
60 Sertularia pelasgica, (Deterville)
61 Tubipora musica, (Ellis)
62 Section and polypes of ditto, magnified, (id.)
63 Flustra earbasea, (id.) .
64 Cells of ditto, magnified, (id.) .
65 Corallium rubrum, (id.)
66 Polypes of ditto, magnified, (id.)
67 Section of Gorgonia Briareus, (id.)
68 Isis hippuris, (id.)
69 Polype of Flustracarbasea, (Grant)
70 Tentaculum of ditto, magnified, (id.)
71 Pennaluia phosphorea, (Ellis)
72 Magnified view ofthe polypes of ditto, (id.)
73 to 76 mode of progression of the Hydra viridis, (Trern
77 Vorticella cyathina, (Muller)
78 Proteus diffluens, (id.) .
79 Volvox globator, (id.)
80 Brachionus urceolaris, (id.) .

bley)

Page 96-96^
101101
. 101 101.101
101 101
101105-
105 105
105-
106 106; 106
106114114
,114 122
122 122122
124125125 125-
125 125
125 125
125 129
129 131
131 13a13613913914»

LIST OF ENGRAVINGS.

XXI

Ft?- Page
81 Medusa Pulm'o, (Macri) . 142
82 Beroe ovalus, (Bruguiere) .... 144
83 Beroe pileus, (id.) . . . . . . 144
84 Velella limbosa, (GtieVin) .... 144
85 Physalia atlantica, (id.) ..... 144
• 86 Actinia rvfa, (original) ..... 146
87 Ditto expanded, (original) ..... 146'
88 Asterias serrulata, (Brugaiere) .... 147
89 Asterias regularis, (id.) . 147
90 Echinus Ananchites ovata, (id.) . . . 147
91 Clypeaster rosaceus, (id.) . 147
92 Ophiura lacerlesa, (id.) ..... 147
93 Euryale muricatum, (id.) ..... 147
94 Penlacrinus europcsus, (Thomson). . . . 147
95 Ambulacra, and feet of Asterias, viewed from the under side,.
(Reaumur) ...... 148
96 Ditto, viewed from the upper side, (id.) . . . 148
97 Vesicles appended to the feet of tho Asterias . . 148
98 Polygonal pieces composing the test of the Echinus . 150
99 Structure of a detached piece of ditto. . . . 150
100 Spine of the Cidaris, (Carus) .... 150
101 Shell of Unio batava, (Goldfuss) ..... 159
102 Adductor muscle of Oyster,. (Hunterian Museum) . 160
103 Shell of Pholas Candida, with abduetor muscle, (Osier)' . 161'
104 Foot of Cardium edule, (Reaumur) . . . 162
105 Planorbus cornutus (Cuvier) .... 166
106 Magnified view of the stria? on the surface of Mother of
Pearl, (Herschel) ..... 169
107 Directions ofthe fibres in the component strata of shells . 170
108 Shell of Achatina zebra, (De Blainville) . . . 176
109 Longitudinal section of ditto, (id.) .... 176
110 Shell of Plerocerus scorpio, at an early stage of growth, (id.)1 178
111 Shell ofthe same when completely formed, (id.) . 178
112 Shell of Cyprcea exanthema at an early period of growth, (id.) 178
178 179181181
181183

113 Shell of the same animal when completed, (id.)
114 Transverse section ofthe shell ofthe Cyprcea exanthema,'
(Hunterian Museum) ....
115 Shell of Conus 
116 Longitudinal section ofthe same, (original)
117 Transverse section ofthe same, (Bruguiere)
118 Inner surface ofthe Epiphragma ofthe Helix pomatia, (De
Blainville) .....

LIST OF ENGRAVINGS.

Fig.
119 Outer surface of the same, (id.)
120 Clio borealis, (Cuvier)
121 Sepia loligo, (De Blainville)
122 Suckers of (he same, (id.)
123 Suckers ofthe Octopus, (original)
124 Shell of Spirula australis, (De Blainville)
125 Longitudinal section ofthe same, (id.)
126 Shell of Nautilus pompilius (id.)
127 Longitudinal section ofthe same, (id.)
128 Pontobdella muricata, (Bruguiere)
129 Nereis, (id.) .
130 Erpobdella vulgaris (Lam.) Hirudo hyalina
131 Diagram illustrating the rings and muscles of Annelida,
(original) .....
132 Gordius aquations .....
133 Serpula opercularia ....
134 Terebella conchilega, (De Blainville) . .
135 Arenicola piscalorum, or Lumbricus marinus
136 Aranea diadema, (Roesel) ....
137 Divisions of the limb of a Crustaceous animal
138 Mandible and palpus of Mysis Fabricii, (Bruguiere)
139 to 141 Feet-jaws belonging to the first, second, and third
pairs, (id.) .....
142 True foot, belonging to the first pair, (id.)
143 Julus terrestris .....
144 Muscles of the trunk of the Melolontha vulgarist (Straus
Durckheim) . . . .
145 Eggs of Bombyx mori ....
146 Larva of the same .....
147 Pupa .of the same .....
148 Imago of the same .....
148*a Caterpillar ofthe Phalenia striaria, (Hubner) ,
b The same in a rigid position, (Lyonet)
149 Calosoma Sycophanta, (Kirby and Sperice)
150 Analysis of skeleton of the same, (Carus)
151 Hind view ofthe segment ofthe head in the same, (id.)
152 Suckers on the font ofthe Musca vomitoria, expanded; rnacr.
nified view, (Bauer) ....
153 Cushions on the foot of the Cimbex lutea, magnified, (id.)
154 Suckers on the under side ofthe foot of a male Dytiscus
marginalis, (id.) .....
155 Cushions and sucker of the Acridium biguttulum, Latr. (id.

Page 183186187187 187191* 191191
191195195195
195 198198 198198
202205205205205
213214 217217217217224 224227
228228235235235235

LIST OF ENGRAVINGS.

xxiii

156 Dytiscus marginalis, upper side, (Rcesel) .
157 Lower side of the same insect, (id.)
158 Notonecta glauca, (Rcesel) .
158* Fore leg of Gryllolalpa, (Kidd)
159 Wing of Gryllus nasutus. Orthoptera
160 Wing of Libellula grandis. Neuroptera
161 Wing of Ichneumon persuasorius. Hymenoptera
162 Wing of Tipula oleracea. Diptera . . >
163 Sting of Anthophora retusa, (original)
164 Separate scales pf the wing of Hesperia Sloanus, (original)
165 Arrangement ofthe scales in the "Wing ofthe same .
172 Longitudinal section of Jhe thigh-bone to show the cancel
lated structure, (Cheselden)
173 Longitudinal section ofthe humerus, (id.)
174 Ossification of the parietal bone, (id.) . f
175 Early stage of ossification ofthe bones ofthe skull, (Cloquet)
176 The same in the adult? showing the sutures .
177 Dorsal vertebra, human .....
178 Junction of vertebras forming the spinal column
179 Longitudinal section ofthe same, showing the spinal canal
180 Elements of structure of a vej-tebra, (Carus)
181 Skeleton of Hog, (Pander and D'Alton)
182 Sternum, clavicle, and scapula; human
184 Skeleton of Cyprinus carpio, (Bonnaterre)
185 Diagram illustrating the progressive motion of Fishes
186 Front view ofthe vertebra of a Cod, (.Gadus morrhua) .
187 Side view of the same ....
188 Vertical and longitudinal section of a part of the spinal co.
lumn in the same . ...
189 A similar section, showing the gradation of structure
190 Similar section in the Squalis cenlrina, (Carus)
191 Bones of the shoulder of the Lophius piscatorius, (id.)
192 Pectoral fin ofthe Raia clavata, (id.)
193 Belt of bones ofthe shoulder of a Ray, (id.)
194 Muscular system of Cyprinus alburnus, (id.)
195 Air bladder of Cyprinus carpio, (Blasius)
196 Eggs ofthe Frog ....
197 Side view ofthe Tadpole magnified, (Rusconi)
198 Upper view of the same, (id.)
199 Adult Frog ....
200 Skeleten of Frog, (Cheselden)
201 Skeleton of the Viper .

237
237 238
242 246 246246246 248 250250262 262265265
265 271
271 271 274 279
279 286 287288288288 288288293 293
294295298303303303
303
306310

XXIV MST OF ENGRAVINGS.
Fig. PaSe
202 Ribs and spine of Boa constrictor, (Home) . . . 312
203 Bones of the foot of the same, (Mayer) . . . 311
204 Muscles moving the claw of the same, (id.) ' . . 311
205 Rudimental bones ofthe foot ofthe Tortryx scytale, (id.) 311
206  of the Tortrix corallinus, (id.) . . . 311
207  of the Anguis fragilis, (id.) . . . 311
208  ofthe Amphisbcena alba, (id.) . . . 311
209  of the Coluber pollutatus, (id.) . . . 311
210 Chalcides pentadactylus, (Bonnaterre) . . . 311
211 Under surface of the foot of the Lacerta gecko, magnified
four times, (Bauer) *. . . . . 319
212 Side view of a longitudinal section ofthe same, (id.) . 319
213 Skeleton ofthe Tortoise, (Carus) . . . .322
214 Section of the thigh bone of the same, (id.) . . 322
215 Hind view of skull of Testudo mydas, (id.) . . 325
216 Bones sustaining the fin ofthe Delphinus phoccena, (Pander
and D' Alton)  336
217 Fore part of the Skeleton of an Ox with the Ligamentum
nucha, (original) ..... 346
218 Skeleton ofthe Stag, (Cheselden) . . r. 350
218* a. Longitudinal section of the horn of an Ox, (original) . 355
b. Ditto of the Antelope, (original) . . . 355
c. Extremity of the same, (original) . . . 355
219 Subcutaneous muscles ofthe Hedge-hog, relaxed, (Carus) 364
220 The same muscles contracted, and drawn over the body,
(Cuvier)  .364
221 Skeleton ofthe Lion, (Pander and D'Alton) . . 365
222 Skeleton of Draco volans, (Tiedemaun) . . . 379
223 Skeleton of Vespertilio Molossus, (Temmink) . . 380
224 Skeleton ofthe swan, (Cheselden) .... 385
225 Lateral section of the cervical vertebra of the Ostrich, (ori
ginal)  .388
226 Fibrils ofthe vane of a feather, magnified, (original) . 393
227 Edges of the fibres, magnified, (original) . . 393
228 Feather, showing its structure,. (F. Cuvier) . . 396
229 Capsule, or Matrix ofthe feather, (id.) . . . 396
230 View of the parts enclosed in the Capsule, when laid open,
(id.)  396
231 Section of the stem, while growing, exhibiting the series of
conical membranes, (id.) .... 396
233 Extensor muscles of the foot and toes of a bird, (Borelli) . 405
234 Position of a bird in roosting, (id.) . . . 405

LIST OF ENGRAVINGS.

VOLUME 11.
Fig. Page
, 239 Cyclosis, or partial circulation in the cells of the Caulinia
fragilis, magnified, (Amici) . . . .42
240 The same in the jointed hair of the Tradescanlia virginica,
(Slack)  42
241 Section of the Hydra vividis, magnified, (Trembley) . 58
242 Hydra vividis seizing a worm, (id.) ... 59
243 The same after swallowing a minnow, (id.) . . .59
244 A Hydra which has swallowed another of its own species (id.) 59
245 Compound Hydra with seven heads, (id.) . . .59
246 Veretilla lutea, showing the communicating vessels of the Po
lypes, (Quoy et Gaimard) . . . . 64
247 Nutrient vessels of the Tcenia solium (Chiaje) . . 64
248 Tcenia globosa, or Hydatid of the Hog, (Goeze) ., . 64
249 Horizontal section of the Rhizostoma Cuvieri, Peron, (Ey-
senhardt) . . . . . . 67
250 Geronia Hexaphylla, Peron Medusa proboscidalis, (Forskal) 67
251 Vascular net-work in margin ofthe disk ofthe Rhizostoma
Cuvieri, (Eysenhardt) .... 67
252 Vertical section of the Rhizostoma Cuvieri, (id.) . . 68
253 Transverse section of one ofthe arms ofthe same, (id.) . 68
254 Transverse section ofthe extremity of a tentaculum ofthe
same, (id.) ...... 68
255 Leucophra patula, highly magnified, (Ehrenberg) . .73
256 Alimentary canal and cseca ofthe same, viewed separately,
(id.)  .73
257 Vertical section ofthe Actinia coriacea, (Spix.) . . 75
258 Digestive organs of the Asterias, (Tiedemann) . . 76
259 Stomachs of the Nais vermicularis, (Ro3sel) . . 77
260 Stomachs of the Hirudo medicinalis, (original) . . 78
261 Mouth ofthe same, showing the three semicircular teeth,
(original) . . . . . .78
262 Tooth of the same, detached, (original) ... 78
263 Glassopora tuberculata; Hirudo complanata, Lin. (Johnson) 78
264 The same seen from the under side, showing the digestive
organs, (id.) . . . . . .78
265 Diagram showing the arrangement and connexions ofthe or
gans ofthe vital functions in Vertebrata, (original) . 81
VOL. I.  D

XXvi LIST OF ENGRAVINGS.
Fig. PaSe
266 Spiral proboscis of Papilio urticce, (Griffith) . . .87
267 Trophi of Locusta viridissima, (Goldfuss) ... 92
268 Filaments composing the rostrum, or proboscis, ofthe Cimex
nigricornis, (Savigny) . . . . .94
269 Sheath ofthe proboscis ofthe same insect, (id.) . . 94
270 Toothed cartilage of the Helix pomatia, (Cuvier) . . 95
271 Mechanism for projecting and retracting the tongue ofthe
Woodpecker, (original) ..... 99
272 Lamina, of Whalebone descending from the palate ofthe Ba-
Icena myslicetus, (Bonnaterre) . . .. . 102
273 Teeth ofthe Delphinus phoccena, (Cloquet). . . 106
274 Skull of Tiger, (Cuvier) . . . . .108
275 Skull of Antelope, (Pander and D' Alton) . . 109
276 Skull of Rat, (id.)  110
277 Longitudinal section of simple tooth, (Rosseau) . ' 111
278 Surface of the grinding tooth of a Horse, (Home), . . Ill
279 Surface of the grinding tooth of a Sheep, (id.) . . Ill
280 Longitudinal section of the incisor tooth of the Rodentia . Ill
281 Vertical section ofthe grinding tooth ofthe Elephant, (Home) 114
282 Grinding tooth of the African Elephant, (id.) . . 114
283 Grinding tooth of the Asiatic Elephant, (id.) . . 114
284 Succession of teeth in the Crocodile, (Carus) . . 120
285 Venomous fang of the Coluber naia, (Smith) . . 121
286 Transverse section of the same, (id.) . . . 121
287 The same tooth, at an earlier period of growth, (id.) . 121
288 The same, still less advanced in its growth, (id.) ¦ . 121
289 Base ofthe former, (id.) ¦ ... 121
290 Base ofthe latter, (id.) . . . . .121
291 Transverse section of the young fang, about its middle, (id.) 121
292 A section, similar to the last, of another species of serpent, (id.) 121
293 Squalus pristus. b. Under side of its snout, (Latham) . 122
294 Interior ofthe Stomach of a Lobster, (original) . 123
295 Gastric teeth of Bullosa aperta, (Cuvier) . . . 123
298 Gizzard of the Swan, (Home) .... 124
299 Crop and gizzard, of the Parrot, (id.) . . 131
300 Crop of the Pigeon, (id.) ...» 131
301 Human Stomach, (id.) ..... 133
302 Interior ofthe stomach ofthe African Ostrich, (id.) . 135
303 Gastric glands ofthe same, (id.) .... 135
304 Gastric glands of the American Ostrich, (id.) . . 135
30| Longitudinal section ofthe gastric glands ofthe Beaver, (id.) 135
306 Stomach of Dormouse, (id.) .... 139

LIST OF ENGRAVINGS. XXvii
Fig- Page
307 Stomach of Hyrax capensis, (Cuvier) . . . 139
308 Stomach of Porcupine, (id.) .... 139
309 Stomach of Kangaroo, (id.) .... 139
310 Stomach of Delphinus phoccena, (id.) . . . 139
311 Cardiac valve of the Hoise, (Gurlt) . . . 140
312 The four stomachs of a Sheep, (Carus) . . . 141
313 Inner surface of the honey-comb stomach, (Home) . . 141
314 Inner surface ofthe many-plies stomach of an Ox, (id.) 141
315 Interior cellular surface of the second stomach of the Camel,
(id-)  141
316 Spiral valve in the intestine ofthe Shark, (Blasius) . 149
317 Digestive organs ofthe mantis religiosa, (Marcel de Serres) 153
318  Melolontha vulgaris, (Leon Dufour) . . 154
319  Cicindela campestris, (id.) . " . . . 154
320 Portion of a hepatic vessel of the Melolontha, highly magni
fied, (Straus Durckheim) .... 155
321 Alimentary canal of the Acrida aptera, (original) . . 155
322 Interior of the gizzard of the samegimagnified, (original) 155
323 Row of large teeth in the same, stilnnore magnified, (original) 155
324 Profile of one of those teeth, still more highly magnified, (ori
ginal) ....... 155
325 Base ofthe same tooth, seen from below, (original) . . 155
326 Alimentary canal of the Larva of the Sphinx Ligustri, (origi
nal)  157
327  of the Pupa of the same, (original) . . . 157
328  of the Imago of the same, (original) . . 157
329  of the Patella, (Cuvier) . . .159
330 Stomachs of the Pleurobranchus Peronii, (id.) . . 159
331 Pyloric appendices in the Salmon, (id.) . . . 160
333 Detached Dorsal vessel of Melolontha vulgaris, (Straus
Durckheim) ...... 172
334 The same, with its ligamentous and muscular attachments,
(id.)  .172
335 Side view of the anterior extremity of the same vessel, (id.) 172
336 Section ofthe dorsal vessel, to show its valves, (id.) . 172
337 Circulation in the antenna of the Semblis viridis, (Carus) 175
338 Course of circulation in the same insect, (id.) . . 175
339 Dorsal vessel of the Caterpillar of the Sphinx ligustri, side
view, (original) . . • . - 177
340 The same in the Chrysalis, (original) . . . 177
341 The same in the Moth, (original) ... 177
342 The same viewed from above, (original) . . .177

XX VIU

LIST OF ENGRAVINGS.

Fig;. 343 Magnified lateral view ofthe anterior extremity o£the dorsal
vessel, (original) . . ...
344 Magnified dorsal view of the same, (original) .
345 Structure ofthe valves ofthe dorsal vessel, (original)
346 Heart and vessels ofthe Aranea domestica, (Treviranus)
346* Circulation in the Planaria nigra, (Duges)
347 Course of circulation in the Erpobdella vulgaris, (Morren)
348 Vessels in abdominal surface ofthe same, (id.)
349 Vascular dilatations, or hearts of the Lumbricus terrestris,
(Morren) .....
350 Cavities and great vessels of the Heart,
351 The Heart laid open to show its Valves,
352 Plan of simple circulation, ....
353 Plan of double circulation,
354 Branchial circulation in Maia Squinado, (Audouin)
355 Organs of circulation in the Loligo sagitalta, (id.)
356 Plan of circulation in Fishes,
357 Plan of circulation in BatMehia,
359 Plan of double or warm-blooded circulation,
360 Heart ofthe Dugong, (Home) .
365 Valves of the Veins, (Cloquet)
366 Heart, branchial artery, and gills of a fish, (Blasius)
367 Branchial apertures in the Squalus glaucus, (Bonnaterre)
368 Branchial apertures in the Petromyzon marinus, (id.)
369 Internal structure ofthe branchia? ofthe same, (Home)
370 Stigmata in the abdominal surface of the Dytiscus margina-
lis, (Leon Dufour) .....
371 Stigmata ofthe Cerambyx heros, (Fab.) magnified, (id.)
372 Longitudinal tracheae of Carabus auratus, (id.)
373 Air vesicles and tracheae of the Scolia hortorum, (Fab.)
highly magnified, (id.) .
374 Respiratory apparatus ofthe Scorpio europceus, (Treviranus)
375 Internal structure ofthe lungs ofthe Turtle, (Bojanus)
377 Air cells ofthe Ostrich, (Parisian Academicians)
378 Lymphatic Absorbents, .
379 Passage of Nerves through a ganglion, .
380 Plexus of Nerves, ......
381 Varieties of forms of antennse of Insects, (Goldfuss)
382 Vertical and longitudinal section of the right nostril in man,
383 Vertical transverse section of the same,
384 Transverse section ofthe nostril of a Sheep, (Harwood)
385 Turbinated bones ofthe Seal, (id.)

Page 177 177 177180 181 183
183184 187
188 189
191 193 195 196
197 199
200 206
216 216
216
216 222222 822 222 225
229233250 255
255 272 283
284 265285

LIST OF ENGRAVINGS. XXIX
Fig. ' Page
386 Turbinated bones ofthe Turkey, (id.) . . .287
387 Nerves distributed to the bill ofthe Duck, (id.) . 288
388 Nasal cavities of the Perea jluviatilis, (Cuvier) . . 291
389 Nasal cavity of the Raia batis or Skate, (Harwood) . 291
390 Human ear, (Cloquet) . . . . .299
391 Posterior surface of the cavit3»of the tympanumj (id.) . 301
392 Ossicula auditus, or small bones of the tympanum, . . 301
393 The position of the latter in the tympanum, . . 301
394 Magnified view ofthe labyrinth detached from the surrounding.
pa,rts, (Breschet) ..... 303
395 Interior structure of the labyrinth, (id.) . . . 304
396 Membranous labyrinth, with its nerves, (id.) . . 304
397 Cretaceous bodies in the labyrinth ofthe Dog, (id.) . 304
398 Ditto in that ofthe Hare, (id.) . . . .304
399 Organ of hearing in the Lobster, (Cams) . . 308
400 Groove in the sac of the former, (id.) . . . 308
401 Organ of hearing in the Astacus Jluviatilis, (id.) . 308
402 Interior view of the same, (id.) . . . !, 308
403 Membranous labyrinth ofthe Lophius piscatorius, (id.) 310
404 -Organ of hearing in the Frog, (Bell) . . . 311
405 Ear ofthe Turkey, (Carus) . . . . ' 311
406 Diagram illustrating one mode of obtaining images of objects,
(original) . . . . . .319
407 Simple Camera Obscura . 320
408 Law ofthe refraction of a ray of light, . . . 321
409 Convergence of rays to a focus, . . . ' . 322
410 Convergence by a double convex lens, . . . 324
411 Spherical Aberration, ..... 324
412 Variations of focal distance consequent upon variations of diver
gence ofthe incident rays, . . . 325
415 Horizontal section of right human eye, magnified, (Home) 326
416 Straight and oblique muscles of the eye-ball, . . 328
417 Lacrymal apparatus, ...... 330
418 Eye of Helix pomatia, (Muller)'. ... 339
419 Stemmata of Caterpillar, (Marcel de Serres) . . 342
420 Eye of the Scorpio tunensis, (Muller) ., . 342
421 Conglomerate eyes of Julus terreslris, (Kirby and Spence). 342
422 External magnified view ofthe compound eye of the Melolon
tha vulgaris, (Straus Durckheim) . . . 344
423 Ditto of that of a Phalena, .... 344
424 Section ofthe compound eye ofthe Libellula vulgata, magnified,
(Duges) .  344

XXX LIST OF ENGRAVINGS.
Fig. Page
425 Highly magnified view of the outer margin of the preceding
section, (id.) ..... 344
426 Portion of the section ofthe eye of the Melolontha vulgaris,
(Muller)  344
427 Portion ofthe section ofthe eye of the Libellula, (Duges) 344
428 Portion of the section of the eye of the Melolontha vulgaris,
(Straus Durckheim) .... 344
430 Interior of the eye of the Perea Jluviatilis, (Cuvier) . 349
431 Fibres of the crystalline lens of the Cod, (Brewster) . 350
432 Denticulated structure of these fibres, (id.) - . 350
433 Section ofthe eye ofthe Goose, (Home) . . 353
434 Nictitating membrane of a Bird, (Petit) . . . 353
435 Muscles of the nictitating membrane, (id.) . . 353
438 Talitrus, (Latreille) . . . . .381
439 Nervous system of the Talitrus, (Audouin) . . 382
440 Nervous system of Cymothoa, Fab., (id.) . . . 382
441 Nervous system of the Maia squinado, (id.) . . 383
442 Nervous system of the Larva of the Spinx ligustri, (New
port,) ....... 386
443 Ditto ofthe Chrysalis ofthe same, (id.) . . . 386
444 Ditto of the Imago ofthe same, (id.) . . . 386
445 Nervous system of the Asterias, (Tiedemann) . . 387
446 Ditto ofthe Aplysia, (Cuvier) .... 387
447  ofthe Patella, (id.) .... 387
448  ofthe Sepia Octopus, (id.) .... 387
449 Brain and spinal marrow of the Columba turtur, (id.) . 389
450 Transverse section of the spinal marrow of the Cyprinus car
pio, ....... 389
451 Brain and spinal marrow of the Trigla lyra, (Arsaky) . 389
452 Brain of the Murcena conger, (Serres) . . . 389
453  Perea Jluviatilis, (Cuvier) . . . 389
454  Testudo my das, (Carus) .... 389
455  Crocodile, (id.) ..... 389
456 -.  Lion, (Serres) ..... 389
457 Lateral view ofthe brain ofthe Perch, (Cuvier) . 389
458  of the Testudo mydas, (Carus) . . . 389
459  of a section of the brain of the Dove, (id.) . 389
460  of the Lion, ...... 389
461 Vertical section of the human brain, (Monro) . . 393
462 Progressive changes in the Manas .... 410
463  Vorlicella ..... 410

OUTLINE OF
CUVIER'S CLASSIFICATION OF ANIMALS,
WITH '*
EXAMPLES OF ANIMALS BELONGING TO EACH DIVISION.

Bimana
Quadrumana Cheiroptera
Insectivora -
Plantigrnda Digitigrada Amphibia -
MarsupialiaRodentiaEdentata
Pachydermata Ruminantia CetaceaAccipitres -
Passeres Scansores -
GallinseGrails
PalmipedesCheloniaSauria
Ophidia
Batrachia .
Acanthopterygii
Malacopterygii

i

. VERTEBRATA. I. Mammalia.
Man.
Monkey, Ape, Lemur.
Bat, Colugo.
Hedge-Hog, Shrew, Mole.
Bear, Badger, Glutton.
Dog, Lion, Cat, Martin, Weasel, Otter.
Seal, Walrus.
Opossum, Kangaroo, Wombat.
Beaver, Rat, Squirrel, Porcupine, Hare.
C Sloth, Armadillo-, Ant-eater, Pangolin, Ornitho-
c. rhyncus.
Elephant, Hog, Rhinoceros, Tapir, Horse.
£ Camel, Musk, Deer, Giraffe, Antelope, Goat,
Sheep, Ox.
Dolphin, Whale.
2. Aves.
Vulture, Eagle, Owl.
Thrush, Swallow, Lark, Crow, Sparrow, Wren.
Woodpecker, Cuckoo, Toucan, Parrot.
Peacock, Pheasant, Grous, Pigeon.
Plover, Stork, Snipe, Ibis, Flamingo.
Auk, Grebe, Gull, Pelican, Swan, Duck.
3. Reftilia.
Tortoise, Turtle, Emys.
Crocodile, Lizard, Gecko, Chameleon.
Serpents, Boa, Viper.
Frog, Salamander, Newt, Proteus, Siren.
4. Pisces.
Perch, Mackerel, Sword-fish, Mullet.
C Salmon, Herring, Pike, Carp, Silurus, Cod,
i Sole, Remord, Eel.

classification OF animals.

Lophobranchi . . Pike-fish, Pegasus.
Plectognathi . . Sun-fish, Trunk-fish-
Chondropterygii . . Lamprey, Shark, Say, Sturgeon.
II. MOLLUSCA.
1. Cephalopoda . . Cuttle-fish, Calamary, Nautilus..
2. Pteropoda . . Clio, Hyalsea.
3. Gasteropoda . . Slug, Snail, Limpet, Whelk.
4. Acephala . . Oyster, Muscle, Ascidia.
S. Brachiopoda . . Lingula, Terebratula.
6. Cirrhopoda . . Barnacle.
III. ARTICULATA. 1. Annelida.
Tubicola . . . Serpula, Sabella, Amphilrite.
Dorsibranchia . . Nereis, Aphrodite, Lob-worm-
Abranchia .... Earth-worm, Leech, Nais, Hair-worm.
2. Crustacea.
l.Malacostraca .
Decapoda . . Crab, Lobster, Prawn.
Stomapoda . . Squill, Phyllosoma.
Amphipoda . . Gammarus, Sand-hopper.
Lsemodipoda . . Cyamus.
Isopoda . . . Wood-louse.
2. Entomostraca . . Monoeulus. 3. Arachnida.
Pulmonalia . . Spider, Tarantula, Scorpion.
Trachealia , . . Phalangium, Mite.
4. Insect a.
Aptera . . . Centipede, Podura.
Coleoptera . . . Beetle, Glow-worm.
Orthoptera . . . Grasshopper, Locust.
Hemiptera . . . Fire-fly, Aphis.
Neuroptera . . Dragon-fly, Ephemera.
Hymenoptera . . < Bee, Wasp, Ant.
Lepidoptera . . Butterfly, Moth.
Rhipiptera . . Xenos, Stylops.
Diptera . . . Gnat, House-fly.
IV. ZOOPHYTA.
1. Echinodermata . Starfish, Urchin.
2. Entozoa . . Fluke, Hydatid, Tape-worm.
3. Acalepha. . . Actinia, Medusa.
4. Polypi . . Hydra, Coral, Madrepore, Pennatula.
S. Infusoria . . Brachionus, Vibrio, Proteus, Manas.

ANIMAL AND VEGETABLE
PHYSIOLOGY.

INTRODUCTION. CHAPTER I.
FINAL CAUSES.
To investigate the relations which connect Man with his
Creator is the noblest exercise of human reason. The Be
ing who bestowed on him this faculty cannot but have in
tended that he should so exercise it, and that he should ac
quire, through its means, some insight, however limited,
into the order and arrangements of creation; some know
ledge, however imperfects of the divine attributes ; and a
distinct, though faint, perception of the transcendent glory
with which those attributes are encompassed. To Man
have been revealed the power, the wisdom, and the good
ness of God, through the medium of the Book of Nature, '
in the varied pages of which they are inscribed in indelible
characters. On Man has been conferred the high privilege
of interpreting these characters, and of deriving from their
contemplation those ideas of grandeur and sublimity, and
those emotions of admiration and of gratitude, which ele
vate and refine the soul, and transport it into regions of a
purer and more exalted being.
VOL. I.  3

18 FINAL CAUSES.
A study which embraces so extensive a range of objects,
and which involves questions of such momentous interest
to mankind, must necessarily be arduous, and requires for
its successful prosecution the strenuous exertions of the hu
man intellect, and the combined labours of different classes
of philosophers during many ages. The magnitude of the
task is increased by the very success of those previous ef
forts : for the difficulties augment as the objects multiply,
and the eminence on which the accumulated knowledge of
centuries has placed us only discloses a wider horizon, and
the prospect of more fertile regions of inquiry; till at length
the mind, conscious of the inadequacy of its own powers to
the comprehension of even a small part of the system of the
universe, is appalled by the overwhelming consideration of
the infinity that surrounds us. The reflection continually
presents itself that the portion of creation we are here per
mitted to behold is as nothing, when compared with the
immensity of space, which, on every side, spreads far be
yond the sphere of our vision, and, indeed, far beyond the
powers of human imagination. Of the planetary system,
which includes this earth, our knowledge is almost entirely
limited to the mathemetical laws that regulate the motions
of the bodies which compose it, and to the celestial me
chanism which patient investigation has at length discovered
to be that most admirably calculated to preserve their har
mony and maintain their stability. Still less have we the
means of penetrating into the remoter regions of the heavens,
where the result of our investigations respecting the myriads
of luminous bodies they contain amounts to little more than
the knowledge of their existence, of their countless num
bers, and of the immeasurable distances at which they are
dispersed throughout the boundless realms of space.
Measured on the vast scale of the universe, the globe we
inhabit appears but as an atom; and yet, within the compass
of this atom, what an inexhaustible variety of objects is con
tained; what an endless diversity of phenomena is presented;
whatwonderful changes are occurring in rapid and perpetual

FINAL CAUSES. 19
succession! Throughout the whole series of terrestrial be
ings, what studied arrangements, what preconcerted adapta
tions, what multiplied evidences of intention, what signal
proofs of beneficent design exist to attract our notice, to ex
cite our curiosity, and to animate our inquiries. Splendid
as are the monuments of divine power and wisdom displayed
throughout the firmament, in objects fitted by their stupen- I
dous magnitude to impress the imagination and overpower
us by their awful grandeur, not less impressive, nor less re- ¦
plete with wonder, are the manifestations of those attributes
in the minuter portions of nature, which are more on a
level with our senses, and more within the reach of our
comprehension. The modern improvements of optical sci
ence, which have expanded our prospects into the more
distant regions of the universe, have likewise brought with
in our range of vision the more diminutive objects of crea
tion, and have revealed to us many of the secrets of their
structure and arrangement. But, farther, our reason tells
us that, from the infinite divisibility of space, there still exist
worlds far removed from the cognizance of every human
sense, however assisted by the utmost refinements of art ;
worlds occupied by the elementary corpuscles of matter,
composing, by their various configurations, systems upon
systems, and comprising endless diversities of motions, of
complicated changes, and of widely extended series of
causes and effects, destined for ever to remain invisible to
human eyes, and inscrutable to human science.
Thus, in whatever field we pursue our inquiries, we are j
sure to arrive at boundaries within which our powers are '
circumscribed. Infinity meets us in every direction, whe-'
ther in the ascending or descending scale of magnitude; and
we feel the impotence of our utmost efforts to fathom the
depths of creation, or to form any adequate conception of
that supreme and Dominant Intelligence, which compre
hends the whole chain of being extending from that which
is infinitely small to that which is infinitely great.
It is incumbent on us, before engaging in a study of such

20 FINAL CAUSES.
vast importance, and extending over so wide a field as that
which lies before us, to examine with attention the nature
of those processes of reasoning, by which we are conducted
to the knowledge of the peculiar class of truths we are seek
ing. Such a preliminary inquiry is the more necessary, in
asmuch as the investigation of these truths is beset with
many formidable difficulties and liable to various sources of
fallacy, which are not met with in the study of other depart
ments of philosophy.
The proper objects of all human knowledge are the rela
tions that exist among the phenomena of which the mind
has cognizance. The phenomena of the universe may be
viewed as connected with one another either by the relation
of cause and effect, or by that of means and end; and, ac
cordingly, these two classes of relations give rise to different
kinds of knowledge, each of which requires to be investi
gated in a peculiar mode and by a different process of rea
soning. The foundation of both these kinds of knowledge
is, indeed, the same; namely, the constant uniformity which
takes place in the succession of events, and which, when
traced in particular classes of phenomena, constitutes what
we metaphorically term the Laws of Nature. It is the pro
vince of philosophy, strictly so called, to discover the cir
cumstances or laws which regulate this uniformity, and to
arrange the observed changes according to their invariable
. antecedents, or causes:' the unknown links by which these
causes are connected with their respective consequents, or
effects, being denominated the powers of Nature. With re
ference to phenomena which are purely mechanical, that
is, to changes which consist in the sensible motions of ma
terial bodies, these powers are denominated forces; and the
intensities, the operations, and the characters of these forces
admit of exact definition, according to the qualities of the
corresponding effects they produce. It is by pursuing the
method of philosophical induction, so well explained by Ba
con, that the physical sciences, which the misdirected efforts
of former ages had failed to advance, have, within the last

FINAL CAUSES. 21
two centuries, been carried to a height of perfection which
affords just grounds for exultation in the achievements of the
human intellect. '
In the investigation of the powers which are concerned in
the phenomena of living beings, we meet with difficulties in
comparably greater than those that attend the discovery of
the physical forces by which, the parts of inanimate matter
are actuated. The elements of the inorganic world are few
and simple; the combinations they present are in most cases
easily unravelled; and the powers which actuate their mo
tions, or effect their union and their changes, are reducible
to a small number of general laws, of which the results may,
for the most part, be anticipated, and exactly determined by
calculation. What law, for instance, can be more simple
than that of gravitation, to which all material bodies, what
ever be their size, figure, or other properties, and whatever
be their relative positions, are equally subjected; and of
which the observations of modern astronomers have rendered
it probable that the influence extends to the remotest regions
of space 1 The most undeviating regularity is exhibited in
the motions of those stupendous planetary masses, which
continually roll onwards in the orbits prescribed by this all-
pervading force. Even the slighter perturbations occasioned
by their mutual influence are but direct results of the same
general law, and are necessarily restrained within certain
limits, which they never can exceed, and by which the per
manence of the system is effectually secured. All the ter
restrial changes dependent on these motions partake of the
same constancy. The same periodic order governs the suc
cession of day and night, the rise and fall of the tides, and
the return of the seasons : which order, as far as we can per
ceive, is incapable of being disturbed 'by any existing cause.
Equally definite are the operations of the forces of cohe
sion, of elasticity, or of whatever other mechanical powers
of attraction or repulsion there may be, which actuate, at in
sensible distances, the particles of matter. We see liquids,
in obedience to these forces, collecting in spheroidal masses,

22 FINAL CAUSES.
or assuming, at their contact with solids, certain curvilinear
forms, which are susceptible of precise mathematical deter
mination. In different. circumstances, again, we behold
these particles suddenly changing their places, marshalling
themselves in symmetric order, and constructing by their
union solid crystals of determinate figure, having all their
angles and facets shaped with mathematical exactness.
The forces by which dissimilar particles are united into
a chemical compound have been termed Chemical affinities;
and the operation of these peculiar forces is as definite and
determinable as the former. They are now known to be
regulated by the law of definite proportions ; a law, the dis
covery of which has conferred on Chemistry the same cha
racter of precision which appertains to the exact sciences,
and which it had never before attained. The phenomena of
Light, of Heat, of Electricity, and of Magnetism have been,
in like manner, reduced to laws of sufficient simplicity to
admit of the application of mathematical reasoning, and to
furnish the accurate results derived from such application.
Thus, to whatever department of physical science our
researches have extended, we every where meet with the
same regularity in the phenomena, the same simplicity in
the laws, and the same uniformity in the results. All is
strictly defined, and subjected to rigid rule: all is subordi
nate to one pervading principle of order. The great Creator
ofthe universe has exercised in its construction the severest
and most refined geometry, has traced with unerring precision
the boundaries of all its parts, and has prescribed to each
element and each power its respective sphere and limit.
Far different is the aspect of living Nature. The specta
cle here offered to our view is every where characterized by
boundless variety, by inscrutable complexity, by perpetual
mutation. Our attention is solicited to a vast multiplicity
of objects, curious and intricate in their mechanism, exhibit
ing peculiar movements, actuated by new and unknown
powers, and gifted with high and refined endowments. In
place of the simple combinations of elements, and the sim-

FINAL CAUSES. 23
pie properties of mineral bodies, all organic structures, even
the most minute, present exceedingly complicated arrange
ments, and a prolonged succession of phenomena, so varied
and so anomalous, as to be utterly irreducible to the known
laws which govern inanimate matter. Let us hasten, with
fresh ardour, to explore this new world that here opens to
our view.
Turning, then, from the examination of the passive ob
jects ofthe material world, we now direct our attention to
the busy theatre of animated existence, where scenes of won
der and enchantment are displayed in endless variety around
us ; where life in its ever-changing forms meets the eye in
every region to which our researches'can extend ; and where
every element and every clime is peopled by multitudinous
races of sensitive beings, who have received from the boun
teous hand of their Creator the gift of existence and the means
of enjoyment. Our curiosity is powerfully excited by pheno
mena in which our own welfare is so intimately concerned,
as are all those that relate to animal life ; and we cannot but
-take a lively and sympathetic interest in the history of be
ings in many respects so analogous to ourselves, like us pos
sessing powers of spontaneous action, impelled by passions
and desires, and endowed with capacities of enjoyment and
of suffering. Can there be a more gratifying spectacle than
to see an animal in the full vigour of health, and the free
exercise of its powers, disporting in its native element, re
velling in the bliss of existence, and testifying by its inces
sant gambols the exuberance of its joy?
We cannot take even a cursory survey ofthe host of living
beings profusely spread over every portion of the globe,
without a feeling of profound astonishment at the inconceiva
ble variety of forms and constructions to which animation
has been imparted by creative power. What can be more
calculated to excite our wonder than the diversity exhibited
among insects, all of which, amidst endless modifications of
shape, still preserve their conformity to one general plan of
construction'? The number of distinct species of insects

24 FINAL CAUSES.
already known and described cannot be estimated at less
than 100,000; and every day is adding to the catalogue.*
Of the comparatively large animals which live on land, how
splendid is the field of observation that lies open to the na
turalist ! What variety is conspicuous in the tribes of Quad
rupeds and of Reptiles ; and what endless diversity exists in
their habits, pursuits, and characters ! How extensive is the
study of Birds alone ; and how ingeniously, if we may so
express it, has nature interwoven in their construction every
possible variation compatible with an adherence to the same
general model of design, and the same ultimate reference to
the capacity for motion through the light element of air.
What profusion of being is displayed in the wide expanse of
the ocean, through which are scattered such various and such
unknown multitudes of animals ! Of Fishes alone the varie
ties, as to conformation and endowments are endless. Still
more curious and anomalous, both in their external form,
and their internal economy, are the numerous orders of
living beings that occupy the lower divisions of the animal
scale; some swimming in countless myriads near the sur
face; some dwelling in the inaccessible depths of the ocean :
some attached to shells, or other solid structures, the pro
ductions of their own bodies, and which, in process of time,
form, by their accumulation, enormous submarine moun
tains, rising often from unfathomable depths to the surface.
What sublime views of the magnificence of creation have
been disclosed by the microscope in the world of infinite
minuteness, peopled by countless multitudes of atomic be
ings which animate almost every fluid in nature ? Of these,
a vast variety of species has been discovered, each animal
cule being provided with appropriate organs, endowed with
spontaneous powers of motion, and giving unequivocal signs
of individual vitality. The recent observations of Profes-
* Four-fifths of the insects at present known have been discovered within
the last ninety years: for in 1743, Ray estimated the total number of species
at 20,000 only. See his work on " The Wisdom of God as manifested in the
Creation," p. 24.

FINAL CAUSES. 25
sor Ehrenberg have brought to light the existence of Mo
nads, which are not larger than the 24,000th part of an inch,
and which are so thickly crowded in the fluid as to leave
intervals not greater than theiriown diameter. Hence, he
has made the computation that each cubic line, which is
nearly the bulk of a single drop, contains 500,000,000 of
these monads, a number which equals that of all the human
beings existing on the surface of the earth.
Thus, if we review every region of the' globe, from' the'
seorchkig sands of the equator to the icy realms of the poles, .
or from the lofty mountain summits to the dark abysses of
the deep; if we penetrate into the shades of the forest, or
into the caverns and secret recesses of the' earth ; nay, if we
take up the minutest portion of stagnant water,- we stilt
meet with life in. some new and unexpected -form, yet
ever adapted to the circumstances of its situation. Where
ver Kfe can be sustained, we find' life produced, k woulrf
almost seem as if Nature* had been thus lavish and' sportive: -
in lier productions with the intent to demonstrate to Man
the fertility of her resource's, and the inexhaustible fund from'
which she has so prodigally drawn' forth the' means requisite
for the maintenance of all these diversified combinations,
for their repetition in endless perpetuity, arid for1 their Su*-
bordin&tion to one harmonious scheme of general' good.-
The vegetable world is no less prolific in wonders than
the animal. In this, as in att other' parts of creation, ample
scope is found for the exercise of the reasoning faculties ;
and at the same time abundant sources are supplied of intel
lectual enjoyment. To discriminate the different characters
of plants?, amidst the'infinite diversity of shape,-of colour, ahd
* In order to avoid' the too frequent, and consequently5 irreverent, intro
duction ofthe Great Name ofthe Supreme BEmo-into familiar discourse on
the operations of hispower, I have, throughout this Treatise,- followed the
common usage of employing the term Nature as a synonym, expressive of
the same power, but veiling from our feeble sight' the too dazzling splendour
<sf its glory.
Vol. I.— 4

26 FINAL CAUSES.
of structure, which they offer to our observation, is the la
borious, yet fascinating, occupation of the Botanist. Here,
also, we are lost in admiration at the never-ending variety
of forms successively displayed to view in the innumerable
species which compose this kingdom of nature, and at the
energy of that vegetative power, which, amidst such great
differences of situation, sustains the modified life of each in
dividual plant, and which continues its species in endless
perpetuity. Wherever circumstances are compatible with
vegetable existence, we there find plants arise. It is well
known that, in all places where vegetation has been esta
blished, the germs are so intermingled with the soil, that
whenever the earth is turned up, even from considerable
depths, and exposed to the air, plants are soon observed to
spring, as if they had been recently sown, in consequence of
the germination of seeds which had remained latent and in
active during the lapse of perhaps many centuries. Islands
formed by coral reefs, which have risen above the level ofthe
sea, become in a short time covered with verdure. From the
materials of the most steril rock, and even from the yet
recent cinders and lava of the volcano, Nature prepares the
way for vegetable existence.. The slightest crevice or ine
quality is sufficient to arrest the invisible germs that are al
ways floating in the air, and affords the means of sustenance
to diminutive races of lichens and mosses. These soon
overspread the surface, and are followed, in the course of a
few years, by successive tribes of plants ef gradually in
creasing- size and strength; till at length the island, or other
favoured- spot, is converted into a natural ahd luxuriant
garden, of which the productions rising from grasses to
shrubs and trees, present all the varieties of the fertile mea
dow, the tangled thicket,, and the widely spreading forest.
Even in the' desert plains of the- torrid zone, the eye of the
traveller is often refreshed by the appearance of a few hardy
plants, which find sufficient materials for their o-rowth in
these arid regions: and in the realms of perpetual snow
which surround the poles, the navigator is- occasionally

FINAL CAUSES. 27
startled at the prospect of fields of a scarlet hue, the result
of a wide expanse of microscopic vegetation.*
But whatever charms the naturalist may find in the oc
cupations in which he is engaged, and however wide may
be the field of his exertions, they still are insufficient to
satisfy the more enlarged curiosity of a philosophic mind.
The passive emotion of astonishment, in which inferior in
tellects are content to rest, serves but to awaken, in him
who has learned to think, a desire of farther knowledge.
Filled with an ardent spirit of inquiry, he cannot but be im
patient under the feeling that, while Nature has placed be
fore his eyes this splendid spectacle of animation, she has
thrown a dense veil over the interior machinery of life, and
has concealed from his view the springs by which she sets
it in motion. With the hope of discovering her proceed
ings, he hastens to explore the several parts which compose
the organized fabric, to examine in minute detail the anato
my of its structure, and to ascertain the nature of the seve
ral actions that take place within it. But overwhelmed by
the multiplicity of objeGts, and lost amidst the compli
cation of phenomena, he soon becomes dismayed by the
magnitude and arduous nature of the investigation. He
finds that his labours will be of no avail, unless, previously
to any attempt at theory, he takes a careful and accurate
account of all the circumstances attending the history and
conditions of life, from the dawn of its existence to its ap
pointed close. On tracing living beings to their origin, he
learns that every individual vegetable and animal takes its
rise from an atom of imperceptible minuteness, and gradu
ally increases in bulk by successive accretions of new matter,
derived from foreign sources, and by some refined, but un
known process, transmuted into its own substance. Then,
* The red show discovered in Baffin's Bay on the 17th of August, 1818,
during the Northern Expedition, under the command of Captain Ross, was
found to owe its colour to minute fungi, or microscopic mushrooms, which
vegetate on the surface of snow, as their natural abode. See Phil. Trans.
for 1820, p. 165.

/
/,

28 FINAL CAUSES.
following the progressive development of the organs, he ob
serves them undergoing various modifications, as they are
assuming new forms, which characterize certain definite
epochs in the general growth of the system. In a great
number of instances, especially among the lower orders of
animals, he witnesses the same individual being acting, in
its time, a variety of different parts ; often reappearing on
the stage of life with new organs, new faculties, and new
.conditions of existence, and undergoing metamorphoses as
^complete as any that have been depicted in the fables of
antiquity. The period al; iengtb arrives when the animal, having
.completed its growth, .attains the maturity of its being, and
acquires the full possession of itg powers. Every organ in
.succession has received its entire development, and has
united its energies with those which ha,d beeri before per
fected. Yet, however complete the arrangements that have
jthus been established, it (:s still necessary, in order to pre
serve the whole .system in a state in which it may be capa
ble of exercising ,the functions of life, fhat the materials
which compose its fabric shoujd undergo a certain, slow, but
constant renovation; and the same circle of actions and re
actions, which have brought it to its state of perfection, must
continue to be repeated, in order that a ,due proportion may
be maintained between the consumption and the supply of
these materials. In ,the course of a certain time, however,
even under the most favourable circumstances, this equili
brium begins to fail: the energies of the system decline,
and the processes of nutrition are insufficient to repair the
waste in the substance of the body. The fluids are dissi
pated faster than they can be renewed; the channels through
which they circulate are more and more obstructed, and at
length cease to be pervjous; and the solids gradually become
hard and rigid. As in a machine of which the wheels are
worn, and the springs have lost their elastic force, so in the
animal body, at an advanced age, the slightest additional
impediment that occurs will stop the movements of the

FINAL CAUSES. 29
whole system: and, when once stopped, their renewal is im-,
possible. Nature has thus assigned to every living being a
certain period as the utmost extent of its duration. Even
when exempt from external interference, all are doomed to
perish, sooner or later, by the slow but unerring operation
of the same internal causes which originally effected their
development and growth, and which are inseparably inter
woven with the conditions of their existence.
Numerous, however, are the extraneous and accidental
causes that may hasten or precipitate their destruction, long
before the period of natural decay. How striking is the
contrast, on those occasions, between the scene we have just
beheld of an animal in the full vigour of its powers, either
rapidly bounding across the plain, or gliding beneath the
wave, or soaring in the elevated regions of air, and the spec
tacle of the same animal lying, the next moment, extended!
at our feet, bereft at once of activity and of sense — of all the
faculties and powers that constitute life,. Can we contem
plate without amazement so complete and instantaneous a
change ; so sudden and awful a catastrophe'? Must we not
be animated by an eager desire to penetrate so great a mys
tery, and resolve the many questions which so striking a
phenomenon must naturally suggest? What, we are led to
ask, is the nature of this extraordinary revolution, extending
over the whole of that frame which had so long delighted
the eye by its beauty, and producing this sudden and irre
trievable extinction of the powers of life ? How comes it
that all those mighty energies' which the animal had so
lately displayed, and which had calfed forth our admiration, <
perhaps even excited our envy, are at once and for ever
annihilated ? What was the bond, thus suddenly dissevered,
which held together the various parts of that compound
frame? What potent spell has been dissolved, which could
retain in combination for so long a period the multifarious
elements of that exquisite organization ; and from the con
trol of which being now released, these elements hasten to
resume their wonted attractions, and entering into new

30 FINAL CAUSES.
forms of combination, are scattered into dust, or dissipated
in air, leaving no trace of their former union ? What me
chanism has been employed in its construction ? What re
fined chemistry has been exerted in assimilating new parti
cles of matter to those previously organized, and in appro
priating them to the nourishment of the parts with which
they become identified? By what transcendent power,
above all, did this assemblage of material particles first be
come animated by the breath of life; and from what elevated
source did they derive those higher energies, apparently so
foreign to their inherent properties, and investing these once
lifeless and inert materials with the exalted attributes of ac
tivity, /of sensation, of perception, of intelligence ? Shall
we ever comprehend the nature of this subtle and pervading
principle, by the agency of which all these wonderful phe
nomena of life are produced, and which combining into one
harmonious system so many heterogeneous and jarring ele
ments, has led to the formation of this exquisite frame, this
elaborate machine, this miraculous assemblage of faculties?
The discovery of a clew, if any such can be found, to the
mazes of this perplexing labyrinth can be hoped for only from
the successful cultivation of the science of physiology. But
before engaging in this arduous study, we ought previously .
to inquire into the methods of reasoning by which it is to be
conducted. The object of physiology is, by the diligent examination
of the phenomena of life, to ascertain the laws which re
gulate those phenomena, both as they apply to the indivi
dual beings endowed with life, and also as they relate to the
various assemblages that constitute the species, the genera,
the families, the orders, and the classes of those beings ; and,
lastly, as they concern the whole collective union of the or
ganized world.
These peculiar laws, which it is the province of physio
logy to investigate, are, as I have before observed, of two
kinds each founded upon relations of a different class. The
first, which depend upon the simple relation of cause and ef-

FINAL CAUSES. 31
feet, are concerned merely with the natural powers of mat
ter. They are the laws that regulate the succession of pheno
mena purely physical in all their stages. These phenome
na consist in changes among material particles, which are
either of a<mechanical or chemical nature; or in the affec
tions of imponderable physical agents, such as heat, light,
electricity, and magnetism ; and they, include also the pheno
mena that take place in organized bodies, and which are re-
ferrible to the operation of certain physical powers, apper
taining to particular structures, such as muscular contraction
and nervous irritation-; phenomena which, as we shall after
wards find, are not reducible to any of the former laws, but
are peculiar to the living, state. The second class of laws
comprise those which are founded on the relation of means
to an end; and which are usually denominated final causes.
They involve the operations of mind, in conjunction with
those of matter. They presuppose intention or design; a
supposition which implies intelligence, thought, motives, vo
lition, — particular purposes to be answered, requiring the
agency of powers and of instruments adapted to the produc
tion of the intended effects: — the knowledge of the proper
ties of matter, the selection and choice of particular means,
and the power of employing them in an effective manner.
These purposes may themselves be subservient to more ge
neral objects, and these objects again may be subordinate to
remoter ends; so that the whole shall comprehend a syste
matic plan of operations, conducive, on the most enlarged
views, to ultimate and general utility.
The study of these final causes is, in some measure, forced
upon our attention by even the most superficial survey of na
ture. It is impossible not to recognise the character of inten
tion, which is so indelibly impressed upon every part of the
structure both of vegetable and animal beings, and which
marks the whole series of phenomena connected with their
history. Microscopic observations teach us that the embryo
of an organic being contains, at a certain period of its forma
tion, the rudiments of the future vegetable or animal, struct

32 FINAL CAUSES.
ture, into which it is gradually transformed by the slow and
successive expansion and development of all its parts. The
subsequent processes of nutrition do nothing more than fill
up the outlines already sketched on the living canvass. Eve
ry organ, nay every fibre, resulting from this development,
contributes its share in the production of certain definite
effects, which we constantly witness taking place around us,
as well as experience in our own persons. But these effects,
though so familiar to us, are not on that account the less in
volved in mystery, or the less replete with wonder. To
say that they are the results of chance conveys no informa
tion; and is equivalent to the assertion that they are wholly
without a cause. Every one who is accustomed to reflect
upon the operations of his own mind, must feel that such a
conclusion is contrary to the constitution of human thought?
for if we are to reason at all, we can reason only upon the
principle that for every effect there must exist a correspond-'
ing cause; or, in other words, that there is an established
and invariable order of sequence among the changes which
take place in the universe.
Bat though it be granted that all the phenomena we be
hold are the effects of certain causes, it might still be alleged,-
as a bar to all farther reasoning, that these causes are not
only utterly unknown to us, but that their discovery is-
wholly beyond the reach of our faculties. The argument is
specious only because it is true in one particular sense, and
that a very limited one. Those who urge it, do not seem-
to be aware that its genera-1 application, irc that very same
sense, would shake the foundation of every kind of know
ledge, even that which we regard as built upon the most
solid1 basis. Of causation, it is agreed that we k-ftow no
thing: all that we do know is, thai; one event succeeds ano
ther' with undeviating constancy. Now, by probing this sub
ject to the bottom, we shall find that, in rigid strictness, we
have no certain knowledge of the existence of any thing,-
save that ofthe sensations and ideas which are actually pass-'
ing in our minds, and of which we are necessarily con--

FINAL CAUSES. 33
scious. Our belief in the existence of external objects, in
their undergoing certain changes, and in their possessing
certain physical properties, rests on a different foundation,
namely, the evidence of our senses ; for it is the result of in
ferences which the mind is, by the constitution of its frame,
necessarily led to form. We may trace to a similar origin
the persuasion, irresistibly forced upon us, that there exist
not only other material objects besides our own bodies, but
also other intellectual beings beside ourselves. We can
neither see5 ndV feel those extraneous intellects, any more
than we can see or feel the cause of gravitation, or the sub
tle sources of electricity or magnetism. We nevertheless
believe in the reality both of the one and of the other; but
it is only because* we' infer their existence from particular
train's of Impressions made upon our senses, of which im
pressions alone our knowledge' can, in meta'phystical strict
ness, be termed certain.
Upon what evidence do I conclude that I a'm not a soli
tary being in the universe ; that all is not centred in myself;
out that there exist other ir/teHe'cts similkr to" my own ? Un
doubtedly no' other than'the observation'that certain effects
are produced, which' the experience 'I have had ofthe ope
rations' of my own mind lead me, by an irresistible analogy,
to" ascribe to a similar agency, emanating from other beings ;
beings,- 'however, 'of whose' actual intellectual presence I can
not be conscious, whose nature I cannot fathom, whose es
sence" I cahno^ understand.' I cart judge of the operations
6f other' minds' only in as far as those operations accord with
what has pass"ed in my own. I cannot divine processes of
thought to which mine have borne rto resemblance, I can-
riot appreciate motives' of which I haVe never felt the in
fluence, nor comprehend the force of passions, never yet
awakened iri my breast meither carl I picture to myself feel
ings to which no sympathetic chord'within me has ever vi
brated. Our own intelligence, our own views, and our own affec
tions, then, furnish the only elements by which, it is possible
vol. i. — 5"

34 FINAL CAUSES.
for us to estimate the analogous powers and attributes of
other minds. The difficulty of applying this scale of mea
surement will, of course, increase in proportion to the dif
ference between the objects compared; and although we may
conceive that there are powers and intelligences infinitely
surpassing our own, the conceptions we can form of such
superior essences must necessarily be indefinite and obscure,
and must partake of the same kind of imperfection as our
notions of the distances of the heavenly bodies, however
familiar we may be with the units of the scale by which
those distances are capable of being expressed. When, on
the other hand, the objects contemplated are more within
the range of our mental vision ; when, for instance, they are
phenomena that we can assimilate to our own voluntary
acts, and in which we can clearly trace the connexion be
tween means and end, then does our recognition of the
agency of intellect become most distinct, and our conviction
of its real and independent existence become most intimate
and assured.
Such is the kind of evidence on which rests our belief of
the existence of our fellow men. Such, also', is the founda
tion of our assurance that there exists a mighty Intellect,
who has planned and executed the stupendous works of cre
ation, with a skill surpassing our utmost conceptions; by
powers to which-we can assign no limit, and the object of
whose will is universal good.*
It will argue no undue presumption, therefore, if, in our
earnest endeavours to form just ideas of the attributes of the
Deity from the examination of nature, we* are led to insti
tute comparisons between His works and those of man; and
strive to gather some faint notions of the divine intelligence
by applying the only standard of admeasurement which we
possess, and are permitted to employ, namely, that derived
from the operationsof human intellect. Our interpretations
of the designs of the Creator must here be obtained through
* The view here taken1 is, of course, limited to Natural Theology,- that
being .the express and exclusive object of these Treatises..

FINAL CAUSES. 35
the medium of human views; and our judgment of his bene
volence can be formed only by reference to our own affec
tions, and by their accordance with those fervent aspirations
after good, which the Author of our being has deeply inter
woven with our frame.
The evidence of design and contrivance in the works of
nature carries with it the greatest force whenever we can
trace a coincidence between them and the products .of hu
man art. If in any unknown region of the earth we chanced
,to discover a piece of machinery, of which the purpose was
manifest, we should not fail to ascribe it to the workman
ship of some mechanist, possessed of intelligence, actuated
by a motive, and guided by intention. Farther, if we had
a previous experience of the operation of similar kinds of
mechanism, we could not doubt that the effect we saw pro
duced was the one intended by the artificer. Thus, if in
an unexplored country, we saw, moving upon the waters of
a lake, the trunk of a tree, carved into the shape of a boat,
we should immediately conclude that this form had been
given to it for the purpose of enabling it to float. If we
found it also provided with paddles at its sides, we should
infer, from our previous knowledge of the effects of such in
struments, that they were intended to give motion to this
boat, and we should not hesitate to conclude that the whole
was the work of human hands, and the product of human
intelligence and design. If, in addition, we found this boat
furnished with a rudder and with sails, we should at once
understand the object of these contrivances, and our ideas
of the skill of the artificer would rise in proportion to the
excellence of the apparatus, and the ingenuity displayed in
its adaptation to circumstances.
Let us suppose that in another part of this lake we found
an insect,* shaped like the boat, and moving through the
water by successive impulses given to that medium by the
action of levers, extending from its sides, and shaped like
paddles, having the same kind of movement, and producing
* Such as the Notonecla glauca, Lin., or water boatman, and the Dylisaus
marginaKs, or water beetle.

36 FINAL CAUSES.
the same effects. .Could we resist the persuasion that the
Artificer of this insect, when forming it of this shape, and
providing it with these paddles, had the same mechanical
objects in view ? Shall we not be confirmed in this idea
on finding that these paddles are constructed with joints,
which admit of no other motion than that of striking against
the water, and of thus urging forwards the animal in its pas
sage through that dense and resisting medium? Many
aquatic animals are furnished with tails which evidently act
as rudders, directing the course of their progressive motion
through the fluid. Who can doubt but that the same inten
tion and the same mechanical principles which guide the
practice of the ship-builder, are here applied in a manner
still more refined, and with a master's hand? If nature has
furnished the nautilus with an expansible membrane, which
the animal is able to spread before the bregze, when propi
tious, and by means of which it is wafted along the surface
of the sea, but which it quickly retracts in unfavourable cir
cumstances, is not her design' similar to that of the human
artificer, when he equips his bark with sails, and provides
the requisite machinery for their being hoisted or furled with
ease and expedition?
The maker of an hydraulic engine places valves in par
ticular parts of its pipes and cisterns, with a view to pre
vent the retrograde motion of the fluids which are to pass
through them. Can the valves of the veins, or of the lym
phatics, or of the heart have a different object; and are they
not the result of deliberate and express contrivance in the
great Mechanist ofthe living frame?
The knowledge of the laws of electricity, in its different
forms, is one of the latest results which science has revealed
to man. Could these laws, and their various combinations,
have been unknown to the Power who created the torpedo,
and who armed, it with an energetic galvanic battery, con
structed upon the most refined scientific prinpjples, for the
manifest purpose of enabling the animal to strike terror into
its enemies, and paralyze their efforts to assail it.
Does not the optician, who designedly places his convex

FINAL CAUSES. 37
lens at the proper distance in a darkened box, for the pur
pose of obtaining vivid pictures of the external scene, evince
his knowledge of the laws of light, of the properties of re
fracting media, and of the refined combinations of those me
dia by which each pencil is brought to a separate focus, and
adjusted to form' an image of remote objects ? Does it not,
in like manner, argue the most profound knowledge and
foresight in the divine Artist, who has so admirably hung
the crystalline lens of the eye in the axis of a spherical
case, in the forepart of whioh He has made a circular win
dow for the light to enter, and spread out on the opposite
side a canvass to receive the picture ? Has no thought been
exercised in darkening the walls of this camera obscura, and
thus, preventing all reflection of the scattered rays, which
might interfere with the distinctness of the image ?
But we farther observe in the eye many exquisite refine
ments of construction, by which various defects, unavoida
ble in all optical instruments of human workmanship, are
remedied. Of this nature are those which render the organ
achromatic, which correct the spherical aberration, and
which provide for the adjustment of its refracting powers to
the different distances of the objects viewedj not to speak
of all the external apparatus for the protection, the preser
vation, and the movements of the eye-ball, and for contri
buting in every way to the proper performance of its office.
Are not all these irrefragable proofs of the continuity of the
same design ; and are they not calculated still farther to ex
alt our ideas of the Divine Intelligence, of the elaborate
perfection impressed upon His works ; and of the compre:
hensive views of His providence ?
These facts, if they stood alone, would be sufficient to,
lead us irresistibly to this conclusion : but evidence of a si
milar kind may be collected in abundance from every part
of living nature to which our attention can be directed, or
to which our observations have extended. The truths they
teach not only acquire confirmation by the corroborating
tendency of each additional fact of the same description, but
the multitude of these facts is so great, that the general con-

38 FINAL CAUSES.
elusion to which they lead must be considered as indubita
ble. For the argument, as it has been justly remarked, is
cumulative ; that obtained from one source being strength
ened by that derived from another ; and all tending to the
same conclusion, like rays converging to the same point,
on which they concentrate their united powers of illumina
tion. The more we extend our knowledge of the operations of
creative power, as manifested in the structure and economy
of organized beings, the better we become qualified to ap
preciate the intentions with which the several arrangements
and constructions have been devised, the art with which they
have been accomplished, and the grand comprehensive plan
of which they form a part. By knowing the general ten
dencies of analogous formations, we can sometimes recog
nise designs that are but faintly indicated, and trace the links
which connect them with more general laws. By render
ing ourselves familiar with the handwriting where the cha?
racters are clearly legible, we gradually learn to decipher
the more obscure passages, and are enabled to follow the
continuity of the narrative through chapters which would
otherwise appear mutijated and defaced. Hence, the utili
ty of comprehending in our studies the whole range of the
organized creation, with a view to the discovery of finaj
causes, and obtaining adequate ideas of the power, the wis
dom, and the goodness of God.

( 3» )

CHAPTER II.
THE FUNCTIONS OF LIFE.
The intentions of the Deity in the creation of the animal
kingdom, as far as we are competent to discern or compre
hend them, are referrible to the following classes of objects*
The first relates to the individual welfare of the animal, em
bracing the whole sphere of its sensitive existence, and the
means of maintaining the vitality Upon which that existence
is dependent. The second comprises the provisions which
have been made for repairing the chasms resulting, in the
present circumstances of the globe, from the continual de->-
structibn of life, by ensuring the multiplication ofthe species,
and the continuity of the race to which each ammal'belongs.
The third includes all those arrangements which have been
resorted to, in order to accommodate the' system to the
consequences that fdlldw from an indefinite increase in the
numbers of each species. The' fourth class relates tb that-
systematic economy in the plans* of organization* by Which'
all the former objects are most' effectually secured. Fshalli
offer some observations on each of. these general heads of
inquiry. With reference to the welfare of the individual animal,.
it is evident that in the" brute creation, the great end to be
answered is the attainment of sensitive enjoyment. To this-
all the arrangements of the system, and all the energies of
its vital powers must ultimately tend. Of what value would
be mere vegetative life to the being in whom it resides, un
less it were accompanied by the faculty of sensation, and
unless the sensations' thence arising were attended with plea
sure? It is only by reasoning analogically from the feelings*

40 THE FUNCTIONS OF LIFE.
we have ourselves experienced that we ascribe similar feel
ings to other sentient beings, and that we infer their existence
from the phenomena which they present. Wherever these
indications of feeling are most distinct, we find that they
result from a particular organization, and from the affec
tions of a peculiar part of that organization denominated
the nervous substance. The name of brain is given to a
particular mass of this substance placed in the interior of
the body, where it is carefully protected from injury.
The sensations', for exciting which th'e brain is the mate
rial, instrument, or immediate organ, are the result of cer
tain impressions made on particular parts of the body, and
conveyed to' that organ by the medium of filaments',- com
posed' of a" Similar substance, and termed nerves. In this
way, then, it has been provided that a' communication shall
be established between the sentient principle and the ex
ternal objects, by which its activity is to be' excited, and on
which it is to be dependent for" the: elements of all its affec
tions, both of sensation and' of intellect. A considerable
portion of this treatise' will be occupied with the develop
ment of the series of means by which impressions from ex
ternal objects are made on the appropriate organs that are
provided to receive and collect them, so as not only to give
rise to Varied sensations, but also to convey a' knowledge of
the Cxisteriee! and different qualities^ of the objects which
produce them. This lattet faculty is termed Perception.
But in the formation'of animals it was not the intention
of Providence to endow them with the mere capacity of
being affected by surrounding objects, and of deriving from
them various sensations of pleasure' and of pain, without
granting them the power of controlling these' effects, and of
acting on those' objects in return.' The faculties of sensation
and perception,' in beings destined to be merely passive,
and the sport of every contingent agency, would have been
riot merely useless, but even baneful endowments.' The same
beneficent power which has conferred these gifts has con
joined that of voluntary motion, by which the animal may
not only obtain possession of such objects as minister to its

THE FUNCTIONS OF LIFE. 41
gratification, and reject those which are useless or hurtful,
but may also move from place to place, and enlarge the
sphere of its perceptions and of its power. The same mass
of nervous substance which, under the name of brain, we
have recognised as the organ of sensation, is also, as will af
terwards be shown, the organ of volition; and the medium,
by which the commands of the will are transmitted from
.the brain to the mechanical apparatus employed for motion,
is again certain filaments of nerves ; but these nervous fila
ments are distinct from those which are subservient to sen
sation. Next in importance, then, to the organs of sensation and
perception, are those of Voluntary Motion. They com
prise two kinds of objects; first, the establishment of a cer
tain mechanism having the cohesion, the strength, and the
mobility requisite for the different actions which the animal
is to perform; and, secondly, the provision of a power, or
agent, which shall be capable of supplying the mechanical
force for setting this machinery in motion. With these ob
jects must be combined various subsidiary arrangements re
lating to the connexions, the support, the protection, and
other mechanical conditions of the organs ofthe body. It will
be convenient to comprehend these under one general head,
considering them as composing the Mechanical Functions
of the animal economy. They Will engage a considerable
share of our attention in this work, as affording the clearest
and most palpable proofs of contrivance and design.
From the peculiar conditions of the living body, not only
with regard to the mechanical properties of its various parts,
and the powers by which their movements are effected, but
also with regard to the chemical laws which regulate the
combinations of elements composing the substance of the
body, there is required, as will be more fully explained in
the sequel, a continual renovation of that substance. For
this purpose new materials are perpetually wanted, and must
be as regularly supplied. Hence arises a new class of func
tions, comprising a great extent of operations, opening a wide
VOL. i.— 6

42 THE FUNCTIONS OF LIFE.
field of curious and interesting inquiry, and furnishing abun
dant evidence of the wise and beneficent operations of na
ture. These may be comprehended under a separate class
bearing the general title of Nutritive Functions. They are
often, also, spoken of under the designation of the Vital
Functions, from their more immediate relation to the con
tinuance of vitalily; that is, of mere vegetative life, as dis
tinguished from the exercise of the higher faculties of sen
sation, perception, and voluntary motion, which are the ul
timate ends of animal existence, and which are emphatical
ly termed the Animal Functions.
The vital as well as the animal functions require for the
execution of their various objects certain instruments of an
appropriate mechanical construction, adapted to those ob
jects. To the contrivances of the mechanist must be added
a refined hydraulic apparatus for the conveyance of fluids,
and for the regulation of their .movements; and with these
must be conjoined the skilful combinations of the laborato
ry, by which the powers of the most subtle chemistry are
exercised in effecting all the transmutations required by this
elaborate system of operations. As far as they involve me
chanical principles, these objects again arrange themselves
under the mechanical functions; and I shall accordingly in
clude them under that head, when giving an account of this
branch ofthe subject.
There is another, and a most important consequence flow
ing from the peculiar chemical conditions of the materials
of which animal structures are composed. The mode in which
their elements are combined is so .complex as to require a
long and elaborate process to accomplish that combination;
and neither the organs with which animals are furnished, nor
the powers with which those organs are endowed, are ade
quate to the conversion of the materials furnished by the
inorganic world into the substances required for the con
struction of their bodies, and the maintenance of their
powers. These inorganic elements must have passed
through the intermediate stages of combination; and must
have been previously elaborated by other organized be-

THE FUNCTIONS OF LIFE. . 43
ings. This important office is consigned to the vegetable
kingdom. Receiving the simple food furnished by nature,
which consists chiefly of water, air, and carbonic acid, to
gether with a small proportion of other substances, plants
convert these aliments into products, which not only main
tain their own vitality, but serve the farther purpose of sup
porting the life of animals. Thus was the creation and
continuance of the vegetable kingdom a necessary step to
wards the existence of the animal world ; as well as a link
in the great chain of being, formed and sustained by Al
mighty power. The physiology of Vegetables presents
many topics of great interest with relation to final causes,
and will in this Treatise be reviewed with special reference
to this important object.
Nutrition, both in the vegetable and animal systems, com
prises a very extended series of operations. In the former
it includes the absorption of the crude materials from the
surrounding elements, — their transmission to organs where
they are aerated, that is, subjected to the chemical action of
the air ; — their circulation in the different parts of the plant,
— their farther elaboration in particular vessels and recep
tacles — their deposition of solid materials — and their con
version into peculiar products, as well as into the substances
which compose the several organs; — and, finally, the growth
and development of the whole plant. Still more various
and complicated are the corresponding functions in animals.
Their objects may be arranged under the following general
heads; each, again, admitting of farther subdivision. The
first end to be accomplished is to animalize the food ; that
is, to convert it into a matter having the chemical properties
of the animal substances with which it is to be afterwards
incorporated. The entire change thus effected is termed
Assimilation, of which Digestion forms a principal part.
The second object is to collect and distribute this prepared
nutriment, which is the blood, to the different organs, or
wherever it may be wanted. The necessary motions for
these purposes are given to the blood by the organs of Cir-
culationf consisting of the Heart, which impels it through

44 THE FUNCTIONS OF LIFE.
a system of pipes called Arteries, and receives it back again
by means of another set of tubes called Feins. In the third
place it is necessary that the circulating blood should con
tinually undergo purification by the chemical action of oxy
gen: a purpose which is answered by the function of Res
piration. The fourth stage of nutrition relates to the more
immediate application of this purified material to the Wants
of the system, to the extension of the organs, to the repara
tion of their losses, and to the restoration of their exhausted
powers. *
Life, then, consists of a continued series of actions and re
actions, ever varying, yet constantly tending to definite ends.
Most of the parts of which the body consists undergo con
tinual and progressive changes in their dimensions, figure,
arrangement, and composition. The materials which have
been united together and fashioned into the several organs,
are themselves successively removed and replaced by others,
which again are, in their turn, discarded, and new materials
substituted, though without any perceptible change of ex
ternal form. Perpetual mutation appears to constitute the
fundamental law of living nature; and it has been farther
decreed by the power which gave the first impulse of ani
mation to this organized fabric, that its movements and its
powers shall be limited in their duration, and that, even
when they are not destroyed by extraneous causes, after
continuing for a certain period, they shall come to a close.
The law of Mortality, to which all the beings that have re
ceived the gift of life are subjected, is a necessary conse
quence of the law of mutation; and the same causes that
originally effected the development and growth of the sys
tem, and maintained it in the vigour of its maturity, by con
tinuing to operate, are certain to lead to the demolition of
the fabric they had raised, and to the exhaustion and final
extinction of its powers. The individual dies; but it is only
to give place to other beings, alike in nature and in form,
equally partaking ofthe blessings of existence, and destined,
after having, in their turn, given rise to a new race of sue-

THE FUNCTIONS OF LIFE. 45
cessors, to run through the same perpetual cycle of changes
and renovations.
Thus the continuance and multiplication of each species
may be assigned as the second of the great ends which are
to be accomplished in the system of living nature. A por
tion of the vital power of the parent is for this purpose em
ployed to give origin and birth to the offspring. The pro
cess itself, by which the germs of living beings originate, is-
veiled in the most impenetrable mystery. But we are per
mitted to trace many of the subsequent steps in the gradual
development both of vegetable and animal organizations;
and certainly no part ofthe economy of animated nature is
more calculated to impress us with exalted ideas of the im
mensity of the scheme of Providence, and the vigilant care
with which the most distant consequences have been antici
pated, than the history of the early periods of their existence:
Nothing can be more admirable than the progressive archi
tecture of the frame; nothing more beautiful than the setting
up of temporary structures, which are required only at an
early stage of growth, and which are afterwards removed to-
give place to more permanent and finished organs.
The utmost solicitude has been shown in every part of
living nature to secure the perpetuity of the race, by the
establishment of laws, of which the operation is certain in-
all contingent circumstances. It has also been manifestly
the object of various provisions to diffuse the races as widely
as possible over a great surface of the habitable globe.
We are next to- advert to the important consequences'
which, in the animal kingdom more especially, flow from
this law of indefinite production. As animals are ultimately
dependent on the vegetable kingdom for the materials of
their subsistence, and as the quantity of these materials is, irr
a state of nature, necessarily limited by the extent of surface
over which vegetation is spread, a time must arrive when
the number of animals thus continually increasing is exactly
such as the amount of food produced by the earth will main
tain. When this limit has been attained, no farther iiv

46 THE FUNCTIONS OF LIFE.
crease can take place in their number, except by resorting
to the expedient which we find actually adopted, namely,
thatof employing the substance of one animal for the nourish
ment of others. Thus the identical combinations of ele
ments, effected by the powers of vegetation, are transferred
in succession from one living being to another, and be
come subservient to the maintenance of a great number of
different animals before they finally, by the process of de
composition, revert to their original inorganic Etate.
" See dying vegetables life sustain,
See life dissolving vegetate again;
All forms that perish other forms supply.
By turns we catch the vital breath, and die." — Pope.
Hence has the ordinance been issued to a large portion of
the animal world that they are to maintain themselves by
preying upon other animals, either consuming their substance
when already dead, or depriving them of life in order to pro
long their own. Such is the command given to the count
less hosts of living beings which people the vast expanse of
ocean; to the unnumbered tribes of insects which every spot
of earth discloses ; to the greater number of the feathered
race; and also to a more restricted order of terrestrial ani
mals. To many has the commission been given to ravage
and to slaughter by open violence; others are taught more
insidious, though no less certain arts of destruction; and some
appear to be created chiefly for the purpose of quickly clear
ing the earth of all decomposing animal or vegetable mate
rials, which might otherwise have filled the air with noxious
exhalations and contaminated the sources of vitality.*
This new law of animal existence must necessarily intro
duce new conditions of organization and of functions. Struc
tures adapted to rapid locomotion must be supplied for the
* As specially appointed for the performance of this useful task may be
cited, among the larger beasts of prey, the hyena, the jackal, the crow, and
the vulture: among marine animals, the Crustacea, and numerous mollusca)
and among the lower orders, innumerable tribes of insects, such as ants,
flesh flies, fee.

THE FUNCTIONS OF LIFE. 47
pursuit of prey, and powerful weapons for attack and de
struction. But nature has not left the weaker animals un
provided with the means of repulse, of defence, or of escape.
For these purposes various expedients, either of force, of
swiftness, or of stratagem, have been resorted to in different
cases. That a large portion of evil is the direct consequence of
this system of extensive warfare, it is in vain to deny. But
although our sensibility may revolt at the wide scene of
carnage which is so generally presented to our view, our
more sober judgment should place in the other scale the great
preponderating amount of gratification which is also its re
sult. We must take into account the vast accession that
accrues to the mass of animal enjoyment from the exercise
of those powers and faculties which are called forth by this
state of constant activity; and when this consideration is
combined, as it ought to be, with that of the immense multi
plication of life which is admissible upon this system alone,
we shall find ample reason for acknowledging the wisdom
and the benevolent intentions of the Creator, who, for the
sake of a vastly superior good, has permitted the existence
of a minor evil.
From this system of hostilities there must also arise new
relations among the different races of animals. It affords a
ready and effectual means of preserving the proper balance
between different races. Each separate species of animals,
far from being isolated arid independent, performs the part
assigned to it in the system of nature, and, however appa
rently insignificant, may have a sensible influence on the
rest of the animal creation. Man, above all other animals,
has effected a most important change in the condition of the
multitude of other races, in every region where his numbers
have multiplied, where the arts of civilization have enlarged
his dominion, and where science has armed him with still
more extensive power.
In every department of nature it cannot fail to strike us
that boundless variety is a characteristic and predominant
feature of her productions. It is only when the object to

48 THE FUNCTIONS OF LIFE.
be attained is dependent upon certain definite conditions, ex
cluding the possibility of modification, that these conditions
are uniformly and strictly adhered to. But wherever that
absolute necessity does not exist, and there is afforded scope
for deviation, there we are certain to find introduced all
those modifications which the occasion admits of. Not only
is this tendency to variety exemplified in the general ap
pearance and form of the body, but it also prevails in each
individual organ, however minute and insignificant that or
gan may seem. Even when the purpose to be answered is
identical, the means which are employed are infinitely di
versified in different instances, as if a design had existed of
, displaying to the astonished eyes of mortals the unbounded
resources of creative power. While the elements of struc
ture are the same, there is presented to us in succession
every possible combination of organs, as if it had been the
object to exhaust all the admissible permutations in the order
of their union.
Some wise purpose, though dimly perceptible to our im
perfect understandings, is no doubt answered by this great
law of organic formation, the Idw of variety. That it is not
blindly or indiscriminately followed, is apparent from its
being circumscribed within certain limits, and controlled by
another law, which we have next to consider — that of con
formity lo a definite type.
The most superficial survey of nature is sufficient to show
that there prevail certain general resemblances among great
multitudes of species, which lead us to class them into more
or less comprehensive groups. Thus in the animal kingdom,
quadrupeds, birds, fishes, reptiles, shell-fish, and insects,
compose natural assemblages or classes, and each of these
is readily divisible into subordinate groups or families. Now
it results from a closer examination of the structure and
economy of plants and animals, that the formation of all the
individual species comprehended in the same class, has been
conducted in conformity with a certain ideal model, or type,
as it is called. Of this general type all the existing forms
appear as so many separate copies, differing, indeed, as to

THfc FUNCTIONS OF LIFE'. 49
particulars, but agreeing as to general characters. Thesame
observation applies to the families, the genera, and other
subordinate groups of living beings.
The more extensive our acquaintance iswith the anatomy
ahd physiology of both plants and animals, the more striking
do these analogies appear; so that amidst endless diversity
in the details of structures and of processes, the same general
purpose is usually accomplished by similar organs and in
similar modes. So firmly is this principle established, that
we may venture with confidence to predict many circum
stances relating to an unknown animal, of which only a few
fragments are presented to us, from our general knowledge
of the characters and economy of the tribe or family, on the
type of which it has been modelled. Thus, the discovery
of a mutilated portion of the skeleton of a fossil animal, gives
to the physiologist, who is conversant with the details of
comparative anatomy, a knowledge of the general structure
and habits of that animal, though all other traces of its exist
ence may have been swept away, amidst the primeval revo
lutions of the globe.*
Not only does this tendency to conform to particular types
obtain in all organic formations, but farther inquiry leads to
the conclusion thatthe deviations from these standard forms,
far from being arbitrary, ai'e themselves referrible to defi
nite laws. The regulating principle of the variations is
subordinate to higher views, and has reference to the respec
tive objects and destination of each particular species in the
general system of created beings. Nature, as far as we can
discern, appears, in conformity with these intentions, first
to have laid down certain great plans of functions to which
she has adapted the structure of the organs ; the minor ob
jects and more subordinate functions being accommodated
to this general design. Hence arises the necessary and re
ciprocal dependence of each organ and of each function on
* See Cuvier's " Discours sur les Revolutions de la Surface du Globe," p*
47, prefixed to the first volume of his " Ossemens Fossiles."
VOL. I. — 7

50 THE FUNCTIONS OF LIFE.
every other ; and hence are deduced what have been termed
the laws of the co-existence of organic forms. By attention
to these laws we may often explain how each variation that
\ is observed in any one organ, common to a natural group
of animals, entails certain necessary and corresponding va
riations in other parts, and extends its influence in modify
ing, in a greater or less degree, the whole fabric. It is in
comparative anatomy as in mechanics, where any alteration
made in the position of one part of a system of bodies occa
sions a change in the centres of gravity, of gyration, and of
oscillation; and evolves new mechanical forces and condi
tions of equilibrium, which render new adjustments in other
parts necessary, in order to restore the equipoise, and pre
serve the harmony of their movements.
We may conclude from these inquiries that the numerous
classes or assemblages of beings, which science has formed,
are by no means arbitrary creations of the human mind, in
vented merely with a view to facilitate the study and to
recognise the identity of species, or calculated only|to sup
ply the imperfections of our memory; but that they have a
real foundation in nature. To regard any of the beings in
the creation as isolated from the rest, would be to take a
very narrow and a false view of their condition; for all are
connected by mutual relations. Even among the leading
types which represent the great divisions of the animal king
dom we may trace several points of resemblance, which show
them to be parts of one general plan, and to have emanated
from the same Creator. In the progress of discovery we
are continually meeting with species which occupy interme
diate places between adjacent types, and appear as links of
connexion in the chain of being.' It often happens, as I
shall hereafter have occasion to point out, that throughout
an extensive series of organic forms, the steps of gradation
by which one type passes into another, are so numerous and
so regular, as to preclude the possibility of drawing a de
cided line of demarcation between those that properly ap
pertain to each other.

THE FUNCTIONS OF LIFE. 51
All these apparent anomalies and gradations of structure
tend still farther to demonstrate the generality of the plans
of nature, and the comprehensiveness of her design, which
embraces the whole series of animated beings. These views
are strongly corroborated by the discoveries that are con
tinually being made of species now no longer in existence,
but which, in former ages of the world, helped to fill up
many of the chasms which now interrupt the continuity of
that series. This knowledge has been revealed to us by the
examination of their fossil remains, those monuments of
former epochs, which have thrown such important light on
the most interesting questions in Geology as well as in Phy
siology. The notion has long prevailed that the beings composing
the vegetable and animal kingdoms, might, if we were
thoroughly acquainted with their structure and economy, be
arranged in a linear series, commencing with the simplest,
and regularly ascending to the most refined and complicated
organizations, till it reached its highest point in man, who is
unquestionably placed at the summit of the scale. Bonnet,
in particular, cherished with enthusiastic ardour the hypo
thesis that all organic beings formed a continuous gradation,
each member of which, like the successive links of a chain,
was connected with that which preceded, and with that
which followed it; and he pursued this idea by applying it
even to the productions of the mineral world. But, divest
ing ourselves of these hypothetical views and^ figurative
images, we find, on sober observation, that instead of one
continuous series, we are presented with only detached frag
ments and interrupted portions of this imaginary system: so
that, if, for the sake of illustration, we must employ a me
taphor, the natural distribution of animals would appear to
be represented, not by a chain, but by complicated net
work, where several parallel series are joined by transverse
and oblique lines of connexion. A multitude of facts, how
ever, tend to show that the real types or models of struc
ture, are more correctly represented by circular or recurring

53 THE FUNCTIONS OF LIFE.
arrangements.* But as the discussion of these and other
topics relating to the plans and designs of nature in the for
mation of organic beings requires a previous acquaintance
with the details of comparative anatomy and physiology, I
shall defer all farther observations respecting them till I
have finished the review I propose to take of the several
structures and functions of the animal and vegetable econo-
my. There are, however, some views that have been en
tertained respecting the procedure of nature in the formation
of the different races of animals, which it will be proper to
notice in this place, as they will occasionally be referred to
when the facts that more particularly illustrate and support
them come to be noticed. i
An hypothesis has been advanced that the original crea
tion of species has been successive, and took place in the or
der of their relative complexity of structure; that the stan
dard types have arisen the one from the other; that each
succeeding form was an improvement upon the preceding,
and followed in a certain order of development, according
to a regular plan traced by the great Author of the universe
for bestowing perfection on his works. This gradation of
structure was necessarily accompanied by a gradation of fa
culties: the object of each change of type being to attain
higher objects, and to advance a farther step towards the ul
timate ends of the animal creation. Many apparent anoma
lies, which are inexplicable upon any other supposition, are
easily reconcileable to this theory. The developments of
structure belonging to a particular type being always pro
spective, are not completed in the inferior orders of the
group formed upon that model, but remain more or less
imperfect, although each organ always fully answers the par
ticular purpose of the individual animal. But it sometimes
happens that the imperfection of an organ is so great, in con
sequence of its development having proceeded to a very small
* Mr. M'Leay is the author of this ingenious theory, which he has de
veloped in his " Horse Entomologies:," and which appears to be verified to,
a great extent by the modern discoveries in comparative anatomy.

THE FUNCTIONS OF LIFE. 53
extent, as to render it wholly useless in that particular spe
cies, although in a higher race of animals it fully performs
its proper function. Thus we shall find that rudiments of
feet are contained within the bodies of various kinds of ser
pents, which can obviously not be serviceable as organs of
progression. In the young of the whale, before its birth,
there is found in the lower jaw, a row of small teeth, which
do not rise above the gums, and can, therefore, be of no use
as instruments of mastication. Their farther growth is ar
rested, and they are afterwards obliterated. This imperfect
or rudimental condition of an organ indicates its relation to
other species belonging to the same type, and demonstrates
the existence of a general plan in their formation. I shall
have occasion to mention several striking instances of this
kind, both in the animal and vegetable kingdom.
In following the transitions from one model" of structure
to another, we often observe that a particular organ has been
very greatly enlarged, or otherwise modified to suit some
particular purpose, foreign to its usual destination, or to qua
lify it for performing some new office, rendered necessary
by the particular circumstances in which the animal is
placed. Thus, the ribs, which in quadrupeds are usually
employed for respiration, are in serpents converted into aux
iliary organs of progressive motion: and in the Draco vo-
lans, or flying lizard, they are extended outwards from the
sides to serve as wings. The teeth, usually intended for
mastication, are in many animals enlarged in order to con
stitute weapons of offence, as in the Elephant, the Boar, the
Narwhal, and the Pristis. In like manner, in the Crustacea,/
organs of the same general structure are converted some
times into jaws, sometimes into feelers, (or palpi,) and some
times into feet; and the transition from the one to the other
is so gradual that it is difficult to draw a proper distinction
between them.
In pursuing the ascending series of animal structures we
meet also with instances of a contrary change, yet still re
sulting from the continued application of the same principle.

54 THE FUNCTION^ OF LIFE.
An organ which has served an important purpose in one
animal, may be of less use in another, occupying a higher
¦station in the scale, and the change of circumstances may
•even render it wholly useless. In such cases we find that
at is gradually discarded from the system, becoming conti
nually smaller, till it disappears altogether. We may often,
however, perceive some traces of its existence, but only in
a rudimental state, and as if ready to be developed, when
the occasion may demand it.
In the greater number of organic structures we may trace
a tendency to the repetition of certain organs, or parts, and
the regular arrangement of these similar portions either
round a central axis, or in a longitudinal series. The for
mer is apparent in the verticillated organs of plants, and in
the radiated forms of zoophytes. The linear arrangement
is exhibited in the similar segments of annulose and other
articulated animals, and also in the pieces which compose
the spinal column of vertebrated animals. In these two
latter classes, also, a remarkable law of symmetry obtains
in the formation of the two sides of the body, which exhi
bits the lateral junction of similar but reversed structures.
The violations of this law are extremely rare; yet some re
markable instances of anomalous formations, in this respect,
will hereafter be noticed.
In treating of the particular functions of the animal and
vegetable economy I shall follow a different order from that
in which I have presented them in the preceding sketch.
As the Mechanical functions depend upon the simpler pro
perties of matter and the well known laws of Mechanism, I
think it best to commence with the examination of these.
Our attention will next be directed to the highly interesting
subjects which relate to the Nutritive or Vital functions
both of vegetable and animal structures: for as they involve
the chemical properties of organized substances, and are,
therefore, of a more refined and intricate nature than the
preceding, I conceive they will be best understood after the
general mechanism of the frame has been explained. These

THE FUNCTIONS OF LIFE. 55-
studies will prepare us for the consideration of living animals
as sentient and active beings, endowed by their bounteous
Creator with the exalted faculties of perception and of vo
lition, which alone give value to existence, and which raise
them so far above the level of the vegetable world. I shall
lastly give a very brief account of the reproductive func
tions and of the phenomena of animal development, in which
the discoveries of modern times have revealed to us so con
siderable a portion of those extensive plans which an all- wise
Providence has beneficently devised for the general welfare
of animated beings, '

PART I.
THE MECHANICAL FUNCTIONS,

CHAPTER I.
ORGANIC MECHANISM.
§ 1. Organization in General.
Life, which consists of a continued series of actions, di«
rected to particular purposes, cannot be carried on but by
the instrumentality of those peculiar and elaborate structures-
and combinations of material particles which constitute or
ganization. All these arrangements, both as respects the
mechanical configuration and the chemical constitution of
the elements of which the organized body is composed,
even when apparently most simple, are, in reality, complex
and artificial in the highest possible degree. Let us take as
a specimen the crystalline lens, or hard central part, of the
eye of a codfish, which is a perfectly transparent, and to all
appearance homogeneous, spherule. No one, unaccustomed
to explore the wonders of nature, would suspect that so
simple a body, which he might suppose to be formed of a
uniform material cast in a mould, would disclose, when ex->
amined under a powerful microscope, and with the skill of
a Brewster, the most refined and exquisite conformation.
Yet, as I shall have occasion to specify more in detail in its

ORGANIC MECHANISM. 57
proper place, this little spherical body, scarcely larger than
a pea, is composed of upwards of five millions of fibres, which
lock into one another by means of more than sixty-two
thousand five hundred millions of teeth. If such be the
complication of a portion only of the eye of that animal,
how intricate must be the structure of the other parts of the
same organ, having equally important offices! What ex
quisite elaboration must those textures have received whose
functions are still more refined! What marvellous work
manship must have been exercised in the organization of the
nerves and of the brain, those subtle instruments of the higher
animal faculties, and of which even the modes of action are
to us not merely inscrutable, but surpassing all our powers
of conception !
It is from the energies of life alone that organic forms are
produced. No fabric achieved by human power ever ap
proached in refinement the simplest of nature's works. The
utmost efforts of the ingenuity or skill of man in the con- ,
struction of the most delicate machinery is infinitely sur-/
passed by the most ordinary of the mechanisms which are!
presented to our view in living bodies. However success
ful may be human artists in their attempts to contrive au
tomata, which shall exactly imitate different animal move
ments, there will always be vvanting that internal principle
of action derived from a higher source than mechanism
can supply, and without which these highly wrought works
of man, like the unvivified statues of Prometheus, must re
main for ever mere masses of insentient and inert materials.
As the living functions imply the mechanical action and
reaction of parts which cohere in some definite order of
arrangement so as to preserve that determinate form to which
they constantly tend to return on being displaced, it is im
possible to conceive that a mere fluid can exercise these
functions; because the particles of a fluid, being equally
moveable in every direction, have no determinate relative
situations, and possess no character of permanence. All or
ganic and living structures, therefore, must be composed of
solid as well as fluid parts;, although the proportion between
vol. i. — 8

58 THE MECHANICAL FUNCTIONS.
these is, in different cases, almost infinitely varied. A dor
mant vitality may, indeed, exist in a system of organs which
have been brought into a perfectly dry state : as is proved
by the examples of vegetable seeds, and also of many spe
cies of animalcules, and even of some of the more highly
developed Annelida, or worms, which may be kept in a
dry state for an indefinite length of time, and when moist
ened with water, resume their activity, as if restored to life.
The germination of seeds under these circumstances is mat
ter of common observation ; but the revivification of animal
cules is a more curious phenomenon, fpr it takes place more
rapidly, and is more striking in its results. The Rotifer
redivivus, or wheel animalcule,* (Fig. 1,) which was first
observed by Lewenhoeck, and was afterwards rendered ce
lebrated by the experiments made upon it by Spallanzani,
can live only in water, and is commonly found in that which
has remained stagnant for some time in the gutters of houses.
But it may be deprived of this fluid, and reduced to perfect

dryness, so that all the functions of life shall be completely
suspended, yet without the destruction of the vital princi
ple; for this atom of dust, after remaining for years in a dry
state, may be revived in a few minutes by being again sup
plied with water. This alternate suspension and restora
tion of life may be repeated, without apparent injury to the
animalcule, for a great number of times. Similar phenome
na are presented by the Vibrio triiici, (Fig. 2,) or the ani-
* Vorlkelh rotatoria of Gmelln, and Furcularia of Lai nark.

ORGANIC MECHANISM. 59
malcule resembling an eel in its shape, which infest dis
eased wheat, and which, when dried, appears in the form
of a fine powder: on being moistened it soon resumes its
living and active state.* The Gordius aquaticus, or hair
worm, which inhabits stagnant pools, and which remains in
a dry and apparently lifeless state when the pond is evapo
rated, will, in like manner, revive, in a very short time, on
being again immersed in water. The same phenomenon is
exhibited by the Filaria, a thread-like parasitic worm, in
festing the cornea of the eye of the horse.-f
Both the composition of the fluid and the texture of the
solid parts of animal and vegetable bodies are infinitely va
ried, according to the purposes they are designed to serve in
the economy. Scarcely any part is perfectly homogeneous ;
that is, composed throughout of a single uniform material.
Few of the fluids are entirely limpid, and none are perfectly
simple in their composition; for they generally contain more
or less of a gelatinous matter, which, when very abundant,
imparts to them vicidity, constituting an approach to the so
lid state. Many fluids -contain minute masses of matter, ge
nerally having a globular shape, which can be seen only by
means of the microscope, and which float in the surround
ing liquid, and often thicken it in a very sensible manner. J
We next perceive that these globules have, in many instances,
cohered, so as to form solid masses ; or have united in lines,
so as to constitute fibres. We find these fibres collecting
and adhering together in bundles ; or interwoven and agglu
tinated, composing various other forms of texture; sometimes
resembling a loose net-work of filaments; sometimes consti
tuting laminse or plates; and, at other times, both plates and
filaments combining to form an irregular spongy fabric.
These various tissues, again, may .themselves be regarded as
the coastituent materials of which the several organs ofthe
body are constructed, with different degrees of complication,
* See a paper on this subject by Mr. Bauer, Phil. Trans, for 1823, p. 1.
i De Blainville, Annales des Sciences Naturelles; X. 104.
t Globules of this description have been found in the lymph, the saliva;
aud.even in the aqueous humour of the eye.

60 THE MECHANICAL FUNCTIONS.
according to the respective functions which they are called
upon to perform.
We shall now examine the several kinds of texture in re
lation to these functions, in the order of their increasing
complexity; beginning with those of vegetables, which are
apparently the simplest of all.

§ 2. Vegetable Organization.
Plants, being limited in their .economy to the functions
of nutrition and reproduction, and being fixed to the same
spot, and therefore in a comparatively passive condition, re
quire for the performance of these functions mechanical con
structions of a very different kind from those which are ne
cessary to the sentient, the active, and the locomotive ani
mal. The organs that are essential to vegetables are those
which receive and elaborate the nutritive fluids they require,
those which are subservient to reproduction, and also those
crimposing the general frame-work, which must be super
added to tlie whole for the purpose of giving mechanical
support and protection to these finer organizations. As
plants are destined to be permanently attached to the soil,
and yet require the action both of air and of light; and, as
they must also be defended from the injurious action ofthe
elements, so we find these several objects provided for by
/three descriptions of parts: namely, first, the Roots, which
/ fix plants jn their situation; secondly, the Stems, which
support them in the proper position, or raise them to the
requisite height above the ground; together with the
branches which are merely subdivisions of the stem ; and
thirdly, the external coverings, which correspond in their of
fice to the integuments, or, skins of animals.
The simplest and apparently the most elementary texture
met with in vegetables is formed of exceedingly minute ve
sicles, the coats of which consist of transparent membranes
of extreme tenuity. Fig, 3 is a highly magnified represen
tation of the simplest form of these vesicles.* But they ge-
* These cells are well represented in the engravings which illustrate Mr.

VEGETABLE ORGANIZATION.

6t

nerally adhere together more closely, composing by their
union a species of vegetable cellular tissue, which may be
regarded as the basis or essential component material of
every organ in the plant. . This cellular structure is repre
sented in figures 4 and 5, as it appears in the Fucus vesicu
losus; the first being a horizontal, and the second a vertical
section of that plant.* The size of these cells differs consi
derably in different instances. Kieser states that the dia
meter of each individual cell varies from the 330th to the
55th part of an inch; so that from 3,000 to 100,000 cells
would be contained in an extent of surface equal to a square
inch. But they are occasionally met with of different sizes,
from even the 1000th part of an inch to the 30th.
3 8. 5
li

In their original state, these vesicles have an oval or glor
bular form; but they are soon transformed into other shapes,
either by the mutual compression which they sustain from
being crowded into a limited space, or from unequal expan?
sion in the progress of their development. From the first
of these causes they often ac'quire angles, assuming the
forms of irregular rhomboidal dodecahedrons, and often of
hexagonal prisms, like the cells of a honey-comb ; and by
Slack's memoir on the elementary tissue of plants, contained in the 49th vo?
lume ofthe Transactions ofthe Society of Arts.
* De Candolle, flffganographie yegetale.

6f THE MECHANICAL FUNCTIONS.
the second, they are elongated in cylinders, or slowly ta
pering cones, thus passing by insensible gradations into the
tubular form.' Figures 6, 7, and 8, are representations of
some of these different states of transition from the one to
the other. These various modifications of the same elemen
tary texture have been distinguished into several classes of
cells, and dignified by separate technical denominations,
which I shall not stop to specify, as it does not appear that
they have as yet thrown any light on vegetable physio
logy. Many of the cells are fortified by the addition of elastic
threads, generally disposed in a spiral course, and adhering
to the inner surfaces of the membranous coats of the cells,
which they keep in an expanded state. (See Fig. 9.) When
the membranes are torn, the fibres; being detached, unrol
themselves, and being loosely scattered among the neigh
bouring cells, give the appearance of fibrous connexions
among these cells, which did not originally exist. Simple
njerabranous cells, containing no internal threads, are often
found intermixed with these fibrous cells. In many of the
cells, again, the original spiral threads appears to have coa
lesced by their edges; thus presenting a more uniform sur
face excepting that a few interstices are left, where the pel
lucid membrane, having no internal lining, presents the ap
pearance of transverse fissures or oval perforations, as shown
.in (Fig. 10.) Cells of this description are said to be reticu
lated or spotted, and, together with those having more regu
larly formed spiral threads, are very abundantly met with in
plants belonging to the tribe of Orchidece.
It has been much disputed whether the cells ofthe vege
table texture are closed on all sides, or whether they com
municate with one another. Mirbel has given us delinea
tions of what appeared to him, when he examined the coats
ofthe cells with the microscope, to be pores and fissures. But
subsequent observations have rendered it probable that these
appearances arise merely from darker portions of the mem
branes, where opaque particles have been deposited in their
substance. Fluids gain access into these cell* by transuding

VEGETABLE ORGANIZATION. 63
through the membranes which form their sides, and not by
any apertures capable of being detected by the highest pow
ers of the microscope.
If all the cells consist*of separate vesicles, as the con
curring pbservations of modern botanists* appear to have
satisfactorily establishedythe partitions which separate them,
however thin and delicate, must consist of a double mem
brane, formed by the adhesion of the coats of the two con
tiguous vesicles. But as these coats can hardly be supposed
to adhere in every point, we may expect to find, that spaces
have been left in various parts between them ; and that com
munications exist to a certain extent betweenall these spaces ;
so as to compose what may be regarded as one large cavity.
These have been denominated the intercellular spaces ; and
they have been supposed to perform, as will hereafter be
seen, an important part in the functions of Nutrition.
Fluids of different kinds occupy both the cells and the
intercellular spaces. The contents of some is the simple
water sap ; that of others consists of pecuhar liquids, the
products of vegetable secretion : and very frequently they
contain merely air. In many of the cells there are found
small opaque and detached particles of the substance termed
by chemists, Fecula, of which starch is the .most common
example. In several parts, and more especially in the
leaves, and in the petals of flowers, the material which
gives them their peculiar colour is contained in the cells in
the form of minute globules. De Candolle has given it the
name of Chromule.\
The cells of the ligneous portion- of trees and shrubs are
farther incrusted with particles of a more dense material,
peculiar to vegetable organization, and termed*Lignine. Ix
is this substance which- principally contributes to the den
sity and mechanical strength of what are called the Woody
Fibres, which consist of collections of fusiform, or tapering
vessels, hereafter to be described, surrounded by assemblages
* In particular, Treviranus, Kieser, Link, Du Petit Thouars, Follini, Amici,
Dutrochet, and De Candolle.
¦j- Organographie, Tom, 1, p. 19.

64 THE MECHANICAL FUNCTIONS.
of cells thus fortified, and the whole cohering in bundles, so
as to present greater resistance to forces tending to displace
them in the longitudinal direction than in any other.
Most of the plants which aro*included in the Linnean
class of Cryptogamiahave a structure exclusively composed
of cells, as has been already shown in the Fucus vesiculosus.
But the greater number of other plants have, in addition to
these cells, numerous ducts or vessels, consisting of mem
branous tubes of considerable length, interspersed through
out every part of the system. These tubes exhibit different
modifications of structure, more especially with regard to
the form of the fibres, or other materials, which adhere to
the inner surface of their membranes ; and these modifica
tions correspond very exactly with those of the vesicles
already described as constituting the simpler forms of vege
table tissue. There can be little doubt, indeed, that the ves
sels of plants take their origin from vesicles, which become
elongated by the progress of development in one particular
direction ;: and it is easy to conceive that where the extre
mities of these elongated cells meet, the partitions which se
parate their cavities may become obliterated at the points
of junction, so as to unite them into one continuous tube
with an uninterrupted interior passage. This view of the
formation of the vessels of plants is confirmed by the grada
tion which may be traced among these various kinds of struc
tures. Elongated cells are often met with applied to each
other endwise, as if preparatory to their coalescence into
tubes. Sometimes the tapering ends of fusiform cells are
joined laterally (as seen in Fig. 12,) so that the partitions
which divide their cavities are oblique. At other times their
ends are broader, and admit of their more direct application
to each other in the same line, being separated only by mem
branes passing transversely ; in which case they present, un
der the microscope, the appearance of a necklace of beads
(Fig. 13.) When, by the destruction of these partitions,
their cavities become Continuous, the tubes they form exhi
bit a series of contractions at certain intervals, marking their
,origin from separate cells. In this state they have received

VEGETABLE ORGANIZATION.

65

the names of moniliform, jointed or beaded vessels.* Traces
of the membranous partitions sometimes remain where their
obliteration has been only partial, leaving transverse fibres.
The conical terminations occasionally observable in the ves
sels of plants also indicate their cellular origin.-.
12 13 14 15 16 17

]

The membrane constituting the tube is sometimes simple,
like those of the simple cells: but it frequently contains
fibres, or other, internal coatings, corresponding to those met
with in the more compound cells. The vessels in which the
internal fibres run in a spiral direction (Fig. 14,) are deno
minated trachea, or spiral vessels; or, from their being found
very constantly to contain air, they are often called air tubes.
Their diameter is generally between the 1000th and the
300th part of an inch. These spiral, or air vessels, pervade
extensively the vegetable system. The threads they con
tain are frequently double, treble, quadruple, or even still
more numerous: they are of great length, and when the ex
ternal membrane of the vessel is divided, they may easily
be drawn out and uncoiled, their elasticity enabling them to
retain 'their spiral shape. The object of this structure ap
pears to be that of keeping the cavity of the tube always
pervious, by presenting resistance to any external force tend
ing to compress and close it.J
* Mirbel gave them the name of " Vaisseaux en chapelet."
f This theory of the derivation of vessels from cells was first advanced by
Treviranus. $ Vessels are sometimes met ,with which appear to be formed simply by
the coils of a spiral fibre in close juxtaposition, and unattached to any ex
ternal envelope, or connecting membrane.
VOL. I. — 9

66 THE MECHANICAL FUNCTIONS.
In many instances the inner fibres of the tube, instead of
forming a continuous spiral, appear in the shape of rings,
succeeding one another at regular intervals, and constituting
what are called annular vessels, (Fig. 15.) They are gene
rally larger than the spiral vessels. In other cases, as was-
first observed by Hedwig, the adjacent coils are found to be
closely coherent throughout the greatest part of their course?
leaving, however, occasional intervals, where the external
membrane, being unprotected, appears from its transparency,
as if spotted or perforated in various places (Fig. 16.) Every
intermediate stage may occasionally be seen in the transi
tion from one of these forms to the other, in consequence of
the various kinds of convolution, of branchings, or of trans
verse junctions of fibres, as well as the greater or less extent
of their lateral adhesions. All these varieties are met with,
not only in different vessels, but, as was observed by Mol-
denhawer and Kieser, even in the different portions of
the same vessel, when followed by the eye throughout a
great extent of its length. Thus, in the course of the same
tube, (as seen in Fig. 17,) we find parts exhibiting spiral
fibres, which in other parts, bifurcate and again unite; and
in others, again, form rings: these may afterwards, by a>
closer junction, present a reticulated appearance, or a series
of transverse lines, which, becoming smaller and smaller,
are at length mere points, arranged in circular rows around.
the cylindrical surface of the vessel.*
What are called the woody fibres originate, like all other
parts of plants, in cells. These are generally fusiform, that
is, of the shape of a double cone, very greatly elongated,
and placed close and parallel to one another, with the nar
row extremities of one set wedged in between those of ano
ther set (Fig. 18.) Their coats are more firm and elastic
than those of ordinary vessels, but do not appear to contain
any internal fibres, although they receive, in the progress of
* Many distinguished botanists, such as Rudolphi, Link, Treviranus, and
Dutrochet, consider these spots as being produced not by the deficiency of
the internal coating, but by the addition of granular bodies. See De Can-
dolle's Organographie Vegetale, torn. 1, p. 56,

VEGETABLE ORGANIZATION. 67
their development, large additions of solid matter. These
fibres are generally collected together into bundles or layers,
and are accompanied by cells and vessels of various descrip
tions, and in different stages of transition. The density of
the woody fibres increases in proportion as these incrusta
tions are formed, till they have become nearly impervious ;
and have acquired a degree of rigidity peculiarly" fitting them
for the office of giving mechanical support to the fabric of
the plant.* Their assemblage thus constitutes a kind of
frame-work for the whole system, which may be regarded
as the skeleton of the plant. Thus, what are called the fibres
of leaves (Fig. 19,) are principally composed of these woody
fibres, distributed in the manner best adapted to support the
expansion of the soft and pulpy substance of those important
organs. Besides the minute cavities of the cellular tissue, there
occur, in various parts of a plant, much larger spaces, appa
rently serving the purpose of reservoirs of particular fluids ;
but sometimes containing only air. Large air cells are, in
particular, met with very commonly in aquatic plants, where
they probably contribute to impart the requisite degree of
buoyancy. There are also contained, in the interior of vegetables,
certain organs, denominated Glands, which are composed
of closely compacted cells, and which perform the function
of secretion, that is, the conversion of the nutritious juices
into particular products required for various purposes in the
economy of the plant.
The external parts of a living plant require protection
against the injurious effects of the atmosphere, and of the
moisture it deposites. For this purpose there is provided a
membrane, termed the Cuticle, which is spread over the
whole surface, investing the leaves and flowers, as well as
* By drying different specimens of wood in a stove, Count Rumford was
led to the conclusion that the specific gravity of the solid matter which con
stitutes timber is nearly the same in all trees. He found that the woody part
of oak, in full vegetation, constitutes only two-fifths of the whole bulk: and
¦that ordinary dry wood contains above one-fourth of its weight of water,
Thomson's Annals of Philosophy, I. 388,

68 THE MECHANICAL FUNCTIONS.
the stem and branches, and interposing a barrier to the ac
tion of fluids, or ether extraneous bodies, on the living or
gans. The cuticle is formed originally by the condensation
of a layer of cellular tissue, of which the cells, being conso
lidated by exposure to the air, and by compression, compose
a thin but impervious pellicle. Amici has distinctly shown,
by means of his powerful microscope, the cellular structure
of the cuticle, and also that the layer of cells of which it con
sists is independent of the subjacent cellular tissue.* Fig.
20 is intended to show this circumstance, the shaded part
representing the cuticle with its series of cells.
Oval orifices, or stomata, as they have been termed, are
discoverable on almost every part of the surface of the cuti
cle, but more especially in those that have a green colour.t
They are placed at nearly equal distances from one another,
and are particularly numerous in the cuticle of the leaves,
where they occupy the intervals between the fibres. These
orifices conduct into the interior of the plant, probably into
the general cavity of the intercellular spaces. It is evident,
from the functions they perform, that they must occasional
ly open and close; but the minuteness of their size precludes
any accurate observation as to the nature of the apparatus
provided for the performance of these motions. Amici de
scribes their margins as formed by two cells, by the move
ments of which, combined perhaps with those of the adjoin
ing cells, he conceives these orifices are opened and closed. J
Great variety, however, is observable in the structure of
the stomata in different species of plants.
Many plants have no stomata, either on the cuticle of the
leaves, or on that of the stem. This is the case with such
aquatic plants as are habitually immersed in water. In those
that are only partially immersed* stomata are met with in
those parts exclusively which are above the water. The
*
* Annales des Sciences Naturelles, II. 211.
f Pig. 22 is a magnified representation of the appearance in the cuticle
ofthe Lycopodium denticulatum, taken in the central part of the lower sur
face of the leaf, from De Candolle. Fig. 21 is a still more magnified view
ofthe stomata in the leaf of the Lilium candidum, from Amici.
$ Annales des Sciences Naturelles, II. 215.

VEGETABLE ORGANIZATION.

69

leaves of the Ranunculus aquaticus, when made to grow in
the air, acquire stomata, but lose them entirely when grow
ing under water. Stomata are wanting in all plants whose
structure is wholly cellular.

Botanists are far from being agreed as to the precise func
tions which the stomata perform. Their usual office un
doubtedly is to exhale water; but they probably also absorb
air under certain circumstances, and in particular exigences.
The principal organs through which the fluids that serve
for nourishment are received into the system of plants, are
those situated at the extremities of the roots, where they are
termed, from their peculiar texture, spongioles.* Of the
functions of spongioles in absorbing fluids I shall have occa
sion to speak when treating of nutrition. But as the roots
exercise a mechanical as well as a nutrient office, we should
here consider them in the light of organs adapted to procure
to the plant a permanent attachment to the soil, upon which
it is wholly dependent for its supply of nourishment. It is
scarcely necessary to point out how effectually they perform
this office. Our admiration cannot fail to be excited when
we contemplate the manner in which a large tree is chained
to the earth by its powerful and widely spreading roots. By
the firm hold which they take of the ground, they procure
* Fig. 23 exhibits the termination of a root of a willbw in a spongiole;
the. arrangement ofthe cells composing which is shown in Fig. 24, from De
Candolle.

70 THE MECHANICAL FUNCTIONS.
1rhe most effectual resistance to the force ofthe winds, which,
acting upon so large a surface as that presented by the
branches covered with dense foliage, must possess an im
mense mechanical power.
The principal seat of the vitality of a plant is the part
which intervenes between the root and the stem. Injuries
to this part are always fatal to the life of the plant.
As the roots penetrate downwards into the earth to dif
ferent distances in order to procure the requisite nourish
ment, so the stem grows upwards for the purpose of obtain
ing for the leaves and flowers an ample supply of air, and
the influence of a brighter light, both of which are of the
highest importance to the maintenance of vegetable life.
The stems of the grasses are hollow tubes; their most solid
parts, which frequently consist of a thin layer of silex, oc
cupying the surface of the cylinder. Of all the possible
modes of disposing a given quantity of materials in the con
struction of a column, it is mathematically demonstrable
that this is the most effective for obtaining the greatest pos
sible degree of strength.*
The graceful continuous curve with which the stem of a
tree rises from the ground, is the form which is best calcu
lated to give stability to the trunk. Evidence of express
mechanical design is likewise afforded by the manner in
which the trunk is subdivided into its branches, spreading
out in all directions, manifestly with a view to procure for
the leaves the greatest extent of surface, and thus enable them
to receive the fullest action of both light and air. The
branches, also, are so constructed as to .yield to the irregular
impulse of the wind, and again, by their elasticity, to return
to their natural positions, and by these alternate inflexions
on opposite sides, to promote the motion of the sap in the
vessels and cellular texture of the liber and alburnum. No
thing can exceed the elegance of those forms which are pre-
• Galileo, the most profound philosopher of his age, when interrogated
toy the inquisition as to his belief in a Supreme Being, replied, pointing to a
straw on the floor of his dungeon, that from the structure of that object
alone he would infer with certainty the existence of an intelligent Creator,

VEGETABLE ORGANIZATION. 71
sented in every part of the vegetable kingdom, whether they
be considered with reference to their direct utility for the
support of individual life, and the continuance of the species,
or whether they be viewed as component parts of that beau
ty which is spread over the scenery of nature, and is so de
lightfully refreshing to the eye of every beholder alive to
its fascinating charms. How enchanting are all the varieties
of flowers, that decorate in gay profusion every part of the
garden of creation; and into which the farther we carry our
philosophic scrutiny, the more forcibly will our hearts be
impressed with the truth of the divine appeal that " Even
Solomon, in all his glory, was not arrayed like one of
THESE." ¦
§ 3. Development of Vegetables.
Farthfr proofs of design may be collected from an ex
amination into the modes in which these structures, so
admirably adapted to their objects, have been gradually
formed. Confining our attention to vascular plants, in which
the process of development has been studied with the great
est attention and success, we find that Nature has pursued
two different plans in conducting their growth.* In the
greater number the successive-additions to the substance of
the stem are made on the exterior side of the parts from
which they proceed. This mode is adopted in what are
called Exogenous plants. In others, the growth is the re
sult of additions made internally ; a plan which is followed
in all Endogenous plants. The Oak, the Elm, the Beech,
the Pine, and all the trees of these northern regions, belong
to the first of these divisions. The Palm tribe, such as the
Date, the Cocoa-nut tree, and indeed, a large proportion of
the trees of tropical climates, together with the sugar-cane,
the bamboo, and all gramineous and liliaceous plants, belong
to the latter. We shall first inquire into the endogenous
* The tribe of Filices, or ferns, the structure of which is vascular, consti
tute an exception to this rule: as they differ in their mode of development,
both from exogenous and endogenous plants.

72 THE MECHANICAL FUNCTIONS.
mode of growth, as being the simplest of these two kinds of
vegetable development.
A Palm tree may be taken as an example of the mode of
growth in endogenous plants. The stem of this tree is usu
ally perfectly cylindrical, attains a great height, and bears on
its summit a tuft of leaves. It is composed of an extremely
dense external cylindric layer of wood; but the texture of
the interior becomes gradually softer and more porous as it
comes nearer to the centre; though with regard to its essen
tial character it appears to be uniform in every part, having
neither medullary rays, nor true outward bark, nor any cen
tral pith ; in all which respects it differs totally from the or
dinary exogenous trees.
The first stage of its growth consists in the appearance of
a circle of leaves, which shoot upwards from the neck of
the plant, and attain, during the first year, a certain size.
The following year, another circle of leaves arises; but they
grow from the interior of the former circle, which they
force outwards as their vegetation advances, and as ligneous
matter is deposited within them. Thus, each succeeding
year brings with it a fresh crop of leaves, intermixed with
ligneous matter, which leaves, exerting an outward pressure,
stretch out the preceding layers that enclose them ; until the
latter acquiring greater density, no longer admit of farther dis
tention, and remain permanently fixed. This happens first to
the outermost layer, which is the oldest: then each succeed
ing layer becomes consolidated in its turn. As soon as the
outer layer has become too hard to yield to the pressure from
within, the growth ofthe inner layers is immediately directed
upwards ; so that they each rise in succession by distinct stages,
always proceeding from the interior ; a mode of develop
ment which has been compared by De Candolle to the draw
ing out of the sliding tubes of a telescope. The whole stem,,
whatever height it may attain, never increases its diameter
after its outward layer has been consolidated. A circle of
leaves annually sprouts from the margin of the new layer of
wood ; these, when they fall off in autumn, leave on the stem
certain traces of their former existence, consisting of a cir-

DEVELOPMEMT OF VEGETABLES. 73
cular impression round the stem. The age of the tree may
accordingly be estimated by the number of these circles, or
knots which appear along its stem. The successive knots
which are seen in the stems of other endogenous plants, as
may be observed in growing corn, and also in various
grasses, may be traced to a similar origin.
The structure of oxogenous trees is more complicated ;
for, when fully grown, they are composed of two principal
iparts, the wood and the bark. The woody portion exhibits
a farther division into the pith, which occupies the centre,
and consists of large vesicles, not cohering very closely,
3but forming a light and spongy texture, readily permeable
to liquids and to air; the harder wood, which surrounds the
pith in concentric rings, or layers; and the softer wood, or
¦alburnum, which is also disposed in concentric layers on the
-outer side of the former. Each of these concentric layers
»of wood and of alburnum may be farther distinguished into
ran inner and an outer portion ; the former being of less den
sity than the latter, and consisting of a lighter cellular tis
sue: while the outer portion is composed of the denser
woody fibres resulting from the union of numerous vessels
with a cellular envelope. The bark is formed by concentric
layers of cortical substanee, of which the innermost are de
nominated the Liber; and the whole is surrounded by an
-outer zone of cellular tissue, termed the cellular envelope.
The exterior surface of this envelope is called the Epider
mis. All these concentric zones may be readily distinguished
in a horizontal section ofthe stem; which also presents a num
ber of lines called Medullary Rays, radiating from the pith
to the circumference. They are composed chiefly of large
-cells, extending transversely, or in the direction of the di
ameter ofthe tree, and composing by their union continuous
vertical planes the whole length of the trunk.
Every vegetable stem, and also every branch which arises
from it, is developed from a germ, or bud, which is origi
nally bf inconceivable minuteness, and totally imperceptible
by any optical means of which we have the command. As
VOL. I.  10

74 THE MECHANICAL FUNCTIONS.
*
soon as it becomes visible, and its structure can be distin
guished, it is found to contain within itself many ofthe parts
which are to arise from it, in miniature, and folded up in the
smallest possible compass. The portion destined to form /the
stem is gradually expanded both in breadth and height, tbut
principally the latter ; so that it rises as it grows, during a
certain period, until the- fibres acquire the solidity and
strength necessary not only for their own support, but also
for sustaining the parts which are to be farther added. In
trees this process generally occupies one whole season;
during which the growth of the first layer of wood, with its
centraj pith, and its covering of a layer of bark, is free and
unrestrained. On the second year, a fresh impulse being'
given to vegetation, a new growth commences from the up
per end of the original stem, as if it were the development
of a new bud : and at the same time a l^yer of cellular tis
sue is formed by the deposition of new materials on the
outside of the former wood, and between it and the bark.
This is followed by a second layer pf wood, enveloping the
new layer of cellular tissue.
The effect of this new growth is to compress the layer of
wood which had been formed during the first year, and to
impede its farther extension in breadth. But as its fibres,
consisting of vessels and cells, are not yet consolidated, and
admit of still greater expansion as long as they are supplied
with nourishment, their growth, which is restrained late
rally, is now directed upwards, and there is no farther en
largement of their diameter. From the same cause the pith
cannot increase in size; and is even found to diminish by
the pressure of the surrounding wood. Thus, the vertical
elongation of the entire stem continues during the whole ef
the second year, and the trunk becomes sufficiently strength
ened by the addition of the second layer on its outside to
bear this increase of its height.
While this process is going on in the wood, porrespond-
ing changes take place in the bark, and a new layer is add
ed on its inner surface, or that which is contiguous to the
wood. This layer constitutes the liber. All these new de
positions must of course tend to stretch the outer portiqns

DEVELOPMENT OF VEGETABLES. > 75
of the bark, which had been first formed, and which yield
to this pressure to a certain extent; but, becoming them
selves consolidated by the effects of the same pressure, they
acquire increasing rigidity ; and, the same cause continuing
to operate, they at length give way, in various places, form
ing those deep cracks, which are observable in the bark
of old trees, and which give so rugged an appearance to
their surface. The cuticle has, long before this, peeled off,
and has been succeeded by the consolidated layers of corti
cal envelope which form the epidermis. But the epider
mis, which is continually splitting by the expansion of the
parts it encloses,' itself soon decays, and is constantly suc
ceeded by fresh layers, produced by the same process of
consolidation in the subjacent cortical substance.
During the third and each succeeding year, the same pro
cess is repeated; new layers of cellular texture and of woody
fibres are deposited around those of the preceding year's
growth, and a new internal coating is given to the liber of
the bark. The compressing, power continues to be exerted
on the internal layers of wood, directing their grpwth verti
cally, while they are capable of elongation, and can be sup
plied with nourishment. In time", however, by continued
pressure, and accumulating depositions of solid matter, the
vessels and the cells become less and less pervious to fluids;
till at length all farther dilatation is prevented. But the tree
still continues to enlarge" its' trunk by the annual accessions
of vigorous and expansible' alburnum, and to take its station
among its kindred inhabitants of the forest; till, arriving at
maturity, its majestic form towers above all the junior or
less vigorous trees.*'
The development of each branch takes place in the same
manner, and by the same kind of process as that of the trunk.
The buds from which they originate, spring from the angle
* It is contended by Dr. Darwin and other writers on vegetable physiology
that each annual shoot should be regarded as a collection of individual buds,
each bud being a distinct individual plant, and the whole tree an aggregation
of such individuals. I shall have occasion to revert to this question when I
come to consider the subjeet of vegetable nutrition.

76 % THE MECHANICAL FUNCTIONS.
formed by the stalk which supports a leaf, and which is
termed by botanists the axilla of that leaf. A law of sym
metry is established by nature in the development of all the
parts of plants. The leaves, in particular, are frequently
observed to arise in a circle, or symmetrically round the pa
rent stem; forming what is termed a whorl, or, in botanical
language, a verlicillated arrangement. In other cases they
are found to have their origins at equal intervals of a spiral
line, which may be conceived to be drawn along the stem,
or the branch from which they grow. When these inter
vals correspond to- the semi-circumference of the stem, the
leaves alternate with one another on its opposite sides.
The stems of most plants, even those which are perfectly
erect, exhibit a tendency to a spiral growth. This is obser
vable in the fibres of the wood of the pine, however straight
may be the direction of the whole trunk. This tendency is
shown even in the epidermis of the cherry tree, for it may
be stripped off with more facility in a spiral direction than
in any other. The primitive direction of the leaves of en
dogenous plants is a spiral one.. It is particularly marked
also in the stems of creepers and parasitic plants, which
are generally twisted throughout their whole length ; a dis
position evidently conducive to the purpose of their forma
tion,, namely, that of laying, hold of the objects with which
they come in contact,, and of twining round them in search
both of nourishment and of support. The twisted stems of
the hop and: of ivy show this structure in a remarkable
degree, and the purpose for which this tendency was given
cannot be mistaken*
A conjecture has beenoffered that this tendency to a spi
ral growth might be owing to the influence of the sun's
light acting successively on different sides of the plant, in
the course of its diurnal motion. In these northern latitudes
the direction of that motion is from east to west; or, to an
observer facing- the south, from left to right. That light has
a powerful influence in determining the direction of the
growth of all the parts of the plant which are above ground,
is manifest to every one who has observed the habits of ve
getables If a growing plant be placed in a situation where

DEVELOPMENT OF VEGETABLES. 77
the light reaches it only on one side, it will always, by de
grees, turn itself to that side, as if eagerly pressing forward
to obtain the beneficial action of that agent. The leaves,
whose functions in a more especial manner require its ope
ration, will always be found turned towards the light. The
branches of a tree, which have naturally a tendency to rise
vertically, have this tendency modified by the superior at
traction of the light, when it can reach them only laterally.
Thus, while those on the upper part spread out in full luxu
riance in all directions, those below them are obliged to ex
pand more in a lateral direction: and this is still more the
case with the lowest branches, which shoot out horizontal
ly to a considerable distance before they turn upwards, and
present their leaves to the light. Often, however, from the
deficiency of this necessary agent, their growth is much
stinted, or entirely prevented. The' operation of this cause
is extensively seen in the interior of a dense forest.
It may be objected to the- theory of the spiral growth be
ing the result of the sun's motion, that were it so, the direc--
tion of the spiral would always be the same, that is, ascend
ing from left to right with reference to the axis. But this
is not found to be the case, for the direction'of the turns,
though generally constant in the same plant, is far from be-*
ing the same in all. Dr. Wollaston ingeniously suggested-
that a verification of the theory would be obtained, were it
found that plants transported from the southern to the nor
thern hemispheres, would havethis direction reversed ;' for
it is evident that the motion of the sun's light in the two
hemispheres is in opposite directions; being, in the southern
hemisphere, from right to left, to a spectator facing the me
ridian position of the sun, which in those regions is to the
north. But the facts are not in accordance with this- view
of the subject; so that we may consider the hypothesis as
untenable. The roots differ considerably from the stems both in their
structure, and in their mode of growth. They exhibit, in-
deed,the appearance of medullary rays and of concentric
layers, but they are destitute of any central pith, and they
have no tracheae; neither does their surface present any ap--

78 THE MECHANICAL FUNCTIONS.
pearance of stomata. They increase in thickness in the same
way as the stem increases. This law obtains both in exo
genous and endogenous plants': they do not, however, grow
in length by the elongation of any of their parts, but simply
by additions made to their extremities. Their ramifications
are not the result of the development of buds, as are the
branches of the stem; but they arise merely from the addi
tional deposites taking different directions. Almost every
part of the surface of the stem or branches may shoot forth
roots if they are covered with earth, and properly moistened,
and if they are supplied with sap from the circulating system
of the plant itself. It is observed, however, that they gene
rally grow from certain points on the surface of the bark>
which appear as dark spots, and are termed Lenticella.*
Great variety exists in the form and dispositions of roots in .
different families of plants, according to the particular pur
poses they are intended to serve, conformably with their ge
neral functions of absorption and of mechanical support.
Both these purposes are promoted by their sending out from
their sides numerous fibrils, or lesser roots, which increase
their firm hold upon the soil, as well as multiply the chan- '
nels for the introduction of nourishment.
Nature has supplied various plants with certain appen
dages to the above mentioned structures, the use of which
are for the most part sufficiently obvious. Of this descrip
tion are the tendrils, which assist in fixing and procuring
support to the stems of the weaker plants; the stipuhn,
which protect the nascent leaves ; and the bractem, which
perform a similar office to the blossom. The different kinds
of hairs,, of down,f of thorns, and prickles, which are found
on the surface of different plants, have various uses; some
of which are easily understood, particularly that of defend
ing the plant from molestation by animals. The sting of
the nettle is of this class;- and its structure bears a striking
• This name was given to them by De Candolle, Annales des Sciences
Naturelles, VII. 1, and Organographie, I. 94
¦J- The finer hairs, and filaments of down, are composed of elongated cells,
either single, or several conjoined end to end.

ANIMAL ORGANIZATION. 79
analogy, as we shall afterwards have occasion to notice, to
that of the poisonous fangs of serpents.
The purposes answered by the down, which covers a great
number of plants, are not very obvious. It, perhaps, serves
as a protection from the injurious effects of cold winds on
the tender surface ; or it may have a relation to the deposi
tion of moisture : or, as it may be farther conjectured, the
number of points which are thus presented to the air may
be designed to convey electricity from the atmosphere, or
to restore the electric equilibrium, which may have been
disturbed by the processes of vegetation.
In the smaller parts of plants, as in the general fabric of
the whole, we find, on examination, the most admirable pro-
, vision made, according to the particular circumstances of
the case, for the mechanical objects of cohesion, support,
and defence. Thus, the substance of the leaf, of which the
functions require that a large surface should be expanded
to the air and light, is spread out in a thin layer upon a
frame-work of fibres, like rays, connected by a net-work of
smaller fibrils, and constituting what is often called the ske
leton of the leaf.
In all these vegetable'structures, while the objects appear
to be the same, the utmost variety is displayed in the means
for their accomplishment, in obedience, as it were, to the
law of diversity, which, as has been already observed, seems
to be a leading principle in all the productions of nature. It
is more probable, however, judging from that portion of the
works of creation which we are competent to understand,
that a specific design has regulated each existing variation
of form, although that design may in general be placed be
yond the limited sphere of our intelligence.
§ 4. Animal Organization.
The structures adapted to the purposes of vegetable life,
which are limited to nutrition and reproduction, would be
quite insufficient for the exercise of the more active func
tions and higher energies of animal existence. The power
of locomotion, with which animals are to be invested, must
aloneintroduce essential differences in their organization, and

.80 THE MECHANICAL FUNCTIONS.
must require a union of strength and flexibility in the parts
intended for extensive motion, and for. being acted upon by
.powerful moving forces.
The animal as well as the vegetable fabric is necessarily
composed of a union of solid and fluid parts. Every animal
texture appears to be formed from matter that was origihal-
Jy in a fluid state ; the particles of which they are composed
having been brought together and afterwards concreting by
a process, which may, by a metaphor borrowed from phy
sical science, be termed animal crystallization. Many of
those animals, indeed, which occupy the lowest rank in the
-series, such as Medusa, approach nearly to the fluid state-;
appearing like a soft and transparent jelly, which, by spon
taneous decomposition after death, or by the application of
heat, is resolved almost wholly into a limpid watery fluid.*
More accurate examination, however, will show that it is
in reality not homogeneous, but that it consists of a large
proportion of water, retained in a kind of spongy texture,
.the individual fibres of which, from their extreme fineness
.and uniformity of distribution, can with difficulty be detected.
Thus, even those animal fabrics which on a superficial view
rappear most simple, are in reality formed by an extremely
artificial and complex arrangement of parts. The progress
•of development is continually tending to solidify the struc
ture of the body. In this respect the lower orders of the
animal kingdom, even when arrived at maturity, resemble:
the conditions of the higher classes at the earliest stages of
their existence. As we rise in the scale of animals, we ap
proximate to the condition of the more advanced states of
development which are exhibited in the highest class.
Great efforts have been made by physiologists to discover
the particular structure which might be considered as the
simplest element of all the animal textures ; the raw mate
rial, as it were, with which the whole fabric is wrought :
* Thus a Medusa, weighing twenty or thirty pounds, will, by this sort of
general liquefaction, be found reduced to only a few grains of solid matter.
Peron, Annales du Musee, torn. XV. p. 43. See also a memoir by Quay and
&aimard, Annales des Sciences Naturelles, torn. I. p. 245.

ANIMAL ORGANIZATION. 81
but their labours have hitherto been fruitless. Fanciful hy
potheses in abundance might be adduced on this favourite
topic of speculation; but they have led to no useful or satis
factory result. Haller, who pursued the inquiry with great
ardour, came to the conclusion that there existed what he
calls the simple or primordial fibre, which he represents as
bearing to anatomy the same relation that a line does to ge
ometry. Chemical analysis alone is sufficient to overturn
all these hypotheses of the uniformity of the proximate ele
mentary materials of the animal organs: for they are found
to be extremely diversified in their chemical composition.
Neither has the microscope enabled us to resolve the pro-
•blem: for although it has been alleged by many observers
that the ultimate elements of every animal structure con
sists of minute globules, little confidence is to be placed in
these results obtained by the employment of high magnify
ing powers, which are open to so many sources of fallacy.
That globules exist in great numbers, not only in the blood,
but in all animals fluids, there can be no doubt ; and that
these globules,'by cohering, compose many of the solids, is
also extremely probable. But. it is very doubtful whether
they are essential to the composition of other parts, such as
the fibres of the muscles, the nerves, the ligaments, the ten
dons, and the cellular texture : for the most recent, and ap
parently most accurate microscopical observations tend' to
show that no globular structure exists in any of these tex
tures.* The element which we can recognise without difficulty as
composing the greater portion of animal structures, is that
which is known by the name of the cellular texture. Al
though bearing the same designation as the elementary ma
terial of the vegetable fabric, it differs widely from it, in its
structure and mechanical properties. It is not, like that of
plants, composed of a union of vesicles; but is formed of a
congeries of extremely thin laminae, or plates, variously con-
* See the Appendix to Dr. Hodgkin and Dr. Fisher's translation of Ed
ward's work on the Influence of Physical Agents on Life, p. 440.
VOL. I. H

82

THE MECHANICAL FUNCTIONS.

nected together by fibres, and by other plates, which cross
25 them in different directions leaving cavities or
cells. (Fig. 25.) These cells, or rather inter
vening spaces, communicate freely with one
another; and, in fact, may be considered as
one common cavity, subdivided by an infinite
number of partitions into minute compart
ments. Hence the cellular texture is through
out readily permeable to fluids of all kinds, and retains these
fluids in the manner, and on the same principle, as a sponge.
The cellular texture is not only the element, or essential
material employed by nature in the construction of all the
parts ofthe animal fabric; but in its simplest form, it con- .
stitutes the general medium of connexion between adjacent
organs, and also between the several parts ofthe same organ-
Like the mortar which unites the stones of a building, the
cellular texture is the universal cement employed to bind
together all the solid structures. Its properties are ad
mirably adapted to the mechanical purposes which are re
quired, in different parts of the frame: and these properties
are variously modified and adjusted to suit the particular
exigencies of the case. When, for instance, different parts
require to be moveable upon each other, the cellular sub
stance interposed between them has its state of condensation
adapted to the degree of motion required. That which con
nects the muscles, or surrounds the joints, and all other parts
concerned in extensive action, has a looser texture, being
formed of broad and extensible plates, with few lateral ad
hesions, and leaving large interstices; while in the more quies
cent organs, the plates of the cellular substances, are thin and
small, the fibres short and slender, and their intertexture
closer and more condensed.
Besides being flexible and extensible, the cellular texture
is also highly elastic, a property which is exceedingly advan
tageous in the construction of the frame. Not only the dis
placement of parts is resisted by this elasticity, but, when
displaced, they tend to return to their natural position. This
property performs a more important part in the mechanism

ANIMAL ORGANIZATION. 83
ofthe animal than of the vegetable system ; as might, indeed,
have been anticipated from the more active and energetic
movements required by the functions of the former.
The cellular texture, in its simple form, admits of the
ready transmission of fluids through it ; but it is necessary,
on many occasions, to interpose a barrier to their passage.
Such barriers are provided in membranes, which are merely
modifications of the same material, spread out into a con
tinuous sheet of a closer texture, after the surfaces of the
plates have been brought to cohere so as to obliterate all the
cellular interstices, and become impervious to fluids. Though
equally flexible and elastic with the original texture of
which it is formed, the membrane has acquired, by this con
solidation, greater strength and firmness, properties which
adapt it to a great number of important purposes.*
Membranes are extensively employed to connect distant
organs, and often serve to determine the direction and ex
tent of their relative motions. They furnish strong cover
ings for the investment, the support, and the protection of
all the important organs of the body. What Paley has
termed the package of the organs is effected principally by
their intervention. Membranes are also employed to line
the interior of all the large cavities of the body, as those of
the chest, and of the abdomen, or lower part of the trunk
containing the organs of digestion. These membranes, af
ter lining the sides of their respective cavities, are reflected
back upon the organs which are enclosed in those cavities,
so as to furnish them with an external covering. Their in
ner sides present every where a smooth and polished sur
face, over which the organs contained in the cavity may
glide without injury. In all these cases, a thin fluid, called
serum, is provided, which moistens and lubricates the sur
faces that are in contact with one another, and obviates the
injury that would otherwise arise from friction. From this
* With a view of ascertaining the actual strength of membranes, Scarpa
stretched a portion of peritoneum, (which is a very thin membrane lining
the abdominal cavity,) over a hoop, and placing weights upon its surface,
found it did not give way till it was loaded with fifteen pounds.

84

THE MECHANICAL FUNCTIONS.

circumstance, the linings of these cavities have been termed
serous membranes. In the neighbourhood of joints, closed ca
vities of the same description, but of smaller size, are met
with, for the obvious purpose of facilitating motion; and
here also friction is prevented by a highly lubricating fluid,
termed synovia, which is poured out between the surfaces-
of the membrane lining the cavities.
Membranes being impermeable to fluids, are extensively
employed as receptacles for retaining them: forming, in the
first place, sacs, or pouches of various kinds for that pur
pose. The ink-bag of the cuttle-fish, the gall-bladder, and
even the stomach itself, are examples of this kind of struc
ture. The coats of these sacs, being very extensible and
elastic, readily accommodate themselves to the variable bulk
of their contents.
In the second place, we find membranes composing tubes
of various descriptions for conducting fluids. Thus, in the
higher classes of animals, the whole of the body is traversed
by innumerable canals, conveying different kinds of fluids.
These canals, when uniting into trunks, or subdividing info-
branches, are called Vessels, (Fig. 26.)

27

The fluids contained in vessels are never stagnant, but are-
almost always carried fowards in one constant direction.
For preventing the retrograde motions of the fluids passing
along these canals, recourse is had to the beautiful con
trivance of valves. The inner membrane of the vessel is
employed to construct these valves; for which purpose it is
extended into a fold having the shape of a crescent; and fixed
by its convex edge to the sides of the vessel, while the other
edge floats loosely in its cavity. Whenever the fluid is im-

ANIMAL ORGANIZATION. 85
pelled in a direction contrary to its proper course, it raises
the loose edge of the valve, which, being applied to the op
posite side of the canal, effectually closes the passage. On
the contrary, it presents no obstacle to the natural flow of
the contents of the vessel, both edges being then closely ap
plied to the same side. Frequently two, or even three
valves are used at the same part, their edges being made to
meet in the middle of the passage, like the flood-gates, or
locks of a canal.* Among the numberless instances of ex
press contrivance which are met with in the examination of
the fabric of animals, there is, perhaps, none more striking
and more palpable, than this admirable mechanism of the
valves* As we ascend from the simpler to the more complicated
systems of organization, adapted to a greater range of facul
ties, we find greater diversity in the mechanical' means em
ployed for carrying on the functions of life. Textures of
greater strength than can be constructed by membranes-
alone become necessary for the security, the support, and
the defence of important organs; and more especially for the
execution of extensive movements. For obtaining these
advantages a peculiar species of fibres is provided, formed
of a much denser substance than even the most consolidated
forms of cellular texture. The animal product termed albu--
men possesses a much stronger cohesive power then gelatin,
which is the basis of membrane. The addition of albumen,
therefore, procures the quality required; and the fibres which)
are produced by its combination with gelatin are opaque,
and of a glistening white colour. By interlacing fibres thus
composed, a close, texture is formed, which is exceedingly
tough and unyielding. These fibrous textures, as they are'
termed, while they retain the flexibility of membranes,
greatly surpass them in strength; but, being at the same time-
incapable of extension, they are necessarily devoid of elasti-
* Fig. 27, representing the section ofa vessel, is intended to show the po
sition ofthe valves when applied to the sides of the vessel, by the stream.
moving onwards in the direction pointed out by the arrow. In Fig. 28, they
are seen closing the passage by the retrograde pressure of the current.

86 THE MECHANICAL FUNCTIONS.
city. Hence, they are adapted to form external tunics for
the investment of such organs as are not intended to vary in
their size. Occasionally, these fibrous capsules, as they are
called, send down processes into the interior of those organs,
for the purpose of giving them mechanical support. This
is the case, for instance, with the membranes surrounding
the brain of quadrupeds, and which form two partitions, the
one vertical, the other horizontal; both being firmly stretched
in their respective positions, and serving to divide the pres
sure. In other cases these sheets of fibrous membrane are
employed as bandages, tightly bracing the muscles, and re
taining them in their relative situations. The joints are sur
rounded by similar bandages, known by the name of Cap
sular Ligaments.
In following the series of animal structures in the orde?
of their increasing density, we find the proportion of albu
men which enters into their composition becoming greater,
while that of the gelatin and mucilage diminishes. Wben
the product is more uniform in its composition, it is in gene
ral less elastic than when it consists of a more complex com
bination of ingredients. A great preponderance of albumen
tends also to diminish the elasticity. Thus the densest
kinds of fibrous texture present, instead of thin and broad-
expansions of elastic membrane, the thick and elongated
form of inextensible cords, constituting the ordinary Liga
ments, and the Tendons. These structures resist with
great power any force calculated to extend them: a proper
ty which of course excludes elasticity, but, when united
with flexibility, implies great toughness. In a word, they
possess all the qualities that can be desired in a rope. It
will hardly be credited how great a force is required to-
stretch, or rather rend asunder a ligament? for it will not
yield in any sensible degree until the force is increased so
enormously as at once to dissever the whole contexture of
its fibres. Nothing can be more artificially contrived than
the interweaving of the fibres of ligaments; for they are not
only disposed, as in a rope, in bundles placed side by side,
and apparently parallel to each other: but, on careful exami-

ANIMAL ORGANIZATION. 87
nation, they are found to be tied together by oblique fibres
curiously interlaced, in a way that no art can imitate. It is
only after long maceration in water, that this complicated
and beautiful structure can be unravelled.
The mechanical properties of these fibrous structures,
which are strictly inextensible ligatures, render them appli
cable to purposes of connexion where motion is to be re
strained. Many cases, however, occur in which a substance^
is wanting, uniting great compactness and strength with a
considerable degree of elastic power. For this purpose a
different texture is fabricated, consisting of twisted fibres,
which impart this required elasticity. Such is the structure
of the elastic ligaments of animals, which are very gene
rally employed for the support of heavy parts that require
being suspended. An instance occurs in quadrupeds, in that
strong ligament which passes along the back and neck to be
fixed to the head, and to support its weight when the ani
mal stoops to graze. This, the ligamentum nucha, as it is
termed, is capable of great extension, and by its* elasticity
reacts with considerable force in recovering its natural length,
after it has been stretched. This ligament is particularly
strong in the Camel, whose neck is of great length.* Ano
ther example of an elastic ligament occurs in that which con-j"
nects the two shells of bivalve mollusca (as those of the oy
ster and muscle.) and which keeps them open when the ani
mal exerts no force to close them. The claws of the Lion,
and other animals of the cat tribe, are retracted within their
sheaths by means of two strong elastic ligaments. Structures
* Many birds are provided with strong elastic ligaments connecting the
vertebrse ofthe neck with those of the back; ligaments ofthe same kind are
also employed for retaining the wings close to the body, where they are not
used in flying: and a similar provision is made in the wings of bats. The
weight ofthe bulky organs of digestion in herbivorous quadrupeds requires
some permanent support of this kind; and this is furnished by a broad, elas
tic fibrous band extended across the lower part of the abdomen. It is par
ticularly strong in the elephant, which remains more constantly in the hori
zontal position than most quadrupeds: and it has been remarked that the ge
neral cellular texture in this animal has an unusual degree of elasticity. —
Hunter on the Blood, &c. p. 112.

•^

88 THE MECHANICAL FUNCTIONS.
of this kind are* employed very extensively in the fabric of
insects.* The animal substance which comes next in the order of
density is Cartilage. The purposes for which this kind of
structure is employed are those in which a solid basis is re
quired for the support of softer or more flexible parts, and
where the mechanical properties that are wanted are firmness
conjoined with some degree of elasticity. Cartilage (or gris
tle) is composed of a finer and more uniform material than
any ofthe preceding structures. It consists almost wholly
of albumen, with a slight proportion of calcareous matter.
Unlike membrane in any of its forms, it contains no fibres,
but, on being cut with a sharp knife, presents the appearances
of a dense homogeneous substance of a white pearly hue.
Its surface is smooth, and where it is exposed to friction, as
in the joints, is often highly polished.
In all the inferior tribes of animals, Nature employs car
tilage to supply the place of bone when rigidity is required
to be given to the fabric. In an extensive order of fishes,
including the shark, the sturgeon, and the ray, we find the
whole skeleton constructed of cartilage. In the fabric of
' very young quadrupeds cartilage is substituted for bone; and
' in the adult animal, various organs, such as the external ears,
the eye-lids, the nostrils, and different parts ofthe apparatus
of the throat and windpipe, are composed of flexible carti
lage, which gives them a determinate shape and firmness. In
all these cases bone, which, besides being three times as hea
vy, is devoid of elasticity, and liable to fracture, would have
been much less suitable. Cartilage is often employed as an
intermedium for connecting different bones, as, for instance,
between the ribs and the sternum,, or breast bone ; whereby,
besides the advantage of greater lightness, the pliancy of
the material diminishes those jars which are incident to the
frame in all its violent actions.
In the construction of cartilage, nature seems to have at
tained the utmost degree of density which could be given
to an internal texture composed merely of the usual animal
* Chabrier, Memoires du Musee, torn. vi. p. 416.

ANIMAL ORGANIZATION. 89
constituents. But substances of still greater hardness, united
with perfect rigidity, are wanted, in numberless instances,
for giving effectual protection to soft and delicate structures,
for supplying a firm basis to the framework of the body, and
for constructing levers of various kinds to be employed in
the more energetic movements of the higher animals. For
all these purposes it was necessary to superadd a material
endowed with stronger cohesive powers, and capable by its
dense concretion of forming solid and inflexible organs.
The substances which nature has selected for this office are
the salts of lime. Sometimes the Carbonate, and sometimes
the Phosphate of lime is employed for forming these hard
and unyielding structures; and often both these calcareous
substances are united together in different proportions in the
same solid fabric. When the carbonate of lime predomi
nates, or is the sole earthy ingredient, it constitutes Shell;
when there is a greater proportion of the phosphate, it is
called a Crust, as is the case with the coverings of the lob
ster and the crab: when the earthy matter consists almost
wholly of phosphate of lime, it composes the different forms
of Bone. I shall have occasion to describe the formation
and properties of each of these structures in the sequel.
The protection of the delicate structure of the fabric from
the injurious influence of external agents is an object of
great importance in the animal economy, and is one which
nature has shown extreme solicitude to secure. For this
purpose she has provided the integuments, under which de
signation are included not merely the skin, but also all the
parts that are immediately connected with it, and are formed
and nourished by the same vessels. No parts of the animal
structure present greater diversity in their form and out
ward appearance than the integuments; yet it is easy to dis
cover, amidst all these varieties, that the same general plan
has been followed in their construction, and that each par
ticular formation is the result of a combination of the same
elementary structures. Of these elements the most important,
and that which generally composes the chief bulk of the
skin, is the Corium, or true skin. The outermost layer is
vol. i. — 12

90 THE MECHANICAL FUNCTIONS.
termed the Epidermis, Cuticle, or scarf-skin; and between
these there is often found an intermediate layer denominated
the Rete Mucosum, or the Pigmenlum.
The corium is generally of considerable thickness, and is
composed of strong and tough fibres, closely compacted to
gether, and pervaded by innumerable ramifications of blood
vessels of every kind. It is endowed with great flexibility,
and is capable of being considerably extended; properties
which fit it for readily accommodating itself to all the move
ments of the body and limbs, and to the variable bulk of the
parts it covers. Being also very elastic, it soon regains its
natural form and dimensions, when left to itself after being
stretched. The skin is connected with the subjacent muscles
and other parts by a large quantity of cellular texture, which,
according to the particular intentions of its formation, some
times binds it tightly over these parts, and on other occa
sions allows of a free and extensive motion. This latter
property is remarkably exemplified in the Raccoon, an ani
mal whose skin hangs loosely on the limbs, and encloses
the body like a wide elastic garment; so that, however
firmly a person may attempt to grasp the animal by the neck,
it can easily turn its head completely round, and bite the
fingers that are holding it. In like manner the skin of the
frog is attached to the body only at a few places, and mav
be readily stripped off. A thin layer of muscular fibres is
often found lying immediately underneath the skin, and is
provided for the purpose of moving it over the subjacent
parts. In animals that roll themselves into a ball, as the
hedge-hog, these muscles are of great size and importance.
We shall see that in the Mollusca, this muscular apparatus
is inseparably blended with the integument, and composes
a peculiar structure, termed the mantle. Immediately co
vering the corium is the Rete Muposum, which is a very
thin layer of soft animal matter, composed of a net- work of
delicate fibres, and containing more or less of the material
from which the colou of the skin is derived.
The Epidermis is a membrane of a very peculiar nature,
consisting of a thin expansion of albuminous matter, appa-

ANIMAL ORGANIZATION. 91
rently homogeneous in its texture and composition. It is
impervious to fluids, although capable of imbibing moisture,
and of slowly transmitting a portion to the subjacent tex
tures. Its thickness varies exceedingly in different parts;
being adapted to the kind of protection it has to afford
against pressure, friction, or other causes of injury. As it
is not nourished by vessels, its outer layer is liable to be
come dry, and unfit for use: and accordingly a separation
of this outward layer generally takes place from time to
time, the loss being speedily repaired by a fresh growth
from the surface in contact with the skin. This process is
often performed periodically, as is most remarkably exem
plified in serpents.
Special provisions are made for preserving the cuticle in
a healthy condition; and more particularly for defending it
from the injurious action of the surrounding element. These
sometimes consist of a supply of oily fluid, prepared in
small cavities which are situated in the skin itself, and have
minute ducts opening upon the surface. These' cavities,
termed sebaceous follicles, are generally interspersed in
great numbers on different parts of the body, abounding
more especially in those places where folds occur, and where
there is the greatest friction. In fishes,' mollusca, and other
aquatic animals, the skin is at all times defended from the
action of the water by a viscid or glutinous secretion, pre- L
pared in this manner, and continually poured out on the sur
face, through ducts, the orifices of which are easily seen with
the naked eye, disposed in a line on each side of the body.
Connected with the skin, and more particularly with the
cuticle, are structures of very various forms, intended for
giving additional protection, occasionally contributing their
aid in progressive motion, and sometimes fashioned into
weapons of offence. In this class should be included all the
varieties of hair, such as wool, fur, feathers, bristles, quills,
and spines, as well as the more ordinary kinds of hair. All
these resemble the cuticle in their chemical composition,
differing only in their degrees of hardness and condensation.
Horn is formed of the same material as hair; as are also the

THE MECHANICAL FUNCTIONS.

nails, the hoofs, and the claws of quadrupeds, and the scales
of fishes, reptiles, and other animals. The integuments of
insects, and especially their more solid and horny coverings,
contain, however, as will hereafter be noticed, a peculiar che
mical principle termed Entomoline.
All these parts seem to be but remotely connected with
the vital actions of the system with which they are associ
ated; and it is doubtful how far they are to be considered as
appertaining to the living portion of the body, or as mere
extraneous appendages. Yet, however they may differ in
their forms, uses, and external appearance, they all are pro
duced by the same kind of vascular structure, variously ar
ranged to suit the particular circumstances in each case: and
the mode of their development and growth is essentially the
same in all.
An extremely delicate and finely organized pulp, com
posed partly of a congeries of minute vessels, and partly of
a gelatinous substance, in which these vessels are imbedded,
constitutes the apparatus by which the nutrient particles are
selected, combined, and elaborated into the materials of the
intended structure. The original form,, situation, and dis
position of this vascular pulp, determines the future figure
and extent of growth -of the production which is to arise
from it. The materials which compose it are deposited
sometimes in masses, as in the scales of the crocodile ; more

generally in layers, as in hoofs and nails, and also in the
scales of fishes;* and occasionally in filaments, as in hair;
* The laminated structure of the scales of fishes is easily distinguished by
applying to them a high magnifying power. As the breath of each new
layer is greater than the last, its edges project , farther, the whole surface

ANIMAL ORGANIZATION.

93

which latter, again, are often agglutinated together by a
strong cement, uniting them into a hard and solid structure,
of which the horn of the rhinoceros is a remarkable exam
ple. In all cases, the portions thus successfully produced,
are no longer susceptible of being nourished, and from the
moment of their deposition, undergo no farther change, ex
cept from the action of external agents. JBy the continual
additions which are made to them at their base, or root, where
the vessels deposite fresh materials-, they gradually increase
in size, protrude through the skin, and continue to grow by
the same process as long as these vessels continue in ac
tivity. The nature of this process is well exemplified in the
growth of hair. Fig. 32 shows the apparatus employed in
its construction, in an imaginary section of the root, on a
magnified scale. Every hair, takes its rise from a minute
vascular pulp, p, of an oval shape, which is implanted below
the corium, or true skin, d.* This pulp is invested by a
sheath or capsule, c, which,
together with the contained
; pulp, and the root of the hair
that grows from it, composes
the bulb of the hair. The
bulb itself is contained in a
small cell formed by con
densed membranes, s, to
which it has no attachment
excepting at the lower part, v,
where the vessels and nerves
of the pulp are passing into

having that con centric striatedappearance which renders it an interesting ob
ject for microscopic examination. Fig. 29 exhibits the striated surface of the
scale of the Cyprinus alburnus, and Fig. 30 that of the Perea Jluviatilis,
The imbricated arrangement of these scales, resembling that of the tiles on
the roof of a house, is shown in Fig. 31. All these figures represent the ob
jects highly magBifiedi
* In the above figure e is a section of the Epidermis, or cuticle;, the dotted
part, -<, represents the situation of the subjacent rete mucosum, and d, the
derm, or corium.

94 THE MECHANICAL FUNCTIONS.
it. The hair, growing by depositions from the inside of the
capsule, which forms the outer part, o, of the shaft, and from
the outside of the pulp, which forms its inner or central
part, i, is forced upwards till it has pierced the skin ; in the
course of its passage a canal is formed for it in the skin it
self, and continuous with that which encloses the bulb ; and
the course of this canal is generally oblique. In the ele
phant, where the thickness and density of the hide, present
considerable obstacles to the passage of the hairs through it,
we may discover, on minute examination, many hairs which
have only penetrated a certain way, as shown at b, without
ever succeeding in reaching the surface.
An opinion has been very commonly entertained that
each hair, on its protruding from underneath the cuticle,
e, at the point q, carries up along with it a portion of this
outward integument, which, stretching as the hair increases
in length, forms over it a very fine external tunic. ' But later
observations have shown that this is not the case, and that
there is simply an adhesion of the edge of the cuticle to the
origin of the hair, without any accompanying prolongation ;
so that if the whole bulb be destroyed, and its pulp absorbed,
the hair may be detached by the slightest force.
From this account it will be seen that a hair is, in its ori
gin, tubular; the inner part being occupied by the pulp.
But as the pulp extends only to that portion of the hair
which is in a state of growth, it never rises above the sur
face of the skin ; and the cavity in the axis of the hair is
either gradually obliterated, or is filled with a dry pith, or
light spongy substance, probably containing air. After a
certain period, the bulb diminishes in size, from the collapse
of the vessels, whose powers of supplying nutriment become
exhausted. The first deficiency in its nourishment appears
in the cessation of the deposite of colouring matter, and the
hair in consequence becomes gray. After a time, the ves
sels becoming quite impervious, the bulb shrivels, the hair
is detached, and the canal which its root occupied in the
skin becomes obliterated.

ANIMAL ORGANIZATION. ' 95
The hair of different animals, and sometimes even of dif
ferent parts of the same animal, varies in its shape, texture,
and mechanical properties. Sometimes, instead of being
cylindrical, the filaments are more or less flattened, striated,
deeply grooved, or even beaded. Instead of being solid,
they may even be tubular: and they exhibit also the great
est diversity in their length, fineness, tenacity, rigidity, and
disposition to curl. All these varieties may be traced to
corresponding differences in the form, and the relative ac
tions of the component parts of the bulb, namely, the pulp
and its capsule.*
The structure of the organs by which hairs are formed
is not easily distinguished, in the ordinary kinds of hair, on
account of their minuteness : it is readily seen, however, in
the large whiskers of the feline species, and also of the seal,
which are subservient to more extended uses than that of
merely covering the body, and which are even supplied
with nerves, converting them into instruments of a sense of
touch. In the quills ofthe porcupiae a still more complicated or
ganization has been detected. Fig. 33 shows a quill with
its bulbous root, detached from the body ; and Fig. 34, a
transverse section magnified. The bulb itself is contained
in a distinct cell, shown at a, Fig. 35, which represents a
longitudinal section of these organs. This cell contains a
portion of fat, in which the numerous vessels supplying its
pulp and capsule are imbedded. The bulb is itself sur
rounded by aw outer sheath, s, into the cavity of which, b,
there opens a duct, d, proceeding from a small cell or fol
licle, f, lodged in the cellular substance on the outside of
the sheath. This upper cell communicates below with ano
ther cavity, c, containing an unctuous matter. During the
formation of the' quill this unctuous matter is supplied through
that channel, and probably enters as an ingredient in its
composition. The capsule of the pulp consists of two mem-
* See F. Cuvier's Memoir on the Formation of the Quills of the Porcu
pine, in the Nouvelles Annales du Museum, I. 429.

96

THE MECHANICAL FUNCTIONS.

branes, the one enveloping the other. Fig. 36 shows the
bulb laid open by dividing the membranes and turning them
aside. The horny portion of the quill is secreted by the

internal membrane, i, and deposited in successive lamina?.'
The external membrane is seen at o. The pulp itself, seen at
p, is still more curiously organized ; its surface being fluted,
or formed into longitudinal processes. The horny matter,
being deposited on these processes, is moulded to their shape,
and concretes into lamina? which converge from the circum
ference of the cylinder towards the centre. The section
(Fig. 34) shows these converging laminse, which, being of
a dark colour, give to the surface of the quill the appearance
of being grooved : this, however, is merely an optical illu
sion, occasioned by the dark laminse being seen through the
transparent exterior covering; as may readily be detected
by viewing the surface with a magnifying glass.* After a
certain period of the growth of the quill, the pulp ceases to
supply the materials for forming the spongy substance which
occupies the interior of the quill. But although it no longer
secretes, it still retains its place ; and the capsule continuing
to deposite horn, the quill becomes a hollow tube of consi
derable diameter. When it has attained a certain size, the
* It is observed by F. Cuvier, that this striated appearance is peculiar to
the quills of porcupines of the old world. Those from America have no
such arrangement of laminre.

MUSCULAR POWER. 97
pulp begins to shrink, and the diameter of the tube dimi
nishes; so that it exhibits a tapering form at both ends. Thus,
mere variations in the bulk and the action of the pulp, ac
companied with changes in that ofthe capsule, are sufficient
to account for every diversity in the form and condition of
the resulting structures.
Among the mechanical uses of the integument, that of
serving as a cushion for relieving the more prominent parts
of the frame, and especially of the bones, from unequal pres
sure ought not to be overlooked. This object is promoted
by the interposition of a layer oi fat, which is another ani
mal substance entitled to be enumerated among the elements
of its structure. It consists of an oily fluid, composed, ac
cording to the analysis of Chevreuil, of two constituent prin
ciples, which he has distinguished by the terms stearin, and
elain.* In warm-blooded animals the temperature of the
body is always sufficient to preserve this compound sub
stance in a fluid form ; but it is prevented from being dif
fused through the cellular texture by being contained in
separate vesicles of extreme minuteness. t Hence, the whole
mass of the fat, which is thus formed of an aggregation of
these vesicles, has not the appearance of being fluid, but
seems to be composed of small grains united by membranous
investments into larger masses; a structure peculiarly adapted
to the purposes of a soft cushion, retaining only a small
share of elasticity, and yielding only in a certain limited de
gree to pressure.
? § 5. Muscular Power.
In Machines contrived by human skill the chief art con
sists in devising expedients for regulating and directing the
* These two constituent principles possess very different degrees of cohe
sion; elain being liquid, and stearin nearly solid, at the usual temperature:
and the consistence of the compound will, therefore, depend altogether on
the proportions in which they are united. Thus a ready expedient has been
provided for varying the mechanical properties of fat, according as circum
stances required.
¦j- Dr. Monro estimated their diameter at between the 800th and 600th oj?
an inch. But their size varies in different animals.
VOL. I.  13

SS THE MECHANICAL FUNCTIONS.
given moving power, so that it may bear, in the proper"
degree, and in the proper order, upon some assigned ob*
jects, and produce some particular effect. The whole of the ,
apparatus employed with this intention, however numerous
may be its parts, however various the forms of its wheels,
its levers, or its pulleys, and however complicated may be
their connexions, resolves itself into a series of intermediate
instruments for the transference of motion from the source
of power, or the point where its action is impressed, to the
parts which are designed ultimately to receive the action of
the force employed. It is an established principle in physics,
that mere machinery is incapable of generating mechanical
force, and that such force must always be originally derived
from some extraneous source. Some impulse from without,
whether it be the pressure of the wind, the fall of a stream
of water, or the action of men or horses, or any other kind
of foreign agency must be resorted to, both to set the engine
in motion, and to continue its movements when they are
once begun. Nor is the case essentially different when the
source of motion apparently resides in some internal part of
the machine itself-, in a watch, for instance, which is ac-
' tuated by the main spring; or in a steam engine, which is
set in motion by the elastic vapour contained in its cylinder :
the spring in the one case, and the vapour in the other, al
though they may in one sense be regarded as impelling
powers, are in reality, but intermediate agents in the distri
bution of a force originating from other sources. In the
watch, the force may be traced to the hand which coiled
the spring: in the steam engine to the fire, which has im*-
parted elasticity to the vapour.
The living body differs from inorganic machinery in con
taining within itself a principle of motion not referrible, as
far as we can perceive, to any of the primary forces which
exist in the inanimate world. This principle has been
termed contractility. In animals of the simplest construc
tion, every part of the' substance of the body seems to be
equally endowed with this contractile property, although ex
hibiting no distinct appearance of a fibrous structure. This

MUSCULAR POWER. 99
is the case with all the lower zoophytes, such as the Infuso
ria, Polypi, Medusas, and the simpler kinds of Entozoa.
Among Polypi and Infusoria we meet with a singular
mode of acting upon the surrounding fluid by means of very
minute and generally microscopic filaments, termed cilia,
which the animal, by some unknown power, causes to vibrate
with great rapidity. Occasionally, these organs are found
even in animals belonging to the higher classes. Wherever
they are met with, they perform, as will hereafter be shown,
very important functions ; sometimes assisting in respiration,
at other times contributing to the supply of food, and very
generally serving as instruments of progressive motion.
In animals placed a little higher in the scale, we begin to
trace the formation of fibres, which at first are irregularly
scattered through the soft substance: but as the organization
becomes more refined, these fibres are collected into bundles,
and compose what are properly called muscles. Muscular
fibres are attached at their extremities to the parts intended
to be moved. In the lower animals, these attachments are
principally to the skin, or other external parts, which are
subservient to the purposes of progressive motion. In the
higher classes, the solid parts, or skeleton, being disposed
more in the centre of the system, the muscles are applied to
them in the interior of the body, and are more distinctly
separated into masses, each having its proper function in
the movements of the frame.
The peculiar property which characterizes the muscular
fibre is that of suddenly shortening itself, so as to bring its
two ends, and the parts to which those ends are attached,
nearer to one another. This contraction is performed with
astonishing quickness and force, and the accumulated effect
of a large collection of these fibres, such as constitutes a mus
cle, is therefore capable of overcoming great resistances, or
of raising enormous weights. Those muscles, which, by
means of their nerves, as will hereafter be noticed, are sub
servient to voluntary motion, are excited into action by an
exertion of tlie will of the animal. There are, however, a
great number of other muscles, the contractions of which}

100 THE MECHANICAL FUNCTIONS.
are involuntary, that is, are produced by other causes than
the will.*
Muscular contractility, of which there exists no trace in
the vegetable kingdom,* has been established by nature as
the primary moving power of the animal machine. This
agent is resorted to on all occasions where considerable me
chanical force is wanted ; just as in a great manufactory,
where an immense quantity of machinery is to be set in mo
tion, and a great variety of work is to be executed, the hu
man mechanist avails himself of some constant rhoving force,
such as that derived from a fall of water, or from the ex
pansion of steam. The laws of inorganic matter furnish no
force which could conveniently have been applied in the ani
mal body for that purpose; but muscular power, from its
high intensity, is adequate to every object, and has been ac
curately adjusted, by the most refined application of the
laws of mechanism, to all the degrees and kinds of effects
intended to be produced.
Although the power be the same, yet the mode of its ap
plication is exceedingly diversified ; and the comparison of
these diversities is the more interesting, inasmuch as there
are few of the animal functions in which the ends to be an
swered are so definite, and the operation of the expedients
employed is so plain and intelligible. For while the intri-
* These two classes of muscles do not differ in their outward appearance:
but Dr. Hodgkin has lately pointed out a. curious difference in the micro
scopic structure of the fibres of some of the involuntary muscles. See Ap
pendix to his Translation of Edwards on the Influence of Physical Agents
on Life, p. 443.
¦j- The principal instances, which have been adduced in support of the
opinion that muscularity occasionally exists in vegetable structures, are the
alternate movements of the leaflets of the Hedysarum gyrims, which have
been fancifully compared to the movements of the ribs in respiration; the
quick motions of tlie stamina of the Berberis, Opuntia, and many plants of
the genera Carduus, and Centaurea,- the closing ofthe leaves of the Dionsea
muscipula; and the shrinking of those of the Mimosa pudica, or sensitive
plant. On a superficial view, it must be acknowledged that these motions
bear a resemblance to the effects of muscular contractility; but I believe
that naturalists are now generally agreed that there is no real analogy be
tween these phenomena, and that there is no substantial evidence for the
existence of that property in the vegetable kindgdom.

MUSCULAR POWER. 101
cate chemical processes of the living system generally elude
our research, and the higher faculties of sensation and per
ception are dependent on still more recondite and mysteri
ous powers of nature, the mechanical functions are effected
by the simple properties of matter, and allow us a clearer
insight into the wonderful art which has been exerted in
their accomplishment.
Muscles, during their contraction, increase in thickness
in the same proportion as they diminish in length.* It is on
this account, more especially, that a knowledge of anatomy
37 38 -43

is so necessary to the painter and the sculptor. In every
movement and attitude of the body, some particular sets of
muscles are in action, and consequently tense and prominent,
while others are relaxed and flattened ; differences which it
is requisite that the artist should faithfully express, in order
to give a correct representation of the living figure.
The dilatation of the muscular fibres in thickness, which
accompanies their contraction in length, would, if these.
fibres had been loose and unconnected, have occasioned too
great a separation and displacement, and have impeded their
co-operation in one common effect. Nature has guarded
against this evil by collecting a certain number of the ele.- -
mentary fibrils, and tying them together, with threads of
* This is illustrated by the annexed figures, 37 and 38, the former showing
the relaxed and elongated, and the latter the contracted and swollen state of
the same muscle.

102 THE MECHANICAL FUNCTIONS.
' cellular substance; thus forming them into a larger fibre ; and
again packing a number of these fibres into larger bundles:
always surrounding each packet with a web of cellular tis
sue; which thus forms a separate investment for each. This
plan of successive reunion into larger and larger assemblages
is carried on through several gradations of size, till- the en
tire muscle is completed.
That we may be the better able to appreciate the excel
lence of the plans adopted in the mechanism of the animal
frame, let us inquire what arrangements would occur to us,.
prior to an acquaintance with those actually adopted, as the
most advantageous dispositions of the muscular power. It
is evident, that the simplest mode would be that of extend
ing the fibres1 of the muscle in a straight line between the
points intended to be brought nearer to each other. This
direct application of. the power, however, is seldom compa
tible with convenience, unless the parts to be moved are of
very small size, ahd require very delicate adjustments.
Straight muscles, accordingly, are employed chiefly for the
* movements of the minuter parts of the apparatus belonging
to the senses, such as the eye, and the ear, and also that of
the voice. In insects, when the hard case, or skeleton, is
wholly external, this direct application of the moving force
is also very generally employed. The shells of the bivalve
mollusca, as of the Oyster and the Cardium, are closed by
one or two straight muscles, the fibres of which pass imme
diately from the .inner surface of the one to that of the other.
In the greater number of cases it is more convenient to
place the muscle in a situation which causes it to act ob- ,
liquely with respect to the direction of the motion produced
in the part to which it is attached. This will, of course, be
attended with a loss of force corresponding to the degree of
obliquity; but there are, at the same time, advantages gained,
not only in point of velocity of motion, but also in the effect
being produced by. a smaller extent of contraction in the
fibres of the muscle. Oblique muscles are frequently em
ployed in pairs, and are made to act on opposite sides of the
line of the intended motion, which is, in this case, the dia
gonal between the direction of the two equal forces. Thus,

MUSCULAR POWER. 103
in order to bring a bone at p, Fig. 39, down to the point q,
the two museles a and b, extending from the fixed points
m and n may be employed ; for as they exert forces in the
directions p m and p n, there will result a force in the inter
mediate direction p o: and the effect desired will be accom
plished more quickly, and with a smaller extent of contrac
tion in the muscles producing it, than if the same power had
been applied by means of a straight muscle in the direction
p o.* It is by means of two sets of muscles, acting thus ob
liquely, that the ribs are brought in closer approximation
every time that the chest is elevated in breathing. Thus
carefully does nature dispose the muscular fibres so as to
obviate the necessity of their being contracted beyond a
certain extent : and thus does she economize, as much as
possible, the expenditure of muscular power, wherever there
is a constant call for its exertion.
The principle which I have just explained, whereby cer
tain advantages result from the obliquity of the action of
muscular fibres, is applied, not only to the entire muscle,
but also to the internal arrangement of its fibres. Thus,
we generally find that, in a flat muscle, its upper and
under surfaces are covered by a thin sheet of fibrous tex
ture, or thin expansion of ligament or tendon; and that
the muscular fibres which are attached to them are di
rected obliquely from the one to the other, in the manner
represented by the section, Fig. 40. There is frequently a
middle tendinous layer interposed between those that are on
the surface (as shown in Fig. 41,) in which case the muscu
lar fibres pass obliquely from the former to the latter, but in
different directions on each side; like the fibres proceeding
from the shaft of a pen. A muscle thus constructed has ac
cordingly been termed a penniform muscle; as is exempli
fied in the straight muscle inserted into the knee-pan (the
rectus extensor cruris,) and also in the muscle which bends
the great toe (the flexor pollicis pedis- longus.) The ar
rangement first described, Fig. 40, forms the semi-penni-
farm muscle; an instance of which occurs in the muscle of
* See a paper by Dr. Monro, in the Transactions of the Royal Society of
Edinburgh. Vol. iii. p. 250.

104 THE MECHANICAL FUNCTIONS.
the leg, which is termed the semimembranosus. Frequent
ly the structure is rendered still more complex, by the in
terposition of several tendinous layers among the fleshy
fibres. This arrangement, which constitutes a complex
muscle, (as shown in Fig. 42) occurs, for example, in the
Solceus, or large muscle, which raises the heel, and forms
the thickest part of the calf of the leg.
It very commonly happens in the animal frame, as it
does in other machines, that the presence of the moving
agent in the place where its action is wanted, would be ex
ceedingly inconvenient. The usual plan adopted for trans
ferring the effect of the moving power to a distant point is
the employment of a rope, or strap. Such is precisely the
office of the tendons, which are long straps, attached at one
end to the muscle, and at the other to the bone, or other
part intended to be moved. (See Fig. 43.) If the hand,
for instance, had been encumbered with all the muscles
which are necessary for the movements of the fingers, it
never could have performed its office as a delicate mecha
nical instrument. These muscles accordingly are disposed
high up on the arm, and their tendons are made to^pass
along the wrist to the joints of the fingers which areto be
moved. /
The employment of tendons is accompanied with this
farther advantage, that by their intervention the( united
power of all the fibres of the muscle may be obta-ihed, and
concentrated upon any particular point. In this respect, like
wise, they resemble a rope, at which a great number of men
are pulling at the same moment, and whose combined strength
is thus brought into action. Another principal use of ten
dons is, that a different direction may, by their means, be
given to the moving power, without altering its position.
Many instances occur of their application in this manner,
Nby their being made to pass round corners of bones, and
along grooves, or channels, expressly formed for their trans
mission, and producing the effect of pulleys.
In a great number of muscles, the fibres, instead of running
parallel to one another, are made either to converge, or to
diverge, in order to suit particular kinds of movements; and

MUSCULAR POWER.

105

we frequently find that different portions of the same mus
cle have the power of contracting independently of the rest,
so as to be capable of producing very various effects, accord
ing as they act separately or in combination. This is exem
plified in the muscle of the back, called the Trapezius, repre
sented in Fig. 44. In many instances, the fibres radiate in
all directions from a common centre : this is the case with the
delicate muscle of the ear-drum, as shown in Fig. 45. In
¦that of the elephant, which is about an inch and a half in
diameter, these radiating fibres are very conspicuous, even
to the naked eye: and they are also visible in the membrane.
of the human ear, when viewed with a good microscope.*
At other times, the muscular fibres run in a circular di
rection, forming what is called an orbicular, or sphincter
muscle, of which an example occurs in that which surrounds
and closes the eye. (Fig. 46.) Very frequently these two
last modes of arrangement are united in some part, as ap
pears to be the case in the membrane of the eye, called the
Iris. (Fig. 47. The circular fibres of the iris surround the
¦central aperture, or pupil, the size of which they diminish
when they contract ; while, on the contrary, the radiating
fibres, acting on the inner circle, and drawing it nearer to,
the outer circumference, which is fixed, lessen the breadth
of the ring, and consequently enlarge the circular aperture.

A similar combination of radiating and circular fibres is
employed in the construction of flat, or slightly concave
muscular disks, which are thus rendered capable of exerting
a strong force of adhesion to the surfaces to which they are
applied. In these organs the circular fibres are placed at

vol. i. — 14

» Home Phil. Trans, for 1800, p. 1.

106

THE MECHANICAL FUNCTIONS.

the circumference, and the radiating fibres in the interior of
the sucker, (see Fig. 48 ;) so that, while the margin of the
disk is closely applied to the object, the force resulting from
the contraction of the circular fibres is exerted to remove
the central portions from the surface of attachment, and
thereby tends to create a vacuum underneath the disk; the
two surfaces remain, therefore, strongly attached by the at
mospheric pressure, which acts on the,ir outer sides. An ap
paratus of this kind, as we shall afterwards find, is met with
very frequently among the lower orders of the animal
kingdom. 49 50 ftl

Another kind of circular disposition of fibres is th.at which
occurs in the muscular coats surrounding canals of various
' kinds, such as the blood vessels and the alimentary tube.
Their action tends to contract the diameter of the canal, and
to exert pressure on its contents, In these cases, there is
generally at the same time provided another layer of fibres,
disposed longitudinally, as shown in Fig. 40; the circular
fibres being seen in Fig. 50. The action of the longitudinal
fibres, is evidently to shorten the canal; while that of the
circular fibres, by tlie yielding of the coats, and the partial
reaction of the contents of the vessel, has a tendency to
extend it. The Ascidia, which is a species of marine worm,
is an example of an animal whose skin contains a union of
straight and circular fibres, by which all its movements are
readily performed. Many instances occur in the cylindrical
envelopes of animals, of the combination of a third series of
fibres, passing obliquely, with those which have transverse
and longitudinal directions. In the muscular skin of the
Leech, for example, besides two internal layers of longitudi
nal fibres, an external one has lately been discovered, which
is composed of oblique or spiral fibres, crossing one another

MUSCULAR POWER. 107
in opposite directions, and greatly facilitating the varied
movements of the animal.*
A variety of still more complicated arrangements may be
traced in the fibres of those muscles which invest hollow
sacs, or receptacles, such as the stomach, (Fig. 51,) and the
heart, (Fig. 52.) We find, in the substance of these organs,
sets of fibres, which pass in a spiral direction, and which,
consequently, unite the effects of both longitudinal and cir
cular fibres; and, when combined with either of these, they
serve to modify and regulate the actions of each organ in a
great variety of ways.t
The infinite mechanical skill, with which the moving
power has been applied to the purposes to be accomplished,
is displayed not only in the larger organs, where great force
is to be exerted, but also, in a still more conspicuous man
ner, in the execution of the smaller motions, requiring the
most accurate regulation,. and the nicest adjustments. We
cannot but be struck with the accordance which may often,
in these instances, be traced with human contrivances, where .
the greater motions are rapidly executed by one set of
agents, acting with considerable power and velocity, while
the minuter approximations to the exact positions are effect
ed by a distinct part of the apparatus, capable of more deli
cate action, though with a smaller force. Thus, while the
astronomer brings- his telescope round by powerful machi
nery, so as to direct it to that part of the heavens, where
the object he wishes to view is situated, a more nice me
chanism is employed tr> direct the instrument accurately to
the exact point; and, again, another is provided for making
the proper focal adjustments. Many parallel cases occur in
«
* Carus, Tabula Anat. Comp. fol. Tab; I. Fig. 6.
+ The muscular fibres of the heart are disposed in' two layers; each set
passing in a spjral course from the basis, or broad part, to the point or apex;
but the direction ofthe turns being different in each, the two layers cross or
decussate, producing the effect and procuring the advantages of a combina
tion of oblique muscles already explained. Thus beautifully is the arrange
ment ofthe muscular 'fibres of th'e heart calculated to produce the rapid and
complete expulsion of its contained blood,, with the smallest amount of con
traction in the individual fibres.

: i

108 THE MECHANICAL FUNCTIONS.
the mechanism of the animal frame; one set of powerful
muscles being employed for the larger movements, and ano
ther set provided for the accurate regulation of the more
delicate inflections and nicer positions. This we shall find
exemplified in the movements of the fingers, and of many
of the organs of the finer senses.
In general, however, we may observe that the mechanical
expedients devised by Nature for effecting each particular
purpose are characterized by the most admirable simplicity.
In this respect, also, as well as in all others, we cannot fail
to recognise their infinite superiority over every correspond
ing invention of man.
" In human works, though labour'd on with pain,
A thousand movements scarce one purpose gain:
In God's, one single can its ends produce,
Yet serves to second, too, some otherwise." — Pope.
We may generally observe, in the mechanism of the
joints, that the muscles are made to act, either directly or*
by means of their tendons, at a point much nearer to the
axis of motion than the resistance to be overcome- With
regard to the direct force, therefore, it is evident that they
must act with a great mechanical disadvantage; and this dis
advantage is still farther increased by the obliquity of the
action with reference to the direction of the motion. But
the contractile power, which is inherent in the muscular
fibre, is so enormous, as amply to afford these losses, great
as they necessarily are ; while, on the other hand, full com
pensation is made by the greater freedom and velocity of
motion thereby obtained. Strength is sacrificed without
scruple to beauty of form or convenience of purpose; and
that disposition of the force is always adopted, from which,
on the whole, the greatest practical benefit results. Every
where do we find the wisest adaptation, of muscular power
to the objects proposed, whether it be exerted in laborious
efforts of the limbs and trunk ; whether employed in ba
lancing the frame, or urging it into quick progression ; or
whether it be applied to direct the delicate evolutions of

MUSCULAR POWER. 109
the fingers, the rapid movements of the organs of speech,
or the more exquisite adjustments of the eye, or of the in
ternal ear. Amidst the endless combinations of machinery
exhibited in different parts of the animal kingdom, although
the mode of application be diversified in ten thousand ways,
the original power is still of the same kind, and is regulated
by the same physical laws ; and similar instruments are em
ployed in effecting this infinite variety of purposes, by the
all-wise and omnipotent Architect of animated creation.

( no)

CHAPTER II.
THE MECHANICAL FUNCTIONS IN ZOOPHYTES.
§ 1. General Observations.
The mechanism of an organized being is designed to ful
fil various important objects. These we may distinguish
into two classes; the one having reference to its internal
welfare, the other to its relations with external bodies. The
different parts of its system must, in the first place, be me
chanically united and supported, as well as protected from
injurious external impressions ; and they must at the same
time be so constructed as to admit of all the internal move
ments, which the performance of their functions renders
necessary. They must, in the second place, be made capa
ble of exerting upon external matter the actions which con
duce to their well being; and, in order to enlarge their
sphere of action, they must have the power of transferring
the whole body from one place to another; or, in other
words, of effecting its progressive motion.
The objects included in the first of these branches of the
mechanical functions are answered by the organization both
of the vegetable and the animal systems : but those of the
latter belong exclusively to the functions of animal life.
The power of locomotion, more especially, constitutes the
most general and palpable feature of distinction between
these two classes of beings. A plant, during the whole pe
riod of its existence, is fixed to the spot where it was first
produced, and is dependent for the continuance of its life on
local circumstances; such as the nature ofthe soil in which
its roots are imbedded, and the qualities of the air and wa-

ZOOPHYTBS. Ill
ter in its immediate vicinity. It is exposed to the action
of the surrounding elements, and affected by their vicissi
tudes, without the means of retreat, and without the power
of reaction. With respect to all external agents, indeed,
vegetables may be regarded as passive beings. Very dif
ferent are the condition and destination of animals. Ex
cepting a few among the lower orders of the creation, such
as Zoophytes and Mollusca, all animals are gifted with the
power of spontaneously changing their situation, according
to their several wants and necessities, and are thus enabled
to seek and to choose those objects which are salutary, and
to avoid or reject those which are injurious. Nature has, for
these purposes, furnished them with a more complex organi
zation, and more varied powers, adap'ted to a greater di
versity of pursuits, and to a higher and more expanded
sphere of existence.
The power of progressive motion is enjoyed in very dif
ferent degrees by different races of animals, according to the
particular model on which they are constructed, and the re
lations which their organization bears to the element assigned
as their residence. All the mechanical circumstances in
their economy, indeed, are so closely linked together, as
scarcely to admit of being considered separately. Thus, we
find, in one animal, a variety of mechanical effects accom
plished by one and the same instrument ; while, in others
they are each produced by a separate and distinct organ. In
some, the leading principle of the construction is simplicity ;
in others, the most elaborate mechanism is displayed. But
the meanshave constant reference to the design, and are ever
varied in exact conformity with the change of purpose, The
relative advantages of each plan of structure appear to have
been carefully estimated, and studiously balanced. Each
quality has been bestowed in different degrees of perfecr
tion ; so, that in following the series of gradation among the
successive tribes of animals, we occasionally meet with far
voured species, endowed with great superiority in some
particular faculty. Some animals excel in swiftness; others
in strength. Some are qualified to dive into the recesses of
the deep; others to flutter in the light regions of air; while,

112 THE MECHANICAL FUNCTIONS.
in many of the inferior ranks, we find all these objects re
nounced for the more certain advantage of security, which
the softer texture of the organs renders one of paramount
importance. That construction of limbs which favours cer
tain movements will necessarily interfere with the ready
performance of others, and must preclude the development
ofthe organs which would be necessary for facilitating them.
Different kinds of prey require dexterity in particular ac
tions for their pursuit and seizure. The animal is, in one
case, formed for climbing trees ; in another, for burrowing
in the earth: in a third, for perforating wood. Some are
provided with organs for penetrating into the bodies of other
animals; others with the means of insnaring their captives;
while others, again, instil into the veins of their victims a
deadly poison. Hence it is necessary, in studying the or
ganization of animals, to bestow particular attention on the
habits and mode of life for which each respective tribe and
species has been destined.
In the examination of the mechanical functions which
will form the first part of this treatise, I shall keep in view,
as the leading object of inquiry, the faculty of progressive
motion, noticing its different degrees of perfection as we fol
low the ascending series of animals; but adverting, also,
occasionally, to the other topics which belong to this class
of functions.
It may be observed, in general, that the mechanical con
struction of animals wiiich constantly inhabit a watery ele
ment is more simple than the construction of those which
live on land, and are encompassed by a lighter medium.
Differing but little in their specific gravity from the fluid in
which they are immersed, aquatic animals are necessarily
•supported, on all sides, by a powerful hydrostatic pressure,
¦which nearly balances the force of gravity, and counter
acts the tendency of their bodies to descend in the fluid.
Many of the obstacles to progressive motion are thus re
moved ; and there is no necessity for the compactness of
frame, and the rigidity and cohesion of substance which are
required in terrestrial animals.

SPONGES. 113
The animals which occupy the lower divisions of the scale
can exist only in a liquid element. Their forms present
many analogies with vegetables ; and hence they have|been
denominated Zoophytes, that is, animated plants : but as it
is now well ascertained that they possess the essential cha
racters of animals, the term of Phytozoa, or plant-like ani
mals, which has been given to them by some modern writers,
would appear to be a more appropriate designation. It is,
•however, scarcely worth while, at the present day, to change
¦a name so generaUy received as that of Zoophytes, and the
application of which is not likely to lead to any misunder
standing.. § 2. .Porifera, or Sponges.
Amo»g Zoophytes, the lowest station in the scale of or
ganization is occupied by the tribes of Porifera, the name
given by Dr. Grant to the animals which form the various
•species of sponge, and which are met with in such multi
tudes on every rocky coast of the ocean, from the shores of
Greenland to those of Australia. Sponges grow to a larger
size within the tropics, and are found to be more diminutive
and of a firmer texture, as we approach the Polar circles.
Dr. Grant observes* that they are met with equally in places
covered perpetually by'the sea, as in those which are left
dry at every recess of the tide. They adhere to, and
spread over the surface of rocks and marine animals, tdl
which they are so firmly attached that they cannot be re
moved without lacerating and injuring their bodies. " Al
though they thrive best," he farther remarks, " in the shel
tered cavities of rocks, they come to maturity in situations
exposed to the unbroken fury of the surge. They cover the
nakedness of cliffs and boulders; they line with a variegated
and downy fleece the walls of submarine caves, or hang in
living stalactites from the roof."
In their general appearance they resemble many kinds of
* Edinburgh Philosophical Journal, vol. xiii. p. 94.
VOL. I.  15

114 THE MECHANICAL FUNCTIONS.
plants, but in their interna] organization they differ entirely
from every vegetable production; being composed of a soft
\ flesh intermixed with a tissue of fibres, some of which are
solid, others tubular; and the whole being interwoven to
gether into a curious and complicated net-work- The sub
stance of which this solid portion, or basis, is formed, is
composed partly of horn, and partly of siliceous or calcare
ous matter. It has been termed the axis of the Zoophyte;
and as it supports the softer substance of the animal, it may
be regarded as performing the office of a skeleton, givingform
and protection to the entire fabric.
The material of which the fleshy portion is composed is
of so tender and gelatinous a nature that the slightest pres
sure is sufficient to tear it asunder, and allow the fluid parts
lo escape ; and the whole soon melts away into a thin oily
liquid. When examined with the microscope the soft flesh
is seen to contain a great number of minute grains, dissemi
nated through a transparent jelly. Every part of the sur
face of a living sponge (as may be seen in fig. 53) presents
to the eye two kinds of orifices; the larger having a rounded

53

55

shape, and generally raised margins, which form projecting
papillae; the smaller being much more numerous, and ex
ceedingly minute, and constituting what are termed the
pores of the sponge.
It was, for a long time, the received opinion among
naturalists that this superficial layer of gelatinous substance
was endowed with a considerable power of contractility: it
was generally believed that it shrunk from the touch, and
that visible tremulous motions could be excited in it by

SPONGE!.. 115
punctures with sharp instruments, or other modes of irrita
tion. These notions are of very ancient date, for they may
be traced even beyond the time of Aristotle; and they have
been handed down by succeeding naturalists, and echoed
from the one to the other, so as to have gained admission with
out being questioned in all the systematic works on Zoology.
The alleged spontaneous palpitation of the flesh, occur
ring in particular parts, had its origin in the views taken of
the nature of sponges, by Marsigli, an Italian naturalist, who,
in the year 1771, announced that he had seen movements of
dilatation and contraction in the round apertures visible on
the surface of sponges. This statement, so confidently ad
vanced, seems to have made a strong impression on Ellis,
who, while pursuing a similar train of observations, came to
persuade himself that he could see, not only the movements
described by Marsigli, but also the passage of water to and
fro, through the same apertures. He communicated this
account to the Royal Society in 1765: it was published in
its Transactions,* and will ever remain an instructive proof
of the degree in which our very perceptions may be influ
enced by preconceived views, and by the force of the ima
gination. Pallas immediately admitted, without examina
tion, the hasty assertion of Ellis, into his " Elenchus Zo-
ophylorum; whence it was copied by succeeding authors,
and the error became at length so widely disseminated, that
for more than half a century it was received as an esta
blished fact in natural history. The more accurate re
searches of Dr. Grant on these subjects have at length dis
pelled the prevailing illusion, and have clearly proved that
the sponge does not possess, in any sensible degree, that
power of contraction which had, for so many ages, been
ascribed to it.f
• Vol. lv. p. 284.
+ See his papers on this subject in the Edinburgh Philosophical Journal,
vol. xiii. p. 95 and 333, from which most ofthe facts mentioned in the above
account are taken. ,

116 THE MECHANICAL FUNCTIONS.
Dr. Grant has also shown the true nature of the currents
of fluid issuing at different points from the surface of these
animals, as well as the absence of all visible movements in
the orifices which give exit to the fluid. Never did he find,
in his experiments, the slightest appearance of contraction
produced in any part of the sponge, by puncturing, lace
rating, burning, or otherwise injuring its texture, or by the
application of corrosive chemical agents. Of jjis discovery
of the fluid currents, he gives the following interesting ac
count : " I put a small branch of the Spongia coalita, with
some sea-water, into a watch-glass, under the microscope,
and, on reflecting the light of a candle through the fluid, I
soon perceived- that there was some intestine motion in the
opaque particles floating through the water. On moving
the watch-glass, so as to bring one of the apertures on the
side of the sponge fully into view, I beheld, for the first time,
the splendid spectacle of this living fountain, vomiting forth;
from a circular cavity ,-an impetuous torrent of liquid matter,
and hurling along, in rapid succession, opaque masses, which
it strewed every where around. The beauty and novelty of
such a scene in the animal kingdom, long arrested my atten
tion, but after twenty-five minutes of constant observation,
1 was obliged to withdraw my eye from fatigue, without
having seen the torrent for one instant change its direction,
or diminish, in the slightest degree, the rapidity of its course.
I continued to watch the same orifice, at short intervals, for
five hours, sometimes observing it for a quarter of an hour
at a time, but still the stream rolled on with a constant and
equal velocity." About the end of this time, however, the
current became languid, and, in the course of another hour>
it ceased entirely. Similar currents were afterwards ob
served by Dr. Grant in a great variety of species. They
take place only from those parts which are under water, and
immediately cease when the same parts are uncovered, or "
when the animal dies.
It thus appears that the round apertures in the surface of
a living sponge are destined for the discharge of a constant

SF0N0BS. 117
stream of water from the interior of the body ; carrying
away particles, which separate from the sides of the canals,
and which are not only seen, under the microscope, con
stantly issuing from these orifices, but may even be per
ceived by the naked eye, propelled occasionally in larger
masses.* For the supply of these constant streams, it is evident that
a large quantity of water must be continually received into
the body of the sponge. It" is by the myriads of minute
pores, which exist in every part of the surface, that this
water enters, conveying with it the materials necessary for
the subsistence of the animaL These pores conduct the fluid
into the interior, where, after percolating through the nu
merous eh-annels of communication which pervade the sub
stance of the body, it is collected into wider passages, ter
minating in the fecal orifices above described, and is finally
discharged. The mechanism by which these currents are
produced is involved in- much obscurity. There can be no
doubt that they are occasioned by some internal movements;
and the analogy of other zoophytes would lead us to ascribe
them to the action of fibrils, or cilia, as they are termed, pro
jecting from the sides ofthe canals through which the streams
pass; but these cilia have hitherto eluded observation, even
with the highest powers of the microscope.
The organization- of sponges is as regular and determinate
as that of any other animal structure-, and presents as syste
matic an arrangement of parts. In some species, such as the
common- sponge, the basis is horny and elastic, and composed
, of cylindric tubes, which open into each other, and thus
form continuous canals throughout the whole mass.
Others have a kind of skeleton, composed of a tissue of
needle-shaped crystals of carbonate of lime, or of silex.
These hard and sharp-pointed fibres', or spicula, are disposed
around the internal canals of the sponge, in the order best
* The currents issuing from the larger orifices are best seen by placing the
living animal in a shallow vessel of sea-water, and strewing a little powdered
chalk on the surface, the motions of which will render- the currents very
sensible to the eye. Fig. SS exhibits these phenomena.

118 THE MECHANICAL FUNCTIONS.
calculated to defend them from compression, and from the
entrance of foreign bodies. Some of these spicula are deli
neated in Fig. 54: but their forms, although constant in each
species, admit of considerable diversity in the different kinds
of sponge.
Although sponges, in common with the greater number
of zoophytes, are permanently attached to rocks, and other
solid bodies in the ocean, and are consequently destined to
an existence as completely stationary as that of plants, yet
such is not the condition of the earlier, and more transitory
stages of their development. Nature, ever solicitous to pro
vide for the multiplication of each race of beings, and for
their dissemination over the habitable globe, has always pro
vided effectual means for the accomplishment of these im
portant ends. The seeds of plants are either scattered in
the immediate neighbourhood of the parent, and take root
in the adjacent soil, or are carried, to more distant situations
by the wind or other agents. In the animal kingdom, the
young offspring of those races which are endowed with a
wide range of activity, are reared on the spot where they
were produced, either by the fostering care of the parent,
or by means of the nourishment with which they are sur
rounded in the egg, and there remain until the period when,
by the acquisition or extension of locomotive powers, they
are enabled, in their turn, to go in quest of food. But in the
tribes of animals at present under our consideration, this or
der is reversed. It is the parent that is chained to the same
spot from an early period of its growth, and it is on the
young that active powers of locomotion have been conferred,
apparently for the sole purpose of seeking' for itself a proper
habitation at some distance from the place of its birth ; and
when once it has made this selection, it there fixes itself un
alterably for the remaining term of its existence.*
* Phenomena, still more curious are presented by a tribe of natural pro
ductions, resembling aquatic plants in all their external characters, but, after
a certain period, giving birth to an immense number of animated globule's,
which, for a time, move briskly in the fluid, like infusory anjmalcules, and
then congregate together, and arrange themselves in linear juxtaposition, as

SPONCIES. 119
The parts of the Spongia panicea, which are naturally
transparent, contain at certain seasons a multitude of opaque
yellow spots, visible to the naked eye, and which, when ex
amined by means of a microscope, are found to consist of
groups of ova, or more properly gemmules,-\ since we can
not discover that they are furnished with any envelope.
In the course of a few months these gemmules enlarge in
size, each assuming an oval or pear-like shape, and are then
seen projecting from the sides of the internal canals of the
parent, to which they adhere by their narrow extremities.
In process of time they become detached, one after the other,
and are swept along by the currents of fluid, which are ra
pidly passing out of the larger orifices. Fig. 55, represents
one of these gemmules detached from the parent sponge.
When thus set at liberty, they do not sink by their gravity
to the bottom of the water, as would have happened had
they been devoid of life; but they continue to swim by their
own spontaneous motions, for two or three days after their
separation from the parent. In their progression through
the fluid they are observed always to carry their rounded
broad extremity forwards. On examining this part with the
microscope, we find that it is covered with short filaments,
or cilia, which are in constant and rapid vibration. These
cilia are spread over about two-thirds of the surface of the
body, leaving the narrower portion, which has a whiter and
more pellucid appearance, uncovered. They are very mi
nute transparent filaments, broadest at their base, and taper-
if by a kind of organic crystallization, thereby forming the stems and branched
filaments of these apparent plants. These singular productions, which have
been recently studied by M. Gaillon, and which he has established into a na
tural family, denominated Nemazoaria, seem in their progressive develop
ments, to possess alternately the characters of vegetables and of animals, and
may perhaps be regarded as connecting links between the two great king
doms of living nature. (See "Appercu d'Histoire Naturclle, et Observa
tions sur les limites qui s^iparent le Regne Vegetal du Regne Animal. Par
B. Gaillon. Boulogne, 1833.")
f Gemmule is a term derived from the Latin word gemma, a bud; and its
meuning, as applied to zoophytes, is that of a young animal, not contained
within an envelope, or egg.

120 THE MECHANICAL FUNCTIONS.
ing to invisible points at their extremities : they strike the
water by a rapid succession of inflections, apparently made
without any regular order, but conspiring to give an impulse
in a particular direction. When the body is attached by its
<tail, or narrow end, to some fixed object, the motion of the
cilia on the fore part ofthe body determines a current of fluid
to pass in a direction backwards, or towards the tail; but
when they are floating in the water, the same action pro
pels them forwards in the opposite direction, that is, with the
broad ciliated extremity foremost. They thus advance, with
out appearing to have any definite object, by a slow gliding
motion, totally unlike the zig-zag course of animalcules in
eearch of prey. Yet they appear to have a consciousness of
impressions made on them; for on striking against each other,
or meeting any obstacle, they retard a little the motion of
their cilia, wheel for a few seconds round the spot, and then,
renewing the vibrations, proceed in their former course.
In about two or three days after these gemmules have quit
ted the body of the parent, they are observed to fix themselves
on the sides or bottom of the vessel in which they are con
tained; and some of them are found spread outjlike a thin cir
cular membrane on the surface of the water. In the former
case, they adhere firmly by their narrow extremity, which is
seen gradually to expand itself laterally, so as to form a broad
base of attachment. While this is going on, the cilia are still
kept in rapid motion on the upper part, scattering the opaque
•particles which may happen to be in the fluid, to a certain
distance around. But these motions soon become languid,
and, in the course of a few hours, cease; and the cilia, being
no longer wanted, disappear. The gemmule then presents
ihe appearance of a flattened disk, containing granules, like
the flesh of the parent sponge; and also several spicula in
terspersed through the central part. In less than twenty-
four hours, a transparent colourless margin has extended
round the whole gemmule, and continues to surround it
during its future growth. The spicula, which were at first
small, confined to the central part, and not exceeding twenty
in number, now become much larger and more numerous;

SPONGES. 121
and some of them shoot into the thin homogeneous margin.
It is a remarkable circumstance that the spicula rnake their
appearance completely formed, as if by a sudden act of
crystallization, and never afterwards increase their dimen
sions. When two gemmules, in the course of their spreading on
the surface of a watch-glass, come into contact with each
other, their clear margins unite without the least interrup
tion ; | they thicken and produce spicula: in a few days we
can detect no line of distinction between them, and they
continue to grow as one animal. The same thing happens,
according to the observation of Cavolini, to adult sponges,
which, on coming into mutual contact, grow together and
form an inseparable union. In this species of animal graft
ing we again find an analogy between the constitution of
zoophytes and that of plants.
#n the course of a few weeks, the spicula are assembled
in groups, similar to those of the parent sponge; assuming..
circular arrangements, and presenting distinct openings at
the points they enclose. The young animal now rapidly
spreads and enlarges in every direction, becoming more
convex, and at the same time more opaque, and more com
pact in its texture ; and, before it has attained the tenth of
an inch in diameter,, it presents, through the microscope, a
miniature representation of its parent.
Thus has a power -ef spontaneous motion been given to
what may be regarded as the embryo condition of animals,
which are afterwards so remarkable for their inertness, and
for the privation of all active powers; and it has been con
ferred evidently for the purpose of their being widely disse
minated over the globe. Had not this apparatus of moving
cilia been provided to the gemmules of such species as
hang vertically from the roofs of caves, they would' have
sunk to the bottom of the water, and been crushed or buried
among the moving sand, instead of supporting themselves
while carried to a distance by the waves and tides of the
ocean. Many species which abound in the Red Sea and
Indian Ocean have, in this way, been gradually transported,
VOL. 1.^-16

122

THE MECHANICAL FUNCTIONS.

by the Gulf stream, from the shores of the east to the cor
responding latitudes of the new world.
§ 3. Polypifera.
The next step in the organic series introduces us to the
extensive family of polypifera. The transition from the
structure of the sponge to that of the polypus may be thus
conceived. Suppose the absorbing orifices of the former to
be enlarged, and their number to be at the same time re
duced; and let these orifices be drawn out into tubes, and
provided with vibratory cilia: in addition to which, let
there be placed around their margin a circular row of larger
filaments, extremely flexible, and capable of twining round
any object that comes within their reach, and of conveying
it to the central orifice, which performs the office of a mouth.
Each tube, thus furnished with a circle of radiating fila
ments, or tentacula, as they are called, is denominated a
Polype* The animal structure thus composed has received
the name of Lobularia (Fig.. 56,) and is the genus among

• For the sake of greater distinctness.! shall employ the term polype to
denote the single tube with its tentacula; and shall designate by tlie Latin-
term polypus the entire animal mass composed of an aggregation of these
polypes. Polypifera, the name ofthe order, expresses animals bearing po
lypes.

POLYPI. 123
this tribe that approaches the nearest in its character to the
•sponge which it resembles in the nature of its internal tex
ture. Each of the polypes with which its surface is studded
ihas eight serrated tentacula. Fig. 57 represents one of these
polypes detached. Polypes may thus be united in immense
numbers into one mass, having mutual organic connexion.
In other cases they may form smaller clusters, or be eve,n
totally unconnected. Sometimes the detached polypes are
still disposed to assemble in groups, as is the case with the
Zoanthus cf Cuvier* (Fig. 58 :) at other times they are al
together isolated, as in the Hydra viridis (Fig. 59.f)
Polypi form a very extensive order of zoophytes, abound
ing in every part of the ocean, but growing in greatest lux-
ariance in the warmer regions of the globe. Their flesh
exhibits the same granular appearance as that ofthe sponge,
but it is generally firmer., and often intermingled with masses
of calcareous matter. The tentacula, which may be com
pared to arms, vary in number and in length in different
species of polypi, and sometimes, instead of a single row,
each ofthe mouths has two or more series of tentacula placed
around it. They are formed of a prolongation of the soft
substance of the polypus, and are sometimes tubular; and
their cavities are then continuous with that of the general
internal cavity into whieh the several mouths open. Be
sides being flexible in every direction, the tentacula are also
capable of being lengthened or shortened at the pleasure of
the animal. Their elongation is produced by the propulsion
of a fluid into their interior, derived from the general cavity
of the body ; and their retraction is effected by the return of
the same fluid.
The whole arrangement of the tentacula on the margin of
the projecting mouths bears a striking resemblance to a
flower, especially to those which, like the daisy, or china-
aster, have the corolla composed of slender radiating petals.
We find, indeed, that as the organs of zoophytes become
more developed, the affinities which these lower departments
* The Hydra sociata of Gmelin; the Actinia sociata of Ellis.
\ In this figure two hydra, are seen" attached to the stem of a plant.

124 THE MECHANICAL FUNCTIONS.
of the animal kingdom retain with plants, are more marked
and more predominant. In the construction of zoophytes,
nature seems still to keep in view the models of vegetable
forms, the characters of whieh, while effecting the transition
from one kingdom to the other, she continues to impress on
her productions. Zoophytes, both in their outward form,
and in the disposition of their internal organs, preserve the
symmetrical arrangement round a common centre so gene
rally exhibited in plants, and especially in flowers, and in
the verticillated leaves and branches.* Hence, the radiated
or star-like forms which predominate in most of the animals
composing this class : and, hence, they have obtained the ti
tle of Radiuta, by which Cuvier has designated them.
Like the animals of the sponge tribe, Polypi are for the
most part attached to some inorganic shell or base,, which
may be either of a horny or calcareous nature. The form
of this shell admits, qf almost infinite variety. In some it
constitutes the external surface of the animal, and encloses
the flesh in a general sheath, leaving only openings at the
extremities of the tubes for the expansion of each set of
tentacula surrounding the respective mouths. Sometimes
these tubes are joined together endwise, like the branches of
a tree, leaving lateral apertures for the protrusion of the
tentacula of each separate polype : this is the casein the
Serlularia. (Fig. 60.) At other times the
tubes are .placed parallel to each other,
like the pipes of an organ, with transverse
partitions at .regular intervals : such is the
structure of the Tubipora musica, as shown
in Fig. 61.. In Fig. 62, a portion of the
tubes is seen highly magnified, and laid
open, to show the pqlypes in their inte-
.riotr.
In some species the horny base is fashioned into a num
ber of cells, each of which serves for the protection of its
respective polype. These cells are generally placed at the
* See page 76.

POLYPI.

125

extremity of the branches, presenting the greatest similitude
to flowers. The Flustra (Fig. 63) is composed of minute

and almost microscopic cells, spread over a flat membra
neous substance, resembling, in the flexibility of its texture,
and its mode of subdivision, the leaves of plants. These
cells are arranged in rows, with great regularity, like those
of a honey-comb, as is seen in the magnified view of them,
Fig. 64.
In other tribes the inorganic base of support is internal,
constituting a kind of skeleton or axis; the polypous mouths
being spread at intervals over the surface of the fleshy layer
which covers this skeleton. This is the case with the Gor-
gonia, Antipalhes and Coral, which exhibit still closer re
semblances to the branched forms of vegetable stems, The
flesh contains granules of calcareous matter, which, in the
dried specimens, adhere to the surface'of the stems. Fig.
65 is a branch of the Corallium rubrum, of which Fig. 66

is a magnified portion, showing the appearance of the poly
pes in their expanded and contracted states. The way in
which the polypes are imbedded in the flesh is seen in Fig,
67, which represents a section of the Gorgonia Briareus.
In many cases the polypes are lodged in cup-like depres-

126 THE MECHANICAL FUNCTIONS.
sions in the surface of the calcareous axis, which affords them
some degree of protection. In Madrepores these depres
sions are crossed by radiating plates, adapted to the form
and number of the tentacula. In Millepores the cells are
closer and more minute, and exhibit none of these star-like
radiations. In some species the plates have more of a pa
rallel arrangement; and in others they form a net- work. '
The material of which this axis, to which the polypes are
attached, is composed, is of various kinds. Sometimes it is
horny, flexible, and elastic, corresponding in its nature to
animal membrane: at other times it is hard and calcareous,
being composed principally of carbonate of lime, with a
small quantity ofthe phosphate; the proportion of this latter
\ ingredient varying in different species. In all cases the
. 3 particles of calcareous matter are united together by some
portion of animal substance which may be obtained by dis
solving out the former by an acid. We always find the ma
terials arranged in concentric layers, indicating that their
deposition has been successive; and the surface is marked
by longitudinal lines, corresponding to the figure ofthe ani
mal covering of flesh. Sometimes the stem consists of horny
and calcareous parts disposed alternately, composing a jointed
structure, which some have fancied might be considered as
making an appr.oach«to an articulated skeleton; for it is ca
pable of considerable flexion, and readily yields to the im
pulse of the waves, without the risk of being broken. This
is the case with the Isis hippuris, commonly known by the
name of jointed coral. (Fig. 68.) There is, in short, hardly
any possible combination of these parts which does not oc
casionally occur amidst the infinite diversities of condition
displayed in this department ofthe animal creation.
These structures are generally attached to submarine
rocks by an expansion of the base into a kind of foot, or
root, which has a strong power of adhesion. In this re
spect, therefore, as in so many others, these animals pre
serve an analogy with plants.
It has been ascertained that, in a great number of instances
these fixed zoophytes are multiplied, like the sponge, by the

POLYPI. .. 127
detachment of gemmules, or imperfectly formed portions of
their soft substance. These gemmules require to undergo
the same kind of metamorphosis in order to bring them to
their perfect state ; and when newly detached from the pa
rent, they exhibit the same singular spontaneous motions,
buoying themselves in the water, and swimming in various
directions, by the rapid vibrations of their cilia, till they
find a place favourable to their growth. On becoming fixed,
they spread out to form a base for the future superstructure;
and, after the foundation has thus been laid, they proceed in
their upward growth, depositing a calcareous or horny axis
in successive layers, until it has acquired the requisite thick
ness; and they then gradually assume the forms character
istic of the particular species to which they belong. The
materials thus deposited are permanent structures, not ca
pable of modification or removal, and not possessing any
vital properties ; for these properties belong exclusively to'
the animated fleshwith which these structures are associated.
The polypes themselves are not developed till after'the for
mation of the root and stem; their growth being in this re
spect analogous to that of the leaves and flowers of a plant.
The gemmules of the Flustra carbasea may be selected
in illustration of these phenomena. These have been ob
served by Dr. Grant,* to swim about in the water as soon as
they have escaped from the cells ofthe parent; each moving
with its narrow end foremost, while the opposite broad
end, which is covered with cilia, expands into a flat circular
zone. These gemmules are very irritable, and are frequent
ly seen to contract the circular margin of their broad ex
tremity; and, while swimming, to stop suddenly in their
course. They swim with a gentle gliding motion: at other
times they appear stationary, all the while revolving rapidly
round their longer axis, with their broad end uppermost: they
often bound forwards, either in straight lines, or describing
circles, with no other apparent object than to keep them
selves afloat, until they shall arrive at a favourable spot for
• Edinburgh Philosophical Journal, XVII. 107 and 337.

128 THE MECHANICAL FUNCTIONS.
fixing their permanent abode, and proceeding in their far
ther development. The time of their remaining in this free
and moving state varies according to circumstances, from a
few hours to about three days. When about to fix, the
slightest agitation of the water causes them to desist, and to
recommence their gliding motions, which they continue for
some time longer. If, when any of these gemmules has be
gun to fix, it be again disturbed, and separated from the
surface to which it had become attached, it generally re
mains free, and perishes. During the process of fixing, it
exhibits no peculiar appearance or change of form; it simply
lies on its side: and the cilia continue to vibrate over the
whole surface, producing a constant current in the water,
apparently for the purpose of cleaning the space immediate
ly surrounding the gemmule. It remains for three days in
this attitude, without undergoing any perceptible change of
form, and without relaxing the vibrations of its cilia. At
the end of this time, the cilia cease to move, and shortly af
ter disappear: then the gemmule begins to swell, the sur
rounding margin becomes more transparent, and the whole
gradually assumes the form of a cell, surrounded by a de
licate white opaque line, which is the rudiment of the calca
reous wall ofthe future cell. Towards the base of this ru
dimental cell, the gelatinous substance in the interior may
be perceived to become more consistent and opaque at a
particular point; from this dull spot within the cell, short
straight tentacula begin to bud, extending upwards in the di
rection of the future aperture. The gelatinous spot, from
which the tentacula originated, assumes the vermiform ap
pearance of the body of a polype; and we may distinctly
perceive the bundles of fibres which connect its head with
the base of the cell. The structure of the polype is per
fected by the addition of a closed capsule; and when it is
first detected protruding from the cell, it possesses all the
parts of an adult polype, and vibrates the cilia of its tenta
cula with as much regularity and velocity as at any future
period. Before the polype is capable of protruding from

POLYPI.

129

the aperture of the first cell, the upper part of the cell has
already extended outwards to form the rudiment of a second:
and so on, in succession, till the whole structure is,completed.
The tentacula of polypi are exquisitely sensible, and are
frequently seen, either singly or altogether, bending their
extremities towards the mouth, when any minute floating
body comes in contact with them. When a polype is ex
panded, a constant current of water is observed to take
place, directed towards the mouth. These currents are >
never produced by the motions of the tentacula themselves-
but are invariably the effects of the rapid vibrations of the
69 70 cilia placed on the tentacula. In the
polypes of the Flustra carbasea, (Fig.
69,) the tentacula have each a single
row of cilia extending along both the
lateral margins, from their base to their
termination.* Each polype has usual
ly twenty-two tentacula; and there are
about fifty cilia on each side of a ten-
taculum, making 2200 cilia on each-
polype. As there are above 1800 cells
in each square inch of surface, and the
branches of an ordinary specimen present about ten square
inches of surface, we may estimate that an ordinary speci
men of this zoophyte presents more than 18,000 polypes,
396,000 tentacula, and 39,600,000 cilia. But other species
certainly contain more than ten times these numbers.t
The vibrations of these cilia are far too rapid to be fol
lowed by the quickest eye, even when assisted by the most
powerful microscope, and can be detected only at the times
when they have become comparatively languid, by the di
minished vigour of the animal: their motions may then be

* A portion of one of these tentacula is represented, highly magnified, in
Fig. 70. The lower figure, (g) is the delineation of one of the gemmules
of the same polypus, also greatly magnified.
¦j- Dr. Grant has calculated that there are about 400,000,000 cilia on a sin
gle Flustra foliacea. Transactions of the Zoological Society of London,
Vol. i. p. 11. k
VOL. I. — 17

130 THE MECHANICAL FUNCTIONS.
seen, ascending on one side of the tentaculum and descend- .
ing on the other. (Fig. 70.) AH the cilia appear to com
mence and to cease their motions at the same moment. The
constancy with which they continue would seem to exelude
the possibility bf their being the result of volition; and they
are, therefore, more probably determined by some unknown
physical cause, dependent, however, on the life of the^ani-
mal. But so retentive are they of the power of motion,
' whatever maybe its cause, that if any one of the tentacula
be cut off, its cilia will continue to vibrate, and will propel
it forward in the fluid for a considerable time, as if it had
become itself an individual animal.
A question arises with regard to the constitution of these
zoophytes, similar to that which has been proposed with re
gard to trees, namely, what limits should be assigned to
their individuality? Is the whole mass, which appears to
grow from one root, and which consists of multitudes of
branches, proceeding from a common stem, to be considered
as one individual animal, or is it an assemblage or aggrega
tion of smaller individuals: each individual being characte
rized by haying a single mouth, with its accompanying ten
tacula, and yet the whole being animated by a common
principle of life and growth? The greater number of natu
ralists have adopted this latter view, regarding each portion,
so provided with a distinct circle of tentacula, as a separate
animal, associated with its neighbours in the construction of
a common habitation, and contributing its quota to the ge
neral nourishment of this animal republic. As the deter
mination of this question involves the consideration of the
function of nutrition, I shall postpone its farther discussion
to a future part of this treatise. As far, indeed, as regards
-the mechanical condition of animals which are so complete
ly stationary, it matters little, whether the whole mass be
regarded as one individual animal, or as an aggregate of dis
tinct individuals. But the question becomes of some impor
tance when applied to detached zoophytes, such as Penna
tula, which are formed of a multitude of polypes connected
jyith ii common stem, but which float at liberty in the sea.

PENNATULA.

131

The Pennatula, (Fig. 71,) has been termed the sea penf
from the circumstance of its calcare
ous axis, or stem, having a double
set of branches, extending in the
same plane from both the sides,- like
the vane of a quill,- and of its series
of polypes being set along one edge
of each branch, like the filainents
which arise from the fibres of the
feather. Some of these polypes are
seen magnified in Fig. 721 Immense*
numbers of these curious animals are
met with in different parts of the ocean. If they possessed
in any degree the power of locomotion, which many natural
ists have ascribed to them, we should be able to ascertain-
whether all their movements- are conducted- by a common'
volition, or Whether they are performed independently of
one' another. It has often, indeed, been asserted, that pen
natula? swim through the water by their own spontaneous
movements, consisting either in the waving up and down of
the lateral branches, or in the simultaneous impulses of the
tentacula of all the polypes. Cuvier even represents the
polypes of the pennatula as having the power of keeping
time, while they are waving the mass through the" water, as
if they were all actuated by a" single undivided volition.
But Dr. Grant, who has watched the motions of these ani
mals with great care, is led by his observations to the con
clusion that pehnatulae are not in reality possessed of any
such locomotive faculty; but that they are carried to and fro
in the ocean, like the gulf weed", without the slightest vo
luntary power of directing their course. Whatever may be
the result of the combined' movements of the tentacula, the
arms are certainly incapable of those inflections wljich have
been supposed to supply the means of progressive motion.
It is only when the contractile flesh of the polypus is re
leased from the restraint which the solid axis imposes upon'
its movements, that the animal becomes capable ef any dis
tinct power of locomotion. Such' is the condition- of the'

132 THE MECHANICAL FUNCTIONS.
animals belonging to the genus Hydra, of which the Hydra
viridis, or fresh water polype, (Fig. 59, p. 122) may be taken
as the type. This singular animal presents us with perhaps
the simplest kind of structure that exists in the animal king
dom. It would almost seem as if nature had formed it with
. the design of exhibiting to us the resources of vitality in
carrying on the functions of animal life without the aid of
the complicated apparatus which she has bestowed upon the
higher orders of the creation. The Hydra consists merely
of a fleshy tube, open at both ends, one of which, being
more dilated, may be regarded as the head, and has for a
mouth the aperture of the tube, which is furnished at its
margin with a single row of tentacula. It thus corresponds
to the general definition of a polypus, and exemplifies its
most simple form.
The whole body may, on the one hand, be considerably
elongated, and, on the other, so much retracted, as to appear
a mere globule;, and these movements are the effect of a vo
luntary power in the animal directed to specific ends. The
number of tentacula varies from six to twelve; they are
slender tubular filaments, capable of being extended to a
great length, and of being bent in all directions. In this
way, they can quickly surround and grasp any small ob
ject which they may happen to touch; and, whenever irri
tated, they instantly retract, so as hardly to be visible with
out the aid of a magnifier. Each tentaculum may be moved
independently of the rest, at the pleasure of the animal.
The remainder of the body tapers gradually from the head
to the other extremity, becoming very slender, and having.
at its termination a flat surface, which has been termed the
foot; for although every portion of the surface has the
power of adhering to the bodies fo which it is applied, it is
principally by this extremity that the animal chooses to at
tach itself to the sides or bottom ofthe vessel in which it is
kept. No trace of the existence of cilia is to be met with
on any part of the surface of these animals.
It is to Mr. Trembley of Geneva that we are indebted for
the discovery this singular animal, the examination of

HYDRA. 133
which has contributed to throw great light on the natural
history of polypiferous animals.* While observing some
aquatic plants, which he had collected and put into water,
his attention was called to the appearance of filaments ad
hering to them, which he at first conceived to be parasitic
vegetables: but farther observation convinced him that they
were endowed with powers of spontaneous motion, and that
they preyed upon small insects: and he, therefore, could no
longer doubt their animal nature. He found that they al
ways placed themselves on the side of the glass next to the
light; and by watching their changes of position, he disco
vered the mode in which they effect their progressive mo
tions. If the hydra be standing in the erect position, its
foot being applied to the bottom of the glass (Fig. 73,) it
slowly bends the body in the direction in which it intends
to advance till its head touches the vessel, as shown in Fig.
74. It then adheres to the surface by the mouth, or by one
or two of its tentacula, and, detaching the foot, bends the
body into a curve, at the same time slightly retracting it,
so that the foot is brought near the head (Fig. 75.) The
foot is then again fixed, preparatory to a new step, which it
takes by detaching the head and projecting, it forwards as
before (Fig. 76.)

73

74 75

76

The progress made by these successive efforts is but slow:
for the hydra often pauses in the middle of a step, as if de
liberating whether it should proceed: so that the traversing
a distance of seven or eight inches is to these animals a very
good day's journey, even in summer. But a mode of tra
velling rather more expeditious than this is occasionally re
sorted to. It consists of a succession of somersets : the hy-
* Memoires pour servir a I'Histoire d'un genre de Polypes d'eau douce,
a bras en forme de cornes. Par A. Trembley, 1744.

134 THE MECHANICAL FUNCTIONS.
dra, while adhering firmly by the mouth, detaches its foot,
and, making it describe a semicircle, throws it over its head,
and places it foremost in the line of progression. Having
attained this situation-, the foot is- then fixed, and a similar
semi-revolution is performed by the head, the body conti
nuing all the: while elongated.
By these and other manoeuvres these animals contrive to
walk with equal facility in any direction, either on the bot
tom or sides of the vessel, or along, the stems of aquatic
plants, to which they are most frequently found attached.
The position in which they appear to take- most delight, is
that of remaining suspended from the surface of the water
by means of the foot alone: and this they effect in the fol
lowing manner. When the flat surface of the foot is ex
posed for a short time to the air, above the surfaee of the
water, it becomes dry, and in this state exerts a repulsive
action on the liquid: so that when dragged below the level
of the surface by the weight of the body, it still remains un
covered, and occupies the bottom of a cup-shaped hollow
in the fluid, thereby receiving a degree of buoyancy suffi
cient to suspend it at the surface. The principle is the same
as that by which a dry needle is supported on water in the
boat-like hollow which is formed by the cohesive force of
the liquid, if care be taken to lay the needle down very gen
tly on the surface. If, while the hydra is floating in this
manner, suspended by the extremity of the foot, a drop of
water be made to fall upon that part, so as to wet it, this
hydrostatic power will be destroyed, and the animal will
immediately sink to the bottom.
While in this state of suspension from the surface, the
hydra is capable of performing several curious evolutions,
and with the assistance of the tentacula, by which it lays
hold of objects within its reach, is able to cross over from
one side of the vessel to the other. It does not appear that
these animals ever employ the tentacula as instruments for
swimming ;- but they frequently use them as cables, or an
chors, to enable them to retain their positions in security,
however violently-the water may be agitated. Great use is

HYDRA. 135
also made ofthe tentacula as-organs of prehension for seizing
and detaining their living prey, and for conveying it to the
mouth, where it is quickly swallowed. On the other hand,
when alarmed, or exposed to irritation, the hydra suddenly
shrinks, by the gradual contraction of all the tentacula, and
of the body also, into a small globule, which might easily
escape notice, unless its .previous situation were accurately
observed. It might be asked by what power is this animal, occupy
ing so low a place in the scale of organization, enabled to
perform these actions? To this question, however, no satis
factory answer has yet been given. The substance of the
hydra, when examined by the microscope, appears to be
nearly homogeneous, except that a number of grains are in
termixed with the pulpy and gelatinous matter composing
the principal bulk of the body. These grains, when pressed
out of the flesh into water, are scattered indiscriminately;
and appear to have been united iij the living animal, by means
of this glutinous material.
No perceptible fibres, either muscular, or of any other
hand, can be detected in the flesh of the polypus : nor is
there the least indication of the formation of transverse
rings, similar to those which exist in worms, and which, in
these latter animals, contribute to progressive motion. Every
portion of the substance of the body is equally irritable and
contractile, and its movements appear to be governed by
some voluntary power belonging to the animal, and direct
ed to the attainment of certain ends. The softness and pli
ancy which it possesses allow of its being closely fitted to
all the inequalities of the surface of the bodies to which it
is applied ; and perhaps this cause alone occasions it to ad
here with great force to these bodies, without the aid of any
glutinous fluid. A conjecture, which has much appearance
of probability, has been offered, that this power of adhesion
is derived from the, presence of a great number of exceed
ingly minute disks, interspersed over every part of the sur
face, constituting so many suckers, and resembling, though
on a very diminutive scale, the sucking apparatus on the
arrns of the cuttle-fish.

136 THE MECHANICAL FUNCTIONS.
. The Zoanthus (Fig. 58) belongs to a tribe of larger poly
pi, which are generally met with assembled in clusters ; on
which account it is termed by Ellis the Actinia sociata, or
cluster animal flower. It consists of a globular body,
having a mouth surrounded by one or two rows of tentacu
la ; and connected below with a firm and fleshy tube, which
adheres strongly to the rocks at the bottom of the sea, so
that it remains permanently fixed in the same place.
The genus Vorticella is formed by a small tribe of ani
mals, which, although they have been usually included un
der the present order, differ from polypi in having no tenta
cula, but only cilia, surrounding the margin of a bell-shaped
body, which is mounted upon a long and slender foot-stalk
(Fig. 77.*) Currents are, as usual, excited by the vibra
tions of the cilia ; which in the simpler species, such as the
77 Vorticella cyathina, here delineated, are
the efficient instruments of progressive
motion. When attached by its foot, the
vorticella advances in search of food, by
the extension of the foot-stalk into a
straight line ; but quickly retreats from
danger, by suddenly throwing it into spi
ral folds. Many of the species of vor-
ticellae are so exceedingly diminutive
as to be imperceptible without the aid of
the microscope. They conduct us, therefore, by a natural
gradation, to the next order we have to notice, and which
is composed wholly of microscopic animals.
§ 4. Infusoria.
The Infusory animalcules, or Infusoria, were so named
by Muller, a Danish naturalist, from the circumstance of
their swarming in all infusions of vegetable or animal sub
stances which have been kept for a sufficient time. They are,
in general, far too minute to be perceptible to the naked eye:
it is to the microscope alone, therefore, that we owe our
* They also differ from polypi in having a distinct intestinal canal, with
numerous stomachs.

INFUSORIA. 137
knowledge of their existence, and of the curious phenomena
they present : yet even the best instruments afford us but
little insight into tneir real organization and physical condi
tions. On this account it is extremely difficultto assign
their true place in the scale of animals. By most systema
tic writers they have been regarded as occupying the very
lowest rank in the series, and as exemplifying the simplest
of all possible conditions to which animal life can be re
duced. Monads, which are the smallest of visible animal-
, cules, have been spoken of as constituting " the ultimate
term of animality;" and some writers have even expressed
doubts whether they really belong to the animal kingdom,
and whether they should not rather be considered as the ele- ;
mentary molecules of organic beings, separated from each!
other by the effects pf chemical decomposition, and retain-i
ing the power of spontaneous, but irregular and indetermi-i
nate motion. It was conceived that all material particles
belong to the one or the other of two classes; the first,
wholly inert, and insusceptible of being organized ; the se
cond, endowed with a principle of organic aptitude, or ca
pability of uniting into living masses, and constituting, there
fore, the essential elements of all organization. According
to this view, all vegetables or animals in existence would
be mere aggregations of infusory animalcules, which gra
dually accumulate by continual additions to their num
bers, derived from organic matter in the food : so that the
body of man himself would be nothing more than a vast
congregation of monads !
This bold and fanciful hypothesis, devised by Buffon, and
recommended by its seductive appearance of simplicity, as
well as by the glowing style and brilliant imagination of its
author, has had many zealous partisans. The new world,
which was disclosed to the wondering eyes of naturalists by
the microscope, abounding in objects and in phenomena of
which no conception could have been formed previously to
the invention of that instrument, was peculiarly calculated
to excite curiosity, and to inspire the hope of its revealing
the secret of the living principle in the arrangement of the
vol. i. — IS

138 THE MECHANICAL FUNCTIONS.
atoms of organic bodies. During the greater part of the last
century, infusory animalcules were the subject of frequent
and laborious microscopical research, and gave rise to end
less conjecture and speculation as to their origin, their vi
tality, and their functions in the economy of nature. Not
withstanding their minuteness, considerable differences of
organization were perceived to exist among them : but many
naturalists still clung to the idea that monads, the most di
minutive of the tribe, and whose very presence can be de
tected only by the application of the highest magnifying
powers, are homogeneous globules of living matter, without
organization, but endowed with the single attribute of vo
luntary motion: and even this property was denied to them
by some authors.
All these fanciful dreams have been dispelled by the im
portant discoveries of Ehrenberg, who has recently found
that even the Monas termo is possessed of internal cavities
for the reception and the digestion of its food ; and who has
rendered it probable that their organization is equally com
plex with that ofthe larger species of infusoria, such as the
Rotifera, in which he has succeeded in distinguishing traces
of a muscular, a nervous, and even a vascular system.
Those animalcules, whose form can be at all distinguished,
exhibit a great diversity of shapes, and variety of modes of
progressive motion. Many, as the Cyclidium, have the ap
pearance of a thin oval pellicle, smoothly gliding in all di
rections through the fluid : some, as the Volvox, are globular;
others, as the Cercaria, are shaped like a pear, tapering at
one end, and often terminating in a slender tail, so as to re
semble a tadpole. In many, this tail is of great length; in
some, as the Furcocerca, it is forked ; in others, it takes spi
ral turns, like a corkscrew. The Kerona has processes like
horns. The shape ofthe Vibrio is cylindrical, and more or
less pointed at one or both ends, like an eel, or a serpent,
which animals it also resembles in its undulatory mode of
swimming.* Some, as the Gonium, have an angular, others,
* Animalcules referrible to this genus are met with in great numbers in
blighted wheat, (Fig. 2, p. 58,) in sour paste, and in vinegar which has lost

INFUSORIA. 139
as the Kolpoda, a waving outline. Some, as the Urceolaria, ,
present the likeness of a bell or funnel, and appear to be
analogous to the Vorticella, in which genus they should pro
bably be included.
Forms still more irregular are exhibited by other infuso:
ria. Of these the most singular is the Proteus (Fig. 78,)
which cannot, indeed, be said to have any determinate shape,

for it seldom remains the same for two minutes together. It
looks like a mass of soft jelly, highly irritable and contrac
tile in every part; at one time wholly shrunk into a ball, at
another stretched out into a lengthened riband; and again,
at another moment, perhaps, we find it doubled upon itself
like a leech. If we watch its motions for any time, we see
some parts shooting out, as if suddenly inflated, and branch
ing forth into star-like radiations, or assuming various gro
tesque shapes, while other parts will, in like manner, be as
quickly contracted. Thus the whole figure may, in an in
stant, be completely changed, by metamorphoses as rapid
as they are irregular and capricious.
The Volvox globator, (Fig. 79) is found in prodigious
numbers at the surface of many stagnant pools. Its figure
is perfectly spherical ; and its movements consist in a con
tinual and rapid rotation, round its axis, frequently remain
ing all the while in the same spot. Another species, the
Volvox confliclor, moves by turning alternately to the right
and to the left.
The progressive movements of infusory animalcules are of
two kinds, the one consisting in a smooth and equable gliding
the whole of its alcohol. In this last fluid they sometimes attain so large a
size as to be visible to the naked eye.

140 THE MECHANICAL FUNCTIONS.
through the fluid, produced apparently by the vibrations of
cilia, which are set on various parts of the body, and often
seem to cover the whole surface : the other, more rapid and
energetic, when the animalcule darts forward in a particu
lar direction, as if in pursuit of prey, and proceeds by sud
den and irregular starts, like a vivacious insect or fish. The
voluntary nature of their motions is evident from the dex
terity they display in avoiding obstacles, while swimming
together in myriads in a single drop.
The great agent in the movements of the animal frame
being the muscular fibre, it was natural to suppose that a
texture analogous to that of muscles might exist in these
latter genera of infusoria. It was not till very recently,
however, that the actual presence of contractile fibres could
be recognised. But this problem has at length been solved
by the discoveries of Ehrenberg, who, in his observations
of the larger and more highly organized species belonging
to the order of Rotifera, has, with a magnifying power of
380, distinctly seen muscular bands running in pairs between
the two layers of transparent membrane which envelop the
body. When the animalcule throws itself into its violent
lateral contortions, these fibrous bands are observed to be
come broader and thicker, as well as shorter, on the side
towards which the contractions take place. There can,
therefore, be no doubt that these are muscular organs, and
that they are the real agents by which the motions wit
nessed are effected.
These Rotifera, or wheel animalcules, are so named from
r their being provided with an apparatus
for creating a perpetual eddy, or circu
lar current in the surrounding fluid.
The remarkable organs, by which this
effect is produced, are . generally two in
number, (Fig. 80, r, r,) and are situ
ated on the head, but do not surround
the opening of the mouth, as is the case
with the tentacula of polypes. They
, consist of circular disks, the margins of

WHEEL ANIMALCULES. 141
which are fringed with rows of cilia, bearing a great resem
blance to a crown wheel. This wheel appears to' be inces
santly revolving, and generally in one constant direction ;
giving to the fluid a rotatory impulse, which carries it round
in a continual vortex. The constancy of this motion would
seem to indicate that it is related to some function of vital
importance, such as respiration. But even considered as
a mechanical action, which is the view we have now to take
of it, this phenomenon is of a nature to excite much curiosi
ty; for the continued revolution round an axis of any part
or appendage to the body, is quite inconsistent with any no
tion we can form of the solid organic attachment of such
appendage ; and we can have no conception of organization
extending through the medium of a fluid, or of any substance,
which, like a fluid, admits of the continual displacement of
its parts. M. Dutrochet has' offered an ingenious solution
of this difficulty. He suggests that the revolution of the
wheels ofthe Rotifera may not be real, but apparent only.*
The indented margin of each wheel being composed of a
material so exceedingly flexible as to be capable of assuming
quickly all kinds of curvatures, may be conceived to be
thrown into undulations, which follow one another round
the circumference ; each part, in succession, becoming al
ternately convex and concave, and thus producing the ap
pearance of the actual advance of the portions that are
raised ; while their real motions are only those of elevation
and depression, by the alternate elongation and contraction
of their perpendicular fibres.
Besides possessing extensive powers of locomotion, the
infusoria manifest in several of the- vital functions, as we
shall hereafter find, a degree of complication, which appears-
to entitle them to a higher station in the animal scale, than
that which most naturalists have assigned to them* They
are certainly superior to the sponges or polypi, doomed by
•nature to be permanently fixed, like plants, to the same spot;
and of which, if we consider them as compound beings, the
* The same opinion was advanced long ago by Vicq. d'Azyr.

142

THE MECHANICAL FUNCTIONS.

individual animals are often so minute as to bescarcely vi
sible without the aid ofthe microscope. Mere size, indeed,
is of all the circumstances attendant on organized beings,
that which should least be assumed as the criterion of com
plication or refinement of structure. An object is great or
small, only in relation to the standard of our own limited
and imperfect senses; but with reference to the operations
of creative power, all such distinctions must vanish. There
is not, as' far as we have the means of judging, in the co
lossal fabric of the elephant, any structure more compli
cated than exists in the minutest insect that crawls un
heeded at our feet.

§ 5. Acalephce.

Floating masses of living gelatinous matter are met with
in every part of the ocean; often in vast numbers, and of va
rious forms ; and having but little the appearance of belong
ing to the animal kingdom. They compose the order Aca
lephce, of which the Medusa
(Fig. 81) may be taken as the
type. They appear, from their
organization, to be raised but a
single step above polypi; and
in point of activity and lo
comotive powers, they rank
among the lowest of those Zoo
phytes which are not perma
nently fixed to the spot where
they were first developed.
They are almost wholly pas
sive beings, floating on the
surface of the sea, or remaining at a small depth below it,
carried to and fro by the motion of every tide and current.
and destined to be the unresisting prey of innumerable
tribes of animals which people every part ofthe ocean.
The usual form of a Medusa is that of a hemisphere, with

MEDUSA. 143
a marginal membrane, like the fold of a mantle extending
loosely downwards from the circumference; together with a
central pedicle descending from the lower surface, like the
stalk of a mushroom, and terminating below in several
fringed laminas, or processes, which have sometimes been
denominated tentacula.
The whole substance of the body of these medusa? is se
mi-transparent and gelatinous, without any distinct fibrous
structure; yet it has considerable elasticity, and possesses
also some degree of contractile power. The animal is seen
alternately to raise and depress the margin of its hemisphe
rical body, and to flap with the fringed membrane or man
tle, which descends from it, in a manner somewhat similar
to'the opening and shutting of a parasol. This pulsatory
movement is performed about fifteen times in every minute,
with great regularity : and by the reaction of the water, the
animal is sustained at the surface ; or by striking the water
obliquely, it may even perform a slow lateral movement.
They descend in the water by simply contracting their di
mensions in every direction. Sometimes, in order to sink
more quickly, they turn themselves over, so that their con
vex part is undermost.
Medusae are met with of very various sizes; the larger
abound in the seas around our coast; but immense numbers
of the more minute and often microscopic species occur in
every part of the ocean.* In some parts of the Greenland
seas they swarm to such an extent that they give a visible
tinge to the colour of the waves for hundreds of miles. The
total number of these animals dispersed over that space sur
passes the utmost stretch of the imagination. In these si
tuations a cubic foot of water, taken indiscriminately, was
found by Mr. Scoresby to contain above 100,000 of these
diminutive medusas.
Belonging to the tribe of Medusaria is a singular genus,
* The luminous property of sea water, pr its phosphorescence, as it is some
times called, generally arises from the presence of minute medusae, which
are met with in greatest numbers at the surface, being specifically lighter
than the surrounding fluid.

144

THE MECHANICAL FUNCTIONS.

denominated the Beroe, (Fig. 82 and 83,) which is remark
able for its organs of progressive motion. Its body is either
globular, or oblong, and it swims with its axis in a vertical
position. Eight longitudinal bands or ridges, which have

been sometimes compared to ribs, extend down its sides,
like those of a melon; and along each of these is attached a
set of little membranes, extended horizontally, and support
ed on radiating fibres; so that they bear a pretty exact re
semblance to the fin of a fish. Their action is not unlike
that of the wings of a bird ; for they are made to flap up and
down, striking the water vertically, and communicating an
ascending impulse to the body. This animal is also pro
vided with two very long and slender processes, which come
out from the sides of the body, and from these a great num
ber of still finer filaments, or cilia, proceed: the whole ap
paratus is highly sensitive and irritable, and on the slightest
touch the filaments are thrown into spiral coils, and retract
rapidly within the body. They thus act the part of tenta
cula, or delicate organs, both of touch and of prehension.*
It wa,s observed by Fabricius, that when a Beroe is cut into
many pieces, each piece continues to live, and to swim about
by the action of the cilia, which still continue their vibra
tory motions.
In two other genera of Acalephse, the Porpita and the
Velella, provision is made for the mechanical support of the
* See a description ofthe Beroe pileus, Lam. by Dr. Grant, in the Trans
actions ofthe Zoological Society of London, vol. i. p. 9. #

* VELELLA. 145
soft gelatinous mass, by means of an internal cartilage. In
the former, this cartilage is of a circular form; in the latter
(Fig. 84,) it is oval, and bears upon its upper edge a thin
pellucid membrane of a triangular shiape, which extends the
whole length of the upper surface of the body. As this
membrane is connected with the cartilage at its middle part
only, while its edges are loose and floating, it is peculiarly
adapted, when above the surface of the water, to catch the
wind and act as a sail. Such, indeed, appears to be the pur
pose fonwhich it was given to the animal ; enabling it to steer
its course by means of the loose edges, and also of the tenta
cula, which extend from the lower side of the body, and act
as a rudder, while the sail is impelled by the wind.
A construction still more artificial is provided in another
family of the same order, denominated the Physalida, or
Hydrostatic Jlcalepha. They have attained this latter ap
pellation from their being rendered buoyant by means of ve
sicles filled with air, which enable them to float without the
necessity of using any exertion for that purpose. The Phy-
salia, or Portuguese Man-of-War, as it is called, (Fig. 85,)
is furnished with a large air-bladder,of an oval shape, placed
on the upper part of the body; and also with a membrane
of a beautiful purple colour, which, as in the Velella, serves
as a sail. These Zoophytes are met with in great numbers
in the Atlantic Ocean, and more especially in its warmest
regions, and at a considerable distance from land. In calm
weather they float on the surface of the sea, rearing their
purple crests, and appearing at first like large air bubbles,
but distinguishable by the vivid hues of the tentacula which
hang1 down beneath them. Nothing can exceed the beauty
of the spectacle presented by a numerous fleet of these ani
mals, quietly sailing in the tropical seas. Whenever the
surface is ruffled by the slightest wind, they suddenly absorb
the air from- their vesicles, and becoming thus specifically
heavier than the water,, immediately disappear, by diving
into the still depths of the ocean. By what process they
effect these changes of absorption and of reproduction of air,
yet remains to be discovered. Other genera, as the Phys-
vol.%— 19

146 THE MECHANICAL FUNCTIONS. •
sophora, have several of these air-bladders; but in other re
spects resemble the ordinary Medusa?, in having no mem
branous crest.
The Actinia; are a .tribe of Zoophytes, which, from the
general resemblance of their forms to those of Polypi, are
by most naturalists included under that order. But they
exhibit a much greater development in their organization;
having very distinct muscular fibres endowed with strong
powers of contraction. Their digestive organs, also, as I
shall have afterwards occasion more fully to notice, are con
structed upon a more complicated plan than in the polypus.
Fig. 86 exhibits an Actinia in its contracted state. When
their tentacula, which surround the mouth, and are very
numerous, are fully expanded, (as shown in Fig. 87,} these

animals present a striking analogy of form to many of the
compound flowers; and accordingly the particular species
are named from these resemblances, the sea-anemone, the
sea-marygold, the -sea-carnation, the sun-ftower, daisy, &c.
Actinias are seen in great numbers on many shores ad
hering by their flat surfaces to rocks, and being generally
permanently fixed to their abode. When the weather is
fine, and the sea calm, it is very amusing to watch the rapid
expansions and retractions of their many-coloured tentacula,
while they are moving in search of food: to observe the
quickness with which they seize on whatever prey comes
within their reach, and to notice the suddenness with which
they collapse into a round contracted mass, on receiving the
slightest injury. t ,
Yet these animals are not of necessity confined to the par
ticular spots where we see them fixed; for they are capable,
when disturbed, of seeking, by a slow progressive motion,
a more secure abode. Reaumur has minutely exanuned the

ECHINODERMATA.

147

arrangements of their muscular fibres, and has described the
actions by which they either attach themselves to the sur
faces of rocks, or effect their sluggish movements.*

§ 6. Echinodermata.
Ascending in the scale of organization we come to the
Echinodermata, a class which comprehends the families of
the Asterida, the Echinida, the Holothurida, and the Cri-
noidea, together with other tribes of less note.

These animals, both in their general form, and in the ar
rangement of their internal organs, retain, in a very marked
.manner, the radiated disposition so characteristic of Zoo
phytes: for we find all their parts symmetrically arranged
either in lines, or in compartments, which proceed from a
common centre, or axis, and which are repeated, in-regular
succession, all round the circumference (See Fig. 88 to 94.)
Besides an external horny, or semi-calcareous covering,
there is also provided, for the support of the softer parts, a
kind of internal skeleton, or jointed frame-work. The or
gans in the interior of the body are farther supported by
* Mfemoires de l'Academie des Sciences, 1710, p. 490.

148

THE MECHANICAL FUNCTIONS.

membranous walls, which impart mechanical firmness to the
fabric. The Asterias, or star-fish (Fig. 88,) is so named from its
star-like form; and the number of rays composing the star
is generally five. Besides the tough coriaceous integument,
which protects the mass of the body, each ray is farther sup
ported by a series of calcareous pieces, resembling those
which compose the spinal column of vertebrated animals, and
forming an articulated axis, constructed with the evident de
sign of combining the greatest strength with a proper degree
of flexibility. Cartilaginous plates are also added for the
more special support of the integument. Thisintegumentitself
is irritable, and has the power of changing its form, although
the muscular fibres by which its motions are effected are not
easily distinguished. Calcareous grains of a solid consist
ence, are thickly interspersed throughout its texture; and
these, in various parts of the body, both in the upper and
the under side, often project from the surface in the form of
spines or prickles. They are particularly large around the
mouth of the animal, which opens at the centre of the under
side. These calcareous masses have a crystalline arrange
ment, and exhibit on fracture the exact oblique angles cha
racteristic of the primitive rhomboid of carbonate of lime.
The under side of each ray (Fig. 95) has a groove, termed,

96

by Linneus, the ambulacrum, or avenue, a name which it
has received from its fancied resemblance to a walk between
rows of trees; for each groove contains a quadruple row of
perforations, like pin holes, through which small fleshy
cylindrical processes pass. These processes extend but a
short distance from the surface; but they admit of being

ECHINUS. 149
elongated or retracted, at the pleasure of the animal, by a
very curious mechanism, which I shall presently describe.
By tending them on either side, in their expanded state,
the Asterias is capable of effecting a slow progressive mo
tion; so that these processes may be regarded as correspond
ing to feet, being levers for the advance of the body. This,
it may be remarked, is the first time that we meet with or^
gans of that description in our progress through the animal
kingdom. Each of these feet is terminated by a concave
disk, which when applied to any flat surface acts as a sucker,
on the principles already adverted to.* Reaumur counted 304
of these feet in each of the five rays of the star fish, making
1520 in all.t Each foot consists of a tube, closed at the
outer end, and the stem of which, after passing through the
aperture in the integument, is dilated into a bag or reservoir
of fluid ; as is shown in Fig. 97. Bv the contraction of this
reservoir, the fluid it contains is propelled into the outer
portion of the tube, which protrudes by being thus distend
ed; the foot fixes itself, by means of its terminal fleshy disk,
to the point it touches, and then, by retracting, draws the
body along for a short distance. By the retreat of the fluid
into its reservoir, the foot is again detached, and ready
to be moved forwards, and is thus made instrumental in
taking another step, by a repetition of the same process.J
From the shortness of these feet, notwithstanding their great

* Page 105.
¦j- Memoires de l'Academie des Sciences, 1710, p. 487.
$ The mechanism by which the feet are protruded and retracted is illus
trated by the diagram, Fig. 97, which exhibits the bladders connected with
them, in different states of distention and contraction. Fig. 96 shows the
upper side of the umbulacra, and of the bladders connected with the feet.
Dr. Grant, from some observations which he made on the structure of' the
cilia of the Beroe pileus, is led to suspect that the rapid vibrations of these
singular organs in the lowest animals may depend on the undulations of
water conveyed through elastic tubes along their bases, in a manner resem
bling the injection of the tubular tentacula of Actinise and Asteri*. If this
conjecture were verified, he remarks, one of the most remarkable pheno
mena of animal motion, though One of the most frequent, would lose much
of its present marvellous character.

150

THE MECHANICAL FUNCTIONS.

number, the advance which this animal can make in any
particular direction is excessively slow.
Besides this movement of creeping, the Asterias is capa
ble of bending and unbending each of its rays; actions, how
ever, which it can perform but very slowly, and not to an
extent sufficient to accomplish its removal from one place to
another.* The skeleton of the Echinus or sea-urchin, (Fig. 91,) is
still more artificially framed than that of the Asterias. It
has a spheroidal form, like that of an orange; the calcareous
material employed in its construction, instead of forming
isolated grains, is accumulated and extended into polygonal
plates (Fig. 98,) the edges
of which are dove-tailed
into each other. The form
of each piece is that of a
lengthened hexagon; and
the whole are regularly ar
ranged in rows, like a mo
saic or tesselated pavement;. Ambulacra are also seen on
the surface of the shell, passing vertically down the sides
ofthe sphere, similar to the meridians of a globe; and con
taining, like those of the Asterias, a double row of perfora
tions.? On the outer spherical surface of the external crust, there
are formed a great number of calcareous tubercles, arranged
with beautiful regularity and symmetry in double lines,
passing, like meridian circles, from the upper to the lower
pole of the sphere. Each appears, when magnified, to be a
smooth and solid ball, projecting from the surface of one of

* In addition to these larger tubes, there exists also a smaller set, which
pierce the skin indifferent places, and are channels for the absorption ofthe
water used in respiration. These I shall have occasion to notice more par
ticularly hereafter.
•(• An architecture of a still more curious description is exhibited in the
calcareous frame- work which has been provided for the support of the teeth,
and other organs of mastication, with which this animal is furnished. The
structure of these organs will be noticed when treating of that function.

ECHINUS. 151
the polygonal plates of the crust. These balls serve for the
support of the spines,* which have grooves or sockets at
their base, allowing of their accurate application to the sphe
rical surface of the tubercles. They thus constitute ball-and
socket joints, allowing of free motion in all directions. Each
joint is connected with the plate on which it turns, by means
ofthe integument, which acts the part of a capsular ligament;
and sets of radiating muscular fibres are provided for effect
ing the movements. of the spines. By employing these spines
as levers, the Echinus advances with great facility along
plane surfaces at the bottom of the sea. This animal is also
aided in its progressive motion by the employment of
suckers, which are placed at the end of the slender tubes,
protruding from the pores of the ambulacra, and analogous
to those of the Asterias.
The Spatangus, a genus belonging to this order, buries
itself in the sand by the action of its spines, which on its
under surface are short, thick, and expanded at the ends, like
the handle of a spoon, with the convexity downwards ; and
which have a limited rotatory motion. Those which grow
from the sides are more slender, and taper towards the ex
tremities, and when not in use they fall flat upon the body
with their points directed backwards. Besides these, there
are a few longer bristles, arranged in a crescent on the back,
and converging till their points meet, but capable of being
erected to a perpendicular position. The animal, when
placed on sand, commences its operations by revolving the
lower spines, thus soon creating a hollow quicksand, into
which it sinks by its own weight so far as to enable the low
est of the lateral spines to cooperate with them, by scatter
ing and throwing up the loosened particles ; while these, at
the same time, contribute, by their re-action, still farther to
depress the body. As the animal sinks, a greater number
of spines are brought into action, and its progress becomes
• It has been ascertained by Mr. Haidinger, that the structure of these
spines is crystalline; arid that their cleavage presents the exact rhomboidal
angles characteristic of carbonate of lime. See his Translation of Mohs's
Mineralogy, vol. ii. p. 91.

152 THE MECHANICAL FUNCTIONS.
more rapid ; while the sand, which had been pushed asfde,
flows back, and covers the body, when it has sunk below
the level of the surface. In this situation the long dorsal
bristles come into play, preventing the sand from closing
completely, and preserving a small round hole for the ad
mission of water to the mouth and respiratory organs.*
Whenever, in following the series of organic structures,
new forms are met with, we always find them accompanied
by corresponding modifications in the processes of develop
ment. The organization of the animals belonging to the
lowest division of the series is not sufficiently perfect to af
ford the means, which are supplied in the higher animals,
of removing or modifying the substances that have at any
time been deposited, and suffered to harden. Hence the
structures composed of these substances remain unchanged
during the life-time of the animal, although they may con
tinue to receive additions of new layers of the same mate
rial, deposited on their surface by the soft parts in contact
with them; for it is through the medium of the soft parts
alone that these materials are supplied. All the solid struc
tures of zoophytes are formed by this process, and they are
subjected to all the consequences of this law of increase. As
these consequences are important in their relation to the
conditions of growth, and to the forms which result, it will
be necessary to direct our attention to them more particu
larly. The influence which this mode of increase by superficial
depositions may have, in changing the form of the original
structure, will depend altogether upon the relative situations
of the soft secreting organ and the hard part on which it is
to deposite new layers : for, as every new layer must occupy
the situation of the soft organ which has formed it, it must
displace the latter, and push it back for a space equal to its
own thickness. In process of time, the addition of numerous
layers having led to successive encroachments of the solid
substance, the latter will have been displaced to an extent
* The account here given is taken from Mr. Osier's papers in the Philo
sophical Transactions for 1826, p. 347.

ECHINUS. 153
which must sooner or later become sensible. If the soft
organs have sufficient room for their expansion, as is the
case when they are external to the hard axis ofthe zoophyte,
the growth of that axis may go on without impediment; and
no change need take place in the general figure of the parts,
since their relative proportions and situations may be pre
served unaltered. But this cannot happen when the new
materials are to be deposited on the internal surface of a
membrane, or a shell, which completely encloses the soft
parts: for the additions thus made to the thickness of the
layer must encroach upon the space within; and, that space
being limited, the soft parts contained in it will not merely
cease to grow, but will be actually contracted in their di
mensions: and if the process of deposition were to go on, the
space occupied by the soft organs would at last be entirely
filled up with solid matter,, and the cavity be obliterated.
Accordingly it is necessary, whenever cells, intended for
the lodgement of soft organs, are to be constructed of hard
materials, that the foundation of these cells should be laid,
and their construction begun, upon a scale ofthe same size
as that which they are intended to have at all future periods ;
because, as we have just seen, after the innermost layer has
been deposited, they admit not of any future enlargement
of their cavity. Thus, we find that, in the case of polypes
which are lodged in cells, the walls of these cells must be
completed before the soft polypous portion has attained its
full expansion ; for were it at first built of a smaller size,
proportioned to that of the young polype, it would prevent
all farther growth.
The globular shell of the Echinus, which is external to
the soft parts that nourish it, and which yet grows from a
very minute sphere to one of large dimensions, keeping pace
With the gradual expansion of the internal organs, might ap
pear to be an exception to the general law. Nature has,
however, accomplished her purpose without deviating from
her Usual plan ; first, by dividing the shell of the Echinus into
a great number of small pieces ; and secondly, by giving to
each piece the polygonal form, which is best adapted to their
VOL. i.— 20

154 THE MECHANICAL FUNCTIONS.
mutual and perfect junction, without leaving any intervening
spaces. Thus, has she provided for the enlargement of the
whole structure, by admitting of additions being made to
the margins of each of the separate polygonal pieces ; fresh
layers of calcareous substance being deposited on the under
side, and on the edges of each, in proportion as the expan
sion of the contents of the shell causes their separation.
That such a succession of deposites has taken place, may
easily be seen, by minutely examining the texture of the
plates, which will be found marked by concentric polygonal
lines. (Fig. 99.)
The spines of the Echinus must be formed by the success
sive deposition of layers on their outer surface, as appears
from the examination of their structure, when a longitudinal
section of them has been made. The lines exhibiting the
succession of layers are seen in Fig. 100, which represents
such a section, Hence, they are probably deposited by the
membrane which covers them during the whole period of
their growth.
There is probably no series of animals that exemplify' in
so marked a manner as the Echinodermata, the gradations
which nature has observed in passing from one model of
construction to another of a totally different aspect, through
every intermediate form. What shapes can be more diver
sified, and apparently irreducible to a common standard,'
than those of the star-like Asterias, (Fig. 88) of the globu
lar Echinus, (Fig. 91,) and of the lily-shaped Pentaerinus;
(Fig. 94,) and yet we find these passing the one into the
other by the most gradual transitions ? Setting out from
the star with five slender rays, which is the standard form
of the Asterias, we find the rays, in succeeding species, as
suming gradually a greater breadth at their base, and their
sides joining at more obtuse angles: the star-like form is
gradually effaced, and the outline is rather a pentagon, with
its sides curved inwards (Fig. 89.) We soon perceive this
curvature giving place to a straight line, so that the shape
becomes an exact pentagon. The next change effected is in
the angles of this pentagon, which by degrees are lost in a

ECHINUS. 155
general rounded outline; still, however, preserving its flat
ness. This stage is attained in the Scutella, and the Cly-
peaster. (Fig. 90.) We next find that, in the Spatangus,
the thickness increases ; though at first with an oval outline,
and with several changes in the situation of the mouth of the
animal. At length, after passing through many intermediate
steps, we arrive at the perfectly circular and spheroidal
Echinus. (Fig. 91.) If we might be permitted to conjec
ture the objects of all these changes, which occur in this con
tinuous gradation, we might not unreasonably suppose them
to be the concentration of the internal organs into one com
pact mass, and the retrenchment of all the external appen
dages. It is also curious to observe, how, amidst all these
modifications, the double rows of perforations, which con
stitute the ambulacra, retain their situations, diverging in
five equidistant lines from one of the extremities of the axis,
and winding round to the other.
Returning to the Asterias, we can trace changes equally
gradual, though in an opposite sense, in another series, which
presents a striking contrast with the former. Here, instead
of the retrenchment of the appendages, we find them great
ly developed, and amplified in every possible degree. The
rays ofthe Asterias become narrower, while their length is
at the same time increased; the vital organs, and also the
tubular feet, are gradually withdrawn from them, and retire
within a central disk, to which the slender rays, 'now bereft
of feet, become mere appendages. Such is the condition of ,
the Ophiura. (Fig. 92.) By the prolongation and taper
ing of these rays to slender filaments, they acquire a greater
prehensile power, and twine with ease round their prey.
We next find their number augmented : it is at first dou
bled, then tripled, and at length indefinitely augmented.
They also become branched, subdividing by simple bifurca
tions, as in the Euryale palmiferum (Fig. 93;) next into
minuter ramifications, as in the Caput Medusa, where the
thousands of filaments have the appearance of a tangled web,
which defies all attempts at unravelling.
The steps are but short from the Caput Medusae to the Cri-

156 THE MECHANICAL FUNCTIONS.
noidea, or lily-shaped tribe, (of which, Fig. 94, representing
the Pentacrinus europceus, is an example;) for they consist
chiefly in the addition of a jointed stalk, which is made to
proceed downwards from the centre of the whole assem
blage of rays, and which is to serve as a common stem for
sustaining the whole mass ; while the branches themselves
are carried up, and folded inwards. The lower joint of the
foot-stalk is a little expanded, in order to procure a more
extensive base of support; and the whole structure thus pre
sents a remarkable resemblance to a liliaceous plant.

(, 157)

CHAPTER III.

MOLLUSCA.

§ 1. Mollusca in general.
The series of animal structures, arranged according to
their mechanical functions, conducts us next to the Mollus
ca; an assemblage of beings which was first recognised as
constituting one of the primary divisions of the animal king
dom by Cuvier, the greatest naturalist of modern times. A
vast multitude of species, possessing in common many re
markable physiological characters, are' comprehended in this
extensive class. In all, as their name imports, the body is
of soft consistence ; ahd it is enclosed more or less com
pletely in a muscular envelope, called the mantle, composed
of a layer of contractile fibres, which are interwoven with
the soft and elastic integument. Openings are left in this
mantle for the admission of the external fluid to the mouth
and to the respiratory organs, and also for the occasional
protrusion of the head and the foot, when these organs exist.
But a large proportion of the animals of this class are ace
phalous, that is, destitute of a head, and the mantle is then
occasionally elongated to form tubes, often of considerable
length, for the purpose of conducting water into the interior
of the body.
Mollusca, with the exception of a few among the higher
orders, are but imperfectly furnished with organs of loco
motion. The greater number, indeed, are formed for an
existence as completely stationary as the Zoophytes attached
to a fixed base. The Oyster, the Muscle, and the Limpet, for
example, are usually adherent to rocks at the bottom of the
sea, and are consequently dependent for their nourishment

158 THE MECHANICAL FUNCTIONS. #
on the supplies of food casually brought within their reach
by the waves and currents of the ocean. This permanent
attachment to the solid body on which they fix their abode,
does not, however, take place till they have arrived at a
certain period of their growth : for at the commencement of
their separate existence, that is, immediately after they are
hatched, they are free to move in the water, and to roam
in search of a habitation. In this respect, therefore, they
preserve an analogy with the gemmules of sponges, and of
polypi, which exercise locomotive powers only in the early
stages of their development.*
The organization of the Mollusca being unfitted for the
Construction of an internal skeleton, Nature has ordained
that the purposes of mechanical support and protection shall '
be answered by the formation of hard calcareous coverings,
or shells, the result of a peculiar process of animal production.
These shells are formed either of one piece, or of several;
• This analogy is strengthened by the circumstance that the movements
of many of these animals, in the first periods of their existence, are effected
by the same mechanism of vibratory Cilia which we found to be instrumental
in the progression of the infusory animalcules, and of the young of polypi.
On observing the first evolution of the ova of the Buccinum undatum, Dr.
Grant found them to consist of groups of spherical gelatinous bodies, which
soon become covered on one side with a transparent envelope, the rudiment
of the future shell; while, on the other side, the gelatinous matter is ex
tended outwards, so as to form the margin of an internal cavity, of which the
entrance is surrounded with vibratory cilia, and in the interior of which a re-
Volution of particles is seen, indicating a constant current of fluid. The vibra
tions of these cilia are perceived long before the pulsations of the heart, and
even before any appearance of that organ is visible; they are, indeed, the
first indications of life in the embryo. The cilia are in activity even before
the animal is hatched; for while confined within the egg, it is seen almost
continually revolving around its centre: a motion which appears destined to
bring a constant supply and renewal of sea water into the interior of the or
ganization, in order to perfect the formation of the shell before the animal
is, as it were, launched into the ocean. Possibly, also, the continued friction
of the cilia against the interior of the egg may tend to abrade it, and open a
passage for the young animal. No sooner has the animal effected its escape,
than it darts rapidly forwards by the motion of its cilia. The same appear
ances have also been observed by Dr. Grant in the young of different Mol
lusca, such as the Doris, Eulis, &c, which have no shell. — Edin. Journal
of Science, Vol. vii.

MOLLUSCA.

159

the separate pieces, in either case, being termed valves; so
that shells may be either univalve, bivalve, or multivalve,
according as they consist of one, two, or more pieces. Uni
valve shells have generally more or less of a spiral form,
and are then called turbinated shells. In a few, the cavity
of the shell is divided by transverse partitions into nume
rous compartments. Some Mollusca have internal shells for
the defence and support of particular organs; and others-
have shells which are partly external, and partly internal.
As respects their shape, colour, and appearance, shells ad
mit of infinite diversity ; yet, as will presently be shown,
all are composed of the same kind of material ; and their
production and increase are regulated by the same uniform
laws.

§ 2. Acephala.
The Mollusca which inhabit bivalve shells, such as the
' Oyster, the Muscle, and the Cockle, are all acephalous.
The two valves of the shell are united at the back by a
hinge joint, often very artificially constructed, having teeth
that lock into each other: and the mechanism of this arti
culation varies much in different species. The hinge is se-,
cured by a substance of great strength. It is seen in Fig.
101, which shows the valves
of the Unto batava,. with the
^^^» connecting ligament. This.
S^^W ligament is composed of two
"|F kinds of texture: the one
which is always external, is
strictly ligamentous; that is,
perfectly inelastic: the other
k has more of the properties of
cartilage, being highly elastic,

y ¦/ and formed of parallel series
WBr of condensed transverse fibres,
101 directed from the hinge of one
valve to the similar part of the others and having generally

160 THE MECHANICAL FUNCTIONS.
a deep black colour, and a pearly lustre. The cartilage is
always situated within the ligament, sometimes in immediate
contact, and forming with it one and the same mass: at other
times, placed at a distance, in a triangular cavity, amongst
the teeth of the hinge. The closing of the valves produces,
in all cases, a compression' of the cartilage, the elasticity of
which tends, therefore, to separate the valves from each
other; that is, to open the shell.
During the life of the animal, the usual and natural state
of its shell is that of being kept open for a little distance,
so as to_ allow of the ingress and egress of the water neces
sary for its nourishment and respiration. But as a security
against danger, it was necessary to furnish the animal with
the means of rapidly closing the shell, and retaining the
valves in a closed state. These actions being only occa
sional, yet requiring considerable force, are effected by
a muscular power : for which purpose sometimes one, some
times two, or even a greater number, of strong muscles are
placed between the valves, their fibres passing directly
across from the inner surface of the one to that of the other,
and firmly attached to both.
— They are named, from
their office of bringing the
valves towards each other,
the adductor muscles. Fig.
102,whichrepresents the sec
tion of an oyster, shows the
situation of the hinge l, the
adductor muscle a, and the transverse direction of its fibres,
with respect to the valves. When these muscles are not in
action, the elasticity of the cartilage attached to the hinge is
sufficient to separate the valves; but as they were not intend
ed to open beyond a certain extent, it was necessary to pro
vide some limitation to the action of the cartilage. The ad
ductor muscle might.it is evident.be called 'into play to
counteract that action ; but this would require a constant
muscular exertion, and a great expenditure, therefore, of
vital force. Nature has always shown a solicitude to econo-

MOLLUSCA ACEPHALA.

161

mize muscular power, whenever a substitute could be had,
and such a substitute she has here provided, by uniting with
the muscle an elastic ligament, of a peculiar construction.
It has a texture similar to that of the ligamentum nucha,
and being placed on the side of the muscle next to the hinge,
allows the valves to separate to the proper distance only.*
When the animal dies, the muscular force ceases, but the li
gament with.which the muscle is associated, retaining its elas
ticity, allows the shell to open, but only to a certain extent ;
and, accordingly, this is the state in which we find bivalve
shells that are cast upon the shore, after the soft flesh of the
animal has decayed and been washed out, provided the car
tilage and the ligament of the hinge are still preserved.!
The simple actions of opening and closing the valves are
capable of being converted into a means of retreating from
danger, or of removing to a more com
modious situation, in the case of those
bivalves which are not actually attached
to rocks or other fixed bodies. Dique-
mare long ago observed that even the
oyster has some power of locomotion, by
suddenly closing its shell, and thereby
expelling the contained water, with a de
gree of force, which, by the reaction of
the fluid in the opposite .direction, gives
a sensible impulse to the heavy mass.

103

* This remarkable structure was first described by Dr. Leach, in a paper
read before the Royal Academy of Paris. Bulletin des Sciences, 1818, p.
14. See also Gray, in Zoological Journal, 1. 219.
¦j- The Pholas is an exception to this rule; for instead of its valves,being
united, as usual, by an elastic ligament, they are connected chiefly by means
of muscles. This departure from the ordinary structure is probably occa
sioned by a new condition introduced into the economy of the animal in con
sequence of its being fitted for excavating passages through hard rocks. It
is furnished, for this purpose, with a complicated boring apparatus moved
by many muscles, and requiring great freedom of action. Fig. 103 repre
sents the shell of the Pholas Candida extremely expanded, in order to show
the hinge, together with the ligament, i; the long and thin process of shell,
p, to the ends of which, on each side, a pair of fan-shaped muscles, more par
ticularly employed in boring, are attached; and the two adductor muscles,
VOL. I. — 21

162 THE MECHANICAL FUNCTIONS.
He notices the singular fact that oysters, which are attached
to rocks occasionally left dry by the retreat of the tide, al
ways retain within their shells a quantity of water sufficient
for respiration, and that they keep the valves closed till the
return of the tide : whereas those oysters which are taken
from greater depths, where the water never leaves them, and
are afterwards removed to situations where they are exposed
to these vicissitudes, of which they have had no previous ex
perience, improvidently open their shells after the sea has
left them, and by allowing the water to escape, sbon perish.*
Many bivalve mollusca are provided with an instrument
shaped like a leg and foot, which they employ extensively
for progressive motion. Its form
in the Cardium, or cockle, is seen
in Fig. 104. This organ is com
posed of a mass of muscular fibres,
interwoven together in a very com
plex manner, and which may be
compared to the muscular structure
of the human tongue : the effect in both is the same, namely,
the conferring a power of motion in all possible ways ; thus
it maybe readily protruded, retracted, or inflected at every
point. The Solen, or razor-shell fish, has a foot of a cylin
drical shape, tapering at the end, and much more resembling
in its form a tongue than a foot. In some bivalves the dila
tation of the foot is effected by a curious hydraulic mecha
nism : the interior of the organ is formed of a spongy texture,
capable of receiving a considerable quantity of water, which
the animal has the power of injecting into it, and of thus in
creasing its dimensions.
The foot of the Mytilus edulis, or common muscle, can
bel advanced to the distance of two inches from the shell,
and applied to any fixed body within that range. By at-
*, a, which retain the valves in contact independently of the ligaments. For
a full description of this apparatus, I must refer to a paper by Mr. Osier, on
burrowing and boring marine animals, contained in the Phil. Trans, for 1826,
p. 342, from which the above figure has been taken.
• Journal de Physique, xxviii. 244.

MOLLUSCA ACEPHALA. 163
taching the point to such body, and retracting the foot, this
animal drags its shell towards it; and by repeating the ope
ration successively on other points of the fixed object, con
tinues slowly to advance.
This instrument is of great use to such shell-fish as conceal
themselves in the mud or sand, which its structure is then
peculiarly adapted for scooping out. The Cardium conti
nually employs its foot for this purpose : first elongating it,
directing its point downwards, and insinuating it deep
into the sand; and next, turning up the end, and forming it
into a hook, by which, from the resistance of the sand, it is
fixed in its position, and then the muscles which usually re
tract it are thrown into action, and the whole shell is alter
nately raised and depressed, moving on the foot as on a ful
crum. The effect of these exertions is to drag the shell
downwards. When the animal is moderately active these
movements are repeated two or three times in a minute^
The apparent progress is at first but small; the shell, which
was raised on its edge at the middle of the stroke, falling
back on its side at the end of it; but when the shell is bu
ried so far as to be supported on its edge, it advances more
rapidly, sinking visibly at every stroke, till nothing but the
extremity of the tube can be perceived above the sand. Mr.
Osier, who has given us this account,* observes that the in
stinct, which directs the animal thus to procure a shelter,
operates at the earliest period of its existence. The Mya
truncata, when fully grown, will not attempt to burrow ;
but on placing two young ones, which were scarcely more
than a Hne in length, and apparently but just excluded, on
sand, in a glass of sea-water, he found that they buried them
selves immediately.
By a process exactly the inverse of this, that is, by dou
bling up the foot, and pushing with it downwards against the
sand below, the shell may be again made to rise by the same
kind of efforts which before protruded the foot. By this
process of burrowing, the animal is enabled quickly toretreat
• Pbilos. Trans, for 1826, p. 349.

164 THE MECHANICAL FUNCTIONS.
when danger presses: and when this is past, it can, with
equal facility, emerge from its hiding-place.
The Cardium can also advance at the bottom of the sea
along the surface of the soft earth, pressing backwards with
its foot, as a boatman impels his boat onwards, by pushing
with his pole against the ground, in a contrary direction. It
is likewise by a similar expedient that the Solen forces its
way through the sand, expanding the end of its foot into the
form of a club. The course of these locomotive bivalves
may readily be traced oh the sand by the furrows which
they plough up in their progress.
These, as well as many other of the bivalve mollusca, are
enabled by the great size and flexibility of this organ to
execute various other movements, of which, from the habit
ual inactivity of animals of this class, we should scarcely
have supposed them capable. The Tellina is remarkable
for the quickness and agility with which it can spring to
considerable distances by first folding the foot into a small
compass, and then suddenly extending it; while the shell is
at the same time closed with a loud snap.
The Pinna, or Marine Muscle, when inhabiting the shores
of tempestuous seas, is furnished, in addition, with a singu
lar apparatus for withstanding the fury of the surge, and se
curing itself from dangerous collisions, which might easily
destroy the brittle texture of its shell. The object of this
apparatus is to prepare a great number of threads, which are
fastened at various points to the adjacent rocks, and then
tightly drawn by the animal; just as a ship is moored in a
convenient station to avoid the buffeting of the storm. The
foot of this bivalve is cylindrical, and has, connected with
its base, a round tendon of nearly the same length as itself,
the office of which is to retain all the threads in firm adhe
sion with it, and concentrate their power on one point. The
threads themselves are composed of a glutinous matter, pre
pared by a particular organ. They are not spun by being
drawn out of the body like the threads of the silk-worm, or
of the spider, but they are cast in a mould, where they hard
en, and acquire a certain consistence before they are em-.

MOLLUSCA ACEPHALA. 165
ployed. This mould is curiously constructed; there is a
deep groove which passes along the foot from the root of the
tendon to its other extremity; and the sides of this groove
are formed so as to fold and close over it, thereby convert
ing it into a canal. The glutinous secretion, which is poured
into this canal, dries, into a solid thread; and when it has
acquired sufficient tenacity, the foot is protruded, and the
thread it contains is applied to the object to which it is to be
fixed; its extremity being carefully attached to the solid sur
face of that object. The canal of the foot is then opened
along its whole length, and the thread, which adheres by its
other extremity to the large tendon ait the base of the foot,
is disengaged from the canal. Lastly, the foot is retracted,
and the same operation is repeated.
Thread after thread is thus formed^ and applied in diffe
rent directions around the shell. Sometimes the attempt
fails in consequence of some imperfection in the thread; but
the animal, as if aware of the importance of ascertaining the
strength of each thread, on which its safety depends, tries
every one of them as soon as it has been fixed, by swinging
itself round, so as to put it fully on the stretch: an action
which probably also assists in elongating the thread. When
once the threads have been fixed, the animal does not ap
pear to have the power of cutting or breaking them off! The
liquid matter out of which they are formed is so exceeding
ly glutinous as to attach itself firmly to the smoothest bo
dies. It is but slowly produced, for it appears that no
Pinna is capable of forming more than four, or at most five
threads in the course of a day and night. The threads which
are formed in hastevwhen the animal is disturbed in its ope
rations, are more slender than those which are constructed
at its leisure. Reaumur, to whom we are indebted for these
interesting observations, states, also, that the marine muscles
possess the art of forming these threads from the earliest pe
riods of their existence; for he saw them practising it, when
the shells in which they were enclosed were not larger than
a millet seed.* In Sicily, and other parts of the Mediter^
* Memoires del'Academie des Sciences: 1711, p. 118 to 123. Poli con-

166 THE MECHANICAL FUNCTIONS.
ranean, these threads have been manufactured into gloves,
and other articles which resemble silk.
§ 3. Gasteropoda.
The Mollusca which inhabit univalve or turbinated shells,
belong to the order of Gasteropoda, and have a more highly
developed organization than the Acephala. The part
which performs the office of a foot is a broad expansion
of fleshy substance, occupying nearly the whole under sur
face of the animal, and forming a flat disk, capable of being
applied to the plane along
which it moves. This is
seen in the Planorbis (Fig.
105, d.) In some species it
is fashioned into a project
ing ridge, which cuts its way,
like a ploughshare, along the surface on which it moves.
The bands of muscular fibres, which compose the principal
part of its structure, are short, and are interlaced together
in a very intricate arrangement. All the columns of their
fibres terminate at the surface ofthe disk; so that when the
animal is crawling, their successive actions produce a visible
undulatory motion of that surface. The effect of these ac
tions is that different parts of the plane on which it moves
are laid hold bf in succession, and each corresponding por
tion of the animal is dragged along, so that the body ad
vances by a slow and uniform gliding motion. The opera
tion of this mechanism may easily be seen in a snail, by
making it crawl on a pane of glass, and viewing the move
ment of its disk from the other side of the glass : the regu
lar undulations which advance in the direction of the mo
tion of the snail, but with twice the velocity, present a cu
rious and interesting spectacle.
A mucilaginous secretion generally exudes from the sur
face of the disk, and tends to increase considerably its
ceived that these threads are dried musculai' fibres;, an opinion, which has
been adopted by Blainville.

OASTEROPODA. 167
power of adhesion, both when the animal is crawling, and
also when it fixes itself on any surface. In the Patella, or
limpet, this adhesion is greatly favoured by the conical form
of the shell, which, having a circular base, enables the mus
cles of the disk, by their efforts to create £ vacuum under
neath it, to command the whole hydrostatic pressure of the
superincumbent water, as well as of the atmosphere above
the water. Besides the muscular bands contained in the
substance of the foot, other sets of fibres are provided for
the purpose of protruding or of retracting the whole mem
ber, and of moving it in different directions.
The foot of the Buccinum undatum, or Whelk, is capa
ble of great dilatation by means of four tubes, which open
from the surface near the gullet, and convey into it a large
quantity of water. It may, by this means, be distended to
a size even greater than the shell itself; so that the opening
which it forms in the sand is large enough to receive the
shell, when the latter is drawn down by the contraction of
the muscles which are attached to the foot.* The foot of
the Scyllcea is grooved, for the purpose of enabling the ani
mal to lay hold of the stems and branches of marine plants,
and advance along them by a gliding motion.
The head is generally furnished with tubular tentacula,
which the animal protrudes for the purpose of feeling its
way as it advances, and which are quickly retracted, by the
reversion of the tube, when they are touched or irritated.
This mechanism is matter of familiar observation in the t&i-
tacula, or horns, of the snail and of the slug, which are ter
restrial mollusca belonging to this order. The former of
these has a turbinated shell of the ordinary structure : the
latter, though extremely similar in its internal structure to
the snail, is destitute of any external shell ; but is furnished,
instead of it, with a small internal plate of cartilage, giving
support to some of the vital organs.
• Osier, Phil. Trans, for 1826, p. 352.

168 THE MECHANICAL FUNCTIONS.

§ 4. Structure and Formation of ihe Shells of Mollusca.
The structure and formation of the shells pf molluscous
animals is a subject of much interest in comparative physi
ology, as presenting many beautiful illustrations of the laws
by which the inorganic parts of the living system are in
creased in their dimensions.
All shells are composed of two portions, the one consist
ing of particles of carbonate of lime, the other having the
character of an animal substance, and corresponding in its
chemical properties either to albumen or to gelatine. The
mode in which these two constituent parts are united, as
well as the nature of the animal portion, differ much in dif
ferent kinds of shell ; and it is chiefly in reference to these
circumstances that shells have been divided into two classes,
namely, the membranous and the porcellaneous shells.
In shells belonging to the first of these classes, the carbo
nate of lime is united with a membranous substance depo
sited in layers, which may be separated from one another,
either by mechanical division with a sharp instrument, or
by the slow actions of air, water, or other decomposing che
mical agents. The shells of the limpet, of the oyster, and
of almost all the larger bivalve mollusca which reside in the
ocean, are of this kind. They are usually covered with a
thick outer skin, or epidermis; and their texture is of a
coarser grain than that of other shells.
If a shell of this description be immersed in an acid capa
ble of dissolving carbonate of lime,- such as the muriatic or
nitric acids properly diluted, at first a brisk effervescence is
produced, but this soon slackens, and the carbonate of lime
contained in the shell is slowly dissolved ; the membranous
layers being left entire, and sufficiently coherent to retain
the figure of the shell, but, having lost the earthy material
which gave them hardness, they assume their natural form
of soft and flexible plates.
Many membranous shells exhibit, on several parts of their
internal surface, a glistening, silvery, or iridescent appear-

Mpfe

STRUCTURE OF SHELLS. 169
ance.* This appearance is caused by the peculiar thinness,
transparency, and regularity of arrangement of the outer
layers of the membrane, which, in conjunction with the par
ticles of carbonate of lime, enter into the formation of that
part of the surface of the shell. The surface, which has thus
acquired a pearly lustre, wa% formerly believed to be a pe
culiar substance, and was dignified with the appellation of
mother of pearl, from- the notion that Was entertained of its
being the material of which pearls are formed. It is true,
indeed, that pearls are actually composed of the same mate
rials, and have the same laminated structure as the mem
branous shells ; being formed by very thin concentric plates
of membrane and carbonate of lime, disposed alternately,
106 and often surrounding a central body,
or nucleus: but Sir David Brewster
has satisfactorily shown that the iri
descent colours exhibited by these sur
faces are wholly the effect of the paral
lel grooves consequent upon the regu-
JlllfiillSSllfl' larity of arrangement in the successive
sB!!!? " S? deposites of shell.t The appearance
^SpV-' y of these grooves or striae when highly
magnified is shown in Fig. 106. J This
iridescent property may be communicated to shell lac,
sealing-wax, gum Arabic, balsam of Tolu, or fusible metal,
by taking an accurate cast or impression of the surface of
mother of pearl with any one of these substances. §
Porcellaneous shells have a more uniform and compact
texture than those ofthe former class. The animal matter
* Examples of this nacreous structure, as it is termed, occur in the shellsr
ofthe Haliotis, or Sea-ear, and ofthe Anodon, or fresh water-muscle.
f Philosophical Transactions for 1814, p. 397.
$ See also a paper on this subject by Herschel in the Edinburgh Philoso
phical Journal, ii. 114, from which the annexed figure is taken.
§ When these shells decay and fall to pieces-, they separate into numerous
thin scales of a pearly lustre. The fine scales thus obtained from the Ph.-
euna, or window oyster, are employed by the Chinese in their water-colour
drawings to produce the effect of silver. Some of this powder has been
brought to England and used for thisipurpose. (Gray, Phil. Trans, for 1833.
p. 794.)
vol. i.— 22

170 , THE MECHANICAL FUNCTIONS.
which unites the carbonate of lime is less in quantity and
not so evidently disposed in layers ; but it is more equally
blended with the earthy particles, with respect to which it
appears to perform the office of a cement, binding them
strongly together, although it has of itself but little cohesive
strength. The Cyprcea and the Volute are examples of por
cellaneous shells.
In shells of this kind the carbonate of lime assumes more
or less of a crystalline arrangement; the minute crystals be
ing sometimes in the form of rhombs, and sometimes in that
of prisms. In the former case they are composed of three
distinct layers, as may be seen by making sections of any
of the spiral univalve shells, or simply by breaking them in
various directions. Each layer is composed of very thin
plates, marked by oblique lines, which
I, show the direction of the crystalline
a fibres.* The direction of the layers
P|| and fibres is also rendered manifest by
^j|lA the planes of cleavage, when they are
tglik broken into fragments. The plates of
^Hpi, the outer and inner layers are always
^j||jl directed from the apex of the cone to
3sk its base, so as to follow the direction of
X the spire : while, on the contrary, those
of the intermediate plate form concentric rings round the
cone parallel to its base. Thus the fibres of each layer are at
right angles to those of the layer which is contiguous to it;
an arrangement admirably calculated for giving strength to
the shell, by opposing a considerable cohesive resistance to
all forces tending to break it, in whatever direction they may
be applied.* We here find that a principle, which has only
* These lines are shown in the diagram,. Fig, 107, which represents a
longitudinal section of a shell of this kind, A is the outer layer, of which
the fibres pass obliquely downwards. B is the middle layer, having fibres
placed at right angles with the former. C is the third, or inner layer, the
fibres of which have a direction similar to the outer layer. Within this lay
er there is frequently found adepos'ite of a hard, transparent, and apparently
homogeneous calcareous material, D. Of this latter substance I shall after
wards have occasion to speak.

STRUCTURE OF SHELLS. 171
of late years been recognised and applied to the building of
ships, namely, that of the diagonal arrangement of the frame
work, and the oblique position of the timbers, is identical
with that which from the beginning of creation, has been
acted upon by nature in the construction of shells.
When the form of the crystals is prismatic, the fibres are
short, their direction is perpendicular to the surface', and
the prisms are generally hexagonal. This structure is ob
servable in the Teredo giganiea from Sumatra,* and also in
many bivalves, such as those belonging to the genera Avi-
cula and Pinna.
When porcellaneous shells are subjected to the solvent
action of acids, the animal matter in their composition offer
ing but little resistance, there is a considerable and long con
tinued effervesdence. The solution of the carbonate of lime
proceeds rapidly, in consequence of the speedy disintegra
tion of the animal substance, which is broken up, and partly
dissolved. The remainder is reduced to minute fragments,
which subside in the form of flakes or scales to the -bottom
of the fluid. Poli has given a minute and elaborate descrip
tion of the appearances of these fragments of membrane,
when seen under the microscope.f
The difference between the textures of these two kinds of
shell is farther illustrated by the impression made upon them
by fire. Porcellaneous shells, when exposed to a red heat,
give out neither smell nor smoke: they lose, indeed, their
colour, but retain their figure unaltered. Membranous
shells, on ihe contrary, emit a strong fetid odour, and be
come black; after which the plates separate, and the struc
ture falls to pieces.
This variety in the composition and structure of different
kinds of shell is accompanied by corresponding modifica
tions of their mechanical properties. The toughness of the
fibrous basis of membranous shells, imparts to them greater
* In this shell the crystalline appearance is so perfect, that when some
fragments were sent to England, they were mistaken for a mineral produce
tion. Home; Lectures, 1.53.
+ See his folio work on the Testacea ofthe Two Sicilies.

172 THE MECHANICAL FUNCTIONS.
strength than is possessed by the porcellaneous shells, which,
in consequence of the tenuity and uniform intermixture of
the animal cement with the calcareous particles, present a
harder and more transparent, but, at the same time, more
brittle compound. It is these qualities, together with their
smooth enamelled surface, often beautifully variegated with
brilliant colours, and presenting altogether a close resem
blance to porcelain, that have procured them the name they
bear. When the transparency and brittleness of these shells are
very great, they have been considered as forming another
class, and they have been termed Vitreous shells, from their
making a nearer approach to glass. Some shells present
intermediate textures between the membranous and the por
cellaneous. All those surfaces of the shell on its outer side which are
not in contact with any part of the animal, are originally
covered with an epidermis:* which, however, is frequently
rubbedioff by friction.
The process employed by nature for the formation and en
largement of the shells of the mollusca was very imperfect
ly understood prior to the investigations of Reaumur, who
maybe considered as having laid the first solid foundations
of the theory of this branch of comparative physiology .f
His experimental inquiries have fully-established the two
following general facts: first, that the growth of a shell is
simply the result of successive additions made to its surface,
and secondly, that the materials constituting each layer, so
added, are furnished by the organized fleshy substance,
which he termed the skin of the animal, but which is now
\ known by the name of the mantle, and not by any vessels
or other kind of organization belonging to the shelf itself.
If a portion of the shell of a living snail, for instance, be
removed, which can be done without injury to the animal,
since it adheres to the flesh only in one point, there is
* This membrane has been termed the Periostracum.
j Memoires de 1'Academie des Sciences, 1709, p. 367, and 1716, p. 303.

FORMATION OF SHELLS. 178
formed, in the course of twenty-four hours, a fine pellicle,
resembling a spider's web, which is extended across the va
cant space, and constitutes the first stratum of the new shell.
This web, in a few days, is found to have increased in thick
ness, by the addition of other layers to its inner surface ; and
this process goes on until, in about ten or twelve days, the
new portion of shell has acquired nearly the same thickness
as that which it has replaced; Its situation, however, is not
exactly the same, for it is beneath the level of the adjacent
parts of the shell. The fractured edges of the latter remain
unaltered, and have evidently no share in the formation of
the new shell, of which the materials have been supplied ex
clusively by the mantle. This Reaumur proved by intro
ducing through the aperture a piece of leather underneath
the broken edges, all round their circumference, so as to lie
between the old shell and the mantle : the result was that no
shell was formed on the outside of the leather; while, on the.
other hand, its inner side was lined with shell.
The calcareous matter which exudes from the mantle in
this process is at first fluid and glutinous; but it soon hardens,
and consolidates into the dense substance of the shell. The
particles of carbonate of lime are either agglutinated toge
ther by a liquid animal cement, which unites them into a
dense and hard substance, resembling porcelain ; or they are
deposited in a bed of membranous texture, having already
the properties of a solid and elastic plate. This explains
the laminated structure possessed by many shells of this
class, such as that of the oyster, of which the layers are easi
ly separable, being merely agglutinated together like the
component leaves of a sheet of pasteboard.
It has long been the prevailing opinion among naturalists
that no portion of a shell which has been once deposited, and
has becomeconsolidated, is capable of afterwards undergoing
any alteration by the powers of the animal that formed it.
Very conclusive evidence has, in my opinion, been adduced
against the truth of this theory, by Mr. Gray.* From a
variety of facts, it appears certain that on some occasions
* Philos. Transactions for 1833, p. 796, et seq.

174 THE MECHANICAL FUNCTIONS.
the molluscous animal effects the removal of large portions
of its shell, when they interfere with its own growth, or are
otherwise productive of inconvenience. We should at the
same time regard these cases in the light of exceptions to the
ordinary rule, that a portion of shell once formed remains
ever after unchanged, while it continues to be connected
with the animal which produced it. In a general way, in
deed, we may consider the connexion between the animal
and the shell as mechanical, rather than vital; and the shell
itself as an extraneous inorganic body, forming no part of
the living system: for whatever share of vitality it may have
possessed at the moment of its deposition, all trace of that
property is soon lost. Accordingly, we find that the holes
made in shells by parasitic worms are never filled up, nor
the apertures of the cavities so made covered over, unless
the living flesh of the animal be wounded; in which case an
exudation of calcareous matter takes place, and a pearly
deposite is produced. The worn edges of shells, and the
fractures, and other accidents which befall them, are never
repaired, except as far as such repairs can be made by the
addition of materials from the secreting surfaces of the man
tle. It is found that shells may be impregnated with poison
ous metallic salts, such as those of copper, without any de
triment to the animals they enclose.
The power of secreting the materials of shell does not
usually extend to the whole of the surface ofthe mantle, but
is generally confined to the parts near the margin, composing
what is termed the. collar. The calcareous substance is al
ways poured out underneath the epidermis,* that is, between
this outermost layer of integument, and the subjacent co
rium, which is incorporated with the mantle, and may be
regarded as forming one and the same organ.f
* Mr. Gray considers the external membrane of the shell, or epidermis, as
formed by the outer edge of the plates of animal substance, which have
scarcely any calcareous matter in their composition, and which are soldered
together into a membranous coat.
¦j- A secreting power is also, in some instances, possessed by the foot, as is
exemplified in some of the gasteropoda, where it forms an operculum, or

FORMATION OF SHELLS. 175
The shape of the shell depends altogether on the extent
and particular form and position of the secreting organ.
The animal, on its exclusion from the egg, has already a
small portion of shell formed, and the simplest case is that
in which this rudiment of shell is a concave disk. We
may conceive the animal, covered by its mantle, to expand
the border of this organ, and extend it beyond the edge of
the shell, where it then forms a new layer of shell; and this
new layer, being applied to the inner or concave surface of
the original shell, will, of course, extend a little way beyond
its circumference. The same happens with the succeeding
layers, each of which being larger than the one which has
preceded it, projects in a circle beyond it ; and the whole
series of these conical layers, of increasing diameters, forms
a compound cone, of which the outer surface exhibits trans
verse lines, showing the successive additions made to the
shell in the progress of its increase. The Patella^ or limpet,
is an example of this form of structure.
But in by far the greater number of mollusca which inhabit
univalve shells, the formation and deposition of the earthy
material does not, as in the preceding instance, proceed
equally on all sides. If the increase take place in front
only, that is, in the fore part of the mantle, the continual
deflexion thence arising necessarily gives the shell a spiral
form, the coils being simply in one plane. This is the* case
in the Planorbis. (Fig. 105) the Spirula, and the Nautilus.
Most, commonly, however, as in the Buccinum, and Acha-
tina (Fig. 108) the deposite of shelf takes place laterally, and
more on one side than on the other; hence the coifs produced
descend as they advance, giving, rise to a curve, which is
continually changing its plane, being converted from a spiral
into a helix, a term of Geometry borrowed' from the Latin
name of the common snail, which, as is well known, has a
calcareous covering, to the mouth of the shell. Mr. Gray also ascertained that
in the Cymbia, and Olivse and" the Ancillarise, shell is deposited, and most
probably secreted by the upper surface of the foot, which is very large, and
not by the mantle, which is small, and does not extend beyond the edge of
the mouth. Phil. Trans, for 1833, p. 805.

176 THE MECHANICAL FUNCTIONS.
/
shell of this form. Fig. 108, which represents the shell ofthe
Achatina zebra, and of which Fig. 109 shows a longitudinal
section, may serve as an example of a shell of this kind.
The axis of revolution is termed the Columella, and the
turns of the spiral are denominated ivhorls. In consequence
of the situation of the heart and great blood vessels relative

ly to the shell, the left side of the mantle is more active than
the right side, so that the lateral turns are made in the con
trary direction ; that is, towards the right.* There are a
few species, however, where, in consequence of the heart
being placed on the right side, the turns of the spiral are
made to the left. Such shells have been termed sinistral
or reversed shells: but this left-handed convolution seldom
occurs among the shells of land or fresh-water mollusca.-
It results from this mode of formation that the apex both
of the simple and of the spiral cone is the part which was
formed the earliest, and which protected the young animal
at the moment of its exclusion from the egg. This portion
may generally be distinguished by its colour and appearance
from that which is formed subsequently. The succeeding
turns made by the shell in the progress of its growth, enlarging
in diameter as they descend from the apex, form by degrees
a wider base. During the growth of the animal, as the body
extends towards the mouth of the shell, its posterior end
often quits the first turn of the spire, and occupies a situa-
* The terms right and left have reference to the position of the animal
when resting on its foot; the head being, of course, in front. See Gray,
Zool. Journal, i. 207.

FORMATION OF SHELLS. 177
tion different from that which it had originally. In these
cases the cavity at the apex of the spire is filled up with
solid calcareous matter of a hardness not inferior to that of
marble. Such is the general form of turbinated shells. It some
times happens, however, as in the Conus, that the upper
surface of ths spiral scarcely descends below the level of the
original portion of the shell, which in the former disposition
of its parts would have been the apex: while the lower por
tions of the spiral turns shoot downwards, so as to form a
pointed process; thus, the whole is still a cone, but reversed
from the former, the part last formed being the outer surface
of the cone, and the circumference of the apparent base, or
flat surface, of which the central part is the one first formed.
Various causes may occur to disturb the regularity of the
process of deposition, by which the shell is enlarged in its
dimensions ; at one time accelerating, and at another retard
ing, or totally arresting its growth. These irregularities are
productive of corresponding inequalities in the surface of
the shell, such as transverse lines, or striae. Whenever an
exuberance of materials has led to a sudden expansion of
growth, which has again soon subsided, a projecting ridge is
produced in the direction of the margin of the mantle at the
time this happens. This change generally recurs at regular
periods, so that these ridges, or ribs, as they are often called,
succeed one another at equal distances along the course of
the spiral turns.
It not unfrequently happens that, at different periods, a
sudden development takes place in particular parts of the
mantle, which become in consequence rapidly enlarged,
shooting out into long slender processes. Every part ofthe
surface of these processes has the power of secreting and
forming shell, so that the portion of shell they construct,
being consolidated around each fleshy process, must neces
sarily have at first the shape of a tube closed at the extre
mity. As fresh deposites are made by the secreting surface,
which are in the interior of the tube, the internal space is
vol. i.— 23

178

THE MECHANICAL FUNCTIONS.

gradually filled up by these deposites ; the process of the
mantle retiring to make way for their advance towards the
axis of the tube. In the course of time, every part of the
cavity is obliterated, the process of the shell becoming en
tirely solid. Such is the origin of the many curious pro
jecting cones or spines which several shells exhibit, and
which have arisen periodically during their growth from
their outer surface. In the Murex these processes are of
ten exceedingly numerous, and occur at regular intervals,
frequently shooting out into various anomalous forms. In
many shells of the genus Strombus these spines are of great
length, and are arranged round the circumference of the
base, being at first tubular, and afterwards solid, according
to the period of growth. This is exemplified in the Ptero-
cera scorpio (Lamarck) of which Fig. 1 10 shows the early,
and Fig. Ill the later period of growth.

A limit has been assigned by nature to the growth of
molluscous animals, and to the shells which they form; and
there is a certain epoch of their existence, when consider
able changes take place in the disposition of the mantle, and
in its powers of secretion. Often we find it suddenly ex
panding into a broad surface, adding to the shell what may
be termed a large lip. Sometimes no sooner has this been
accomplished than the same part again shrinks, and the
mantle retires a little way within the shell, still continuing
to deposite calcareous layers, which give greater thickness
to the adjacent part of the shell ; and at the same time nar-

FORMATION OF SHELLS. 179
row its aperture, and materially alter its general shape and
aspect. Thus it happens that the shells of the young and of
the old individuals of the same species are very different,
and would not be recognised as belonging to the same tribe
of mollusca. This is remarkably the case with the shell of
the Cyprcea, or Cowrie, which, in the early stage of its
growth, (Fig. 112) has the ordinary form of an oblong tur
binated shell: but from the process just described taking
place at a certain period, the mouth of the shell (as shown
in Fig. 113,) becomes exceedingly narrow, and the edges of
the aperture are marked by indentations, moulded on cor
responding processes of the mantle.* But in this instance
the change does not stop here ; for both edges of the mantle
next take a wider expansion, turning over the outer surface
of the shell, and passing on till they meet at the upper convex
part, or back of the shell, forming what has been termed the
dorsalline. They deposite, as they proceed, adense and high
ly polished porcellaneous shell, beautifully variegated with
coloured spots, which correspond exactly with the coloured
114 parts of the mantle that deposites
them. This new plate completely
envelops the original shell, giving it a
new covering, and disguising its for
mer character. A transverse sec
tion, (Fig. 114,) at once shows the
real 'steps by which these changes
have taken place.t
Changes equally remarkable are observed to occur in the
interior of the shell at different stages of its growth. On
* Similar changes occur in the shells ofthe Ovula (spindles,) Erato (tear-
shells) and Marginella, (dates.) 'Gray, Phil. Trans, for 1833, p. 792.
¦j- According to Bruguiere, there is reason to believe that the animal ofthe
Cyprxa, after having completed its shell, in the manner above described, still
continuing to grow, and being incommoded for want of space, quits its shell
altogether, and sets about forming a new one, better suited to its enlarged
dimensions. It is stated also that the same individual is even capable of
forming in succession several shells. Blainville, however, considers it im
possible that the living animal can ever quit its shell. Malacologie, p. 94.

180 THE MECHANICAL FUNCTIONS.
the inner surface of the Milra, the Volute, and other shells
of a similar kind, there is deposited a layer of a hard semi-
transparent calcareous material, having a vitreous appear
ance.* The thickness of the layer, which thus lines the ca
vity of the shell, is greater as it approaches the apex; and
where the spire is much elongated, or turrited, as it is called,f
this deposition entirely fills the upper part, which, in the
early condition of the shell, was a hollow space with thin
sides. The purpose answered by this deposite is evidently
to give solidity and strength to a part which, by remaining
in its original state, would have been extremely liable to be
broken off by the action of the sea.
In other cases a different expedient is adopted. The
animal, instead of fortifying the interior of the apex by a
lining of hard shell, suddenly withdraws its body from that
part, and builds a new wall or partition across the cavity,
so as to protect the surface thus withdrawn. That portion
of the shell which is thus abandoned, being very thin and
brittle, and having no support internally, so'on breaks off,
leaving what is termed a decollated shell ; examples of this
occur in the Cerithium decollatum, the Bulimus decollatus,
&c. The young of the genus Magilus has a very thin shell
of a crystalline texture; but when it has attained its full
size, and has formed for itself a lodgement in a coral, it
fills up the cavity of the shell with a glassy deposite, leaving
only a small conical space for its body ; and it continues to
accumulate layers of this materia], so as to maintain its
body at a level with the top of the coral to which it is at
tached, until the original shell is quite buried in this vitreous
substance. I
The forms of the Cone an^ Olive shells are such as
to allow but a small space for the convolutions of the
body of the animal, which accordingly becomes 'in the
progress of its enlargement, excessively cramped. In or
der to obtain more space, and at the same time lighten the
* This is the substance represented at ,>, Fig. 107, p. 170.
f As in the genera Turritella, Terebra, Cerithium, and FaSciolaria.

FORMATION OF SHELLS.

181

shell, the whole of the two exterior layers of the inner
whorls of the shell are removed, leaving only the interior
layer, which is consequently very thin when compared with
the other whorl, that envelops the whole, and which, re
taining its original thickness, is of sufficient strength to give
full protection to the animal. That this change has actually
been effected is very distinctly seen in the Conus (Fig. 115)
by examining a vertical section of that shell, as is represented
in Fig. 116. All the inner partitions of the cavity thus laid

117

open are found to be extremely thin and transparent, and
to consist only ofthe innermost lamina of the original shell;
as will appear on tracing them up to that outer portion of
the section b b, which lies on each side of the proper apex
of the shell, and which forms the apparent base. The lines
on this part of the section indicate the thickness which
each successive whorl had originally, and when it was itself
the outermost whorl. The section also shows the vitreous
deposite which lines the upper parts of the cavity, and which
completely fills up the smaller turns of the spire, near the
apex.* There are, indeed, instances among shells of the total re
moval of the interior whorls. This is found to occur in that
of the genus Auricula, which are molluscous animals, re-
* Fig. 117, which is a transverse section ofthe same shell, shows the spi
ral convolutions, and the comparative thinness ofthe inner portions. It also
forms a striking contrast with a similar section of the shell of the Cypr<ea,
Fig. 114. •

182 THE MECHANICAL FUNCTIONS.
spiring by means of pulmonary organs. In the young shell
of this tribe, the partitions which separate the cavities of the
whorls are incomplete, and twine parallel to each other; but
they wholly disappear as the animal approaches to maturity.
In other cases, the animal is found to remove exterior por
tions of shell formerly deposited, when they lie in the way
of its farther growth, and when the mouth of the spire is
advancing over the irregular surface of the preceding whorls.
Thus we often find that the ridges, ribs, or processes which
had been deposited on the surface of the shells of the Tri
ton, Murex, &c. are removed to make way for the succeed
ing turn of the spire. In other cases, however, no such
power of destroying portions of shell previously deposited
seems to exist; and each successive whorl is moulded upon
the one which it covers.
It may also be observed, that some mollusca have the
means of excavating the shells of other animals on which
they may choose to fix, for the purpose of forming a conve
nient lodgement for themselves. The Pileopsis (or fool's
cap) has this faculty in a remarkable degree; and it is also
met with occasionally in Siphonariw and Patella.. The
common Patella, or limpet of our, own coasts, often, indeed,
forms for itself, by some unknown process, a deep cavity
out of a calcareous rock.
When the animal which inhabits a spiral shell retires
within it, the only part of its body that is exposed to injury
is that which is situated at the mouth of the shell. With
a view to its protection, it constructs, in many instances, a
separate plate of shell, adapted to the aperture, and denomi
nated an Operculum. This piece is con
structed by a process similar to that by
which the rest of the shell is formed ; that
is, by the deposition of successive layers
on the internal surface, sometimes in an
annular, and sometimes in a spiral form.
Figure 118 exhibits the lines which ap
pear on the inner side of the operculum
of the Turbo, and which indicate the>

FORMATION OF SHELLS. 183
succession of deposites by which it has been formed.
If an operculum were to be constructed of a consi
derable size, and were connected to the shell itself by a
regular hinge, it would be entitled to be considered as a
distinct valve. Here, therefore, we perceive, as was re
marked by Adanson, a connecting link between the univalve
and the bivalve testacea. A Clausium is another kind of
covering, serving also for protection, and consisting of a thin
spiral plate of shell, attached to the columella by an elastic
spring, by which the plate is retracted when the animal re
tires into its shell. It thus corresponds, exactly in its office
to a door, opening and closing the entrance as occasion re
quires. An Epiphragma is a partition of a membranous or
calcareous nature, constructed merely for temporary use.
It is employed for closing the aperture of the shell during
certain periods only, such as the winter season, or a long
continued drought.
It is remarkable in how short a time this species of Helix
will construct this covering, when circumstances occur to
urge its completion. On the approach of winter, the animal
prepares itself for passing that season in a state of torpidity,
first, by choosing a safe retreat ; and next by retiring com
pletely within its shell, and then barricading its entrance by
constructing the epiphragma just described, and of which
the outer surface is represented in Fig. 119. Having formed
this first barrier, the animal afterwards constructs a second,
of a membranous nature, situated more internally than the
first, and at a little distance from it. If at any other season,
while the snail is in full vigour, the experiment be made of
surrounding it with a freezing mixture, it will immediately
set about constructing a covering for its protection, against
the cold; and it works with such diligence, that in the course
of an hour or two, it will have completed its task, and formed
an entire epiphragma.* When the genial warmth of return
ing spring has penetrated into the abode of the snail, the
animal prepares for emerging from its prison, by secreting
a small quantity of a mucous fluid, which loosens the adhe-
* Gray, Zoological Journal, i. 214.

184 THE MECHANICAL FUNCTIONS-
sion that had taken place between the epiphragma and the
sides ofthe aperture; and the former is, by the pressure of
the foot of the snail, thrown off. The whole of this process
of construction has to be renewed, on every occasion when
another covering is required.*
One great use of these coverings is to prevent evaporation
from the surface of the body of the animal. It is thus that
Snails, Bulimi, &c. may be preserved for months, and even
years in a torpid, but living state, ready to be restored to
the active functions of life, when sufficient water is supplied.f
The enlargement of bivalve shells is conducted on the
same principles as that of univalves; the augmentation of
bulk taking place principally at the outer margin of each
valve, and corresponding with the growth of the included
animal. The order of succession in which the layers are de
posited is clearly indicated by the lines on the surface, which
frequently appear of different hues from the addition of co
louring particles secreted at particular periods by the mantle.
The shells of Oysters and other acephalous mollusca which
adhere to rocks, are often moulded, during their growth, to
the surfaces to which they are applied. The mantle, being
exceedingly flexible, accommodates itself to all the inequali
ties it meets with, and depositing each successive layer of
shell equally on every part, the figure of the surface is as
sumed, not only by the valve in contact with it, but also by
the other valve, which is formed by the opposite surface of
the mantle.J and which during its formation was immediate
ly superposed on the thin edge of the other valve, while it
was deflected by the irregular surface on which it grew.
As the enlargement of the shell proceeds, it was necessary
that the muscle, which closes the valves, and is attached to
* An epiphragma differs from true shells in having no adhesion in any part
to the animal which formed it.
f A remarkable instance of this apparent reviviscence of snails, which had
lain for many years in a dormant state in a cabinet of shells, and which
crawled out on being accidentally put into warm water, is recorded in the
Philosophical Transactions for 1774, p. 432.
t Defrance, Annales des Sciences Naturelles, ii. 16.

MOLLUSCA PTEROPODA. 185
I their inner surface, should be gradually removed to a great
er distance from the hinge, so that it may preserve its rela
tive situation with regard to the whole shell, and retain un
diminished its power of acting upon the valves. For this
purpose its adhesions are gradually transferred, by some un
known process, along the surface of the valves; and the pro
gress of the removal may generally be distinctly traced by
the marks which are left in the shell at the, places before oc
cupied by the attachments of the muscular fibres. The same
process takes place when there are two or three muscles in
stead of one.
A few genera of Mollusca, such as the Pholas, have, in
addition to the two principal valves, small supplementary
pieces of shell. They have been accordingly comprised in
the order of Multivalves, which also comprehends Cuvier's
order of Cirrhopoda, including the several kinds of Barna
cles, (the genus Lepas of Linnasus,) which are furnished
with a great number of jointed filaments, or cirrhi, and form
an intermediate link of connexion between the Mollusca
and the Articulata. But the limits of this treatise will not
allow me to dwell on the endless diversities of structure
which this subject presents. § 5. Pteropoda.
In the Mollusca belonging to the two orders which have
now passed under our review, namely, the Acephala and
Gasteropoda, the mantle, while it folds over the principal
viscera of the body, leaves apertures for the admission of
water to the gills, or organs of respiration. But there exist
a few genera having the sac formed by the mantle closed
on every side; a structure which renders it necessary to
adopt a different arrangement with regard to the gills, and
to place them externally, and we then findi them spreading
out like a pair of wings, on each side of the neck. Since
this general closing of the mantle precludes, also, the for
mation of any organ of progressive motion corresponding
to a foot, advantage is taken of the projection of the gills to
vol. i. — 24

186 THE MECHANICAL FUNCTIONS.
employ them as oars for the purpose of enabling the animal
to swim through the water.
Mollusca of this description are found in great abundance
in the colder regions of the ocean surrounding both the
120 north and south poles; and other species
#%^|§|§ are also met with, though in smaller num
bers in the tropical seas. The Clio borea-
lis, of which Fig. 120 is a representation,
is the most perfect specimen of this form
of construction. It swarms in the Arctic
seas, and constitutes the principal food of
the whale. The position of its gills, which
perform the office of oars or feet, at the
same time that they resemble in their shape and action the
wings of an insect, are characters which have suggested the
title of Pteropoda, given by Cuvier to this order of Mol
lusca. § 6. Cephalopoda.
Following the progress of organic development, we now
arrive at a highly interesting family of Mollusca, denomi
nated the Cephalopoda, and distinguished above all the pre
ceding orders by being endowed with a much more elabo
rate organization, and a far wider range of faculties. The
Cephalopoda have been so named from the position of cer
tain organs of progressive motion, which are situated on the
head, and like the tentacula of the Polypus, surround the
opening of the mouth. (See Fig. 121.) These feet, or arms,
or tentacula, if we choose so to call them, are long, slender,
and 'flexible processes exceedingly irritable, and contractile,
in every part, and provided with numerous muscles, which
are capable of moving and twisting them in all directions
with extraordinary quickness and precision. They are thus
capable of being employed as instruments, not only of pro
gressive motion, but also of prehension. For this latter pur
pose they are in many species peculiarly well adapted, be
cause being perfectly flexible as well as highly muscular,

MOLLUSCA CEPHALOPODA.

187

ffiey twine with ease round an object of any shape, and grasp
it with prodigious force. In addition to these properties
they derive a remarkable power of adhesion to the surfaces

of bodies from their being furnished with numerous suckers
all along their inner sides. Each of these suckers, as shown
separately in Fig. 122, is usually supported on a narrow
neck, or pedicle, and strengthened at its circumference by
a ring of cartilage. Their internal mechanism is more ar
tificial than the simple construction already described, (p.

106;) for when the surface ofthe disk is fully expanded, as
shown in Fig. 123* b, we find that it is formed of a great
number of long slender pieces, resembling teeth closely set
together, and extending from the inner margin ofthe cartila
ginous ring in the form of converging radii, to within a short
distance of the centre, where they leave a circular aperture.
In the flattened state of the sucker, this aperture is filled by
the projecting part of a softer substance, which forms an in
terior portion, capable of being detached from the flat cir
cle of teeth, when the sucker is in action, and of leaving an

188 THE MECHANICAL FUNCTIONS. I
intervening cavity. The form of this cavity is exhibited in
Fig. c, which represents a perpendicular section of the whole
organ, and where the central portion or principal mass of the
sucker is drawn away from the circular disk, the inner mar
gin of which appears like a row of teeth. It is evident that
by this mechanism, which combines the properties of an ac
curate valve, with an extensive cavity for producing rare
faction, or the tendency to a vacuum, the power of adhesion
is considerably augmented.*
So great is the force with which the tentacula of the cut
tle-fish adhere to bodies by means of this apparatus, that
while their muscular fibres continue contracted, it is easier
to tear away the substance ofthe limb, than to release it from
its attachments. Even in the dead animal I have found that
the suckers retain considerable power of adhesion to any
smooth surface to which they may be applied.
Our attention must first be directed to the remarkable fa
mily of Sepia, which comprehends three principal genera,
namely, the Octopus, the Loligo, or Calamary, (depicted in
Fig. 121,) and the common Sepia, or Cuttle-fish. The first
of these, the Octopus, which was the animal denominated
Polypus by Aristotle, has eight arms of equal length, and
contains in its interior two very small rudimental shells,
formed by the inner surface of the mantle. This shell be
comes much more distinct in the Loligo, where it is carti
laginous, and shaped like the blade of a sword. (Fig. 123.)
The internal shell ofthe common Sepia is large and broad,
and composed wholly of carbonate of lime : it is well known
by the name of the cuttle-fish bone. Its structure is ex
tremely curious; and deserves particular attention, as estab
lishing the universality of the principles which regulate the
formation of shells, whether internal or external, and from
* The description I have here given is the result of my own examination
of a large Octopus, which I had lately an opportunity of dissecting: and the
annexed figures 123,* a, b, c, are copied from drawings I made on that oc
casion. * represents tlie sucker in its usual form when not in action: b
shows the sucking surface fully expanded: and c is a section of tlie whole,
which had become somewhat flattened by the operation of dividing it.

MOLLUSCA CEPHALOPODA. 189
which structures differing much in their outward appearance
may result. It is composed of an immense number of thin
calcareous plates, arranged parallel to one another and con
nected by thousands of minute hollow pillars of the same
calcareous material, passing perpendicularly between the ad
jacent surfaces. This shell is not adherent to any internal
part ofthe animal which has produced it; but is enclosed in
a capsule, and appears like a foreign body impacted in the
midst of organs, with which, at first sight, it would appear
to have no relation. It, no doubt, is of use in giving me
chanical support to the soft substance of the body, and, espe
cially to the surrounding muscular flesh; and thus probably
contributes to the high energy which the animal displays
in all its movements. It has been regarded as an internal
skeleton; but it certainly has no pretensions to such a desig
nation; for, although enveloped by the mantle, it is still
formed by that organ; and the material of which it is com
posed it still' carbonate of lime.' On both these accounts it
must be considered as a true shell, and classed among the
productions of the integuments. It differs, indeed, alto
gether from bony structures, which are composed of a dif
ferent kind of material, and formed on.principles of growth
totally dissimilar.*
Besides tentacula, the Sepia is also furnished with a pair
of fleshy fins, extending along the two sides of the body.
The Loligo has similar organs of a smaller size, and situated
only at the extremity of the body which is opposite to the
head. They have been regarded as the rudiments of true
fins, which are organs, developed in fishes, and which are .
supported by slender bones, called rays; but no structure of
this kind exists in the fins of the Cephalopoda.
4 In swimming, the organs principally employed by cuttle-
* Some analogies have, indeed, been attempted to be traced between the
cartilaginous lamina of the Loligo, and the spinal column ofthe lowest order
of cartilaginous fishes: these I shall have occasion to point out in the sequel.
Solid cartilaginous structures also exist in the interior of the body of the ce
phalopoda, which are considered by some naturalists as indicating an approach
to the formation of an internal skeleton, analogous to that of vertebrated ani
mals.

19Q THE MECHANICAL FUNCTIONS.
fish for giving an effective impulse to the water, are the ten
tacula. These they employ as oars, striking with them from
behind forwards, so that their effect is to propel the hinder
part of the body, which is thus made to advance foremost,
the head following in the rear. They also use these organs
as feet for moving along the bottom of the sea. In their pro
gress, under these circumstances, the head is always turned
downwards, and the body upwards, so that the animal may
be considered as literally walking upon its head. The ne
cessity of this position for the feet, arises probably from the
close investment of the mantle over the body; for although
the mantle leaves an aperture in the neck for the entrance of
water to the respiratory organs, yet, in other respects, it
forms a sac, closed in every part, except where the head,
neck, and accompanying tentacula protrude.
In the Calamary, as well as in the common Sepia, two of
the arms are much longer than the rest, and terminate in a
thick cylindrical portion covered with numerous suckers,
which may not unaptly be compared to a hand. These pro
cesses are employed by cuttle-fish as anchors for the purpose
of fixing themselves firmly to rocks, during violent agitations
of the sea; and accordingly we find that it is only the ex
tremities of these long tentacula that are provided with
suckers, while the short ones have them along their whole
length. The other genera of cephalopodous Mollusca are, like the
Sepise, provided with tentacula attached to the head. They
comprehend animals differing exceedingly in their size:
some being very large, but a great number very minute, and
even microscopic* The shells of these animals are often
found to contain partitions dividing them into a number of
chambers; hence they have been termed camerated, or muj^
tilocular, or polythalamous shells. The Spirula (Fig. 124)
is a shell of this description, of which the cellular structure
and numerous partitions are rendered visible by making a
* A particular account has been given of the shells of these microscopic
cephalopoda by M. D'Orbigny, in the Annales des Sciences Naturelles,- vii.
96.

MOLLUSCA CEPHALOPODA. 191
section through it: (Fig. 125.) Some, however, as the Ar
gonaut, or Paper Nautilus, have shells undivided by par
titions; and are accordingly termed unilocular or mono-
thalamous. The shell of the Argonaut is exceedingly thin,
ahd almost pellucid, probably for the sake of lightness, for
it is intended to be used as a boat. For the purpose of ena
bling the animal to avail itself of the impulses of the air,
while it is thus floating on the waters, nature has furnished

it with a thin membrane, which she has attached to two of
the tentacula, so that it can be spread out like a sail to catch
the light winds which waft the animal forwards on its course.
While its diminutive bark is thus scuddinsr on the surface of
the deep, the assiduous navigator does not neglect to ply its
tentacula as oars on either side, to direct, as well as acce
lerate its motion. No sooner does the breeze freshen, and
the sea become ruffled, than the animal hastens to take down
its sail, and quickly withdrawing its tentacula within its
shell, renders itself specifically heavier than the water, and
sinks immediately into more tranquil regions beneath the
surface.* The common Nautilus, which is provided with a similar
sailing apparatus, is an inhabitant of a polythalamous shell,
(Fig. 126,) of which Fig. 127 represents the section. The
formation of this, as well as of other shells of this descrip
tion, presents very curious phenomena. The animal at cer
tain periods of its growth, finding itself cramped in the nar-
* It must be confessed, however, that the habits of the Argonaut are still
very imperfectly known. Considerable doubts are entertained whether the
shell it inhabits is formed by the animal itself, or whether it is the production
of some other, but unknown species of Mollusca, and is merely taken pos
session of by the Argonaut as a convenient habitation, which it can quit and
enter again at pleasure.

192 THE MECHANICAL FUNCTIONS. '
row part of the spire, draws up that portion of the mantle
which occupied it, thus leaving a vacant space. ' The sur
face of the mantle which has receded, immediately begins
to secrete calcareous matter, which is deposited in the form
of a partition, stretching completely across the area of the
cavity. As the animal proceeds to increase in size, and to
occupy a wider portion of the external shell, the same ne
cessity soon recurs, and the same expedient is again resort
ed to. It withdraws its mantle from the narrower into the
wider part of the shell; and then forms a second partition,
at a little distance from the first, corresponding to the space
left by the receding of the mantle. This process is repeat
ed at regular intervals, and produces the multitude of cham
bers contained in polythalamous shells, of which the living
animal occupies only the largest, or that which continues
open.* The partitions are in general perforated either in the
centre or at one side, for the purpose of giving passage to a
tube, which extends to the apex of the shell. This tube is
often surrounded either entirely or partially by shell, which
forms what is denominated the syphon; portions of which
are seen in the section Fig. 127.

* This structure is extremely prevalent in fossil shells: some of which are
spiral, such as the Cornu Ammonis, while others are straight cones, such as
the Bacculite and Orthoceratite. In most of these the partitions are very
numerous, and have undulating surfaces.

( 193 )

CHAPTER IV.

ARTK3ULATA.
§ 1. Articulated Animals in general.
From the Cephalopoda,- the transition is easy to the low
est order of vertebrated animals. But previously to pur
suing the analogies which connect these two divisions of the
animal kingdom, we have to pass in review a very exten
sive series of ariimal forms,- constructed upon a peculiar sys
tem, and occupying, as well as the Mollusca, a place inter
mediate between Zoophytes and the more highly organized
classes'. We have' seen that even in those Zoophytes which are
distinguished from the rest by a more elaborate conforma
tion- of organs, the powers of progressive motion are always
extremely limited. Nor are the Mollusca in general more
highly favoured with respect- to the degree in which they
enjoy this faculty. But the greater number of the animals'
composing- the series we are now to' examine are provided
with a complete apparatus for motion-, and endowed with
extensive- capacities for using and applying it in various
ways. While nature has preserved in the construction of
their vital organs the simplicity which marks the primitive-
modes of organization, and- has adhered to a definite model
in the formation of the different parts of the system, she has
nowhere displayed more boundless variety in the combi
nations of: the forms which she has impressed upon the
mechanical instruments, both of prehension and of progres
sion. All the tribes; of Zoophytes,- and by far the greater num-
vol.- 1. — 25

194 THE MECHANICAL FUNCTIONS. ¦ »
ber of Mollusca, are limited by the constitution of their
system, to an aquatic existence. But in following the series
of Articulated animals, we very soon emerge from the
waters, and find structures'1 adapted to progression on land.
For this we see that preparation is early made in the de
velopment of the nascent structures. A farther design, also,
soon becomes manifest; and instruments are given for ele
vating the body above the ground, and for traversing with
rapidity the light and scarcely resisting atmosphere. This
prospective design may be traced in the whole system of in
sects; every part of which is framed with reference to the
properties of the medium through which these movements
are to be performed. § 2. Annelida.
The lowest division of articulated animals comprehends
those which have a vermiform shape,' and which compose
the class of Annelida, or Annulose animals; of which the
earth-worm may be taken as the type, and most familiar ex
ample. In the series of structures which constitute this di
vision of the animal kingdom,,we may trace remarkable gra^
dations of development, through which nature appears to
pass in attaining the higher and more perfect conformations.
. It may be remarked that, in effecting the transition from
Zoophytes to the new model of construction here presented,
nature seems to have wholly abandoned that radiated dispo
sition of parts, and those star-like forms, so characteristic
of the beings which are placed on the confines of the ani
mal kingdom, and which still retain an analogy with vege
table structures. She now adopts a more regular law of
symmetry; by which all the parts are referrible to one lon
gitudinal axis, and also to a vertical plane* passing through
that axis, and which has been termed the mesial plane. As
a direct consequence of this law, we shall find that in the
forms which are hereafter to pass under our review, as far
as the external organs and general outline of the body are
concerned, all that exists on one side is an exact counterpart,
like a reflected image, of what is found on the other side.

ANNELIDA.

195

While in the Star-fish, and Echinus, nothing in point of si
tuation was definite excepting the upper and the lower sur
face, and there was no side which could be exclusively de
nominated either the right or the left side, and no end that
could be properly said to be the front or the back, in Ar
ticulated as well as in Vertebra ted animals, all these dis
tinctions are clearly marked and easily defined.
In all the Annelida the firmest parts of the body, or those
which give mechanical support to the rest, are external, and
may be regarded either as appendages to the integuments,
or as modifications of the integuments themselves. They
consist of a frame-work, composed of a series of horny
bands or rings: their assemblage having -more or less of a
lengthened cylindric shape, and constituting a kind of ex
ternal skeleton, which encloses all the other organs. This
is exemplified in the earth-worm; in the Pontobdella, (Fig.
128,) which is a species of leech; and in-ithe Nereis, (Fig.

129.) These rings give rise to the division of the body into
as many different segments? In some cases, however, we
find all these rings compressed into the form of a flat oval
disk. This is the case in the Erpobdella, of which Fig. 130
is an enlarged representation.
In general, the first of the segments into which the body
is divided contains the principal organs of sense, and is suf
ficiently distinct from those which follow to entitle it to the
appellation of the head; while the lengthened prolongation
of the opposite extremity, when such a form is present, may
be denominated the tail.

196 THE MECHANICAL FUNCTIONS.
The rings which encircle the body are connected lateral
ly by a looser and more flexible portion of integument, and
also by layers of muscular fibres curiously collected into
bands. . The muscular flesh of insects, and other animals of
this class, differs much from that of the larger animals, being
soft and gelatinous in its texture, though endowed with a
high degree of irritability, and contracting with great force.
The fibres composing each band are all parallel to one ano
ther, and have seldom any tendinous attachments; being
generally inserted directly on the parts they are destined to
move. Thus, the adjacent margins of the rings of worms,
(as shown in the diagram, Fig. 131,) are connected together
by these muscular bands, which pass transversely from the
one to th'e other, immediately under the skin, and parallel
to the axis of the body. There are generally four distinct
bands provided; two running along the back, and two along
the lower part of the body.
The effects which result from the action of these muscles
are such as might easily be anticipated. T|ie lower set
must, when contracting, bring the rings nearer to one an
other at that lower part; and when the whole series occur
pying that situation are exerted in concert, they raisp
the body in the form of an arch. An opposite curvature
will be produced by the contraction of the upper bands,
which, by raising both ends of the body, bend , the back
downwards. In proportion as the two bqnds, situated on
each, side, act in concert, while* the others are relaxed, the
body will be bent laterally towards that side, When all the
four muscular bands contract together equally, their joint
effect will be to bring the rings near to each other, and to
contract the length of the worm; the skin being ,at the same
time wrinkled and swelled out between the rings.
Other muscular bands, also, attached to the rings, pass
from the one to the other in oblique directions. By means
of these muscles the rings may be made to recede at some
points, while they approach at others ; so that the body may
be either twisted laterally on its axis, or wholly elongated,
according as the actions of these oblique muscles are par
tially or generally exerted.

ANNELIDA. 197
The skin on the surface of the earth-worm is furnished,
at the parts where it covers the rings, with very minute bris
tles, called Setce, by means of which the animal is enabled
to fix those parts on the ground, while the other portions of its
body are in motion. Both in the anterior and posterior segs
ments, these hairs are directed towards the centre of the anir
mal; while those on the. middle segments are perpendicular.*
We almost constantly find, in animals belonging to the or
der of Annelida, some provision of this kind. Often con
sisting of Jufts of hair regularly disposed in rows on each side
of the under surface. In the Nereis (Fig. 129,) a genus of
sea-worms, there are often above a hundred pair of little tufts
of strong bristles : and between these we find tentacula to pre
vent the ariimal from running against any thing by which it
might be injured. They also raise the body from the ground,
for which purpose, as they are used under water, very little
support is necessary.t Sometimes the whole body is'covered
with hair; at other times these appendages are in the form
of hooks, which, of course, give greater power of clinging to
the objects, on whihh they fasten. In some, again, they as-.
sume more the nature of feet, of which they exercise during
progression all the functions; being furnished with several
sets of muscles for adjusting and strengthening their actions.
The mode by which an animal of this description advances
along the ground is very simple. It first protrudes the
head by the elongation of the foremost segments of the body,
while the others cling to the earth by means of the rings,
and also of the bristles and other appendages to the integu
ments. The head is then applied to the ground, and made
the fixed point, and the segments next to it, which had been
elongated, are now contracted by the action of their longi
tudinal muscles; in doing which, equal portions of the suc-
* As an instance of the extraordinary multiplicity of species existing in,
eveTy department of living nature; I may here notice, that of the common
earth-worm, apparently so uniform in its shape, Savigny has lately, by a
closer examination, been able to distinguish no less than twenty-two differ
ent species among those found in the neighbourhood of Paris alone.
f Home; Lectures, &c. Vol i. p. 115.

198

THE MECHANICAL FUNCTIONS.

ceeding segments are necessarily elongated: these are next
contracted; and so on, in succession, till the whole is brought
forwards to the head : after which the same series of actions
is repeated, beginning with the advance of the head. Worms
often reverse this motion, and are thus enabled to move back
wards, or with the tail foremost.*
Great variety exists in the forms of the animals referrible
to the type of Annelida. The Gordius, or hair-worm, (Fig.
132,) is that which exhibits the greatest development in
length compared with the breadth of the body. It has the
form of a very long and slender thread: the annular structure
being indicated only by very slight transverse folds of the
integuments. No external members, nor even tentacula,
have been given to this simplest of vermiform animals.

Many of the animals of this class being soft and defence
less, are obliged to consult their safety by retreating into
holes and recesses, or by burrowing in the sand or mud.
One genus only, the Serpula (Fig. 133,) forms for itself an
external shell, which is shaped into a spiral tube. Others,
as the Sabella and the Terebella, accomplish the same ob
ject by collecting grains of sand, or fragments of decayed
shells, or other substances, which they agglutinate together
by means of a viscid exudation, so as to form a firm defen
sive covering, like a coat of mail. Fig. 134 shows this
rude architecture in the Terebella conchilega. These co
verings, however, composed as they are of extraneous ma-
* See Home; Lectures on Comparative Anatomy, Vol. i. p. 114.

ANNELIDA. 199
terials, and not being organic productions of the animals
themselves, are structures wholly foreign to their Systems.
These inhabitants of tubes, the Tubicolce of Cuvier, are gene
rally furnished with tentacula, issuing from the head, which,
when the rest of the body has retired within the tube, is the
only part exposed.
The expedient resorted to for progressive motion by the
Lumbricus marinus of Linnaeus {Arenicola piscaforum of
Lamarck,) is very remarkable.* This worm, depicted in
Fig. 135, swarms on all sandy shores, and is dug up in great
numbers as bait by the fishermen. It bores its way through
the sand by means of the peculiar construction of the rings
of its head, which, when elongated, has the shape of a re-
.gular cone. As each ring is so much smaller than the one
behind it as to admit of being received within it, the whole-
head, when completely retracted, presents a flat surface.
When this disk is applied to the sand, the animal, by gradu
ally projecting the cone, and successively dilating the rings
of which it is composed, opens for itself a passage through the
sand, and then secures the sjdes ofthe passage from falling in
by applying to them a glutinous cement, which exudes from
its skin, and which unites the particles of sand into a kind of
wall, or coating. This covering does not adhere to the body,.
but forms a detached coherent tube, within which the animal
moves with perfect freedom, and which it leaves behind it
as it progressively advances; so that the passage- is kept per
vious throughout its whole length by means of this lining,
which may be compared to the brick work of the shaft of
a mine, or tunnel.
An apparatus of a more complex description is provided
in the Terebella conchilegd, belonging to a tribe of marine
worms, which, from the peculiar circumstances of their situ-^
ation, inhabiting parts of the shore nearly midway between
high and low water, are obliged often to prolong their tubes
to a great length through the sand; for. in consequence of
the frequent shifting of the sands in storms, these animals are
* See the account given by Mr. Osier, Philosophical Transactions for
1826, p. 342.

200 THE MECHANICAL FUNCTIONS.
sometimes buried to a considerable depth, and at others have
several inches of their tubes exposed. In the one case, they
must work their way speedily to the surface; in the other,
they must dive deeper below it. The manoeuvres of the
terebella are best observed by taking it out of its' tube and
placing it under water upon sand. It is then seen to unfold
all the coils of its body, to extend its tentacula in every di
rection; often to' a length exceeding an inch and a half, and
to catch, by their means, small fragments of shells, and the
larger particles- of sand. These it drags towards its head,
carrying them behind the scales which project from the an
terior and lower part of the head, where they are immediate- ,
ly cemented by the glutinous matter which exudes from that
part of the surface. Bending the head alternately from side
to side,- while it continues to apply the materials of its tube,
the terebella has very soon formed a complete collar, Which
it sedulously employs itself to lengthen at every part of the
Circumference with an activity and perseverance highly in*-
teresting. For the purpose of fixing the different fragments
compactly, it presses them into their places with the erected
scales, at the same time retracting its body. Hence the
fragments, being raised by the scales, are generally fixed by -
their posterior edges, and thus overlaying each other,- often
give the tube an imbricated appearance.
Having formed a tube of half an inch, or an inch in length*
the terebella- proceeds to burrow ; for. which purpose it directs
its head against the sand, and contracting some of the poste
rior rings, effects- a slight extension ofthe head, which thus
slowly makes its way through the mass before it,- availing
itself of the materials which it meets with in its course, and
so continues to advance till the whole tube is completed.
After this has been accomplished, the animal turns itself
within the tu-be^-so that its head is next to the surface, ready
to receive the water which brings it food,- and is instrumental
in its respiration: In summer, the whole task is completed
in four or five hours; but in cold weather* when the worm is
more sluggish, and the gluten is secreted more scantily, its
progress is considerably slower.

ANNELIDA. 201
Tentacula of various kinds are also met with in several of
the more active and vivacious kinds of Annelida, such as the
Nereis, (Fig. 129,) proceeding from the margin of the mouth
and other parts of the head. This animal swims with great
facility by rapid, undulating inflections of its body ; and by
practising a similar succession of movements in the loose sand
at the bottom of the water, it quickly buries itself, and even
travels to considerable distances through the sand, first ex
tending th'e anterior rings, and then bringing up the poste
rior part ofthe body; its progress being also much assisted
by the action of its numerous bristly feet.*
Facilities for progression are also given by the addition
of tubercles, arranged in pairs along the under side of
the body, which serve the purposes of feet, arid are often
furnished with bristles or hooks. In the Amphitrite, and
many other genera, tufts of hair occupy the place of feet
on each side, and being moved by muscles specially provided
for that purpose, serve as levers for effecting progressive
motion. We find the same object accomplished by very different
means in other animals of this class. The leech, for instance,
having the rings which encircle its body very numerous and
close to each other, could not well have advanced by the or
dinary modes of vermiform progression. As a substitute,
accordingly, it has been furnished with an apparatus for suc
tion at the two extremities of the body, which are formed
into disks for that purpose. By fixing alternately the one
and the other, and contracting or elongating the body as the
occasion requires, the leech can move at pleasure either for
wards or backwards. Thus, while the tail is fixed, the head
may be advanced by lengthening the whole body, and when
the head is fixed, the hinder sucker can be brought forwards
by the contraction of the body, and applied to the ground
near to the head, and preparation may thus be made for
taking another step.
Most of the parasitic animals which inhabit the interior
* Osier, Phil. Trans. 1826, p. 342.
vol. i. — 26

202 THE MECHANICAL FUNCTIONS.
cavities of the body, and especially the alimentary canal,
correspond in external form, as well as in many circum
stances of internal conformation, to the Annelida. They
compose an order denominated the Enlozoa.

§ 3. Arachnida.
In passing from the Annelida to the Arachnida, an order
which comprehends all the species of spiders, together with
animals allied to them in conformation, we find that a conside
rable advance has been made in the progress of development.
The frame-work of the body is more consolidated, and the
instruments provided for progressive motion are shaped into
longer and more perfect levers, are united by a more refined
system of articulation, and are moved by more distinct and
more powerful muscles; so that the body is elevated from
the ground, and enjoys a greater range of action, and a wider
sphere of perception.
The rings, which always compose the frame- work of the
Annelida, are here consolidated so as to form two principal di
visions of the body ; the one in front, which is termed the Ce-
phalo-thorax, contains the organs of sensation, and of masti
cation, and also the principal reservoir of circulating fluids;
the other, which is behind, and contains the organs of diges
tion, is termed the abdomen. In the spider (Fig. 136, where
c is the cephalo-thorax, and a
the abdomen) these two por
tions of the body are separated
by a deep groove, which leaves
only a slender pedicle, or tube of
communication between them.
There are usually in the male
four pairs of legs, constantly articulated with the cephalo-tho
rax; but the female is furnished with an additional pair to ena
ble her to carry her eggs. For the purpose of obtaining an ex
tensive base of support, the feet of the spider are spread out in
diverging rays, so as to include a very wide circle. They are
divided into several joints, those next to the body being termed

ARACHNIDA. 203
the haunches, and the succeeding ones the leg, and the tarsus,
and each foot is terminated by two, or sometimes three hooks.
Besides these, there are other members resembling feet,
which are placed in front of the head, and have affixed to
them either a moveable hook, or pincers, which are employed
as organs of prehension, and of offence. Through the larger
branches of these a canal passes, which opens near the point,
and conducts a poisonous fluid into the wounds inflicted by
this formidable weapon.
In common with all articulated animals, spiders, in the
progress of their growth, cast their outer skin several times,
and at regular periods. In the earlier stages of their ex
istence, although they have the general form of the mature
insect, yet they have a smaller number of legs: the last pair
not making their appearance till after the spider has at
tained a certain size. We may here trace the commence
ment of that system of metamorphosis, which, as we shall af
terwards find, is carried to so great a length in winged insects.
Spiders are endowed with extensive powers of progres
sive motion, and display great activity and energy in all
their movements. The long and elastic limbs on which the
body is suspended, being firmly braced by their articulations,
enable the muscles to act with great mechanical advantage
in accelerating the progression of the body. Hence, these
animals are enabled to run with great swiftness, and to
spring from considerable distances on their prey ; powers
which were necessary to those tribes that live altogether by
the chase. The greater number of speciesj however, as is
well known, are provided with a curious apparatus for
spinning threads, and for constructing webs to entangle flies
and other small insects. Every species of spider weaves
its web in a manner peculiar to itself: and, besides the prin
cipal web, they often construct in the neighbourhood a
smaller one in the form of a cell, in which they conceal
themselves, and lie in ambush for their prey. Between
this cell and the principal web they extend a thread of com
munication, and by the vibrations into which it is thrown, on
the contact of any solid body, the spider is immediately ac-

204 THE MECHANICAL FUNCTIONS.
quainted with the event, and passes quickly to the spot, by
the assistance ofthe same thread.
Some species have the power of conveying themselves to
considerable distances through the air by means of threads
which they dart out, and which are borne onwards by the
wind, while the spider is clinging to the end of the thread
which is next to it. In this manner these spiders are often
carried up to a great height in the air : and it has been sup
posed that during their flight they often seize upon gnats and
other flies; because the mutilated remains of these insects are
often seen adhering to the threads: this point, however, is
still open to much doubt.
The Natural History of the spider is, in many points of
view, highly interesting, not only from the great extent to
which the organic development is carried, and the energy
with which all the functions of animal life are performed ;
but also with reference to the wonderful instincts displayed
in the construction of its web, in the surprise and destruc
tion of its victims, and in the zealous guardianship of its
young. It would be impossible, in so brief an outline as the
one I am now tracing, to enlarge upon so fertile a topic,
without being led too far from the object I have at present
more particularly in view; namely, the development of or
ganization with reference to the organs of progressive mo
tion. § 4. Crustacea.
The plan which Nature appears to have commenced in
the construction ofthe Arachnida, is farther pursued in that
of the Crustacea. The portions into which the external
frame-work of the body was divided in the former, are still
farther consolidated in the latter: they are composed of
denser materials, and endowed with greater rigidity; thus
not only offering more resistance to external forces, but also
giving a firmer purchase to the muscles which are the moving
powers. The limbs, as well as the whole body, are incased
in tubes of solid carbonate of lime : they are articulated with

CRUSTACEA.

205

great care, and almost always compose hinge joints. The
muscles, by which these solid levers are moved, are lodged
in the interior, and their fibres either pass directly from one
point to another, across the joint, or else they are attached
to cartilaginous plates, which, for the purpose of receiving
the muscles, are made to project into the interior of the up
per portion of the limb, being themselves immoveably con
nected with the lower portion. By this expedient, not only
is the employment of a tendon dispensed with, but a larger
surface is presented for the attachment ofthe muscles, which,
by acting also upon a longer lever, obtain great mechanical
advantage. It would be superfluous to occupy more time in
explaining the minutiae of structure in these joints, because
the simple inspection of the limbs of a crab or lobster will
give clearer ideas of this mechanism than can be conveyed
by any laboured description. I shall therefore only give a
brief sketch of the principal constituent parts of these ex
ternal members of the Crustacea.
The number of pairs of legs is either three or four: each
leg is divided into five pieces. The piece h, (Fig. 137.)

next the trunk, is termed the haunch, to which is united
the trochanter, t ; after which comes in succession the fe
mur or thigh, f; two portions of the leg, l; and the tarsud,
p. The haunch is usually short, being interposed merely as
a base for increasing the extent of motion of the pieces
which follow; and sometimes it is itself composed of more

206 THE MECHANICAL FUNCTIONS.
than one piece. The leg is usually divided by a joint into
two pieces. The tarsus is terminated by a single or double
hook, and sometimes by a pincer, or claw.
New organs, not met with among the Arachnida, are here
for the first time developed, namely, the Antennae, of which
there is one on each side of the head. They are denomi
nated, in popular language, the feelers; although it is more
than probable that they perform some function of higher
importance than that of conveying perceptions of mere
touch. The antennae consist of slender filaments, composed
of a great number of pieces articulated together: and they
are infinitely diversified in their form in the different genera
and species, both of Crustacea and of Insects.
The jaws, and other parts connected with the mouth, pre
sent a great complication of structure; and many of these
parts are employed in various uses besides those of masti
cation ; such as the seizing of objects, turning them in va
rious ways for examination, and, according to their suita
bleness as articles of food, conveying them into the mouth.
These organs are called the Palpi, and sometimes the false
feel. They always exist in pairs, and take their rise from
the lower lip, or some adjacent part of the head. The por
tions of which each is composed are articulated together and
moved by muscles in the same manner as the ordinary or
proper feet. It is worthy of notice, however, that some
times the foremost pairs of palpi are shaped more like jaws,
and actually perform the office proper to jaws, of compress
ing and dividing the food previously to its introduction into
the mouth. These auxiliary jaws are then called mandibles.
In other instances, we see them assuming every variety of in
termediate form between that of mandibles and of false feet,
so Jhat it is often difficult, amidst these gradual transitions
of structure, to decide to which of these two kinds of organs
a specimen we meet with properly belongs. It is apparent
ly with a view to evade this difficulty that a term has been
invented which shall include them all, namely, that of feel-
jaws. These transitions are illustrated by the annexed
figures of several of these members in the Mysis Fabricii;

CRUSTACEA. 207
Fig. 138, being that of a mandible, with its feeler, or palpus;
Figures 139, 140, and 141, representing the first, second,
and third pairs of feet-jaws; and l£ig. 142, the first pair of
true feet. It would thus seem as if the same constituent ele
ment of the fabric is converted by nature into the one or other
of these organs, according as best suits the exigencies of each
particular case.*
In the lobster, the crab, and many other Crustacea, the
foremost pair of true feet are also modified to suit a parti
cular purpose ; the pincers which terminate them being ex
panded into a claw, and constituting a powerful organ of
prehension, and a formidable weapon of offence. It resem
bles a finger and thumb in its power of grasping and strong
ly compressing any object on which it seizes; and, to enable
it to do this with more effect, the inner edges of both parts
of the claw are notched or serrated.
The large portion of shell which is consolidated into one
piece, and covers the upper part of the body, is termed the
shield, or carapace. The tail of the crab is very short, and
is united with the body, appearing as if it had been folded
under it. The feet-jaws are particularly large, but short:
the articulations of the feet are such as to allow of scarcely
any motion but in a transverse plane. This is the cause of
the greater facility the crab finds in walking side-ways,
which it can do with great quickness when urged by a sense
of danger. The lobster, on the contrary, is better formed
for swimming than for walking. The hinder part of its
body is divided into segments, which play upon each other
by a remarkable kind of mechanism, the margins of each
portion overlapping the succeeding segment, and partly en
closing it. The'tail is the principal agent used in swim
ming, and the whole force of the muscles is bestowed upon
its movements. As it strikes the water from behind for-
* The labours of Savigny, Audouin and Latreille appear to have estab
lished a complete analogy in the respective component parts, not only ofthe
feet, feet-jaws, jaws and mandibles, but also of the palpi and other appen
dices attached to the head, in all the articulated animals, whether belonging
tot he classes of arachnidai Crustacea, myriapoda, or winged insects.

208 THE MECHANICAL FUNCTIONS.
wards, the lobster can only swim backwards ; and it is as
sisted in this action by five pairs of false feet, which are at
tached to the under side, of the body, behind the true feet,
and which terminate in a fin-shaped expansion, acting like
an oar. The extremity of the tail is still more expressly
formed for giving effect to the stroke, being terminated by
a number of flat scales, which, when expanded, present a
broad surface to the water.
The calcareous coverings of these Crustacea are analogous
to shell both in structure and composition. They contain,
however, some phosphate of lime, in addition to the carbo
nate. The calcareous particles are deposited on a membrane
of considerable firmness; and they together compose a dense,
but thin and fragile structure, which, in order to distinguish
it from the shells of the mollusca, has been denominated a
crust. A solid structure of this kind, as we have already
seen, does not admit of increase by the extension of its own
parts: so that, in order to allow of the growth of the parts
which it encloses, it is necessary that it be cast off, and ex
changed for a new shell of larger dimensions.
The process by which this periodical casting and renewal
of the shell are effected, has been very satisfactorily investi
gated by Reaumur. The tendency in the body and in the
limbs to expand during growth is restrained by the limited
dimensions of the shell, which resists the efforts to enlarge
its diameter. But this force of expansion goes on increasing,
till at length it is productive of much uneasiness to the ani
mal, which is, in consequence, prompted to make a violent
effort to relieve itself; by this means it generally succeeds
in bursting the shell ; and then, by dint of repeated struggles,
extricates its body and its limbs. The lobster first with
draws its claws, and then its feet, as if it were pulling them
out of a pair of boots : the head next throws off its case, to
gether with its antenna? ; and the two eyes are disengaged
from their horny pedicles. In this operation, not only the
complex apparatus of the jaws, but even the horny cuticle
and teeth of the stomach, are all cast off along with the shell :
and, last of all, the tail is extricated. But the whole process

CRUSTACEA. 209
is not accomplished without long-continued efforts. Some
times the legs are lacerated or torn off, in the attempt to
withdraw them from the shell; and in the younger Crustacea
the operation is not unfrequently fatal. Even when success
fully accomplished, it leaves the animal in a most languid
state : the limbs, being soft and pliant, are scarcely able to
drag the body along. They are not, however, left altoge
ther without defence. For some time before the old shell
was cast off, preparations had been making for forming a
new one. The membrane which lined the shell had been
acquiring greater density, and had already collected a quan
tity of liquid materials proper for the consolidation of the
new shell. These materials are mixed with a large propor--
tion of colouring matter, of a bright scarlet hue, giving it
the appearance of red blood, though it differs totally from
blood in all its other properties. As soon as the shell is
cast off, this membrane, by the pressure from within, is sud
denly expanded, and by the rapid growth of the soft parts-,
soon acquires a much larger size than the former shell.
Then the process of hardening the calcareous ingredient
commences, and is rapidly completed; while an abundant
supply of fresh matter is added to increase .the strength of
the solid walls which are thus constructing for the support
of the animal. Reaumur estimates that the lobster gains,
during each change of its covering, an increase of one-fifth
of its former dimensions. When the animal has attained its
full size, no operation of this kind is required, and the same
shell is permanently retained.
A provision appears to be made, in the interior of the ani
mal, for the supply of the large quantity of calcareous mat
ter required for the construction of the shell at the proper
time. A magazine of carbonate of lime is collected, pre
vious to each change pf shell, in the form of two rounded
masses, one on each side of the stomach. In the crab these
balls have received the absurd name of crab's eyes; and
during the formation of the shell they disappear.
1 It is well known that when an animal of this class has
been deprived of one of the claws, that part is in a short
vol. i. — 27

210 THE MECHANICAL FUNCTIONS.
time replaced by a new claw, which grows from the stump
of the one which had been lost. It appears from the inves
tigations of Reaumur, that this new growth takes place more
readily at particular parts of the limb, and especially at the
joints ; and the animal seems to be aware of the greater fa
cility with which a renewal of the claw can be effected at
these parts; for if it chance to receive an injury at the ex
tremity of the limb, it often, by a spontaneous effort, breaks
off th.e whole limb at its junction with the trunk, which is
the point where the growth more speedily commences. The
wound soon becomes covered with a delicate white mem
brane, which presents at first a convex surface: this gradu
ally rises to a point, and is found on examination to conceal
the rudiment of a new claw. At first this new claw en
larges but slowly, as if collecting strength for the more vigo
rous effort of expansion which afterwards takes place. As it
grows, the membrane is pushed forwards, becoming thin
ner in proportion as it is stretched; till at length.it gives
way, and the soft claw is exposed to view. The claw now
enlarges rapidly, and in a few days more acquires a shell as
hard as that which had preceded it. Usually, however, it
does not attain the same size; a circumstance which ac
counts for our frequently meeting with lobsters and crabs
which have one claw much smaller than the otheri In the
course of the subsequent castings, this disparity gradually
disappears. The same power of restoration is found to re
side in the legs, the antennas, and the jaws.
We must naturally be curious to learn, if possible, from
what source these astonishing powers of regeneration are de
rived . Reaumur hazarded the conjecture, that there might be
originally implanted in each articulation a certain number of
embryo limbs, ready to be developed as occasion might re
quire; somewhat in the way in which the rudiments of the
secondary teeth remain concealed in the jaw, in preparation
for replacing the first set when these have been removed.
But this hypothesis is overturned by the fact that if the ani
mal loses only part of the limb, it is the deficient portion
alone, and not the whole limb that is regenerated. The

CRUSTACEA. 211
sprouting of the new claw bears a strong analogy to the
budding of a plant; both having their origin from an imper
ceptible atom, or germ, which is either formed on the oc
casion, or had pre-existed in the organization. We are,
however, totally destitute of the means of deciding which
of these alternatives is nearest to the truth. It is but too
probable that the agents which can effect such wonderful
operations will ever baffle our most scrutinizing inquiries,
and that they are of too refined an order to come within
the reach of the most subtle conjectures that human imagi
nation can devise.

( 212 )

CHAPTER V.
INSECTS.
§ 1. Aptera.
Apterous, or wingless insects, form the next term in the
series of articulated animals. Closely allied in their organi
zation to many of the preceding families, they differ from
them in being essentially formed for a terrestrial instead of
an aquatic life. Most of the lower tribes of this order are
parasitic, that is, derive their nourishment from the juices
of other animals, the skin of which they infest and penetrate,
and into which they insert tubes for suction. The various
tribes of Acari, or mites, of Pediculi, or lice, of Ricini, or
ticks, of Pulices, or fleas; together with the Podura, or
spring- tail; the Lepisma, and, the family of Myriopoda, or
millepedes, are comprehended in this order. I shall be
obliged to pass over these tribes very cursorily, noticing
only a few of the more remarkable circumstances attending
their mechaniqal conformation.
The Pulex is the only apterous insect which undergoes
complete metamorphoses in the course of its development.
In the first stage of its existence, it has the form of a long
worm, without feet, frequently rolling itself into a spiral
coil. It consists of thirteen segments, having tufts of hair
growing upon each. In its mature state, it has six articu
lated legs, the hindmost of which are of great size, for tHte
purpose of enabling the insect to take those prodigious leaps
which astonish us in beings of so diminutive a size, and af
ford a striking proof of the exquisite mechanism pervading
even the lowest orders of the animal creation.
The Podura leaps into the air by a mechanical contri
vance of another kind ; employing for this purpose the tail,
which is very long, and forked at the end. In its ordinary

APTERA. 213
state this organ is kept folded under the abdomen, where it
is concealed in a groove. The pieces of which it is com
posed are articulated together in such a manner as to admit
of their being rapidly unbent by the action of its muscles,
the whole mechanism conspiring to produce the effect of a
powerful spring, by which the body is propelled forwards
to a considerable distance. In some species, this flexible
tail has a flattened form, for the purpose of enabling, the
insect to leap from the surface of water, an action which it
performs with apparently as much ease as if it sprung- from
a solid resisting plane.
The Lapisma leaps by means of moveable appendages,.
placed in a double row along the under side of the body, and
acting like springs. There are eight pairs of these members,
corresponding in situation and structure to the false feet of
the Crustacea, and, like them, terminating in. jointed fila
ments. The Julus and the Scohpendra-i which compose the fa
mily of the Myriapoda, so called from the immense num
ber of their feet„undergo, to a certain extent, a. kind of me-
taphorphosis in the progress of their development. When
first hatched they have often no feet whatever, and resem
ble the simpler kinds of worms. Legs at length make their
appearance ; but they arise in succession, and it is. not until'.
the later periods of their growth that these-animals acquire
their full complement of segments, with their accompanying'.
legs. The Julus terrestris, for example, (Fig* 143) has, at
143 its entrance into; the world, only
eight segments and six feet; but.
acquires in the course of its deve
lopment, fifty segments and about two hundred feet. The-
anterior legs are directed obliquely forwards, and the rest
more or less backwards. The mandibles have the form of
small feet; as we have seen is frequently the case in crusta-
ceous animals.

214 THE MECHANICAL FUNCTIONS.

§ 2. Insecta alata.
Our attention is now to be directed to the more highly
developed Insects, which have been formed with a view to
progression through the air. On these, which compose the
most extensive class of the whole animal kingdom, Nature
has lavished her choicest gifts of animal powers, as far as
they are compatible with the diminutive scale to which she
has restricted herself in their formation. The model she
has chosen for their construction is that which combines the
greatest security against injurious impressions from without,
with the most extensive powers of locomotion ; and which
also admits of the fullest exercise of all those faculties of
active enjoyment which are characteristic of animal life.
She has provided for the first of these objects by enclosing
the softer organs in dense and horny coverings, which per
form the office of an external skeleton, sustaining and pro
tecting the viscera, and furnishing extensive surfaces of at
tachment to the muscles, from the action of which all the
varied movements of the system are derived.
The muscular system of perfect insects is exceedingly
complex. Lyonet has described and delineated an immense
number of muscular bands in the caterpillar of the Cossus,
and the plates he has given have been copied in a variety
of books in illustration of this part of the structure of in
sects. The recent work of Straus Durckheim affords an

equally striking example of admirable arrangement in the
muscles of the Melolontha vulgaris, or cockchaffer, the ana-

WINGED INSECTS. 215
tomy of which has been minutely investigated by that dis
tinguished entomologist. These muscles are represented in
Fig. 144, which has been carefully reduced from his beauti
fully executed plates. The largest mass of muscular fibres
is that marked a, constituting the muscles which depress the
wings, and which are of enormous size and strength.
On examining the different structures which compose the
solid frame-work of insects, we find them conforming in eve
ry instance to the general type of annulose animals,ihasmuch
as they consist of thickened portions of integument, encir
cling the body; but variously united and consolidated, for
the manifest purpose of obtaining greater mechanical strength
and elasticity than if they had remained detached pieces,
joined only oy membranous connexions. A long flexible
body, such as that possessed by the Myriapoda, could not
easily have been transported through the air; for every bend
would have created a resistance, and have impeded its ad
vance during flight. Hence the body of the insect, which
is to be ultimately adapted to this mode of progression, has
been shortened by a reduction in the number of its segments,
and rendered more simple and compact. The segments des
tined to support the wings have been expanded for the pur
pose of lodging the powerful muscles which are to move
'them; and rendered dense and unyielding in order to support
their action.
Nature has farther provided insects with instruments
adapted to different kinds of external actions. They con
sist of articulated levers, variously combined together, and
forming legs, claws, pincers, oars, palpi, and, lastly, wings,
calculated for executing every variety of prehension, of
progression, or whatever other action their wants and ne
cessities require. § 3. Development of Insects.
It would appear as if the final accomplishment of objects
so numerous, so widely different, and so liable to mutual in
terference, could be attained only by the animal being sub-

216 THE MECHANICAL FUNCTIONS.
jected to a long series of modifications, and passing through
many intermediate stages of development. The power of
flight is never conferred upon the insect in the earlier periods
of its existence : for before its structure can obtain the light
ness which fits it for rising in the air, and before it can ac
quire instruments capable of acting upon so light an ele
ment, it has to go through several preparatory changes, some
of which are so considerable as to justify the term of meta
morphoses, which has been generally given to them.* But
transient is the state of perfection in every thing that re
lates to animal existence. When the insect has, by a slow
development reached this ultimate elaboration of its organs,
its life is hastening to a close; and the period of its perfect
¦state is generally the shortest of its whole existence.
The history ofthe successive stages ofthe development of
inseets opens a highly interesting field of philosophical inqui
ry. For a certain period of the early life of these animals,
the growth of all the parts appears to proceed equably and
uniformly : but at subsequent epochs, some parts acquire a
great and sudden increase of size, and others that were in a
rudimental condition become highly developed, and consti
tute what appear to be new forms of organs, although their
elements were in existence from a much earlier period. The
modifications which the harder and more solid structures of
insects exhibit in the progress of these changes, are particu
larly remarkable, as illustrating the principles on which the
development is conducted. The researches of modern en
tomologists have led to the conclusion that the frame-work,
or skeleton of insects, is always formed by the union of a
certain determinate number of parts, or elements, originally
distinct from one another, but which are variously joined
and soldered together in the progress of growth : frequently
exhibiting a great disproportion in the comparative expan
sion of different parts. The enlargement of any one part,
however, exercises a certain influence on all the neighbour-
* Transformations quite as remarkable occur in several tribes of animals
belonging to other classes: such as those of the Frog among reptiles, and of
the Lemsea among parasitic worms.

DEVELOPMENT OF INSECTS.

217

ing parts, and thus are the foundations laid of all the endless
diversities which characterize the several species belonging
to' each tribe and family.
In the progress of development, we may recognise two
principles, which, though apparently opposite to-each other,
concur and harmonize in their operation: these' are expan
sion and concentration. Thus, while those segments of body
which follow the head are greatly enlarged, in order to sup
port the more recently developed organs of progressive mo
tion, they are also more consolidated, and rendered stronger
by the union of several pieces which were before separate.
The posterior segments, having no such appendages to sup
port, are less dilated, and- the whole body is much shortened
by the approximation of the segments, which,- in this way,
compose the abdomen, or hinder division of the insect,
The progress of the metamorphoses of insects is most
strikingly displayed in the history of the Lepidopterous, or
butterfly and moth tribe.* The egg, which is deposited by
the butterfly, gives birth to a caterpillar; an animal, which,

in outward shape-, Bears not the slightest resemblance to its
parent, or to the form it is itself afterwards to assume. It
has, in fact, both the external appearance, and the mechani
cal structure, of a worm.- The same elongated cylindric
* The four periods of the existence of the Bombyx mori, or the moth of
,the silk- worm,. are shown in the annexed engravings: Fig. 145 are the eggs?
Fig. 146'; the Larva, or caterpillar; Fig. 147, the Pupa, or chrysalis* and*
Fig. 148, the Imago, or perfect insect.
VOL. I.— 28-

218 THE MECHANICAL FUNCTIONS.
shape, the same annular structure of the denser parts of its
integument, the same arrangements of longitudinal and ob
lique muscles connecting these rings, the same apparatus of
short feet, with claws, or bristles, or tufts of hairs, for faci
litating progression; in short, all the circumstances most
characteristic of the vermiform type are equally exemplified
in the different tribes of caterpillars, as in the proper An
nelida. But these vermiform insects have this peculiarity, that
" they contain in their interior the rudiments of all the or
gans of the perfect insect. These organs, however, are con
cealed from view by a great number of membranous cover
ings, which successively invest one another, like the coats of
an onion, and are thrown off, one after another, as the in
ternal parts are gradually developed. *These external in
vestments, which hide the real form of the future animal,
have been compared to a mask; so that the insect, while
wearing this disguise, has been termed larva, which is the
Latin name for a mask.
This operose mode of development is rendered necessary
in consequence of the greater compactness of the integu
ments of insects, as compared with those of the annelida. In
proportion as they acquire density, they are less capable of
being farther stretched, and at length arrive at the limit of
their possible growth. Then it is that they obstruct the di
latation of the- internal organs, and must be thrown off- to
make way for the farther growth of the insect. In the
mean time a new skin has been preparing underneath,
moulded on a larger model, and admitting of greater exten
sion than the one which preceded it. This new skin, at
first, readily yields to the distending force from within, and
a new impulse is given to the powers of development; un
til, becoming itself too rigid to be farther stretched, it must,
in its turn, be cast off in order to give place to another skin.
Such is the process which is repeated periodically, for a
great number of times, before the larva has attained its full
size. These successive peelings of the skin are but so many

DEVELOPMENT OF INSECTS. 219
steps in preparation for a more important change. A time
comes when the whole of the coverings of the body are at
once cast off, and the insect assumes the form of a pupa or
chrysalis; being wrapt as in a shroud, presenting no appear
ance of external members, and retaining but feeble indica
tions of life. In this condition it remains for a certain pe
riod: its internal system continuing in secret the farther
consolidation of the organs; until the period arrives when
it is qualified to emerge into the world, by bursting asun
der the fetters which had confined it, and to commence a
new career of existence. The worm, which so lately crawled.
with a slow and tedious pace along the surface ofthe ground,
now ranks among the sportive inhabitants of air; and ex
panding its newly acquired wings, launches forward into
the element on which its powers can be freely exerted, and
which is to waft it to the objects of its gratification, and to
new scenes of pleasure and delight.
Thus do the earlier stages of the development of insects
exhibit a recurrence of those structures which are found in
the lowest department of this series of animals. The larva,
or infantile stage of the life of an insect, is, in all its me
chanical relations, a mere worm. The imago, or perfect
state, on the other hand, exhibits strong analogies with the
erustaceous tribes, not only in the-general form of the body,
but also in the consolidated texture of its organs, (especially
of those which compose its skeleton) and in the possession
of rigid levers, shaped into articulated limbs, and furnished
with large and powerful muscles, from all which circum
stances great freedom and extent of motion are derived. To
this elaborate frame, nature has added wings, those refined
instruments of a higher order of movements, subservient
to a more expanded range of existence, and entitling the be
ings on which they have been conferred to the most elevated
rank among the lesser inhabitants of the globe.
The mechanical functions of insects scarcely admit of be
ing reduced t<5 general principles, in consequence of the
great diversity of forms, of habits, and of actions, that is met

220 THE MECHANICAL FUNCTIONS.
with among the innumerable hosts of beings which rank un
der this widely extended department of the animal creation.
In these minute creatures may be discovered all the me
chanical instruments and apparatus required for the execu
tion of those varied movements which we witness in the
larger animals, and which, though almost peculiar to the
different classes of those animals, are here frequently united
in the same individual. Insects swim, dive, creep, walk,
run, leap, or fly, with as much facility as fishes, reptiles,
quadrupeds, or birds. But besides these, a great number
have also movements peculiar to themselves, and of which
we meet with no example in other parts of the animal king
dom. In attempting to delineate a sketch of the movements of
insects, and of the mechanism by which they are performed,
I am compelled, by the great extent ofthe subject, to confine
myself to very general views; and must refer such of my
readers as ar.e desirous of fuller information on this subject
to the works of professed entomologists.
The mechanical conditions of an insect in its several states
of larva, pupa, and imago, are so widely different, that it
will be necessary to consider each separately. In many tribes,
however, the difference between the larva and the perfect
insect is much less considerable than in others. Those be
longing to the orders of Hemiptera and Orthoptera, for ex
ample, come out of the egg with nearly the same form as
that which they have in the mature state; excepting that
they are without wings; these organs being added in the
progress of their growth, and constituting, when acquired,
their perfect or imago condition.
§ 4. Aquatic Larva.
Many insects, which, when fully developed, are the most
perfectly constructed for flying, are, when in the state of
larvae, altogether aquatic animals. Some of them are destitute
of feet, or other external instruments of motion, swimming
only by means of the alternate inflections of the body from
side to side, in the same manner as the Nais, and the Leech.

AQUATIC LARVA?. 221
Sometimes, these actions are performed by abrupt strokes,
giving rise to an irregular zig-zag course: this is the case
with the larva ofthe gnat, and with many others which have
no feet. In the structure of the larva of the Libellula, or
dragon-fly, a singular artifice has been resorted to for giving
an impulse to the body, without the help of external mem
bers. It is that of the alternate absorption of water into a
cavity in the hinder part of the body, and its sudden ejec
tion from that cavity, so that the animal is impelled in a con
trary direction, upon the same principle that a rocket rises
in the air by the reaction of that fluid. It has, at various
times, been proposed to apply the power of steam to the pro
duction of an effect exactly similar to that of which Nature
here presents us with so perfect an example, for the purpose
of propelling ships, instead of the ordinary mode of steam
navigation. Some larva?, such as that of the Siratiomys, eollect a
bubble of air, which they retain within a tuft of hair at the
extremity of the tail, evidently with a view of diminishing
the specific gravity of the body, and thus giving greater effi
cacy to the muscular actions which they employ in their pror
gression through the water. Another use is also made of these
tufts of hair; for, by repelling the water, they allow of the
insect's suspending itself from the surface of the fluid, in the
manner already noticed, in giving the history of the evolu
tions of the hydra.*
The impulse given by the lateral inflections of the body
are in many cases assisted by short legs ; but the larvse of
the Ephemera, though furnished with legs, do not use them
for this purpose, and swim simply by the action of the tail.
Those of the Dytiscus are furnished with a pair of very long
members, projecting to a considerable distance from the
sides, and flattened at the ends, to serve as oars. The larvce
of the Hydrophilus are also admirably formed for swim
ming; and they not only dart forwards with surprising velo
city, but also turn in all directions with the utmost facility.
* Page 133.

222 THE MECHANICAL FUNCTIONS.
r
§ 5. Terrestrial Larvae.
The movements of larvae that are not aquatic are perfectly
analogous to those of the Annelida, which they much resem
ble in their outward form and mechanical structure. The
muscles by which the annular segments of the body are
moved, are exceedingly numerous, and beautifully arranged
with reference to the motions they are intended to effect.
The investigation of the structure of these minute organs
has long exercised the talents of the most skilful entomo
logists, and still offers much that remains to be explored.
The researches of Lyonet, already alluded to, on the anato
my of the larva of the Bombyx Cossus,* of which he has
published an elaborate description, accompanied by admi
rable engravings, will ever remain a splendid monument of
patience and ingenuity in overcoming the difficulties which
impede this kind of inquiry. In the body and the limbs of
this caterpillar, Lyonet counted above 4000 separate muscu
lar bands, all arranged with the most perfect symmetry, and
adapted, with wonderful precision, to the performance of the
required effects.
In these larvse, as in the simpler forms of the Annelida,.
progression is often accomplished solely by the alternate
contraction and extension of the annular segments, aided, in
many cases, by short hairs, and frequently, also, by a slimy
secretion which exudes from their bodies. Many larvae,
which are destitute of feet, move onwards by first, coiling
the body into a circle, making the head and the tail meet, and
then springing forwards by a sudden extension of the back,
producing an effect like the unbending of a bow. By an
artifice of the same kind, some larvae contrive to leap to a
considerable distance, by the violent effort which they make
in unfolding the curvatures of their bodies.
Some larvae avail themselves of their jaws in order to fix the
head, and drag the rest ofthe body towards it. In this manner
do ,the larvse of the Cerambyx, or Capricorn beetle, advance
* Cossus ligniperda. Fabricius.

TERRESTRIAL LARVAI. 223
along the winding passages which they have themselves exca
vated, holding by the jaws, and dragging themselves forwards.
These movements are assisted by the resistance afforded by
short tubercles, which project from different parts of the
back and under surface of the body ; so that these insects
advance in the passage by an act similar to that by which a
chimney-sweeper, exerting the powerful pressure of his el
bows, shoulders, and knees, manages to climb up a chimney.
For the purpose of enabling insects to take stronger hold
of the surfaces they pass over, we often observe them fur
nished with spines, or hooks, which are m,oved by appro
priate muscles, and they occupy different situations on the
body. Modifications without end occur with regard to these
and other external parts, subservient in various degrees to
progressive motion* Every possible gradation is also seen
between the short tubercles already mentioned, and the more
regularly formed feet or legs. Those which are regarded as
spurious legs, or prolegs, as they have been called, occupy
an intermediate place between these two extremes. They
consist of fleshy and retractile tubercles, and are often very
numerous; while the number of the true legs, as they are
called, is limited to six. These last are the representatives
of the legs of the future perfect insect; for they are attached
to the first three segments ofthe thorax; and are formed of
those' portions articulatedto each other, corresponding to the
three principal joints of the imago. The true legs are gene
rally protected by horny scales; but the coverings ofthe pro-
legs are wholly membranous. The office of these spurious
legs is merely to serve as props to support the body while
the insect is walking, and to prevent its hinder part from
trailing on the ground. They are frequently terminated by
single or double hooks; and also by a marginal coronet of
recurved spines. These hooks, or spines, enable the insect
to cling firmly to smooth surfaces ; and also to grasp the most
slender twig, which could not have been laid hold of by legs
of the usual construction.
The speed with which these larvae can advance, is regu
lated by many circumstances, independently of the mere

224 THE MECHANICAL FUNCTIONS.
possession of legs : for some caterpillars move slowly, while
others can run very nimbly. The following is the order in
which the legs are usually moved: namely, the anterior and
the posterior leg on the same side are advanced at the same
moment, together with the intermediate one on the other
side; and this takes place alternately on both sides.
There is one tribe of caterpillars called Surveyors or
Geometers; (Fig., 148,,* a) which walk by first fixing the
148* B

fore feet, and then- doubling the -body into a vertical arch ;
this action brings up the hind part of the catarpillar, which
is furnished with prolegs, close to the head. The hind ex
tremity being then fixed by means of the prolegs situated at
that part, the body is again extended into a straight line ;
and this process being repeated, the caterpillar advances
by a succession of paces, as if it were measuring the distance,
by converting its body into a pair of compasses. At the
same time that they employ this process, they farther pro
vide for their security by spinning a thread, which they
fasten to different points of the ground as they go along.
The great force exerted by the muscles of many cater
pillars is exemplified by their often fixing themselves to an
object, and extending the body to a distance, as if it were
a rigid cylinder. This attitude is shown in Fig. 148* b.
Many other species of caterpillar practise the same art of
spinning fine silken threads, which especially assist them in
their progression over smooth surfaces, and also in descend
ing from a height through the air. The caterpillar of the
cabbage butterfly is thus enabled to climb up and down a
pane of glass, for which purpose it fixes the threads Which it
spins in a zig-zag line, forming so many steps of a rope lad
der. The material of which these threads are made is a glu-

treatment of larvae. 225
tinous secretion, which, on being deposited on glass, adheres
firmly to it, and very soon acquires consistence and hard
ness by the action of the air.
Other caterpillars, which feed on trees, and have often oc
casion to descend from one branch to another, send out a
rope made with the same material, which they can prolong
indefinitely; and thus either suspend themselves at pleasure
in the air, or let themselves down to the ground. They
continue, while walking, to spin a thread as they advance, so
that they can always easily retrace their steps, by gathering
up the clew they have left, and reascend to the height from
which they had allowed themselves to drop.

§ 6. Imago, or Perfect Insect.
The process which nature has followed in the develop
ment of the structure of insects, has for its object the gra
dual hardening and consolidation of texture, and the union
and concentration of organs; for we find that the segments
which were at a distance from one another in the larva, are
approximated in the perfect insect, and often closely tied to
gether by ligaments; and in other cases, adjoining segments
cohere so as to form but a single piece. Thus, the number of
separate parts composing the solid fabric is considerably di
minished. Other segments, again, fold inwardly, forming
internal processes, and adding to the extent and complica
tion of the skeleton. L
The integuments of perfect insects, being designed to be
permanent structures, are thicker and more rigid than those
of their larva?, and are formed of several layers, in which
the component parts of the integuments of the larger ani
mals may readily be distinguished. Their rigidity does
not, like that of shells, arise from the presence of carbo
nate of lime ; for they contain but a small proportion of this
material : and whatever calcareous ingredient enters into
their composition is in the form of phosphate of lime. In
external appearance their texture approaches nearer to that
vol. i. — 29

226 THE MECHANICAL FUNCTIONS.
of horn than to any other animal product; yet in their che
mical composition they differ from all the usual forms of al
buminous matter. The substance to which they owe their
characteristic properties is of a very peculiar nature; it has
been termed Chitine by M- Odier,* and Entomoline by M.
Lassaigne.f This substance is found in large quantity in
the wings and elytra of coleopterous insects. It is remark
able for not liquefying, as horn does, by the action of heat;
and accordingly the integuments of insects, even after having
been subjected to a red heat, and reduced to a cinder, are
found to retain their original form.J
With this substance there is blended a quantity of colour
ing matter, which has usually a dull brown or black hue.
But the colour of the external surface is generally owing to
another portion of this matter, which is spread over it like
a varnish, and being soluble in alcohol and in ether, may be
removed by means of these agents. The colours which are
displayed by insects, and which arise from the presence of
this latter substance, are often very brilliant, and, as is the
case with many other classes of animals, the intensity of the
tints is heightened by the action of light. The elytra of
tropical insects display a gorgeous metallic lustre depend
ing on the reflection of the prismatic colours ; and the same
variegated hues adorn the scales of the butterflies of those
regions. Hair grows in various parts of the surface of insects.
Where the integument is membranous and transparent, these
hairs may be distinctly perceived to originate from enlarged
roots, or bulbs, and to pass out through apertures in the skin ;
as is the case with the hair of the larger animals. Their
chemical composition, however, is very different, for they
are formed of the same substance as the integuments, name-
* Annates de Chimie, torn. 76.
¦J- See the work of Straus Durckheim, p. 33.
i M. Odier had concluded from his experiments that no nitrogen enters
into the composition of this substance. That this conclusion has been too
hastily adopted has been proved by Mr. Children, who, by pursuing another
mode of analysis, found that the chitine of cantharides contains not less than
nine or ten per cent, of nitrogen. See Zoological Journal, i. 111—115.

STRUCTURE OF INSECTS. 227
ly, entomolinc. The purposes served by the hairs are not
always obvious. In many cases they seem intended to pro
tect the integuments from the water, which they repel from
their surfaces. They also tend to prevent injury arising
from friction ; and are accordingly found to be more abun
dant in those parts, as the joints, which are liable to rub
much against one another.
The divisions of the body are frequently marked by deep
incisions, whence has originated the term insect, expressive
of this separation into sections. It is, however, a character
which they possess in common with all articulated animals,
the typical form of which consists, as we have seen, of a se
ries of rings, or segments, joined endwise in the direction of
a longitudinal axis. The principal portions into' which the
body is divided are the head
the trunk; and the abdomen :
each of which is composed of
several segments. I have here
^,aBBKg% given, in illustration, the an
nexed figures, showing the suc
cessive portions into which the
solid frame-work, or skele
ton, of one of the beetle tribe,
the Calosoma sycophanta,*
may be separated. The en
tire insect, which presents the
most perfect specimen of a
complete skeleton' in this class of animals, is represented in
Fig. 149; and the several detached segments, on an en
larged scale, in Fig. 150. The head, c, as seen' in the lat
ter figure, may be regarded as being composed of three
segments: the trunk, x, v, z, of three; and the' abdomen, b,
of nine. Fig. 151 , is a view of the head separated from the
trunk, and seen from behind, in order to show that its form
is essentially annular, and that it resembles in this respect
the rings of which the thorax consists, and to which it forms-
a natural sequel. * Varabus sycophanta. Linn.

228

THE MECHANICAL FUNCTIONS.

The head contains the brain, or principal enlargement of
the nervous system, and the organs of sensation and of mas
tication. Its size, as compared with the rest of the body,

varies much in different insects, and is in general propor-
tionably larger than it is in the larva state. Its integument,
which, from analogy with- vertebrated animals, has been)
called the skull, or cranium, (c, Fig. 150,) is usually the
hardest part of the general crust. Although it may appear,
on a superficial examination, to consist of a single undivided
piece, yet, on tracing its gradual formation, it is found to be
in reality composed of a union of several of the segments of
the larva. Audouin and Cams distinguish three component

STRUCTURE OF INSECTS. 229
segments in the cranium of insects ; while Straus Durckheim
considers it as formed by the consolidation of no less than
six segments ofthe vermiform larva. According to this
theory, the same elements which in the thoracic segments
are developed into feet, are here employed to form parts
having other destinations. From the segment adjacent to
the thorax the antennae are supposed to be developed. The
two anterior segments belong properly to the face; the one
giving origin to the mandibles, (m,) to the maxillae, or proper
jaws, (j,) and also to the palpi, (p;) the other produeinglhe
processes called the labial palpi, (l.)
The mode in which the head is connected with the trunk
varies much in different insects. Sometimes it is united by
a broad basis of attachment, forming a joint between the ad
jacent surfaces : but' usually it is only appended by a narrow
filament, or neck; so that the articulation is effected by liga
ment alone. Occasionally, it is placed at the end of a long
pedicle, which removes it to a considerable distance from
the trunk. In the Hymenoptera and Diptera, the head
moves upon a pivot, so as to admit of its being turned com
pletely r&und.
The trunk, or Thorax, is composed, as shown in the figure^
of three segments, termed respectively the Prothorax (x;)
the Mesothorax (y;) and the Metathorax (z.*) The first
of these, the prothorax, carries the first pair of legs; the se
cond, or mesothorax, gives origin to the second pair of legs,
and also to the first pair bf wings, or to the Elytra (e,) as in
the example before us; and the third, or metathorax, supports
the third pair of legs, and the second pair of wings (w.)
These last two segments are closely united together, but
the original distinction into two portions is marked by a
* In these denominations I have followed the nomenclature of Victor Au
douin (Annales des Sciences Naturelles, torn. p. i. 119,) as being the sim
plest and clearest: but other entomologists have applied the same terms to
different parts. The first segment is termed by Straus Durckheim and
other French writers, the Corselet. Mr. Kirby calls it the Manitrunk, and
restricts the term Prothorax to its upper portion. The united second and
third segments are the Thorax of Straus Durckheim, the Tronc alifire of
Cnabrier, and the Alilrunlc of Kirby.

230 THE MECHANICAL FUNCTIONS.
transverse line. Each of these three segments is divisible
into an upper, a lower, and two lateral portions, which are
joined together at the sides ofthe trunk; these again admit
of farther subdivision; but for the names and descriptions of
these smaller pieces I must refer the reader to works on
Entomology. The parts of the thorax to which the wings
are attached indicate the situation of the centre of gravity of
the whole insect; a point which being in the line of the re
sultant of all1 the forces concerned in the great movements
of the body, requires to be sustained by the moving powers
under all circumstances either of action or repose.
Victor Audouin, who has made extensive researches on
the comparative forms of all these parts in a great variety
of insects, appears to have satisfactorily established the ge
neral proposition that, amidst the endless diversity of forms
exhibited by the skeleton of insects, they are invariably
composed of the same number of elements, disposed in the
same relative situations and order of arrangement; and that
the only source of difference is a variation in the proportion
al development of these elements. He has also observed that
the great expansion of one part is generally attenaed by a
corresponding diminution of others.
The third division of the body is termed the Abdomen
(b;) it is composed of all the remaining segments, which join
to form a cavity enclosing the viscera subservient to nutri
tion, respiration, and reproduction. The number of these
abdominal segments is very various in different genera of
insects. Sometimes there appear to be but three or four;
while, in other cases, there are twelve or even a greater
number. In the Calosoma (Fig. 150, b,) the abdomen has
six complete, followed by three imperfect segments. Not
feeing intended to carry any of the organs of progressive
motion, they retain the form of simple hoops, which is the
primitive type of the segments of annulose animals. Each
segment has a ligamentous connexion with the next, which
is often so close, as hardly to admit of any molion between
them; but in other instances it is more lax, and allows of
the abdomen being flexible. In the former case, which is the

STRUCTURE OF INSECTS. 231
construction in all the Coleoptera, or beetles, the rings have
an imbricated arrangement; that is, each overlaps the next,
often to the extent of two-thirds of its breadth: so that they
present a succession of spheroidal hoops, capable of being
drawn out, to a certain extent, like the tubes of a telescope,
This very artificial construction is manifestly designed to
allow of a great variety of movements, determined by the
position ofthe muscles they enclose: for since the surfaces
which receive, as well as those which are received, are seg
ments of spheroids, this structure admits of a twisting mo
tion ; and the latter segment may be pushed more or less
into the cavity of the former, either generally, or on one
side. Each segment, besides being separate from the rest, is far
ther divided into an upper, or dorsal, and a lower, or ventral
portion; each portion having the form of a semicircle, or ra
ther of an arch of a circle. These are connected at the sides
by a ligamentous band, which runs the whole length of the
abdomen. Great advantage results from this division of the
circles, allowing of the upper and lower portions of the ab
dominal covering being at one time separated, and at ano
ther brought nearer together; for thus the cavity is capable
of being enlarged or contracted in its dimensions, and adapt
ed to the variable bulk of its contents. It is deserving of
notice that, during the process of transformation, some of
the abdominal segments, which are present in the larva, dis
appear entirely, or leave only imperfect traces of their for
mer existence. Sometimes the posterior segments become
so exceedingly contracted in their diameter as to give rise
to the appearance of a tail: this is exemplified in the Pa-
norpa. The junction of the abdomen with the trunk is effected
in various ways. In all the Coleoptera, it is united by the
whole margin of its base, without having a narrower part ;
in other tribes there is a visible diminution of diameter, form
ing a groove all round, or an incision, as it is technically
termed. In the Hymenoptera, this incision is so deep as to
leave only a narrow pedicle like a neck, connecting these

232 THE MECHANICAL FUNCTIONS.
two divisions of the body. In some this pedicle is short, in
others long: in the former case, an exceedingly refined me
chanism is resorted to for effecting the necessary movements
in a part so bulky, compared with the narrowness of the
surface of attachment.*
Insects in their perfect state have constantly six legs,
which are the developments of the six proper legs of the
same animal in its larva condition; all the spurious legs
having disappeared during its metamorphosis. We have
seen that in the myriapoda, the result of development is an
increase in the number both of segments and of legs; the
.reason of which is that, being terrestrial animals, a length
ened form was more useful and accordant with their desti
nation ; but in winged insects, where the object is to procure
the means of flight, the organs require to be concentrated,
and all superfluous parts must be retrenched and discarded
from the fabric. The multiplication of organs, which in the
former case indicated the progress of a higher development,
would in the latter have been the source of imperfection.
As long as the insect remains in its larva stage, its condition
is analogous to that of the myriapode ; but in the more ele
vated state of its existence, its structure is subject to new
conditions, and regulated by new laws.
While the number of members is thus reduced, ample
compensation is given by their increased activity and power,
derived from their augmented length, and the more distinct
lever-like forms of the pieces which compose them.
These pieces (see Fig. 150) are named, from their sup
posed analogy to the divisions of the limbs of the higher or
ders of vertebrated animals, the haunch (h,) the trochanter
(t,) the femur (f,) the tibia (s,) and the tarsus (r.) In ge
neral the femur (or thigh) has nearly a horizontal, and the
tibia (or legs) a vertical position, while the whole tarsus (or
foot) is applied to the ground.
The haunch (h,) which is supposed to correspond to the
* For the details of this structure I must refer to writers on entomology,
and in particular to Kirby and Spence's "Introduction to Entomology,"
vol. iii. p. 701.

STRUCTURE OF INSECTS. 233
hip bone of quadrupeds, is a broad, but very short truncated
cone. The mode of its articulation with the trunk admits
of great variety; sometimes it is united by a ball and socket
joint, as in the Curculio and Cerambyx; and it then has, of
course, great freedom of motion: at other times the joint is
of the hinge kind, as in the Melolontha. The trochanter
(t,) and the femur (f,) though in reality distinct pieces, are
usually so firmly united as to compose only one division of
the limb. The articulation of this portion with the haunch
is always effected by a hinge-joint. Joints of this descrip
tion, when formed, as they are in insects, by the apposition
of two tubular pieces, are constructed in the following man
ner. One of the tubes has, at the end to be articulated, two
tubercles, which project from the margin, and are applied to
the adjacent end ofthe other tube at two opposite points, of
its circumference; the line which passes through those two
points being the axis of motion. On the side where the
flexion is intended to be made both tubes are deeply notched,
in order to admit of their being bent upon one another at a
very acute angle; and the space left by these notches is filled
tip by a pliant membrane, which performs the office of a li
gament. These articular tubercles and depressions are so
adjusted to one another, that the joint cannot be dislocated
without the fracture of some of its parts. As the different
axes of motion in the successive joints are not coincident,
but inclined at different angles to one another, the extent of
motion in the whole limb is very greatly increased. Thus,
in the cases where the articulation of the haunch with the
trunk is a hinge-joint, the axis of this joint and of the next
are placed at right angles to each other; so that there results,
from the combination of both, a capability in the thigh of
executing a circular motion in a manner almost as perfect
as if it had revolved in a spherical socket. The principle
of this compound motion is the same as that employed on
ship-board for the mariner's compass, and other instruments
which require to be kept steady during the motion of the
ship. For this purpose what are called gimbals are used,
the parts of which have two axes of rotation, at right angles
VOL. 1. — 30

234 THE MECHANICAL FUNCTIONS.
to each other, so as to enable the compass to take its proper
horizontal position, independently of any inclination of the
ship. The tibia, or shank (s,) is joined at an acute angle with
the femur; and is frequently either beset with spines, or else
notched or serrated.
The tarsus, or foot'(R,) is the last division of the limb: it
is divided into several joints, which have been supposed to
represent those of the toes of quadrupeds. The joints are
generally of the hinge kind, but some are met with of a
more rounded form, and approaching to that of the ball and
-socket. The whole structure is most admirably adapted to
its exact application over all the inequalities of the surfaces
on which the insect treads. But as the habits and modes of
life of this numerous class are exceedingly diversified, so
the form of the feet admits of greater variety than that of
any other part of the limb.
The feet of insects diverge, and spread over a wide sur
face; thus extending the base of support so as to ensure the
stability of their bodies in the most perfect manner. When
the legs are very long, as in the Tipula,* the body seems,
indeed, more to be suspended than supported by them; con
trary to what obtains in quadrupeds, where the feet are
more immediately underneath the points at which they are
connected with the trunk.
The last joint of the tarsus is generally terminated by a
claw, which is sometimes single and sometimes double, and
which contributes to fasten the foot, under a variety of cir
cumstances, both of action and of repose. By means of feet
thus armed, the insect can ascend or descend the perpendicular
sides of a rough body with the greatest ease; but it is scarce
ly able to advance a single step upon glass, or other polished
surfaces, even when horizontal. The hooks at the ends of
the anterior pair of feet are directed backwards, those of the
middle pair inwards, and of the posterior pair forwards; thus
affording the greatest posssible security against displacement.
* It has been conjectured that the object in furnishing this insect with legs
of so great a length is that of enabling it to walk among blades of grass.

STRUCTURE OF INSECTS. 235
Many insects are provided with cushions, at the extremity
of the feet, evidently for the purpose of breaking the force
of falls, and preventing the jar which the frame would other
wise have to sustain. These cushions are formed of dense
velvety tufts, of hair, lining the underside of the tarsi, but
leaving the claw uncovered; and the filaments, by insinu
ating themselves among the irregularities of the surfaces
to which they are applied, produce a considerable degree of
adhesion. Cushions are met with chiefly in large insects
which suddenly alight on the ground after having leaped
from a considerable height : in the smaller species they ap
pear to be unnecessary, because the lightness of their bodies
sufficiently secures them from any danger arising' from falls.
Some insects are furnished with a still more refined and
effectual apparatus for adhesion, and one which even enables
them to suspend themselves in an inverted position from the
under surfaces of bodies. It consists of. suckers, the ar
rangement and construction of which are exceedingly beau
tiful;' and of which the common house-fly presents us with
aii example. In this insect that part of the last joint of the
tarsus which is immediately under the root of the claw, has
two! suckers appended to it by a narrow funnel-shaped neck,
moveable' by muscles in all directions. These suckers are
shown in Fig.- 152, which represents the under side of the
foot of Musca vomitoria, or blue-bottle fly, with the suckers
expanded. The sucking part of the apparatus consists of a
membrane, capable of contraction and extension, and the
edges of which are serrated, so as to fit them for the closest
application to any find of surface. In the Tabanus, oi
153

horse-fly, each foot is furnished with three suckers. In
the Cimbex lutea, or yellow saw-fly, there are four, of which-

236 THE MECHANICAL FUNCTIONS.
one is placed upon the under surface of each of the first four
joints of the toes, (Fig. 153 :) and all the six feet are pro
vided with these suckers. In the Dytiscus marginalis,
suckers are furnished to the feet of the male insect only.
The first three joints of the feet of the fore-legs of that in
sect have the form of a shield, the under surface of which is
covered with suckers having long tubular necks: there is
one of these suckers very large, another of a smaller size,
and a great number of others exdeedingly small. A few of
the latter kind are represented highly magnified in Fig. 154.
In the second pair of feet, the corresponding joints are pro
portionally much narrower, and are covered on their under
surface with a multitude of very minute suckers. The Acri
dium biguttulum, which is a species of grasshopper, has one
large oval sucker, under the last joint of the. foot, imme
diately between the claws. On the under surface of the
first joint are three pairs of globular cushions, and another
pair under the second joint. Fig. 155 shows these parts.
The cushions are filled with an elastic fibrous substance ;
which, in order to increase the elasticity of the whole struc
ture, is looser in its texture towards the circumference.*
The mode in which these suckers operate may be dis
tinctly seen, by observing with a magnifying glass the ac
tions of a large blue-bottle fly in the inside of a glass tum
bler. A fly will, by the application of this apparatus, remain
suspended from the ceiling for any length of time without
the least exertion ; for the weight of the body pulling against
\ the suckers serves but to strengthen their adhesion: hence,
' ) we find flies preferring the ceiling to the floor, as a place of
rest. Insects w_hich, like the gnat, walk much upon the surface
of water, have at the ends of their feet a brush offine hair,
the dry points of which appear to repel the fluid, and pre
vent the leg from being wetted. If these brushes be moist
ened with spirit of wine, this apparent repulsion no longer
takes place; and the insect immediately sinks and is drowned.
* Philosophical Transactions for 1826, p. 324.

AQUATIC INSECTS.

237

§ 7. Aquatic Insects.
Although many insects are inhabitants of water while
in their laVva state, few continue to reside in that element
after they have undergone all their metamorphoses. When?
they have attained the imago state, indeed, every part of
their bodies becomes permeated by air, which forms alto-'
gether a large portion of their bulk, and gives to the insect,
when it is immersed in water, a strong buoyant force. As-
the largest volume of air is contained in' the abdomen, this
part is comparatively lighter than either the trunk or head;;
and the natural position of the insect in the fluid is oblique'
to the horizon, the head being depressed,- and the abdonien'
elevated. Any force impelling the body forwards in the'
direction of its axis tends, therefore, to make it also descend.
The effect of this downward force is counteracted by the'
sustaining pressure of the water, which is directed vertically
upwards : so that the real operation of the force in question-1
is to carry the body forwards nearly in a horizontal di
rection. In insects destined to move in water, sometimes all the
legs, but occasionally only one pair, are lengthened and ex
panded into broad triangular surfaces, capable of acting as-

156

oars : and these surfaces are farther extended by the addi-"
tion of marginal fringes of hair, so disposed as to project5
and act upon the water every time the impulse is given, but'
to bend down when the leg is again drawn up, preparatory

238 THE MECHANICAL FUNCTIONS.
to the succeeding stroke; thus imitating the action which is
called feathering an oar. The impulses are given with
great regularity, all the feet striking the water at the same
moment. (
Of all the coleopterous insects, the Dyliscus, or water-
beetle (of which Fig. 156 represents the upper, and Fig. 157
the, under side,) is the one best constructed for swimming:
its body having a flattened form, very much resembling a
boat, narrower before than behind, and its surface present
ing no projecting parts. The upper surface in particular is
extremely smooth, to enable it to glide under the water with
the least possible friction. Its centre of gravity is placed
Very near the under surface. The posterior legs, which act
as powerful oars, are attached to very large haunches, for
the purpose of containing the thick muscular bands which
are inserted into the trochanter, and by which these joints
are moved with great power. As the motion of these oars
is to be performed in a plane nearly parallel to the axis of
the body, the haunches are not required to be moveable;
and accordingly they are firmly united to the thorax; a
structure which renders the motion ofthe other joints more
regular and uniform. When the Dytiscus wishes to rise, it
need only desist from all action, and abandon itself to the
buoyant force of the fluid, which quickly carries it to the
surface. The Nolonecta, or water-boatman (Fig. 158,) is remarka
ble for always swimming on its back, a peculiarity depend
ing on the form of its body, which is
semi-cylindrical, with the legs affixed to
the flat surface; so that, when lying on
>its back in the fluid, the centre of gravity
is below the centre of the whole figure,
or the metacentre, as it is termed, and
the equilibrium is maintained. It is evident that, under these
circumstances, if it were placed in the water with its legs
Undermost, it would unavoidably tilt over, and resume its
usual position. Its long legs extending at right angles to
the body, present a striking resemblance to the oars of a boat;

PROGRESSIVE MOTION OF INSECTS. , 239
and they act, indeed, in the same manner, and on the same
principles.

§ 8. Progressive Motion of Insects on Land.
The actions of the limbs of insects in walking are quite
different from what they are in swimming, and are very
similar to those of the caterpillar, in which we have seen
that the motions of the anterior and posterior legs on one
side are combined with that of the middle one on the other
side; and the two sets of legs are moved alternately. In
consequence of their relative positions with the trunk, the
anterior legs are advanced by the extension, and the poste
rior legs by the flexion of the corresponding joints. When
the feet have fixed themselves on the ground, the contra
ry actions take place, and the body is brought forwards.
During_.ih.is period the legs which compose the other set are
called into play, and are advanced; and the same succession
of actions takes place with these as with the former. This
can easily be seen when the insect walks very leisurely; but
in a more quickened pace, the succession of actions is too
rapid to be followed by the eye.
The action of leaping is performed by the sudden exten
sion of all the joints of the limb, which are previously folded
as close as possible. The joints principally concerned in
this action, are those of the thigh and tibia, as they furnish
the longest and most powerful levers. Preparatory to the
effort, the tibia is brought down as close as possible to the
ground, by bending it over the tarsus; and the thigh also
is bent upon the tibia, so as to form with it a very acute
angle. In order to enable it to take this position with most
advantage, we find in many of the Coleoptera, that the thigh
has a longitudinal groove for the reception of the tibia, with
a row of spines on each side of the groove. While the
limb is in this bent position, the extensor muscles are vio
lently exerted, and by producing a sudden unbending of this
apparatus of folded springs, they project the whole body,

240 THE MECHANICAL FUNCTIONS.
Jby the accumulated impulse, to a considerable height in the
air. The leaps of insects being generally forwards, all the
legs do not participate equally in the effect; for the fore legs
contribute much less to it than the hind legs, and are more
useful in modifying the direction of the leap, than in adding
to its force. The power of leaping is derived principally
from the great size and strength of the extensor muscles of
the legs, which, being contained within the femur, necessa
rily swell that division of the limb to an unusual thickness;
and in order to procure sufficient velocity of action, both the
femur and tibia are much elongated. Thus the locust, which
is so constructed, leaps with ease to a distance two hundred
times the length of its own body. We may in general, in
deed, infer the particular kind of progressive motion for
which the insect is intended by observing the comparative
length of the different pairs of legs. When they are of equal
size, the pace is uniform: — swiftest in those that have the
longest legs, — slowest when they are short. When the an
terior legs are much longer than the posterior, the power of
prehension may be increased, but that of progression is im
peded. The great prolongation of the posterior legs is ge
nerally accompanied by the power of jumping, unless, in
deed, they are at the same time much bent, for such curvature
disqualifies them from acting advantageously as levers.
Many insects have the extremity of the tibia armed with
a coronet of spines, which assist in fixing this point against
the plane from which they intend to spring, and which give
to the limb a steady fulcrum. The Cicada Spumaria has
been known to leap to a distance of five or six feet; which
is two hundred and fifty times its own length : this, if the
same proportions were observed, is equivalent to a man of
ordinary stature vaulting through the air the length of a
quarter of a mile, When the same insect is laid on glass,
on which the spines cannot fasten, it is unable to leap far
ther than six inches,*
The insects belonging to the genus Elater are provided
* De Geer, III. 178, quoted by Kirby and Spence.

PROGRESSIVE MOTION OF INSECTS. 241
with a peculiar mechanism for the special purpose of accom
plishing a singular mode of leaping, independently of any
action of the legs. The legs of this insect are so short, that
when it is laid on its back, it cannot turn itself, being unable
to reach with its feet the plane on- which it is lying, and
procure a fulcrum for the action of its muscles. It is appa
rently with the design of remedying this inconvenience,
that nature has bestowed on this tribe of insects the faculty
of springing into the. air, and making a somerset, so as to
light on the feet; an effect which is accomplished by an ex
ceedingly curious mechanism. The prothorax is prolonged
beyond the length it usually has in other coleoptera, and it
is articulated with the mesothorax on the dorsal side by
two lateral tubercles, which form a hinge joint, limiting'its
motions to a vertical plane.- The sternum, or pectoral por
tion ofthe prothorax, is also extended backwards, and ter
minates in an elastic spine, which is received into a cavity
in the mesothorax, and which, while the insect is lying on
its back, with- the prothorax bent upon- the mesothorax, re
coils with the force of a spring, and communicates to the
body an impulse which carries it upwards to a considera
ble height. If the el'ater should fail in its* first attempts to
recover its feet, it repeats its leaps till it succeeds. We
find no example of a similar structure in any other part of
the animal kingdom. -
The express adaptation of structure to the mode of life-
designed for each species' of insect is nowhere more strongly
marked than in those which are intended to burrow in the
earth : and of these the Gryllo-talpa, or mole cricket, pre
sents a remarkable example. A minute account ofthe ana
tomy of this insect has been given by Dr. Kidd,* from
which it appears that being destined, like the mole, to live
beneath the surface of the earth, and to excavate for itself a
passage through the soil, it is furnished with limbs peculiar
ly calculated for burrowing, with a skin which* being. co
vered with a fine down, effectually prevents the adhesion of
* Phil. Trans, for 1825, p. 203.
VOL. I. — 31

242 THE MECHANICAL FUNCTIONS.
\
the moist earth through which it moves, and with a form of
body enabling it to penetrate with least resistance the op
posing medium. By being endowed with the powerof moving
as easily in a backward as in a forward direction, it is ena
bled quickly to retreat in the narrow channel it has exca
vated: and as a safeguard in these retrograde movements, it
is provided with a pair of posterior appendages, which are
supplied with large nerves, and may be regarded as serving
the purpose of caudal- antennae.
The fore-legs, (one of which is represented in Fig. 158*)
are the burrowing implements, and they are admirably cal
culated for their peculiar office,
both in their shape and in the
mode of articulation of their
several divisions, which bear a
considerable analogy to the
corresponding member1 of the mole. Dr. Kidd observes,
that, compared with the other legs, and with the general size
of the animal, they are as if the brawny hand and arm of a
robust dwarf were set on the body of a delicate infant ; and
the indications of strength which their structure manifests,
fully answer to their extraordinary size. For a more par
ticular description of the mechanism of this instrument, I
must refer the reader to the paper above quoted.
§ 9. Flight of Insects.
If the excellence of a mechanic art be measured by the
difficulties to be surmounted in the attainrrient of its object,
none surely would rank higher than that which has accom
plished the flight of a living animal. No human skill has
yet contrived the construction of an automaton, capable, by
the operation of an internal force, of sustaining itself in the
air in opposition to gravity, for even a. few minutes; and
far less of performing in that element the evolutions which
we daily witness even in the lowest of the insect tribes.
to the ultimate attainment of this faculty it would appear
that all the transformations they undergo -in external appear-

FLIGHT OF INSECTS. 243
ance, and all the developments of their internal mechanism,
are expressly directed. Wings are added to the frame only
in the last stage of its completion ; after it has disencum
bered itself of every ponderous material that could be spared, *
after it has been condensed into a small compass, and after
it has been perforated in all directions by air-tubes, giving
lightness and buoyancy to every part. Curiously folded up
in the pupa, the wings there attain their full dimensions^
ready to expand whenever the bandages which surround
them are removed. No sooner is the. insect emancipated
from its confinement, than these organs, which are composed
of duplicatures of a dense, but exceedingly fine membrane,
identical in its composition with the general integuments,
begin to separate from the sides of the body, and to unfold
all their parts. Their moisture rapidly evaporates, leaving
the delicate film dry and firm, so as to be .ready for imme
diate action. The fibres, or nervures, as they are called, form
a delicate net-work, for the support of this fine membrane,
like the frame of the arms of a windmill, which supports
the canvass spread over them. The microscope shows that
these fibres are tubular, and contain air; a structure the most
effectual for gonjoining lightness with strength; and many
entomologists are of opinion that the insect has the power,
during, the act of flying, of directing air into the nervures,
so as to dilate them to the utmost, and render them quite
tense and rigid.
In the great majority of insects, the wings are four in
number; of which the first pair are, as we have seen, affixed
to the mesothorax, and the second to the metathorax. These
two segments -of the thorax, composing what has been
termed the alitrunk, constitute the most solid portion of
the skeleton, and are frequently strengthened by ridge?,
and other mechanical contrivances for support. The su
perior extremities of these supports, which have been com
pared to the clavicles, or furcular bones of birds, are always
curved inwards. This part of the trunk requires to be al
ternately dilated and contracted during flight; and, hence,

244 THE MECHANICAL FUNCTIONS.
the several pieces of which its dorsal portion is composed, -
are loosely connected together by ligaments.*
The shape of the wings is more or less triangular. They
are moved by numerous muscles, which occupy a large
space in the interior of the trunk, and consist' of various
kinds of flexors, extensors, retractors, levators, and depress
ors; the whole forming a very complicated assemblage of
moving powers. The largest, and consequently most pow
erful of these muscles, are those which depress, or bring
down the wings. They form a 'large mass, marked a, in
Fig. 144. All these muscles exert great force in their con
tractions, which are capable of being renewed in very rapid
succession; for, indeed, unless they had this power, even so
light a body as that of an insect could not have been sus
tained for a moment in so rare a medium as the atmosphere,
far less raised to any height by its resistance.
The simple ascent and descent of the wings would be suf
ficient, without any other movement being imparted to them,
to carry forwards the body of the insect in the air. The
aetion in which the muscles exert the greatest force is in
striking the air during the descent of the wing; an impulse
in the opposite direction being the result of Ijie reaction of
the air. The axis of motion of the wings is a line inclined
at a small angle to the axis ofthe body, and directed, from
before, backwards, outwards, and r downwards; and they
move in a plane which is not vertical, but inclined forwards.
The angle which the plane of the wing foj: ms with the hori
zon varies continually in the different positions of the wing;
but the general resultant of all these successive impulses is a
force directed forwards and upwards; the first part of this
force produces the horizontal progression of the insect, while
the second operates in counteracting the force of gravity;
and, during the advance of the insect, either maintains it at
the same height, or enables it to ascend.
When the insect wishes to turn, or to pursue an oblique
* See Chabrier's "Essai sur le Vol des Insectes," Memoires du Museum
d'Histoire Naturelle; vi. 410; vii. 297, and viii. 47 and 349. See, also, Zoolo-
gicalJournal, i. 391.

FLIGHT OF INSECTS. 245
course, it effects its purpose very easily by striking the air
with more force on one side than on the other ; or, by em
ploying certain muscles which bend the body to one side,
it shifts the situation of the centre of gravity, so that the re
action of the air on the wings is exerted in a different direc
tion to what it was before; and the motion of the body is
modified accordingly.
By exerting with the wings a force just sufficient to ba
lance that of gravity, insects can poise themselves in the air,
and hover for a length of time over the same spot, without
rising or falling, advancing or retreating ; and the body may,
all the while, be kept either in the horizontal, or in the erect
position. In the latter case the motions are similar to those
which take place in ordinary flying, only they are more
feebly exerted, since all that is required is to sustain the
weight of the body without urging it to a greater speed.
Libellula, Sphinxes, and a great number of Diptera, exhibit
this kind of action : among the latter, the Stratiomys is most
remarkable for its power of remaining long in the same fixed
position. The number, form, and structure of the wings of insects
have furnished entomologists with very convenient charac
ters for their classification: on these are founded the orders
of the Coleoptera, Orthoptera, Rhipiptera, Hemiptera, New
roptera, Hymenoptera, Diptera, and Lepidoptera. To enter
into any detail in a field of such vast extent as is presented
by the infinitely diversified mechanism of the insect crea
tion, would, it is obvious, far exceed the proper limits of
this treatise. I must, therefore, confine myself to a few lead
ing points in their structure and modes of progression.
In the Coleoptera, an order which comprehends by far the
largest number of genera of insects, the lower pair of wings
(w, Fig. 150, p. 228) are light and membranous, and of a
texture exceedingly fine and delicate.' They are of great
extent, compared with the size of the body, when fully ex
panded ; and are curiously folded when not in use. For the
protection of these delicate organs, the parts which cor
respond to the upper pair of wings of other insects, are here

246

THE MECHANICAL FUNCTIONS.

Converted into thick, opaque and hard plates, (e,) adapted to
cover the folded membranous wings when the insect is not
flying, and thus securing them from injurious impressions,
to which they might otherwise be exposed from heat, mois
ture, or the contact of external bodies. These wing-cases,
or elytra, as they are termed, are never themselves em
ployed as wings, but remain raised and motionless during
the flight of the insect. They probably, however, contri
bute to direct the course of flight, by variously modifying
the resistance of the air.*
In the Orthoplera (Fig. 159,) the coverings of the wings,
or tegmina, instead of being of a horny texture, are soft and
flexible, or semi-membranous. The wings themselves, being
broader than their coverings, are, when not in use, folded
longitudinally, like a fan.
In the new .Order of Rhipiptera of Latreille,t which in
cludes only two genera, the tegmina are anomalous both in

160

159

162

their situation and shape ; being fixed at the base of the an
terior legs, very long and narrow, and apparently incapable
of protecting the wings. The wings themselves are of am-
* The Elytra of insects hay,e been regarded, by Oken, as corresponding to
the bivalve shells ofthe Mollusca, .a notion which seems to be founded upon
a fanciful and strained analogy.
f The Strepsiptera of Kirby. See Transactions of the Linnsan Society,
XI. 86.

FLIGHT OF INSECTS. 247
pie extent, forming, when' expanded, a quadrant of a circle,
with five or six nervures radiating from their base, and folded
longitudinally. In the Hemiptera, the tegmina, or . as they are here
called, the hemi-elytra, are coriaceous towards their base,
but membranous towards their extremity, and the true
wings are folded transversely, so as to cross one another.
These hemi-elytra are employed to strike the air in flight,
and their movements accompany those of the wings.
Insects having four thin membranous and transparent
wings are arranged under two orders ; namely, the Neurop-
tera (Fig. 160,) in which the lesser nervures form an inter
lacement of fibres, crossing one another nearly at right
angles, like net-work, or lace ; and the Hymenoptera (Fig.
161,) in which they are disposed like the ramifications of
arteries or veins, diverging at acute angles from the main
trunks. T,he insects belonging to these two orders enjoy
extensive powers of flight. Libellula, and JEschnae, which
are included in the first of these orders, never close their
wings, but,, when they are not flying, keep them constantly
expanded, and ready for instant action. They fly with the
greatest ease in all directions, sideways, or backwards, as
well as forwards, and can instantly change their course with
out being obliged to turn their bodies. Hence they possess
great advantages bothin chasing other insects, and in evading
the pursuit of birds. Bees, which are hymenopterous in
sects, have often been observed\ to fly to great distances
from their hive in search of food. The humble bee adopts
a very peculiar mode of flight, describing, in its aerial course,
segments of circles, alternately to the right and to the left.
The velocity with which these insects move through the
air, in general, much exceeds that of a bird, if estimated
with reference to the comparative size of these animals.*
* I have been favoured by Mr. George Newport with the following ac
count of the structure of the sting of the Wild Bee. [Anihophora retusa,
Kirby) which he has lately carefully examined, and from whose drawings
of the dissected parts the annexed figures ( 1 63) have been engraved. " The
sting of this bee, a, is formed of two portions placed laterally together, but

248

THE MECHANICAL FUNCTIONS.

Although the greater number of insects have four wings,
there are many, such as the common house fly, and the gnat,
which have only two. These compose the order Diptera,
(Fig. 162.) In these insects we meet with two organs, con
sisting of cylindrical filaments, terminated by a clubbed ex
tremity ; one arising from each side of the thorax (as seen in
the above figure,) in the situation in which the second pair
of wings originate in those insects which have four wings.

capable of being separated. The point, p, is directed a little upwards.and
is a little curved: the barbs, seen still more highly magnified at 4,. are about
six in number, and are placed on the under surface, and their points directed
backwards. At the base of the sting, e, there is a semicircular dilatation
apparently intended to prevent the in
strument frpm being thrust too far out
of the sheath (shown separately at v,)
in which it moves: it has also a long ten
don, to which the muscles are attached.
It is between these plates, when ap
proximated, that the poison flows from
the orifice of the somewhat dilated ex
tremity of the poison duct, d, which
comes from the anterior part of the'
poison bag, 11. This bag is of an oval
shape, and is not the organ which se
cretes the poison, but merely a recepta
cle for containing it; for it is conveyed
into this bladder by means of a long con
voluted vessel, c, which receives it from
the secreting organs, s: These organs
consist of two somewhat dilated vessels
resembling cseca, but which have each a
slender secretory vessel extending from
them. The sting moves in » tubular
sheath, v; which is open at its base,'' and
along its upper surface, as far as the
part where the sting is prevented from
being thrust out any farther. The mus
cles which move the sheath are distinct from those of the sting, and are at
tached to an elongated and curved part on each side of its base, and to an
arched and moveable part which is apparently articulated with it. Swam-
merdam has delineated these parts as eaca in his dissection of the 'common
hive bee, but has not noticed the secretory vessels. The sting of the hive
bee resembles that of the Anthopliora retusa."

FLIGHT OF INSECTS. 249
They are named the halteres, or poisers, from their sup
posed use in balancing the body, or adjusting with exactness
the centre of gravity when the insect is flying. Whatever
may be their real utility, they may still be regarded as rudi
ments of a second pair of wings ; and they afford, therefore,
when thus viewed, a striking instance of the operation of
the tendency which prevails universally in the animal king
dom, and modifies the structure of each individual part so
as to preserve its conformity to one general type.
The innumerable tribes of butterflies, sphinxes, and moths,
are all comprehended in the order Lepidoptera, and are dis
tinguished T>y having wings covered with minute plumes or
scales. These scales are attached so slightly to the mem
brane of the wing as to come off when touched with the
fingers, to which they adhere like fine dust. When examined
with the microscope, their construction and arrangement ap
pear to be exceedingly beautiful, being marked with parallel
and equidistant stria?, often crossed by still finer lines, the
distinct visibility of which, in many kind's of scales, as those
of Pontia brassica, or cabbage butterfly, and the Morpho
Menelaus of America, constitutes a good criterion of the ex
cellence of the instrument. The beautiful colours which
these scales possess may perhaps generally be owing to the
presence of some colouring material; but the more delicate
hues are probably the result of the optical effect of the striae
on the surface; and in some cases they result from' the thin
ness of the transparent plate of which they consist; for I
have observed in several detached scales that the colours
they exhibit by transmitted light are the complementary co
lours to those which they display when seen by reflected light.
The forms of these scales are exceedingly diversified, not
only in different species, but also in different parts of the
wings and body of tft% same insect ; for the surface of the
body, generally, as well as the limbs, and even in some spe
cies the antennae are more or less covered with these scales.*
* In the posthumous work of Lyonet, which has lately appeared, nearly
the whole of six quarto plates are crowded with the delineations of the dif
ferent forms of the scales found in the Bombyx Cossus.
VOL. I. — 32

250 THE MECHANICAL FUNCTIONS.
Fig. 164 exhibits some of the more usual shapes as they ap
pear when viewed with high magnifying powers.
Each scale is inserted into the membrane of the wing by
a short pedicle, or root, and overlaps the adjoining scales ;
and the whole are disposed in rows with more or less regu-

i \
£ A
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iiii

larity ; one row covering the next, like tiles on the roof of a
house.* This imbricated arrangement, together with the
marks that are left on the membrane of the wing where the
scales have been rubbed off, are shown in Fig. 165, which
is a faithful delineation of the appearance of the wing of the
Hesperia Sloanus, seen through a powerful microscope.
The membrane of the wing itself, when 'stripped of its
scales, is as perfectly transparent as that of the bee, and is,
in like manner, supported by diverging nervures. Many
butterflies exhibit in some parts of the wing, smooth pearly
spots, called by entomologists, ocelli, or eyes, which arise
from those parts being naturally destitute of scales. The
number of these scales necessary to cover the surface of the
wings must, from their minuteness, be exceedingly great.
The moth of the silk worm (Bombyx mori, Fig. 148,)
* The scales on the abdominal rings ofthe I,episma are of two kinds; one
set being arranged in rows, as usual, and the others, which are of a different
shape, being inserted between and over the former, so as to fasten each firmly
in its place.

FLIGHT OF INSECTS. 251
which has but a small wing, contains, according to Levven-
hoeck, more than two hundred thousand of these scales in
each wing.
These scales doubtless contribute to the protection of the
wing; but they at the same time add considerably to their
weight, and impede the velocity of their action. This in
convenience appears to have been in a great measure com
pensated by the greater size of the wings, and by the extent
of the surface with which they strike the air. Still, how
ever, it is sufficiently obvious that insects of this order fly
with less rapidity and steadiness than most others. But this
unsteadiness, again, is turned to good account; for the but
terfly, by its irregular and apparently capricious movements,
alternately dipping and rising in the air, so as to describe a
series of zigzag lines,. more easily eludes capture when pur
sued, not only by naturalists, but also by birds that are ea-
.gerly seeking to secure them. It is astonishing to what a
distance the silk worm moths will fly ; some have been
known to travel more than a hundred miles in a short time.
The Papilio Iris often rises to so great a height in the air
as to be quite invisible.
A mechanical contrivance is adopted in many of the Le-
pidoptera for keeping their wings steady during flight, con
sisting of a hook covered with hair and scales attached to
the under side of the upper wings near their base, and con
nected also by means of bristles to the base of the lower
wing: by this attachment all the wings are locked together,
and brought into action at the same time. Insects of the
Sphinx tribe are also provided with a kind of rudder formed
by the expansion of the tail, enabling them to steer their
course with more certainty. The Lepidoptera in general
fly with the body nearly upright, contrary to the habits of
most other winged insects, whose bodies, while flying, are
nearly in a horizontal position.
The feats of agility and strength exhibited by insects have
often been the theme of admiration with writers on natural
history ; and have been considered as affording incontrovert
ible proofs of the enormous power with which their muscles

252 THE MECHANICAL FUNCTIONS.
must be endowed. We have already had occasion to notice
a remarkable instance of the force and permanence of mus
cular contraction in- those caterpillars which frequently re
main for hours together in a fixed attitude, with their bodies
extended from a twig, to which they cling by their hind
feet alone.* Ants will carry loads which are forty or fifty
tim.es heavier than their own bodies; and the distances to
which many species, such as the Elaier, the Locust, the
Lepisma, ind above ajl the Pulex, are capable of leaping,
compared with the size of the insects themselves, appear
still more astonishing. Linnaeus has computed that the
Melolontha, or chaffer, is, in proportion to its bulk, more -
than six times stronger than the horse: and, has asserted that
if the same proportional strength as is possessed by the Lu-
panus, or stag-beetle, had been given to the elephant, that
animal would have been capable of tearing up by the roots
the largest trees, and of hurling huge rocks against his as
sailants, like the giants of ancient mythology.
But while we must admit that all these facts indicate a re
markable degree of energy in the contractile power of the
muscular fibres of insects, we should at the same time re
collect that the diminutiye size of the beings which display
those powers is itself the source of a mechanical advantage
not possessed by larger animals. The efficacy of all me
chanical arrangements must ultimately depend on a due pro
portion between the moving and the resisting forces: hence
mechanism of every kind must be adjusted with reference
not merely to the relative, but to the absolute dimensions
of the structures themselves. This will be evident when
we consider that the forces which are called into action are
resisted by the cohesion of the particles composing the solid
parts of the machine; and this cohesion being hot a variable,
but a constant and definite force, must necessarily limit the
dimensions of every mechanical structure, whether intended
for stability or for action. An edifice raised beyond a cer
tain magnitude, will not support itself, because the weight
* See Fig. 148*. p. 224.

FLIGHT OF INSECTS. 253
of the materials increases more rapidly than the strength.
How often has it been found that a machine which works
admirably in a sma^l model, will totally fail in its perform
ance when constructed on a larger scale? Any lever, of
whatever form, may be increased in its dimensions until the
force of gravity becomes superior to the cohesion of its own
particles ; and consequently any structure, like a vegetable
or animal body, composed of a combination of levers, would,
if its size were to exceed a certain limit, fall to pieces mere
ly by its own weight. This can be prevented either by em
ploying materials of greater cohesive strength, or by in
creasing, at the points where the strains are greatest, the
thickness of the parts compared with their length : but the
choice of materials is necessarily restricted within narrow
limits, and the latter expedient would entirely alter the re
lative proportions of the parts, and would require a com
plete change in the plan of their construction. In passing
from the smaller to the larger animals, we find, according
ly, that new models are adopted, a new order of architec
ture introduced, and new laws of development observed.
We have next, then, to direct our attention to the procedure
of nature in the execution of this more enlarged and compre
hensive scheme of animal organization.

(254 )

CHAPTER VI.
VERTEBRATA.
*
§ 1. Vertebrated Animals in general.
If it be pleasing to trace the footsteps of nature in con
structions so infinitely varied as those of the lower animals,
and to follow the gradations of ascent from the zoophyte to
the winged insect, which exhibits the greatest perfection com
patible with the restricted dimensions of that class of beings,
still more interesting must be the study of those more ela
borate efforts of creative power, which are displayed, on a
wider field, in the higher orders of the animal kingdom. In
the various tribes of beings which are now to come before
us, we find nature proceeding to display more refined deve
lopments in her system of organization, resorting to new
models of structure on a scale of greater magnitude than be
fore, devising new plans of economy, calculated for more ex
tended periods of duration, and adopting new arrangements
of organs, fitted for the exercise of a higher order of facul
ties. The result of these more elaborate constructions is
seen in the vast series of Vertebrated Animals, which com
prises a well-marked division of Zoology, comprehending
all the larger species that exist on the globe, in whatever
climate or element they may be found; and including man
himself, placed, as he unquestionably is, at the summit of
the scale ; — the undisputed Lord of the Creation.
A remarkable affinity of structure prevails throughout the
whole of this extensive assemblage of beings. Whatever
may be the size or external form of these animals, whatever
the activity or sluggishness of their movements, whether
they be inhabitants of the land, the waters, or the air, a
striking similitude may be traced, both in the disposition of
their vital organs, and in the construction of the solid frame-

VERTEBRATED ANIMALS, 255
work, or skeleton, which sustains and protects their fabric.
The quadruped, the bird, the tortoise, the serpent, and the
fish, however they may differ in subordinate details of or
ganization, are yet constructed upon one uniform principle,
and appear like varied copies from the same original model.
In no instance do they present structures which are alto
gether isolated, or can be regarded as the results of sepa
rate and independent formations.
In proceeding from the contemplation of the structures of
articulated to those of vertebrated animals,we appear to pass,
by a rapid excursive flight, from one great continent to ano
ther, separated by an immense gulf, containing no interme
diate islands from which we might gather indications of these
tracts of land having been originally connected. At the
very first sight, indeed, the general fabrics of these two de
scriptions of animals appear to have been constructed upon
opposite principles; for, in the one, as we have already seen,
the softer parts are internal, and are enclosed in a solid crust,
or shell, or horny covering,, answering, at once, the purposes
of protection and mechanical support, and furnishing exten
sive surfaces for the attachment of the organs of motion.
But, in the Vertebrata, the solid frame-work which serves
these purposes, occupies, for the most part, an internal situa
tion, constituting a true-jointed skeleton, which is surround
ed by the softer organs, and to which the muscles, destined
to move their several parts;, are attached. The office of ex
ternal defence is intrusted solely to the integuments, and
their different appendages. Such is the general character of
the arrangements which nature has here adopted; from
which, however, she has occasionally deviated with respect
to some important organs of extremely delicate texture, and
which require to. be shielded from the slightest pressure.
This occurs with regard to the brain, and the spinal marrow,
which, we shall presently find, are especially guarded by a
bonv structure, enclosing them on every side, and forming
an impenetrable case for their protection. The solid mass
of bone, thus provided to defend the brain, gives also the
opportunity of lodging safely the delicate apparatus subser-

256 THE MECHANICAL FUNCTIONS.
vient to the finer senses, namely, those of sight, of hearing,
and of smell. The security which these organs derive from
this protection allows of their being carried to a higher de
gree of improvement than could be attained in the lower or
ders. There is also another advantage, of considerable moment,
which results from the internal situation of the skeleton,
namely, that it admits of an indefinite extension by growth,
without interfering with the corresponding enlargement of
the softer organs ; for we have seen that in all the instances
in which this arrangement is reversed, that is, whenever the
enclosing surfaces become solid, and can no longer yield to
the dilatation of the contained organs, no alternative remains
but that of breaking up the exterior case, and wholly cast
ing it off, to make room for the farther growth of the ani
mal; after which operation, it has to be replaced by another
covering of larger dimensions. This operation is generally
required to be performed a great number of times, before
the animal can acquire the size it is destined to attain.
Hence the perpetual moul tings ofthe caterpillar; hence the
repeated castings of the shells ofthe Crustacea; and hence
also the successive metamorphoses of the insect. Nothing '
of this kind takes place among the Vertebrata; where all the
organs are developed in regular and harmonious succession,
without the slightest mutual interference, and without those
vicissitudes of action, and of torpidity, which we witness in
the chequered existence of the insect.
§ 2. Structure and Composition of the Osseous Fabric.
The process employed for the formation and extension of
the solid frame-work of the Vertebrata differs totally from
that which we have seen exemplified in the growth of shells,
or of the hard coverings of insects and of crustaceous ani
mals. These latter structures, and the modes adopted for their
increase, are suited only to animals in which the functions
of the economy have not reached that perfection to which
they are carried in the higher classes. In the more elabo-

CHEMICAL COMPOSITION OF BONE. 257
rate system of the vertebrata, the skeleton is composed of
true bones; that is, of solid pieces, which, although they are
dense calcareous structures, yet continue organized during
the whole period of development, and form as much a- part
of the living system as any other organ of the body. We
have formerly seen that the membrane in which the calca
reous matter of the shell is deposited, should properly be
.classed among the integuments ; being analogous to them1 not
only in being situated externally, but also in their structure
and in their function. It is not so with bone, which is
essentially an internal structure.*
In their chemical composition, likewise, bones are striking
ly contrasted with the calcareous products of the Mollusca f
for in the former, the earthy portion consists almost wholly
of phosphate of lime: a material which appears to have been
selected for this purpose from its forming much harder com
pounds with animal membrane than the carbonate. Where
ver great strength and rigidity are required, this is the ma
terial depended upon for imparting these qualities? and it
has, accordingly, been employed for the osseous structures*
which are among the most elaborate results of organiza
tion. The densest and hardest of these structures are those
* De B-lainville regards the hard coverings of insects, together with the
shells of the Crustacea, as structures derived altogether from the integuments,
and as perfectly analogous, in this respect, to the scales, hoofs, ot Other horny
productions of the skin in vertebrated animals. Geoffr6y St. Hilaire con
tends, on tlie contrary, that the former constitute the true sfceleton of the
lower classes, and that a perfect analogy may be traced between the rings,
which are the essential constituents of the frame-work of annulose animals,
and the vertebra:, which enclose the spinal cord of the higher classes. Pro
fessor C'arUs appears, in his system of organic formations, to haVe kept in' view
both these analogies-; giving to the former class of structures the denomina
tion of Dermo-skeleton, and to the latter that of Neuro-akeleton, (See his Ta->
bulas Anatomiam Comparativam illustrantes,- edited by Thienemann.) Ana
logies have also been imagined to exist between the external and internal
situations ofthe woody fibres of plants belonging respectively to the endoge
nous and exogenous classes, and that of the corresponding relative situations
pf the skeletons of invertebrated and vertebrated animals. See a Memoir by
Dumortier, in the Nova Acta Pbysico-Medica Acad. C<esar. Leopold. Caro
lina: Natur. Curios. XVI. 219.)
vol. i. — 33

258 THE MECHANICAL FUNCTIONS.
in which the proportion of phosphate of lime is the greatest,
when compared with that of the animal substance which
cements them together; the force of mutual cohesion among
its own particles being much greater than that imparted by
the cementing ingredient. The internal bony portions of
the ear, where, in order perfectly to transmit the sonorous
vibrations, the greatest solidity is required; are the densest
parts of the skeleton; and phosphate of lime enters most
largely into the composition of these bones. The tympanic
portions of the temporal bone of the whale and the cachalot,
where the great size of the organ gives us advantages in ex
amining them, are as dense and as hard as marble. The
bony portions of the teeth, likewise, afford instances of very
hard calcareous formations: but the enamel, which consists
almost wholly of phosphate of lime, is harder still, and re
sembles the siliceous stones, being, like flint, capable of
striking fire with steel. 'It is scarcely necessary to point
out the obvious intentions which are fulfilled by this pecu
liarity of structure, conferring extraordinary hardness on a,
part, of which the appropriate office is that of breaking
down hard bodies subjected to their mechanical action.
But this extreme degree of crystalline hardness would be ill-
suited to other parts of the frame. In ordinary bones, ab
solute rigidity is not the quality which is alone wanted; for,
in general, the hardest bodies are also the most fragile.
An excess of rigidity, therefore, would have been attended
with brittleness, and been productive of the worst conse
quences to parts exposed to sudden and violent concussions.
It is in order to guard against this evil that an elastic ani
mal matter is employed as the basis ofthe structure, acting
as a strong cement interposed between the calcareous par
ticles. This composition of bone is rendered evident by subject
ing it to certain chemical processes. On exposure to heat,
we find it first becoming black, from the development of
the charcoal attendant upon the destruction of the animal
membrane. The oil contained in the cavities exudes, and,
taking fire, is soon totally consumed. The bone then reco
vers its whiteness, and undergoes no farther change by the

CHEMICAL COMPOSITION OF BONE. 259
action of the fire. If it be now examined, it will be found
to have lost nearly half its original weight, and to have be
come exceedingly brittle; this, as already mentioned, being
the natural property of phosphate of lime, when deprived of
its animal cement. We may perceive on the surface of a
bone so treated, a number of minute crevices, showing where
this animal substance had been situated, in its original state.
On breaking the bone across, we may also discover the size
and shape of the cavities which contained the marrow, or
oily fluid above mentioned.
It is easy to reverse this process by steeping the bone in
an acid sufficiently diluted to prevent its injuring the animal
membrane, but yet "sufficiently powerful to dissolve the
phosphate and carbonate of lime. Diluted nitric or muria
tic acids maybe used for this purpose, and will, in this way,
gradually separate the earthy particles from the membranous
portion of the bone. During the action of the acid a few
bubbles of carbonic acid gas make their appearance, indi
cating the.presenc.e of a small quantity of carbonate of lime,
which always exists in bones, intermixed with the phos
phate. The phosphate may be recovered from its solution
in the acid by precipitation with a pure alkali, such as a so
lution of ammonia. This precipitate is readily dissolved,
without effervescence, by nitric, muriatic, or acetic acids.
A small quantity of sulphuric acid may also be detected in
the fluid by the addition of nitrate of barytes. Iron, in
small quantity, is also found in the composition of human
bones. The substance which remains, after the earth has been
thus abstracted, retains the exact figure and dimensions of
the original bone, but has lost all its other mechanical pro
perties. It is soft, flexible, and elastic ; resembling in every
respect the muscular or fibrous structures, and being, like
them, resolvable into gelatin and albumen by long boiling
in water. This substance has sometimes, but erroneously,
been considered as identical with cartilage; for it has nei
ther the whiteness, nor the elasticity, nor the texture of carti
lage, nor is it at all similar to that substance in its chemical

260 THE MECHANICAL FUNCTIONS.
composition; for while cartilage is formed almost wholly of
albumen, the animal basis of bone is almost entirely resol
vable into gelatin.
Thus may a bone be analyzed into its two constituent
parts: by the process first described we obtain its earth de
prived of its animal constituent; by the second, we obtain
its membranous basis free from earth. The first of these
gives it hardness ; the second, tenacity : and thus, by the in
timate combination of these elements, two qualities, which,
in masses of homogeneous and unorganized matter, are
scarcely compatible with one another, are skilfully united.
The mechanical structure of bone is no less worthy of ad
miration, as evincing the skill with which every part is
adapted to its destined uses. The animal membrane, which,
as we have seen, is the bed in which the calcareous phos
phate is deposited, partakes of the reticular structure belong-'
ing to the ordinary cellular texture; and a bone, when mi
nutely examined, exhibits also the same appearance of plates
intermixed with fibres. In the outer compact portion, in
deed, the fibrous arrangement of the particles is not so easi
ly distinguished: but it may be detected in young bones
while they are becoming ossified : and also in bones which
have been long exposed to the weather, or long macerated
in water. The interior of most bones, in the higher classes
of animals, presents distinctly the appearance of irregular
cavities, resulting from the partial separation of the plates,
and their mutual crossings, and fibrous connexions.
The different mechanical purposes for which bones are
employed in the animal economy require them to be of dif
ferent forms. Where a part is intended to have compact
ness and strength, with a very limited degree of motion, it
is divided into a great number of small pieces, united toger
ther by ligaments, and the separate bones are short and com
pressed, approaching more or less to a cubical shape. Of
such is the column of the spine composed, as also the joints
of the wrist and ankle. Where the principal object is either
extensive protection, or the provision of broad surfaces for
the attachment of muscles, we find the osseous structure ex-

STRUCTURE OF BONE. 261
panded into flat plates; as is exemplified in the bones of the
skull, in the shoulder blade, and still more remarkably in the
bony shield which surrounds the body of the tortoise. On
the other hand, where a system of levers is wanted, as in
the limbs, which have to sustain the weight of the trunk,
and to confer extensive powers of locomotion, the bones are
modelled into lengthened cylinders, generally somewhat ex
panded at the extremities, for greater convenience of mutual
connexion. In the form, the structure, and the arrangement of these
levers, which allow of the regular and accurate application
of the moving power, and are calculated, in circumstances
so various, to give effectual support to the fabric, and also
to execute a great diversity of movements, we discern most
palpable manifestations of profound design, and the most
exquisite refinements of mechanic skill. All the scientific
principles of architecture and of dynamics are more or less
exemplified in the construction of this part of the animal
fabric. Levers of various kinds are most artificially com
bined in the formation of the fins of fishes, the wings of
birds, and the limbs of quadrupeds. The power of the arch
in resisting superincumbent pressure is exhibited in various
parts of the osseous systems of vertebrated animals; such as
the human foot, the spine, the pelvis, and more especially
in the vaulted roof of the skull, and in the carapace, or upper
shell, of the tortoise.
The construction of these levers evinces that a minute at
tention has been bestowed on every condition by which me
chanical advantage could be gained. In the more perfect de
velopments of structures, such as those which obtain in the
higher orders of mammalia, and also in the class of birds, all
the long bones are hollow cylinders, and their cavity is
largest in the middle of their length. This is shown in Fig.
172, which represents a longitudinal section of a human
thigh bone, and in Fig. 173, which is a similar section of
the humerus, or bone of the arm. The walls of these bones
consist of a dense and compact substance, formed by the
close cohesion of the osseous plates. These walls are of

262

THE MECHANICAL FUNCTIONS.

greater thickness in the middle of the shank or shaft of the
column, and become thinner as we follow them towards
172 either of the ends. This gradual diminu
tion in the thickness of the walls arises
from the continual separation of the plates,
which bend inwards, and crossing each
other, leave a multitude of irregular
spaces or cells, which are termed cancelli
The plates, proceeding from each side
obliquely inwards, at length meet each
other in the axis of the cylinder, so as to
close the middle cavity near the extremi
ties ofthe bone, where this spongy or can
cellated structure is found to occupy its
whole diameter.
Now if we consider that the principal
mechanical property required in every
cylindrical lever is rigidity, and more
especially the power of resisting forces
applied transversely, that is, tending to
break the cylinder across, we shall soon
perceive that a given quantity of materials .
could not possibly have been disposed in a manner better cal
culated for such resistance than when in the form of a tube,
or hollow cylinder.* To this mechanical principle I have
already had occasion to advert, when speaking ofthe hollow
stems of vegetables, which derive their chief strength from
their possessing this form;f and we now find it again ap
plied in the structure of bones, which, by having been made
hollow, are rendered considerably stronger than if the same
materials had been collected into a solid cylinder of the
same length. We may farther remark, that as it is in the
middle of the shaft that J.he strain is greatest, so it is here
that the cavity is largest, and the resistance most effectual.
* An elaborate mathematical demonstration of this proposition was long
ago given by Dr. Porterfield, in a paper contained in the first volume of
Medical Essays and Observations, published by a Society in Edinburgh,
p. 95.
f P. 70.

OSSIFICATION. ' 263

§ 3. Formation and Development of Bone.
But it is not enough to contemplate the purposes so admi
rably answered by these arrangements. Our curiosity'can-
not but be powerfully excited to learn what processes and
refined series of means are employed by nature to raise and
to perfect all these artificially contrived structures. It fortu
nately happens that in this- instance we are permitted to
penetrate a little farther than usual into the secrets of or
ganic evolution: for the succession of changes can be better
followed by the eye in the slow development of the harder
parts, than in the quicker growth of more yielding and ex
pansible textures. The peculiar material, also, of which
bone is formed, is easily distinguished by its hardness, its
whiteness, and its opacity, from the softer and more trans
parent animal substance with which it is intermixed. Hence
we are allowed an opportunity of observing ihe earliest stages
of its deposition, and of accurately following the subsequent
changes it undergoes.
The parts of the embryo animal, which are destined to
become bone, partake of the soft and gelatinous consistence,
which, at that early period, characterizes all the textures of
the body; and they can hardly, indeed, be distinguished »
from the semi-fluid portions which surround them. In pro
cess of time, when the vascular circulation of the blood has
been established , and the newly formed arteries have extend
ed their branches over every part of the nascent organiza
tion, those vessels which are appropriated to the task of
forming the bones, arrive at the pulpy masses where their
work is to commence. As sculptors, before working upon
the marble, first execute a model of a coarser and more plas
tic material, so the first business of these arteries is to pre
pare a model of the future bone, constructed, not with the
same material of which it is afterwards to consist, but with
another of a simpler and softer nature, namely, cartilage.
In every case, then, cartilage is first formed-,.and becomes
visible by its greater opacity when compared with the adja-

264 THE MECHANICAL FUNCTIONS.
cent jelly. It is an exact representation, in miniature, of the
bone, which is, in due course, to take its place. It is evident
that until the other parts of the fabric have proceeded so far
in their development as to have acquired a certain degree of
solidity and firmness, and to bear, as well as to require, the
support of more massive and rigid structures, this flexible
and elastic cartilage maybe employed with great advantage
as its substitute : for a hard and unyielding structure would,
in the early stages of its formation, have even been injuri
ous. But in proportion as the fabric is enlarged, the ne
cessity for mechanical support increases, and farther provi
sion must be made for resistance to external violence.
When, at length, all is prepared for the construction of
\ the bone, the next step to be taken is the removal of the
cartilage, which had been erected as the scaffolding for the
intended building. But in taking down this scaffolding, the
whole must not be removed at once; each part must be car
ried away, piece by piece, while the operation of fixing in
their position the beams and pillars of the edifice proceeds.
The way is cleared at first by the absorption of the central
part of the cartilage, and a few particles of ossific matter
are deposited in its room. While this process is going on,
greater activity is displayed in the arteries; they rapidly
. enlarge in diameter, so as to admit the colouring globules
of the blood ; and they thus become visible to the eye, which
can now follow their course without difficulty. From being
at first red points, they soon spread out into lines, of which
we trace the branches to a certain extent, although we can
not pursue them to their minuter ramifications. They now
assume more active functions, and hasten to exeeute their
task by depositing granules of calcareous phosphate: these
are laid down, particle by particle, in a certain determinate
order, and in regular lines, so as to form continuous fibres.
When a great number of these delicate fibres are gathered
together, and connected by other fibres, which shoot in va
rious directions across them, a texture composed of an as
semblage of long spicula, and thin plates, is constituted.
In the cylindrical bones, the spicula prevail, and they are

OSSIFICATION.

265

arranged longitudinally, and parallel to one another, and to
the axis of the bone. They first constitute a ring in the
middle of its length: this ring enlarges in all its dimensions,
but principally in its length; the spicula becoming larger,
not by the stretching of their parts, in consequence of the .
insinuation of fresh materials between those already depo
sited, but by the addition of new particles at both their ex
tremities. In like manner, the ring increases in thickness,
not by the deposition of phosphate of lime between the ori
ginal layers, but by the application of fresh layers on the ,
outside of those already existing. £-""
In the flat bones, the process of ossification is very simi
lar to what I have just described; only the fibres have a ra
diated arrangement, shooting out from the spot where the
first deposite took place, as from a common centre. This is
seen in Fig- 174, which represents the parietal bone of the
176

*>""

human skull,- in an early stage of its ossification', and shows
very distinctly the radiating fibres. In the cubical, and
more irregularly shaped bones, the process is, doubtless,
conducted wi^i the sa:me order and regularity, although it
cannot so readily be followed by the eye.
The same process is repeated in- different parts of the bone,, .y
wherever nature has, in conformity with determinate laws
of development,, appointed particular centres of ossification.
The bone continues to extend from each of these centres,
proceeding gradually towards the circumference, or the re^
moter parts of the cartilage, on which the ossific materials
are moulded, and by the form of which that of the future
vol. i. — 34

266 THE MECHANICAL FUNCTIONS.
bone is regulated. The process of ossification has, however,
' this peculiarity, that the cartilage is progressively absorbed
I to make room for the deposites of bony substance. When
the bone is long, separate points of ossification appear in the
extremities, before the central portions are ossified; and the
ends, thus formed into bone, are afterwards united to the
shaft, so that the whole shall form a continuous bony mass.
In the flat bones, also, if the surface be extensive, an addi
tional number of arteries are engaged to perform the work,
which is begun from several auxiliary centres of ossification,
and the completion of which is materially accelerated by
their co-operation.
This mode of increase often gives rise to a curious result,
of which a striking example is presented in the bones of the
\ skull. The brain, which these bones are designed to pro
tect, requires this protection at a very early period of life.
The- growth of so large a surface of bone, as would be re-
. quired for covering the brain, could not have proceeded
with sufficient quickness for the exigencies of the occasion,
\if it had' originated from a single point. Therefore it is
that,. besides feeing commenced at a very early age, the pro-
cess; goes on from a great number of separate points at the
same time. The ossification is evidently hurried on in order
to complete the roofing in of the edifice by the time at which
the animal is to be ushered into the world, and exposed to
dangers from the contact of external-bodies. The divergent
fibres shoot out rapidly, coalescing with those in their im
mediate neighbourhood', which co-operate to form an exten
sive bony plate. When they have reached" the prescribed
fine, they have become so much expanded' as to have lost
the power of coalescing with the fibres which have origi
nated' from other centres, and are proceeding in a contrary
direction. Yet tlie arteries still continuing to deposite ossific
matter, each set of fibre's insinuate themselves between those
ofthe opposite" set, for some little distance, and until' their
farther progress is stopped by the increasing resistance they
encounter. The consequence is that the edges of trie bones,.
which have thus met, are irregularly jagged, like the teeth
of a saw, presenting externally the zig-zag line of junction

OSSIFICATION. 267
which is a called a suture. This is seen in Figures 175 and
176, the former of which represents the upper side of the
skull of an infant; and the latter, the same bones when com
pletely ossified.
The union of bony fibres proceeding from different cen
tres of ossification is not indiscriminate, but is found to-be
regulated by definite laws, and to have certain relations to
the general plan of conformation originally established.
Each distinct bone is formed from a certain number of ossi-
fic centres, which altogether constitute a system appertain
ing to that bone only, and not extending to the adjacent
bones. These pieces unite- together, as if by a natural affi
nity; and they refuse to unite with the bony fibres proceed
ing from neighbouring centres, and belonging to other
groups. The groups themselves are not arbitrary, but are
pre-established parts of the original design. Circumstances
occasionally, indeed, arise, which may overrule this inhe
rent tendency to preserve the line of separation between
two bones ; and we then find them coalescing to form a sin
gle piece. Such unions are technically called anchyloses.
Were this the whole of what takes place in the formation
of a bone, the process would not, perhaps, differ very mate
rially from that by which a shell is produced; for a shell,
as we have seen, is the result of successive depositions of
calcareous matter, forming one layer after another, in union
with a corresponding deposite of animal membrane. But
the subsequent changes which occur, show that the constitu
tion of bone is totally dissimilar to that of shell : for no por
tion of the shell that is once formed, and has not been re-
moved,issubjectto anyfarther alteration. Itis adead, though
perhaps not wholly inorganic mass; appended, indeed, to the
living system, but placed beyond the sphere of its influence.
But a bone continues, during the whole of life, to be an in
tegrant part of the system, partaking of its changes, modi
fied by its powers, and undergoing continual alterations of
shape, and even renewals of its substance, by the actions of
the living vessels.
The form which had at first been rudely sketched, slow
ly advances towards perfection in the course of its growth;

260 THE MECHANICAL FUNCTIONS.
and the general proportions ofthe parts are still preserved;
the finished bone exhibiting prominences and depressions in
the same relative situation as at first; and not only having si
milar internal cavities, but being frequently excavated in
parts which had before been solid. During all these gradual
alterations of shape, however, there is no stretching of elastic
parts ; for all the osseous fibres and laminae are rigid and un
yielding, and in this respect retain an analogy with shell.
The changes thus observed can have been effected in no other
way than by the actual removal of such parts of the young
bone as had occupied the situations where vacuities are found
to exist in the old bone. We find, for instance, that in the
early state of a hone there are no internal cavities, but the
whple is a uniform solid mass. At a certain stage of ossifi
cation, cells are excavated by the action of the absorbent
vessels, which carry away portions of bony matter lying in
the axis of the cylindrical or in the middle layer of the flat
bones.* Their place is supplied by an oily matter, which is
the marrow, as the growth proceeds, while new layers are
deposited on the outside of the bone, and at the ends of the
long fibres, the internal layers near the centre are removed
by the absorbent vessels, so that the cavity is farther en
larged. In this manner the outermost layer of the young
bone gradually changes its relative situation, becoming more
and more deeply buried by the new layers which are suc
cessively deposited, and which cover and surround it; until
by the removal of all the layers situated nearer to the cen
tre, it becomes the innermost layer; and is itself destined in
its turn to disappear, leaving the new bone without a single
particle which had entered into the composition of the ori
ginal structure.
It has been found that by mixing certain colouring sub
stances with the food of animals the bones will spon become
deeply tinged by them. This fact was discovered acciden
tally by Mr. Belchier, who gives the following account of
* The bones of the lower class of vertebrated animals, as of Fishes and
Reptiles, seldom reach this stage of ossification, but remain solid throughout;
corresponding to the bones ofthe higher classes at the early periods of their
development.

SKELETON OF THE VERTEBRATA. 269
the circumstances that led him to notice it.* Happening
to be dining with a calico printer on a leg of fresh pork, he
was surprised to observe that the bones, instead of being
white as usual, were. of a deep red colour ; and on inquiring
into the circumstances, he learned that the pig had been fed
upon the refuse of the dying-vats, which contained a large
quantity of the colouring substance of madder. So curious
a fact naturally attracted much attention among physiolo
gists, and many experiments were undertaken with a view
to ascertain the time required to produce this change, and
to determine whether the effect was permanent or only tem
porary. The red tinge was found to he communicated much
more quickly to the bones of growing animals than to those
which had already attained their full size. Thus the bones
of a young pigeon were tinged of a rose colour in twenty-
four hours, and of a deep scarlet in three days; while in the
adult bird, fifteen days were required merely to produce the
rose colour. The dye was more intense in the solid parts of
those bones which were nearest to the centre of circulation,
while in bones of equal solidity, but more remote from the
heart, the tinge was fainter. The bone was of a deeper dye
in proportion to the length of time the animal had been fed
upon madder. When this diet was discontinued, the co
lour became gradually more faint, till it entirely disap-
peared.t § 4. Skeleton ofthe Vertebrata.
The purposes to be answered by the Skeleton, in verte
brated animals, resolve themselves into the three following:
first, the affording mechanical support to the body generally,
fend also to different portions of the body ; secondly, the pro-
* Philosophical Transactions, for 1736, vol. xxxix. 287 and 289,
-(- These experiments by no means prove, as was once supposed, that the
substance ofthe bone is renewed with every change of hue; but merely that
the colouring particles of madder, when present in the blood, readily attach
themselves to the phosphate of lime in the bones, and are as quickly washed
out again by the circulating fluid, when restored to its usual state. (See a
paper by Mr. Gibson, in the memoirs of the Lit. and Phil. Soc. of Manches
ter. Second series, i. 146.)

270 THE MECHANICAL FUNCTIONS.
viding a solid basis for the attachments of the muscles which
are to effect their movements ; and, thirdly, the giving pro
tection to the vital organs, but more particularly to the cen
tral parts of the nervous system. Of these, the last is the
circumstance that has the greatest influence in determining
the principles on which the osseous frame-work has been
constructed. In the nervous system of all the animals
coming under the denomination of vertebrata, the spinal
marrow, together with the brain, which may, indeed, be
considered as the anterior extremity of the spinal marrow,
only much enlarged by an additional mass of nervous sub
stance are the most important parts of that system, and the
organs which stand most in need bf protection from every
kind of injury. These two portions of the nervous system,
when viewed as composing a single organ, have been deno
minated the spino-cerebral axis, in contradistinction to the
analogous parts ofthe nervous system of articulated animals;
for, amidst great differences of structure and of functions, an
analogy is still retained among the several forms of the ner
vous system, characterizing these two great divisions of the
animal kingdom. In the embryo state ofthe vertebrata, the
central parts of that system consist of two separate filaments,
running parallel to each other the whole length of the body :
but, jn process of time, these two filaments unite, and con
stitute a single spinal cord : and the primary type of the ske
leton is determined by the peculiar form of this, the central
organ of the nervous system.
In laying the foundations of the skeleton, then, the first
object is to provide for the security of the spinal cord : and
this is accomplished by enclosing it within a series of carti
laginous rings, which are destined to shield it during ifls
growth, and, by their subsequent ossification, to protect it,
most effectually, from all injurious pressure. It is this part
of the skeleton, accordingly, of which the rudiments appear
the earliest in the embryo animal. These rings form a co
lumn, extending, in a longitudinal direction, along the trunk;
retracing to us the series of horny rings, in which the bodies
of worms, of insects, and, indeed, of all the Articulata, are

VERTEBRAL COLUMN.

271

incased. When ossified, these several rings are termed ver
tebrae; and the entire column which they compose is the
Spine. Fig. 177 shows the form of one of the vertebrae of
the back in the human skeleton. Fig. 178 is a side view of
four vertebrae joined together, and Fig/ 179 is a vertical sec

tion of the same part of the spine, showing the canal formed'
by the rings. From the constancy with which the spinal
column is found in all animals of this type, and from the
uniformity of the plan on which, amidst endless variations,
it is modelled, it liars been chosen as the distinctive charac
ter of this great assemblageof animals, which have, accord
ingly, been denominated the Vertebrata or Vertebrated Ani
mals. Nor is the spine of less importance when viewed in its
mechanical relations to the rest of the skeleton. It is the
great central beam- cf the fabric, establishing points of union
between all its- parts, and combining them into one conti
nuous frame- work : it is the general axis of all their motions,
or the common fulcrum on which the principal bones ofthe
extremities are made to turn : it furnishes fixed points of at
tachment to all the large muscles which act upon these bones
as levers, and, also, to those which move the trunk itself..
'/ If this column had been perfectly rigid, the whole frame
work would have been exposed to inconvenience, and1 even
danger, amidst the shocks it must encounter during all the
quick and sudden movements of the body. Not only must
its mechanism be framed to sustain these shocks, but alsoto*

272 THE MECHANICAL FUNCTIONS.
accommodate itself to various kinds of flexions, and twist-
ings of the trunk. While these objects are provided for,
care must at the same time be taken that the spinal marrow
it encloses shall, amidst all these motions, remain secure from
pressure ; for so delicate is its structure that the least degree
of compression would at once interrupt its functions, and
lead to the most fatal consequences. A safe passage is like
wise to be afforded to the nerves, which issue from the spi
nal marrow, at certain intervals, on each side throughout
its whole length.
No where has mechanical art been more conspicuously
displayed than in the construction of a fabric capable of ful
filling these opposite, and apparently incompatible functions.
The principal difficulty was to combine great strength with
sufficient flexibility. This we find accomplished, first, by
the division of the column into a great number of pieces,
each of which being locked in with the two adjoining
pieces, and tightly braced by connecting ligaments, is al
lowed but a very small degree of flexion at the point of junc
tion. This slight flexion at each single joint, however, by
becoming multiplied along the series, amounts to a consi
derable degree of motion in the whole column.
The oroad basis of each bone is connected with the next,
not by a joint, but by a plate of equal breadth (m, m, Figures
178 and 179,) composed of a peculiar substance, interme
diate in its texture to ligament and cartilage, and possessing
in a remarkable degree the qualities of toughness and ad
hesion, united with compressibility and elasticity. By yield
ing for a certain extent to a force tending to bend it to either
side, it diminishes the quantity of motion which would other
wise have been required in each individual joint; and by
acting at the same time as a spring, it softens all the jars
and concussions incident to violent action: for we find that
however the spine may be bent, no chasm is left by the flex
ions of the vertebrae upon one another, nor is the continuity
of the column in the smallest degree interrupted.
The motions of the vertebrae upon each other are farther
regulated by the mode in which their articular processes,

VERTEBRAL COLUMN. 273
which are the pieces that project obliquely on each side,
play upon each other. These processes, which are seen at
a, a, in the preceding figures (177 and 178) are of great use
in preventing the sudden displacement ofthe vertebra; for
this effect cannot be produced by any force short of that
which would occasion fracture. Any one who will try to
dislocate by sheer force, the spine of a hare or rabbit will
find reason to admire the art with which its bones have been
locked together, and the skill displayed in combining great
flexibility with such powerful resistance to every effort that
can be made to separate them.
For the purpose of allowing a passage to the spinal mar
row, the bodies of the vertebras (b, Figs. 177 and 178,) are
hollowed* out behind, into a groove, over which a broad
plate of bone is thrown from the sides of the vertebrae, like
the arch of a bridge. The succession of arches, when the
vertebrae are joined together, forms a continuous canal,
which is occupied by the spinal marrow. Notches, corre
sponding to each other, are left in the sides of each of the
arches, forming apertures for the secure passage of the
nerves as they issue from the spinal marrow. All these cir
cumstances are visible in the figures, particularly in the sec
tion, Fig. 179, where c, c, is the canal for the spinal mar
row, and in which the apertures just mentioned are distinct
ly seen, at o, o.
In order to give an advantageous purchase to the muscles
which are attached to the spine, each vertebra has, besides
the parts above described, a projecting piece of bone, ex
tending upwards from the crown of the arch, and denomi
nated the spinous process (s, s.) The sharp ridge that runs
along the middle of the back of a quadruped, is formed by
the continued series of these processes. There are also, on
the sides of the vertebrae, two other projecting pieces, which
are denominated the transverse processes (t,) and which serve
as levers for bending the column laterally, that is, either to
the right or to the left. All these component parts of the
spine are subject to considerable modifications, in different'
tribes of animals, according to the particular mechanical
vol. i. — 35

274

THE MECHANICAL FUNCTIONS.

circumstances of the system, and to the particular intentions
of their formation.
There is scarcely any part of the osseous fabric of which
the variations better illustrate the strict unity of plan and
the beautiful law of gradation observed by nature in all her
operations, than the spine. In studying the various modifi
cations which this part of the skeleton undergoes, it will be
useful to bear in mind the principles which appear to regu
late its formation, and which Geoffroy St. Hilaire has de
duced by following the history of its early growth, and no
ticing the order in which its several parts are developed.*
In common with all bones, the vertebrae take their rise from
certain determinate points, or centres of ossification, where,
at first, detached pieces of bone are formed, destined to
unite together so as to compose the entire bone. An accu
rate knowledge of the general forms and relative situations
of these elementary pieces is of much importance, because
we find that particular circumstances determine the deve
lopment of some of these parts much earlier, and to a greater
extent than other parts, and thus lead to great differences
in the shapes and proportions of various bones, at different
periods of their growth, although their origin and composi
tion are essentially the same.
The number of elements which enter into the composi
tion of a vertebra has been differently estimated by different
physiologists ;, but the following are
certainly entitled to that character.
They are represented in their relative
situations in Fig. 180. The first is
the part which forms the nucleus, or
body (b) of the vertebra; and its ossi
fication begins at the centre. Next
in importance are the two bony plates,
or leaves, as they may be called (l,
l,) which proceed from the sides of
the body, and embrace the spinal
marrow which is situated between
them. The fourth essential element

* Memoires du Museum, ix. 79 and 89.

STRUCTURE OF VERTEERjE. > 275
is the spinous process, (s,) which unites the two leaves, and
thus completes the superior arch, of which it may be re
garded as the key stone, for the protection of the spinal
marrow. Then come the two transverse processes (t, t)
which extend outwards from the sides, and with which the
arches of bone, constituting the ribs (r, r) are generally
connected. These are the six parts which may be consi
dered as the elements that are most essential, and most con
stantly present in the composition of the vertebrae. But
some other parts may*also be noticed as of very frequent
occurrence: such are the bony plates which cover the two
flat portions of the bodies of the vertebras, forming the sur
faces immediately contiguous to the intervertebral ligament;
which surfaces, in some of the lower orders of the verte
brata become articular. There is frequently, also, a deve
lopment of processes, (f,) forming arches and spines at the
lower surface of the vertebrae, or the one opposite to that
which gives rise to the superior arches already mentioned.
This structure is very generally met with in fishes, and it
is observed also in the cetacea. The arches thus formed
enclose a large artery, which is the continuation of the aor
ta, or the main artery running along the back, immediately
under the spinal column.
There are still other processes, less constantly present and
more variable in their shape. They form articular surfaces
for the purpose of being connected with the surfaces of cor
responding processes in the contiguous vertebra. Of these
there are four (a, a, a, a) belonging to each vertebra, two
in front, and two behind. These, however, should not be
included among the primary elements of the vertebras, be
cause we findvthem, in different instances, occupying differ
ent positions, and formed sometimes by extensions of the
bodies, and at other times of the leaves. In following them
through the several tribes of animals, we observe them shift
ing their places, in various ways, and not even preserving
any constancy in their number. They are wholly absent in
fishes : in the crocodile, and other reptiles, they approximate
so as to form- three articular surfaces, namely, two close to

276 THE MECHANICAL functions.
one another, and a third posterior to these. In the Orni-
thorhynchus, while the latter retains its situation in the mid
dle, the other surfaces have separated from each other, and
have travelled outwards, taking their stations upon the
leaves. In the Mammalia, the middle surface has wholly
disappeared, and the outer surfaces have risen into what are
termed the oblique processes.
In addition to these, accessory bones are often developed
to suit particular occasions. Thus, in fishes, we see that one
or two additional pieces (i) are affixed to the ends of each
spinous process. In many cases, instead of being thus placed
in a line with these processes, they appear at a little distance,
as if they had slipped from their proper situations : they are
then, found between the spinous processes, and receive the
name of interspinous bones.
The spinous processes have a tendency, when their de
velopment proceeds, to divide into two branches, and this bi
furcation frequently takes place also in the interspinous bones.
The transverse processes, likewise, occasionally develope ac
cessory pieces, as is found to be the case in some reptiles;
but, in other instances, they undergo a gradual change of po
sition, as we follow them backwards along the spinal column,
where they descend towards the abdominal region.
The flexibility of particular portions ofthe spinal column
is regulated by the size and form of its processes. When
these are much developed, they necessarily obstruct the flex
ion ofthe vertebra? in the directions in which they are situ
ated : when they are small, no such hinderance arises, and the
spine is free to move in all directions. Thus, when we see
the spinous processes much enlarged, while the transverse
processes are small, we may infer that the spine is incapable
of any bending in that direction; but that it has the power
of free lateral flexion. This is the condition of the spine of'
fishes, where this latter kind of motion is the one principally
wanted. In dolphins, and other cetacea, on the contrary,
where the actions are required to be vertically upwards and
downwards, the spinous processes are small, and the trans
verse processes very long and broad.

structure of the spine. 277
Every instance of variation in the forms of these impor
tant parts of the osseous system, will, in like manner, be
found to have a relation to some particular circumstance in
the living habits of the animal, and to be subordinate to the
general plan of its economy. But, in order to understand
the mode in which nature has effected these changes, it is
necessary to study the elements of each part of the osseous
system; for these constitute the alphabet by which the com
binations she presents to us become legible, and by which
their origin and progress are unfolded to our comprehension.
According as each of these elements of ossification receives
different degrees of development, so the different bones
they compose acquire their particular shapes and relativedi-
mensions. Sometimes, indeed, we find that one or other of
these elements has disappeared ; or, at least, we can discover
no trace of its development ; in other cases, we see it ex
ceedingly expanded, and appearing under forms of greater
complication, so as to be with difficulty identified: on some
occasions, as we have just seen in the spinous bones of fishes,
its accessory structures are multiplied, as if continued efforts
were made by the system to repeat the same structures.
Amidst all these modifications, the parts that preserve the
greatest constancy of form are those which are of most im
portance, and which are constituent parts ofthe primordial
type of the class to which the animal belongs.
The spinal column is generally prolonged 'at its posterior
extremity into a series of vertebras, which are sometimes
exceedingly numerous; decreasing in their size as they ex
tend backwards, and having continually smaller processes,
the one disappearing after the other, till all of them are lost,
and nothing remains in those at the extremity of the series
but the cylindrical bodies of the vertebras. Even these be
come stinted in their growth and ossification, until we find
the terminal pieces generally remaining in the estate of car
tilage. Such is the structure ofthe osseous support of the tail,
as seen in many quadrupeds in its most developed forms. It
illustrates the law, that when in any system there occurs a
frequent repetition of the same structure, the evolution, in

278 THE MECHANICAL functions.
the latest of those repetitions, becomes less perfect, and ends
by being abortive. In the present instance, the consequences
of this law are highly advantageous, since it provides for
the flexibility of the tail, and qualifies it for being applied
to a great variety of useful purposes, as we find more espe
cially exemplified in the Ateles, or spider monkey, and in
the' Kanguroo.
Next in importance to the spine is the cranium, or osse
ous covering of the brain; together with the bones of the
face, which protect the organs of the finer senses. An ac
curate investigation of the mode in which these bones are
formed has led many modern anatomists to the opinion
t|jpt they were originally parts of the spinal column, and
that they are, in fact, developments of vertebras, much al
tered, indeed, in shape, in consequence of the new condi
tions to which they have been subjected; but still possessing
all the essential elements of vertebras. In the embryo con
dition of these organs, and while the brain is yet undeve
loped, the resemblance of the bony circles which enclose it
to vertebras is certainly very striking; but in proportion as
the brain becomes expanded, the similarity diminishes ; for
the rapid growth of the brain in the higher orders of animals
is necessarily attended with an equally sudden expansion of
the bones of the skull. Hence, their sevefal elements are
thrown into unusual positions, and being variously distorted
and disfigured, can hardly be recognised under the strange
disguises they assume.
The extensive researches that have been recently made in
this branch of comparative anatomy, have supplied many
facts, which tend to support the hypothesis that the bony
coverings of the brain are the result of the development of
three vertebras. According to this theory, the first of these
supposed cranial vertebra, beginning our enumeration from
the neck, is the origin of the occipital bone, of which the
lower part, or that which immediately supports the cerebel
lum, corresponds to the body ofthe vertebra; the two lateral
portions to the leaves; and the upper flat plate, to the spinous
process. The body of the second cranial vertebra becomes,

SKELETON OF VERTEBRATA.

279

in process of time, the posterior half of the sphenoid bone,
which lies in the middle of the basis of the skull ; the tem
poral bones being formed by its leaves, and the parietal bones
by the lateral halves of its spinous process. The third cra
nial vertebra is constituted by the anterior half of the sphe
noid bone, which is its body, and the frontal bones, which
are its leaves. This theory, which originated with Dumerily
and /was extended by Oken, has been farther applied to the
bones of the face, by Geoffroy St. Hilaire, who conceives
them to be likewise developments of several other supposed
cranial vertebras;* but the analogies by which the hypothe
sis is supported become more feeble and confused, as we
recede from the middle of the spinal column.
All the other parts of the skeleton may be regarded as ac
cessory to the spine; and they are far from exhibiting the
same constancy either in form or number, as the vertebral
column. In some instances, as in serpents, these accessory
parts are altogether wanting; in others, they exist only in
rudimental states; and it is but in a few that they can be
considered as having reached their full development. In or
der to obtain a standard of comparison by which to estimate
all their gradations of evolution, it will be best to consider

* In this theory of G. St. Hilaire, the number of cranial vertebra is seven,.
each composed of nine elementary pieces.

280 ' THE MECHANICAL functions.
them first in their more perfectly developed forms, as they
are presented in the higher classes of quadrupeds. In the
following descriptions, the skeleton of the Hog (Fig. 181)
will be taken for the purpose of reference.
The ribs consist of arches of bone affixed at their upper
ends to the bodies of the vertebras, and also, by a separate
articulation, to their transverse processes; where, in general,
they are allowed a slight degree of motion. Their primary
use is to defend the vital organs situated in the region of the
chest, or thorax, (namely, the heart and the lungs ;) but they
are subservient also to the function of respiration, by the al
ternate movements which are given to them by their mus
cles. The two parts, of which they are composed, often form
an angle by their junction, and at this angle a process occa
sionally extends, for the purpose of forming connexions with
the neighbouring ribs.
The ribs are connected in front ^th the breast bone, or
sternum (s,) often by the intervention of cartilages, which,
from their similarity of form to the ribs, appear as continua
tions of them, and are provided apparently to eke out the re
mainder of the semicircle. These cartilages, which have
been termed the sterno-costal appendices, often become os
sified, either wholly or in part.
The sternum is formed of nine elementary pieces, each pro
ceeding from a separate centre of ossification. Two of these
occupy the end which is nearest to the head; four are lateral,
and two are situated at the opposite extremity; one only be
ing central and surrounded by the rest. Few subjects in
comparative ostelogy are more curious and instructive than
to trace the development of these several elementary parts
in the different classes of animals, from the rudimental states
of this bone as it occurs in fishes, to its greatly expanded con
ditions in the tortoise and the bird, which exhibit the most op
posite proportions of these elements.
Last in the order of constancy come the bones of the ex
tremities. As we ascend in the scale of animals we may
observe the prevalence of a tendency to the concentration
of organs, and consequently to the diminution of their num-

' SKELETON OF VERTEBRATA. 281
ber. While in animals of the inferior orders, which are
possessed of extremities, we find a considerable number of
legs; in all the animals comprised in the class of true insects
nature has limited the number to six; and in the vertebrata
it never exceeds four. As in insects, we observed that all
the legs are divided into the same number bf parts, so we
find among quadrupeds a striking correspondence in the
bones of the fore and the hind extremities. Both the one
and the other are connected with the spine by the interme
dium of large and broad bones, which are intended to serve
as a basis for their more secure attachment, and for giving,
at the same' time, extensive and advantageous purchase to
the muscles,- which are to move the limbs. The two bones
by which the anterior extremity is connected with the
trunk are the blade-bone, or Scapula, (b,) which sends out
a process called the coracoid bone; and the collar-bone, or
the Clavicle* which extends from the scapula to the ster
num. The' corresponding connecting bones of the posterior
extremity are three in number, and constitute, together with
the part of the spine to which they are attached, what is
called the Pelvis (p.) The part of the spine which is thus
included in the pelvis, is termed the Sacrum^ In its com
plete' state' of ossification it is a single bone; but it was ori
ginally composed of a number of separate vertebras, which
"have afterwards become consolidated into a single bone, and
which bea'r the marks of having been compressed from be
hind forvvards during their growth, so that they could only
expand laterally. The vertebras which succeed to these,
and* which: are not consolidated with the sacrum, compose
what is called the os coccygis, (q,) or more properly the
coccygeal vertebra: when they are' sufficiently numerous to
compose a tail, they come under the denomination of caudal
vertebrae. The three bones of the pelvis, are the ilium, the
* This bone does not exist in the skeleton of the hog; but its form and
connexions with the sternum and scapula in the human skeleton are shown
in Fig. 182, where s is the sternum; x, the xiphoid cartilage; c, the clavicle?
n, the scapula; a, the acromion; k, the cpracoid process; and g, the glenoid-
cavity for the articulation ofthe humerus.
vol. i. — 36

282 THE MECHANICAL FUNCTIONS
ischium, and the pubis. They all concur in the formation-
of a large cup-like cavity, called the acetabulum, which
receives the round head ofthe thigh bone (f,) and constitutes,-
generally, the largest joint in the body.
A single bone composes the first division of each limb,
both in the fore and hind extremities. In the fore leg. it is
termed the humerus (h,) in the hind leg, the femur (f.).
The next division contains two bones, placed parallel- to each
other; they are, in the former, the radius (r,) and the ulna
(u ;), in the latter, the tibia (t,) and fibula (f.) These are
followed by a number of small, rounded, or cubical bones,
collected together in a group, which constitutes the Carpus
(w,), in the fore leg, and the Tarsus (v) in the hind leg.
Next come; a set of long cylindrical bones, composing the
metacarpus (m,). in the former, and the metatarsus (m>) in
the latter case. In the most complete forms of development,
these are always five in number, in each limb; they are
placed generally parallel to each other, but are enveloped in
one common covering of, integument. The Phalanges, or
- toes (z,) are cylindrical bones, continued in a line from each
ofthe former: they are generally three in number in each
toe. To the last joint, which is often termed the ungual
bone, there is usually attached either a nail, a claw, or a
hoof. Small, detached bones are frequently found at the ex
terior part of the- angles which they form by their junction,
serving the purpose of giving a more advantageous position
to the tendons of the muscles which extend those joints.
The patella,, or knee pan (p,) is the largest of these, and is
pretty constantly present. Smaller bones of this description
are met with on- the joints-of the fingers, and are termed se
samoid bones.
On comparing; these divisions of the limbs of quadrupeds
with those of insects, we cannot fail to perceive that there
exists between them a marked analogy; and that naturalists
were not led away by mere fancy when they applied to the
latter the same names as those borne by the former. This,-
however, is not the only instance of analogy that may be
discovered between the structures of articulated and of ver-

SKELETON OF VERTEBRATA. 283
tebrated animals, however strong may be the contrast which
they offer in all the essential features of their conformation.
The rings which compose the skeleton of the insect, and
which enclose its principal nervous chords, have been sup
posed to have an analogy with the circles of bone which con
stitute the primary forms of the vertebras, and which con
tain the spinal chord; although, in the former case, it is true,
other viscera are included within the arches, whereas none
are contained in the latter. They agree, also, in having
the head placed at one extremity, distinct from the trunk,
and containing the principal organs of the senses. Farther
correspondences have been likewise traced in the minuter
anatomy of these parts, which it would here occupy too
much space to examine in detail.
An approximation is apparently made towards an internal
skeleton in the cephalopodous mollusca; where we find a
central body, cartilaginous in some species, calcareous in
others. In the Lo'ligo, it has a long and slender shape, and
is pointed at the end like the blade of a sword; it bears, as
we shall hereafter notice, some resemblance to the cartila
ginous spine of the fish called the Myxine, or Gastrobran-
chus, which does not enclose the spinal marrow, but only
admits it to pass along a groove in its upper edge.
All these multiplied instances, when weighed together,
and united in a comprehensive view, are sufficient to prove,
that there exist very perceptible links of connexion among
all the classes of created beings, even in those apparently
the most remote from one another. They render it clear to
the discerning eye of the philosophic naturalist, that all the
races of animated beings are members. of one family, and the
offspring ofthe same provident Parent, who has matured all
his plans on a deeply premeditated system, and who dis
penses all his gifts with the most salutary regard to the ge
neral welfare of his creatures.

( 284 )

CHAPTER VII.
/
FISHES.
In reviewing the series of animals which compose each
great division of this kingdom of nature, we constantly find
that the simplest structures and modes of progression are
those belonging to the aquatic tribes. Among vertebrated
animals, the lowest rank is occupied by Fishes, a class com
prehending an immense number of species, which are all
inhabitants of the water, which exhibit an endless variety
of forms, and open to the physiologist a wide field of in
teresting research. We cannot fail to perceive, on the most
cursory glance, the beautiful adaptation of the form and struc
ture of all these animals to the properties of the element in
which they are destined to reside. In order that the fish
might glide through the fluid with the least resistance, all
its vital organs have been collected into a small cdmpass,
and the body has been reduced into the shape of a compact
oval, compressed laterally, and tapering to a thin edge, both
before and behind, for the purpose of readily cleaving the
water as the fish darts forward, and also of obviating the re
tardation which might arise from the reflux of the water col
lected behind. With a view to diminish friction as mueh
as possible, the surface ofthe body has been rendered smooth,
and the skin impregnated with oil, which defends it from
injurious impressions, and at the same time prevents the
water from penetrating into its substance.
The body of a fish is nearly of the same specific gravity
as the water it inhabits ; and the effect of gravity is therefore
almost wholly counterbalanced by the buoyant force of that
fluid ; for the weight of a mass of water, equal in bulk to the
body itself, is the exact measure of this buoyant force. If
this weight were precisely the same as that of the fish, the
animal would be able to remain suspended in any part of the

FISHES. 285
fluid without the necessity of employing any voluntary mo
tion or exertion for that purpose; but as the body of a fish
is generally a little heavier than the fluid medium, especial
ly if it be fresh water, it is necessary for the animal to give
its body some degree of motion, in order to prevent its
sinking. In land quadrupeds, the limbs have to perform the double
office of supporting the body, and of effecting at the same
time its locomotion ; but as nearly the whole of the weight
of a fish is already sustained by the element in which it is
immersed, its instruments of motion may be employed ex-
clusively for progression, and the powerful hydrostatic pres
sure, which supports the body on all sides, supersedes the
necessity of that cohesive rigidity of frame, which is essen
tial to the safety of terrestrial animals, Hence we find that
in one whole tribe of fishes, the skeleton is composed mere*
ly of cartilage ; and, in all, it exhibits much less of the osse
ous character than in the higher classes. The frame-work
pf the skeleton, even of osseous fishes, has not the compact
ness possessed by that of quadrupeds or reptiles: the pieces
which compose it are joined together less firmly; many of
them, indeed, remain in an imperfectly ossified condition,
their elementary pieces being detached from one another, as
if the usual process of consolidation had been arrested at an
early stage. The texture of the bones of cartilaginous fishes
corresponds to this primeval condition ; for it is composed
merely of granules of calcareous phosphate, interspersed
amidst the cartilaginous substance in detached masses, or
presenting the appearance of coarse fibres, thinly scattered
through the semitransparent bone, Compared with the
quantity of gelatin which enters into their composition, the
bones of fishes contain but a small proportion of earthy in
gredient, a circumstance which explains the pellucidity of
the mass, and the readiness with which the osseous fibres it
contains can be distinguished. Another consequence ofthe
want of density in the bones of fishes is, that their articula
tions are less regular and perfect than the corresponding

286 THE MECHANICAL FUNCTIONS.
joints of terrestrial animals ; for it is evident that where the
parts are soft and flexible, joints are not required.
In the osseous fishes, the bony structures are more finished;
and they even arrive at a degree of hardness, equal to that
ofthe higher classes. But this development is not uniform in
all the bones; in the head of the pike, for instance, while
some of the bones have acquired a great hardness, others
remain wholly and permanently in a cartilaginous condition.
The bones of fishes, however advanced in their ossification,
never reach that stage of the process in which cavities are
formed ; thus there is no space for marrow, nor even for the
cellular or cancellated structure which we have noticed in
the more perfect bones/* The general disposition of the

bones which compose the entire skeleton will be understood
from Fig. 184, which represents that of the Cyprinus Carpio,
or carp. The muscular flesh of fishes is likewise softer than
that ofthe higher classes; and the cellular substance more
attenuated and more gelatinous; so that the membranes
which it forms are of a looser and more pulpy texture.
Progressive motion in fishes is effected by the simplest
means, the principal instrument employed for this purpose
being the tail ; for the fins, as we shall presently find, are
merely auxiliary organs, serving chiefly to balance the body-
while it receives its propulsion from the tail. A fish moves
in the water upon the same principle as a boat is impelled
* Cuvier, sur les Poissons. Tom. i. p. 218.

FISHES. 287
in paddling; for the action of the tail upon the water is late-'
ral, like that of an oar, which it resembles in the vertical poj
sition of its plane ; and the effect is transferred by the resist
ance of the water to the body where the
impulse originates. Let us suppose, for
example, that the tail is slightly inclined to1
}^ the right, as shown in Fig. 1 85-. ]ft in this-
situation, the muscles on the left side, tend*-
ing to bring the tail in a right line wkh the
body, are suddenly thrown into aetion, the
resistance ofthe water, by reacting against
the broad surface of the tail in the direc
tion p r, perpendicularly to that surface,
/ M \ -will cause the muscular action to give the
whole body an impulse in that direction; and the centre of
gravity, c, will move onwards in the direction c b, parallel-
to p r. This impulse is not destroyed by the farther flexion
of the tail towards the left side, because the principal force
exerted by the muscles has already been expended in the
motion from r to m, in bringing it to a straight line" with
the body ; and the force which carries it ofi to l is much
weaker, and, therefore, occasions a more feeble reaction.
When the tail has arrived at the position l, indicated by
the dotted outline, a similar action of the muscles on the
right side will create a resistance and an impulse in the di
rection of k l, and a motion of the \\4iole body in the same
direction, c a. These impulses being repeated in quick suc
cession, the fish moves forwards in the' diagonal c d, inter
mediate between the directions of the two forces. By bend
ing the whole body almost in a circle, and then suddenly
straightening it, fishes are often able to leap to the top of a
high cataract, in ascending against the stream of a river.
Such being the plan upon which progression is to be ef'
fected, we find that every part of the mechanism of the fish-
is calculated to promote its execution. The principal mus
cular strength is bestowed upon the movements, of the tail;
and the largest assemblage of muscles consistsof those which-
give it the lateral flexions that have been just described.-

288

THE MECHANICAL FUNCTIONS.

For this purpose, all the important viscera are placed for
wards, and crowded towards the head. No room is allowed
for a neck; and the abdomen may be almost regarded as
'continuous with the head, there being, properly, no inter
vening thorax; for the respiratory organs are situated rather
beneath than behind the head. All this has been done with
a view to leave ample scope for the prolonged expansion of
the coccygeal vertebras, and of their muscles, which com
pose more than half the bulk of the animal.
Having seen how all impediments to the free motion of
the tail have been carefully removed,- let us next inquire into
the mechanism by which mobility has been given to that
organ. The first peculiarity we meet with in the structure
of the spine of fishes is the mode in which the vertebrae are
connected together. The bodies of each vertebra, as may
be seen in Figures 186 and 187, are hollowed out, both be

fore and behind', (considering the spinal column as extended
horizontally,) so as to form cup-like hollows : by which
means,where the concave surfaces of two adjacent vertebras
are applied to one another, a cavity, having the shape of a
double cone, is formed by the junction of the margins of
these conical hollows. These cavities are distinctly seen
laid open in Fig. 188, which represents a vertical section of
three adjacent vertebras of a cod. The edges that are in
contact, are united all round by an elastic ligament, which
readily yields to the bending of the vertebras upon one ano-

( SKELETON OF FISHES. 289
ther by the application of any force to one side of the spine,
and restores it to its former state, when the force has ceased
to act. The extent of motion in each joint is but small; but
being multiplied in the whole series, the resulting effect is
considerable. The cavity itself is filled with a gelatinous,
but incompressible fluid substance, which constitutes a sphe
rical pivot for all the motions of the joint.
This singular kind of articulation would appear framed
with a view to allow of motion in all directions. Here,
however, the motions are restricted by the extension of the
spinous processes (s, s, in the preceding figures,) which in
fishes are of great length; so that they effectually prevent all
flexions either upwards or downwards, and limit it to those
from side to side. It is precisely these latter kind of motions
which are wanted in the fish, for striking the water laterally,
with the broad vertical surface ofthe tail. Processes of a si
milar form and appearance, (f, f,) and which impede any
flexion downwards, are generally also met with in the lower
surface of the spine, and more especially in the hinder por
tion of the column. These are the inferior spinous pro
cesses, and, like the superior, they also form an arch, through
which there passes the continuation of the abdominal aorta,
or great artery which proceeds down the back. The num
ber of vertebra is very various in different fishes: in some
they are multiplied exceedingly, as in the shark, where there
are more than two hundred.
There are few parts of the structure of animals that ex
hibit more remarkable instances of the law of gradation than
the spine of fishes, in which we may trace a regular progress
of development from- the simplest and almost rudimental
condition in which it exists in the Myxine and the Lam
prey, to that of the most perfect of the osseous tribes. Its
condition, in the former of these animals, presents a close
analogy with some structures that are met with in the mol
luscous, and even in annulose animals. So near is the resem
blance of the spinal column of the myxine, moref specially,
to the annular condition of the frame-work of the vermes,
vol. i. — 37

290 THE MECHANICAL FUNCTIONS.
that doubts have often arisen in the minds of naturalists
whether that animal ought not properly to be ranked among
this latter class. Its pretensions to be included among the
vertebrata are, indeed, but slender and equivocal; for, in
place of a series of bones composing the vertebral column,
it has merely a soft and flexible tube of a homogeneous and
cartilaginous substance, exhibiting scarcely any trace of divi
sion into separate rings, but appearing as if it were formed
of a continuous hollow cylinder of intervertebral substance,
usurping the place of the vertebrae, which it is the usual office
of that substance to connect together, and having in its axis
a continuous canal filled with gelatinous fluid. This, how
ever, is not the channel intended for containing the spinal
marrow, for that nervous cord is on the outside of this
column. The cartilage, indeed, sends out no processes to
bend round the spinal marrow, and forms no canal for its
passage and protection. The nervous matter here consists
merely of two slender cords, which run parallel to one ano
ther in a groove on the upper part of the spinal column; and
these cords are covered only by a thin membrane, the pre
sence of which it requires very minute attention to detect.
The partial protection thus afforded to so important an
organ is not greater than that given by the cartilaginous
lamina of the cuttle-fish, which in form, texture, and situa
tion, is very analogous to the spine ofthe myxine.
As we ascend from this rudimental condition of the spine,
we find it, in the lamprey, more distinctly divided into
rounded portions, appearing like beads strung together.
These rudimental bodies of vertebras have not yet completed
the cup-like hollows on their two ends, but are shaped like
rings, being perforated in the centre, so as still to form a
continuous canal throughout the whole column.
Proceeding to more advanced developments, we find, in
the sturgeon and other cartilaginous fishes, a greaterconden-
sation of substance produced by the deposition of granules
of osseous!! matter ; the central canal becomes divided into
lozenge-shaped compartments by the closing in of the sides

STRUCTURE OF FISHES. 291
of the body of each vertebra.* Frequently the sides do not
quite meet, and the leaves, which are developed from the
upper surfaces of the vertebras, now form arches over the
spinal cord, and are united above by spinous processes. Yet
the whole skeleton in these fishes remains in the incipient
stage of ossification, being more or less cartilaginous; and
where the ossific process has begun, it has not advanced the
length of producing union between the pieces formed from
the separate centres of ossification. Where they meet with
out uniting, they form no sutures, but overlap one another.
Thus the bony structures are detached, and often complete
ly isolated ; affording to the physiologist an opportunity of
studying the earlier stages of this interesting process, and
marking with distinctness the number of the elements of
each bone, and the relative situations of their centres. This
knowledge is more especially of importance towards under
standing the formation and connexions of the bones of the
head, which are very numerous and complicated ; and the
investigation of which has been prosecuted with extraordi
nary diligence by Geoffroy St. Hilaire and other continental
zootomists. It is here, more especially, that we obtain the clearest evi
dence of the derivation of the cranial bones from vertebras
analogous to those of the spine. The occipital bone, in par
ticular, corresponds to a spinal vertebra in all its essential
elements. In many fishes, the body of this bone, being
lengthened out to form the posterior part of the basis of the
skull, becomes the basilar portion. We find, on its posterior
surface, the same cup-like cavity as in the true vertebrae, '
and it is joined to the next vertebra in the same manner as
the spinal vertebras are joined to each other. Its crest has
* A small aperture still remains, establishing a communication between
the cavities the whole length of the spine. This is supposed to be designed
to obviate the compression ofthe fluid in the different cells or cavities during
the motions of the spine. The vertical sections, Fig. 189 and 190, of two
contiguous vertebrae in different fishes, will convey an idea of this gradation
of development.

292 THE MECHANICAL FUNCTIONS.
the exact shape of a spinous process. In front, the basilar
bone is united to the sphenoid bone, which, with the vaulted
roof that springs from the sides of both of these bones, like the
leaves and spinous processes of the vertebras, form together
a long cranial cavity. This cavity is placed in a direct line
with the spinal canal, and contains the nervous tubercles
which constitute the brain. Yet the brain does not com
pletely fill this cavity; for a space is still left, which is occu
pied by a pulpy substance. In like manner, the accordance
of the other cranial bones with vertebras, has been attempted
to be traced ; but in proportion as we recede from the cen
tral parts of the spine, this correspondence is less distinct, in
consequence of the various degrees of development which
these several elements have received, in order to adapt them
to particular purposes relating to sensation, to the prehen
sion and deglutition of food, and also to aquatic respira
tion. It is impossible, however, without exceeding the li
mits within which I must here confine myself, to enter into
the details of structure which would be requisite in order to
render this subject sufficiently intelligible.
The rest of the skeleton of fishes is extremely simple. In
many, as in the Ray and Tetrodon, there are no' ribs.
When the bones exist, they are articulated with the ex
tremities of the transverse processes of tjie vertebras, of
which they appear to be merely continuations, or appendi
ces. There is generally no sternum to which they can be
attached below : in a few fishes only, such as the herring
and the dory, we find rudiments of this bone, consisting of
a few pieces placed in a line on the lower part of the trunk.*
The parts of the skeleton of fishes, which correspond to
the arms and legs of quadrupeds, are the pectoral and ven
tral fins (marked respectively by the letters p and v in Fig.
184.) The former are met with, with but few exceptions,
* The bony arches arising from the skull, which support the branchiae, or
gills, have been considered as the bones corresponding to the ribs of terres
trial quadrupeds; and if this view were taken of them, it would tend to con
firm the analogy ofthe cranial bones to the spinal vertebrae.

STRUCTURE OF FISHES.

293

in all fishes; and they consist of a series of osseous pieces,
in which we may often recognise with tolerable precision
the analogous bones composing the anterior extremities of
a quadruped; such as the scapula, clavicle, humerus, ulna,
and radius.* These two latter bones are very distinctly
marked in the Lophius piscatorius, or Angler, as may be
seen in Fig. 191, where b is the scapula; c, the clavicle; u,
the ulna ; and r, the radius. The carpus may also be recog

nised in a chain of small bones, w, interposed between the
radius and the Phalanges, z. In the Ray these phalanges
are very numerous, and each is divided into several pieces
by regular articulations: these are shown in Fig. 192: they
are arranged close to one another in one plane, and form an
effectual base of support to the integument which covers
them. The scapula, according to Cuvier, is sometimes de
tached from the rest of the skeleton, and at other times con
nected with the spine: in most cases, however, it is sus
pended from the cranium ; a fact which may be cited in
* Those anatomists who are fond of pursuing the theory of analogies,
maintain that all these bones are merely developments of certain ribs, pro
ceeding from the spine in its anterior parts. A similar origin has been as
signed to the pieces of bone to which the ventral fins are attached: but it is
difficult to reconcile this theory with the fact that these bones do not pro
ceed from the spine, and are quite detached from the rest of the skeleton.
It is evident, therefore, that if they are to be considered as analogous to the
bones ofthe hinder extremities in the mammalia, they must be in a condition
of very imperfect development.

294 THE MECHANICAL FUNCTIONS.
farther corroboration ofthe analogy which the cranial bones
have to vertebras.
In the ray and the shark tribes, both the anterior and pos
terior extremities are sup
ported by arches., of bones,
forming a sort of belt. This
structure is an approach to
that which obtains in many
reptiles, and indicates a farther step in the regular progress
of development. This belt in the ray is shown in Fig. 193.
In examining that part of the skeleton of fishes which
corresponds to the posterior extremity, we observe the total
absence of both femur and tibia; but the .bones of the toes
are attached to a set of small bones, which appear to act the
part of a pelvis, but which, in consequence of their not being
connected with the spine, have no determinate situation, and
are found at various distances from the head in different
fishes. They appear emancipated from the restraints to
which they would have been subjected had they been fixed
to a sacrum, or to any particular part ofthe spine: we 'find
them, accordingly, often placed considerably forwards;
and in some instances, as in the Subbrachieni, even anteri
orly to the pectoral fins, which are the true arms ofthe ani
mal. But in one whole order of fishes, the Apodes, there is
not even a vestige of ventral fins, nor are any pelvic bones
provided for their support. This is the case with the Eel,
the Gymnotus, &c. In a few species there is also a total
absence of pectoral as well as ventral fins.
The dorsal fins are supported by a series of slender bones
(d, Fig. 184,) which are joined to the spinous processes of
the vertebras, and are formed from distinct centres of ossi
fication. These rays, as they are called, are sometimes
destined to grow to so considerable a length, as to require
being subdivided into many pieces, in order to lessen the
danger of fracture, to which a very long filament of bone
would have been exposed, and also to allow of a greater de
gree of flexibility. These rays assume branched forms from

MUSCULAR SYSTEM'S^ FISHES. 295
the farther subdivision of their parts, and when, for the pur
pose of adding strength to the fin, it becomes necessary to
multiply the points of support, intermediate bones are de
veloped, serving as the basis ofthe rays. Convenience re
quires tha^ they should be detached from the ends of the
spinous processes, which is their usual position, and placed
Between them : when in this situation, they bear the name
of interspinous bones; and when a still greater length of
osseous support is wanted, new centres of ossification are
developed at their extremities, giving rise to a series of ad
ditional pieces, joined end to end, and carrying out the in
terspinous bone, and the ray which terminates it, to a con
siderable distance. This structure is distinctly seen in the
small dorsal fins of the Mackerel. The anal fins, which
are situated on the lower side of the body, in the vertical
plane, and next to the»tail, are, in like manner, supported
by rays, having the sarne parallel, or fan-like arrangement
as the preceding. The caudal fin, or terminal expansion of
the tail, has also a similar structure.
The muscles of fishes compose a large portion ofthe bulk
of the body, but they are arranged in a less complex man
ner than those of the animals of the higher classes. Those
which appear immediately underneath the integuments are
shown in Fig. 194, where m, m, are the great lateral muscles,

producing the flexion of the body and tail : d is the dorsal fin,
which is raised by the muscle d ; p, the pectoral fin, expanded
by the muscle p : v, the ventral fin, moved by the muscles
situated at v.: a, the anal fin, in like manner moved by mus
cles at its base a: and c, the caudal fin, the muscles for
moving which are seen at c: o is the operculum, or flap,

296 THE MEt/S-kN

ICAL FUNCTIONS.

which covers the gills; and n, the nasal cavities, or organs
of smell. The form of the body, and disposition of the ske
leton, allow of their being inserted immediately on the parts
which they are intended to approximate. Hence the use of
long tendinous chords is dispensed with.*
The actions of the muscles are easily understood from the
nature of their insertions. In general, the direction of the
fibres is, in some degree, oblique, with reference to the mo
tion performed. Two series of muscles are provided for the
movements of the tail, which consist almost exclusively of
lateral flexion, the whole spine in some degree participating
in this motion. These muscles occupy the upper and lower
portions of the trunk; their limits being strongly marked by
a line running longitudinally the whole length of the body on
each side. The inclination of their fibres is somewhat diffe
rent in each. The advantage in point of velocity of action
winch results from this obliquity has already been pointed
out. Those fins which are in pairs are capable of four motions ;
namely, those of flexion and extension, and also those of ex
panding and closing the rays ; for each of which motions ap
propriate muscles are provided: and, indeed, each ray is fur
nished with a distinct muscular apparatus for its separate mo
tion; and these smaller muscles regulate with great nicety
all the movements of the fins, expanding or closing them
like a fan, according as their action is to be strengthened
or relaxed. This feathering of the fin, as it may be called,
takes place in most fishes, and is particularly observable in
the tail of the Esox, or pike tribe. Each ray of these fins,
indeed, is furnished with a distinct muscular apparatus, for
its separate motion.
Whatever analogy may exist in the structure of the fins
* Between the layers of flesh, however, there occur slender semi-transpa
rent tendons, which give attachment to a series of short muscular fibres pass
ing nearly at right angles between the surfaces of the adjoining plates.
See Sir A. Carlisle's account of this structure ip. the Philosophical Transac
tions for 1806.

SWIMMING BLADDER OF FISHES. 297
of fishes and the feet of quadrupeds, there is none in the
manner in which they are instrumental in effecting pro
gressive motion. The great agent by which the fish is im
pelled forwards is the tail : the fins, which correspond to the
extremities of land animals, are useful chiefly for the pur
poses of turning, stopping, or inclining the body, and for
retaining it in its proper position. The single fins, or those
which are situated in a vertical plane, passing through the
axis of the body, (the mesial plane,) prevent the rolling of
the body, while the fish darts forwards in its course. The
fins which are in pairs (that is the pectoral and the ventral
fins,) by their alternate flexions and extensions, act like
oars; while they are capable, at the same time, of expanding
and of closing the rays, like the opening and shutting of a
fan, according as their action is required to be effective, or
the contrary. All these auxiliary instruments are chiefly
serviceable in modifying the direction, and adjusting the
variations of force derived from the impulse of the tail.
They are employed, also, in suddenly checking or stopping
the motion, and giving it a more rapid acceleration. But
still the tail is the most powerful of the instruments for pro
gression, being at once a vigorous oar, an accurate rudder,
and a formidable weapon of offence.
Independently of these external instruments of progres
sion, mostfishes are provided with internal means of changing
their situation in the water. The structure by which this
effect is accomplished is one of the most remarkable in
stances that is met with of an express contrivance for a spe
cific purpose, and of the employment of an agency of a class
different from that of the mechanical powers usually resorted
to for effecting the same object. We have seen that if the
body of a fish were heavier than an equal bulk of water, and
if no muscular exertions were made, it must necessarily de
scend in that fluid. If, on the contrary, it were specifically
lighter, it would as necessarily rise to the surface. Were
the animal to acquire the power of altering, at pleasure, its
specific gravity, it would then possess the means of rising
vol. i.— 38

298 THE MECHANICAL FUNCTIONS.
or sinking, without calling into action either the fins or the
tail. Such is precisely the object of a peculiar mechanism,
which nature has provided in the interior of the body of the
fish. A large bladder, filled with air, has been placed im
mediately under the spine, in the middle of the back, and
above the centre of gravity. This is known by the name
of the air-bladder, or the swimming bladder, and in the
cod-fish it is called the sound. It frequently, as in the
Carp, consists of two bladders (a, b, Fig. 195) joined end

wise, and communicating with each other by a narrow neck.*
When distended with air, it renders the whole fish specifi
cally lighter than the surrounding water ; and the fish is
thus buoyed up, and remains at the surface without any ef
fort of its own. On compressing the bladder, by the action
of the surrounding muscles, the included air is condensed,
the specific gravity of the whole body is increased, and the
fish sinks to the bottom. On relaxing the same muscles,
the air recovers its former dimensions, and the fish is again
rendered buoyant. Can there be stronger evidence of de
sign than the placing of this hydrostatic apparatus, acting
upon philosophical principles, in the interior of the organi
zation, for a purpose so definite and unequivocal 1
In several tribes of fishes there is a canal (c d) establish
ing a communication between this bladder and the stomach,
or the gullet (o ;) so that by compressing the bladder, a quan
tity of air may be forced out, and a very sudden increase of
* There is great variety in the form and structure of the air-bladder in
different fishes. Sometimes it contains a large glandular body of a peculiar
structure, which has been conjectured to be an apparatus for secreting air
from the blood: but this is by no means very generally met with.

SWIMMING BLADDER OF FISHES. 299
specific gravity produced ; followed, of course, by a quick
descent. When, by any accident, the air bladder has been
opened, or has burst, so that all the air has escaped, the fish
is seen to grovel at the bottom, lying on its back, and can
never afterwards rise to the surface. On the other hand, it
occasionally happens that a fish which has remained too long
at the surface of the sea, exposed to the scorching rays of a
tropical sun, suddenly finds itself retained against its will at
the surface, because the bladder has become over distended
by the heat, and resists all the efforts which the animal can
make to compress it. It thus continues' floating, until the
coolness of the night has again condensed the air in the blad
der to itsformer bulk, and restored'the power of descending.
Some tribes of fish are totally unprovided with an air-
bladder. This is the case with the flounder, the sole, and
other genera of a flat shape, forming the family of Pleuro-
nectes. They are chiefly inhabitants of sand-banks, or other
situations where they are comparatively stationary, seldom
moving to- a distance, or rising much in the water ; and
when they do so, it is with manifest effort, for their ascent
must be accomplished entirely by the continued beating and
flapping of the water with their expanded pectoral fins. It
is only the larger fish of this form, such as rays, which have
very voluminous and powerful pectoral fins for striking the
water down-wards with considerable force, that can rise with
facility without the assistance of an air-bladder. In these,
the lateral fins, whieh are enormous expansions of the pec
toral fins, may be compared to wings,- their vertical action
on the water being similar in effect to the corresponding
movements of a bird, when it rises vertically in the air.
Those fishes whieh swim rapidly, and frequently ascend and
descend in the water, are, in general, provided with the
largest air-bladders.
In studying the varieties presented by the forms of the
fins in different tribes of fishes, we find the same constant
relation preserved with the particular situations and circum
stances in which they are placed. The dorsal fins, which

300 THE MECHANICAL FUNCTIONS.
are more especially useful for steadying the body, are long
est in those fishes which inhabit the most stormy seas. The
most voracious tribes, which incessantly pursue their prey,
are furnished with most powerful muscles, and possess the
greatest means of rapid progression. On the other hand,
many of the more pacific, and weaker species are studiously
guarded by a dense and hard integument, serving as a shield
against the attacks of enemies, and often armed with sharp
points, which are sufficient to repel the most daring assail
ant. The Balistes is covered with scales of singular hard
ness, closely set together, and frequently having rough
edges. The Ostracion, or trunk fish, instead of these scales,
is provided with a kind of coat of mail, composed of osseous
plates, curiously joined together, like a tesselated pavement,
and reminding us of the arrangements we have seen adopted
in the calcareous coverings of the echinida.
Some of the cartilaginous fishes are, in like manner, pro
tected by calcareous plates, appended to the integuments.
There is a row of plates of this kind,, of a quadrangular
shape, which passes along, the middle of the back in the stur
geon: and the whole body ofthe Ostracion, or Trunk-fish,
is covered with osseous scales. All these have no imme
diate relation to the skeleton, but are apparently remnants
of inferior types, of which one of the prevailing characters
is the external situation of the protecting organs.
Diodons and Tetrodons are remarkable for being provided
with the means of suddenly assuming a globular form, by
swallowing air,which, passing into the crop or first stomach,
blows up the whole animal like a balloon. The abdominal
region being thus rendered the lightest, the body turns over,
the stomach becoming the uppermost part; and the fish floats
upon its back, without having the power of directing itself
during this state of foreed distention. But it is while lying
thus bloated and passive, at the mercy of the waves, that this
animal is really most secure; for the numerous spines, with
which the surface of the body is universally beset, are raised
and erected by the stretching out of the skin, thus present-

MOVEMENT OF FISHES. 301
ing an armed front to the enemy, on whatever side he may
venture to begin the attack.
There is a numerous family of fishes, found in the seas of
India, so constructed as to be able to crawl on land to some
distance from the shore. One of these, the Perea scandens,
is even capable of climbing on the trees which grow on the
coast.* If we consider the density of the medium which fishes
have to traverse, the velocity with which they move will ap
pear surprising. They dart through the water with ap
parently as much ease and rapidity as a bird flies through
the air. Although this may partly be accounted for by the
size of their muscles, and the advantageous mode of their in
sertion, yet these advantages would avail but little, were it
not for the sudden manner in which their power is exerted.
Where the great length and flexibility of the spine tend to
impair the force with which the tail strikes the water, the
resulting motion is slow and desultory, as is the case with
eels, and other fishes of the same elongated construction.t
Most fishes, however, move with the utmost rapidity, and
with scarcely any visible effort; and perform long journeys
without apparent fatigue. The Salmon has been known to
travel at the rate of sixteen miles an hour for many days to
gether. Sharks often follow ships across the Atlantic, not
only outstripping them in their swiftest sailing, but playing
round them on every side, just as if the vessel were at rest.
* See the account given by Lieutenant Daldorff; Linnean Transactions, III.
62. I shall have occasion to notice, in the sequel, the, remarkable conforma
tion of the respiratory organs of these and other fishes, which enable them
to live, for a time, out of their natural element.
f Carlisle, Phil. Trans, for 1806, p. 9.

( 302 )

CHAPTER VHI.

RF.PTILIA.

$ 1. Terrestrial Vertebrata in general.
The numerous tribes of vertebrated animals which are
strictly terrestrial, or destined to mOve on land, differ widely
in their modes of progression, and in the mechanical advan
tages of their formation. The .greater number are quadru
peds; some formed for climbing trees, others for burrowing
in the earth; some for treading on sandy plains, some for
scaling precipices. A few seem scarcely capable of ad
vancing ; others outstrip the winds in fleetness. Some fami
lies of reptiles are entirely destitute of any external organs
of motion, the whole trunk of the body restingon the ground:
while man occupies a place where he stands, alone, being
distinguished by the exclusive faculty of permanently sus
taining himself on the lower extremities.
In reviewing the developments and the mechanical func
tions exhibited by so great a diversity of structures, I shall
commence with an examination of those amphibious reptiles
which appear to form an intermediate link in the chain con
necting the strictly aquatic, with the terrestrial vertebrated
animals: then, taking up this latter series, I shall consider
the more simple conformation, and less perfect motions of
terrestrial animals destitute of limbs; and gradually ascend
to those in which the support and progression of the body is
effected by extremities, more and more artificially formed :
concluding with the human structure, which terminates this
extensive series.

BATRACHIA.

303

§ 2. Batrachia.
The order of Batrachia, or Amphibious Reptiles, con
stitutes the first step in the transition from aquatic to terres
trial vertebrata. It is more particularly the function of res
piration that requires to be modified, in consequence of the
change of element in which the animal is to reside; and as
if it had been necessary, conformably to the laws of animal
creation, that this change should not be abruptly made, we
find that Batrachian reptiles, with which this series com
mences, are constructed, at -first, on the model of fishes;
breathing the atmospheric air contained in the water by
means of gills, and moving through the fluid by the same
instruments of progression as fishes, which, indeed, they ex
actly resemble in every part of their mechanical conforma
tion. The tadpole, which is the young of the frog, is, at
first, not distinguishable in any circumstance of its internal
skeleton, or in the disposition of its vital organs from the
class of fishes. The head, indeed, is enlarged, but the body

immediately tapers to form a lengthened tail, by the pro
longation of the spinal column, which presents a numerous
series of coccygeal vertebras, furnished with a vertical ex
pansion of membrane to serve as a caudal fin, and with ap
propriate muscles for executing all the motions required in
swimming. The appearance of the tadpole, in its early
stage of development, is seen in Fig. 197 and 198, the

304 THE MECHANICAL FUNCTIONS.
former being a side, and the latter an, upper view of that
animal. Yet, with all this apparent conformity to the structure of
a strictly aquatic animal, the tadpole contains within its
organization the germs of a higher development. Prepara
tions are silently making for a change of habitation, for the
animal's emerging from the waters, for the reception of at
mospheric air into new cavities, for the acquisition of limbs
suited to new modes of progression; in a word, for a terres
trial life, and for all the attributes and powers which belong
to quadrupeds. The succession of forms, which these meta
morphoses present, are in themselves exceedingly curious,
and bear a remarkable analogy to the progress of the trans
formations of those insects, which in the first stages of their
existence are aquatic. To the philosophic inquirer into the
marvellous plans of creation, the series of changes which
mark these singular transitions cannot fail to be deeply in
teresting; and occurring, as we here find them, among a tribe
of animals allied to the more perfect forms of organization,
they afford us a better opportunity of exploring the secrets
of their development by tracing them from the earlier stages
of this complicated process, so full of mystery and of won
der. The egg of the frog (Fig. 196) is a round mass of trans
parent nutritive jelly, in the centre of which appears a small
black globule. By degrees this shapeless globule exhibits
the appearance of a head and tail, and in this form it emerges
from its prison, and moves briskly in the water. _ From
the sides of the neck there grow out feathery tufts, (Fig.
198, b, b,) which float loosely, and without protection, in
the surrounding fluid. These, however, are mere tempo
rary organs, for they serve the purposes of respiration only
until the proper gills are formed, and they then shrink and
become obliterated. The true gills, or branchia are con
tained within the body, and are four in number on each
side, constructed on a plan very similar to those of fishes.
Retaining this aquatic constitution, the tadpole rapidly in-

DEVELOPMENT OF THE BATRACHIA. 305
creases in size and in activity for several weeks. In the mean
time the legs, of which no trace was at first apparent, have
commenced their growth. The hind legs are the first to
make their appearance, showing their embryo forms with
in the transparent coverings of the hinder part of the
trunk, just at the origin of the tail. These are soon suc
ceeded by the four legs, which exactly follow the hind
legs, in all the stages of their development, until they
have acquired their due proportion to the size of the
trunk. The animal, at this period, wears a very ambiguous
appearance, partaking of the forms both of the frog and of
the lizard, and swimming both by the inflections of the tail,
and the irregular impulses given by the feet. This inter
val is also employed by this amphibious being, in acquiring
the faculty of respiring atmospheric air. We observe it
rising every now and then to the surface, and cultivating its
acquaintance with that element, into which it is soon to be
raised ; occasionally taking in a mouthful of air, which is re
ceived into its newly developed lungs, and afterwards dis
charging it in the form of a small bubble: When the ne
cessary internal changes are at length completed, prepara
tions are made for getting rid of the tail, which is now a
useless member, and which, ceasing to be nourished, dimi
nishes by degrees, leaving only a short stump, which is soon
removed. The gills are by this time shrunk, and rapidly
disappear, their function being superseded by the lungs,
which have been called into play; and the animal now
emerges from the water, and begins a new mode of existence,
having become a perfect frog, (Fig. 199.) It still, however,
retains its aquatic habits, and swims with great ease in the
water by means of its hind feet, which are very long and
muscular, and, of which th'e toes are furnished with a broad
web, derived from a thin extension of the integuments.
No less curious are the changes'Which take place in all the
other organs for the purpose of effecting the transformations
rendered necessary by this entire alteration in all the ex
ternal circumstances of that animal, — this total reversal of
vol. i.— 39

306

THE MECHANICAL FUNCTIONS.

its wants, of its habits, of its functions, and of its very con
stitution. I shall have occasion to notice several of these
transitions when reviewing the other functions of the animal
economy: but at present our concern is chiefly with the
structure of the frame in its mechanical relations to progres
sive motion. In order to form a correct idea of these re- y
lations, it will be necessary to notice the leading peculiari
ties of the skeletons of this tribe of animals.
The skeleton of the adult frog is shown in Fig. 200; from
which it will be seen that the spinal column is comparatively

much shorter than that of fishes, or, indeed, of any other
class of animals; for.it consists of only eight vertebras, ex
clusive of those-which have united to form the os coccygis.
It was evidently the intention of nature to consolidate the
frame-work of the trunk, in which flexibility was not re
quired for progressive motion: the performance of that func
tion being transferred to the hind extremities, which are ex
ceedingly large in proportion to the rest of the body. There
is a tendency in every part of the skeleton to develope itself
in a transverse direction, while the trunk is shortened as
much as possible.
The mode in which the vertebras are articulated together,

SKELETON OF TIIE BATRACHIA. 307
differs widely from what we have seen in fishes, and ap
proaches to the structure of the higher classes of vertebrata.
The body of each vertebra, instead of having at its posterior
surface a cup-like cavity, terminates by a projecting ball,
which is received into the qavity in the anterior surface of
the next vertebra, so as to compose a true ball and socket
joint, capable, when other circumstances permit, of a rota
tory motion. But the vertebras of the tadpole, as we have
seen, are constructed on the model of those of a fish ; that is, ^
have cup-like cavities on both their surfaces, which play on
balls of soft elastic matter, interposed between them. We
should naturally be curious to learn the mode in which the
transition from this structure to that of the frog is accom
plished. By carefully watching the progress of ossification,
while this change is taking place, Dutrochet found that the
gelatinpus ball, on which both the adjacent vertebras play in
the tadpole, becomes gradually more solid, and is converted
into cartilage. This cartilage afterwards becomes united by ,
its anterior surface to the vertebra which is in fronj; of it ;
and the whole then becomes ossified, so as to compose only
one bone, its posterior surface remaining distinct, and con
tinuing to play within the cup-like hollow of the vertebra
which is behind it. The cartilaginous coccygeal vertebras
of the tadpole are lost long before there is time for their
being ossified; but those nearest to the body are consolidated
into one long and straight os coccygis, which, being joined
to the sacrum at an angle, gives rise to the strange deformi
ty observable at that part ofthe back of a frog; for it here
looks as if it had been broken. The spinal cavity is, at the
same time, obliterated ; that portion of the spinal marrow
which had passed through it, in the aquatic life of the ani
mal, being now withdrawn.
The theory of the spinal origin of the cranial bones re-
' ceives considerable support from their structure and relative
position in the skeleton of the frog. The cavity for the
lodgement of the brain, which is enclosed by these vertebras,
is perfectly continuous in the same fine with the spinal ca- v

308 THE MECHANICAL FUNCTIONS.
nal, which, indeed, it scarcely exceeds in its diameter. -The
bones of the face, are, at the same time, expanded laterally,
so as to bear no proportion to the cranial cavity. The head
plays on the vertebral column by two lateral articular sur
faces, formed upon the root of each leaf of the occipital bone,
while its body, or basilar portion, is scarcely eonnected with
the first cervical vertebra, and has no articular' surface.
In place of -ribs, we find only small, slender, detached
bones,, or rather cartilages, affixed to the extremities of the
transverse processes of some of the vertebras. They may
be regarded as rudimental ribs.*
The pelvis consists of two slender and elongated iliac bones,
which are extended backwards, and which, at their anterior
extremities, merely touch the points qf the transverse pro
cesses ofthe lastvertebra of the back. This vertebra is much
broader than the rest, and, although it consists but of a single
'bone, must he considered as a sacrum. The two pubic and
ischiatic bones are exceedingly small, but still contribute to
form the acetabulum, or cavity for the reception of the thigh
bone, at the hinder extremity of the slender bones above
mentioned. This is the simplest possible form to which the
pelvis can be reduced, while it preserves its attachments to
the spine. It presents, in this respect, a more advanced
stage of development than that of fishes.
The connexion of the bones of the anterior extremities
with the spine is analogous to that which takes place in rays
and sharks : there being an osseous belt formed by the sca
pula, clavicle, and coracoid bone, with the latter of which
the humerus is connected. The sternum is large and con
siderably developed; making some slight approach to the
expansion it receives in the Chelonia. The radius and ulna
are united into one bone : the bones of the arm and leg. in
* The plan of reproduction, jn these animals requires that the ovary, or or
gan which contains the eggs, should be capable of enormous dilatation, in
order to contain the immense bulk to which these eggs are expanded, pre
viously to their being brought forth. It was probably in order to make room
for this dilated ovai-y that the ribs have not been developed.

PROGRESSIVE MOTION OF BATRACHIA. 309
general, resemble, in their figure and connexions, those of
the higher orders of Mammalia, to the type of which this
order of reptiles evidently approximates. There -are five
toes in the foot, with sometimes the rudiment of a sixth:
the anterior extremity has only four toes, which are with
out claws.
The necessity of employing the same instruments for pro
gression in the water and on land, is probably the cause
which prevents their having the form best adapted for ei
ther function. The hind feet of the frog, being well con
structed for striking the water backwards in swimming, are,
in consequence, less capable of exerting a force sufficient to
raise and support the weight of the body in walking ; and
hence this animal is exceedingly awkward in .its attempt
" to walk. On a short level plane it can proceed only by
leaps; an action which the length and great muscularity of
the hind legs particularly fit it for performing. The toad,
on the other hand, whose hind legs are short and feeble,
walks better, but does not jump or swim so well as the
frog.* The Hyla, or tree-frog, has the extremities of each
of its toes expanded into a "fleshy tubercle, approaching in
the form of its concave surface to that of a^ucker, and by
the aid of which it fastens itself readily to the branches of
trees, which it chiefly inhabits, and along which it runs with
great agility.
The Salamander is an animal of the same cla-ss as the
frog, undergoing the same metamorphoses from the tadpole
state. It differs much, however, in respect to the develop
ment of particular parts of the skeleton. The anterior ex^
. * It is singular that the frog, though so low in the scale of vertebrated ani
mals, should bear a striking resemblance to the human conformation in its or
gans of progressive motion. This arises from the exertions which it makes
in swimming being similar to those of man in walking, in as far as they both
result from the strong action of the extensors of the feet. Hence, we find a
distinct calf in the legs of both, produced by the swelling of similar muscles.
the muscles of the thigh present, also, many analogies with those of man;
particularly in the presence of the long muscle called the sartorius, the use
of which is to turn the foot outwards, both in stepping and in swimming.

310 THE MECHANICAL FUNCTIONS.
tremities of the salamander make their appearance earlier
than the hind legs, and the tail remains as a permanent part
ofthe structure. The rudimental ribs are exceedingly small,
and the sternum continues cartilaginous. The pelvis has no
osseous connexion with the spine, but is merely suspended
to it by ligaments. The land salamanders have a rounded
tail, but the aquatic species, or Tritons, have it compressed
vertically ; thus retaining the fish-like form of the tadpole,
and the same radiated disposition of the muscles.
\ 3. Ophidia.
In the class of serpents we see exemplified the greatest
possible state of simplicity to which a vertebrated skeleton
can be reduced; for, as may be seen in Fig. 201, which
shows the skeleton of a viper, it consists merely of a length
ened spinal column, with a head but little developed, and a
series of ribs; but apparently destitute of limbs, and of the

bones which usually connect those limbs with the trunk;
there being neither sternum, nor scapula, nor pelvis. Pro
fessor Mayer has, however, traced obscure rudiments of
pelvic bones in the Anguis fragilis, the Anguis ventralis, and
the Typhlops crocotatus, and is of opinion that they exist
much more generally in this order of reptiles than has been
commonly imagined. Some serpents, as the Boa, Python,
Tortryx, and Eryx, have claws, which may be considered as
rudiments of feet, visible externally. In others, as the An
guis, Typhlops, and Amphisbana, they exist concealed under

SERPENTS.

311

the skin. In others, he has discovered cartilaginous fila
ments, which he conceives to correspond to these parts.*
In the conformation of the skull and bones of the face,
serpents present strong analogies with batrachia n reptiles,
and also with fishes, one tribe of which, namely, the apodous
or anguiliform fishes, they greatly resemble by the length
and flexibility of the spine. These' peculiarities of confor
mation may be traced in a great measure to the mode of
life for which they are destined. The food assigned to them
is living. prey, which they must attack and vanquish before
they can convert it into nourishment. The usual mode in
which the boa seizes and destroys its victims is by coiling
the hinder part of its body round the trunk or branch of __
a tree, keeping the head and anterior half of the body
* Some of these rudimental parts are represented in the following figures.
Fig. 203 exhibits the claw of the Boa constrictor, placed at the termination
of a series of bones, representing very imperfectly the bones of the lower
extremities. Fig. 204 shows the muscles attached to these small bones.

203

The three following figures, 205, 206, and 207, represent the daws and ru
dimental bones of the Tortrix scytale, Tortrix corallinus, and Anguis fragi
lis, respectively. Thoseof the Amphisboena alba, Fig. 208, and the Coluber
pullaius, Fig. 209, are still less developed. The Chakides, or snake lizard,
which has four minute feet, is represented in Fig. 210. (Ann. des Sc. Nat.
vii. 170.)

312

THE MECHANICAL FUNCTIONS.

disengaged; and then, by a sudden spring, fastening upon
the defenceless object of its attack, and twining round its
body, so as to cpmpress its chest, and put a stop to its res
piration. Venomous serpents, on the other hand, coil them
selves into the smallest possible space, and suddenly darting
upon the unsuspecting or fascinated straggler, inflict the
quickly fatal wound.
It is evident, from these considerations, that, in the ab
sence of all external instruments of prehension and of pro
gressive motion, it is necessary that the spine should be
rendered extremely flexible, so as to adapt itself to a great
variety of movements. This extraordinary flexibility is
given, first, by the subdivision of the spinal column into a
great, number of small pieces; secondly, by th'e great free
dom of their articulations ; and thirdly, by the peculiar mo
bility and connexions of the ribs.
Numerous as are the vertebras of the eel, the spine of
which consists of above a hundred pieces, that of serpents is
in general formed of a still greater number. In the rattle
snake (Crotalus horridus) there are about two hundred ver
tebras; and above three hundred
have been counted in the spine
of the Coluber natrix. These
vertebrae are all united by ball
and socket joints, as in the
adult batrachia; the posterior
rounded eminence of each ver
tebra being received into the
anterior surface of the next.
Fig. 202 is a view of this por
tion of the skeleton in the Boa
constrictor, showing also the ar
ticulation of the ribs with the
vertebras.
While provision has thus been made for extent of mo
tion, extraordinary care has at the same time been bestowed
upon the security of the joints. Thus, we find them effectu-

SERPENTS. 313
ally protected from dislocation by the locking in, above
and below, of the articular processes, and by the close in
vestment of the capsular ligaments. The direction of the
surfaces of these processes, and the shape and length of the
spinous processes, are such as to allow of free lateral flex
ion, but to limit the vertical and longitudinal motions; and
whatever degree of freedom of motion may exist between
the adjoining vertebras, that motion being multiplied along
the column, the flexibility of the whole becomes very great,
and admits of its assuming every degree and variety of
curvature. The presence of a sternum, restraining the mo
tions of the ribs, would haye impeded all these movements,
and would have also been an insurmountable bar to the di
latation of the stomach, which is rendered necessary by the
habit of the serpent of gorging its prey entire.
The mode in which the boa exerts a powerful pressure on
the bodies of the animals it has seized, and which it has en
circled within its folds, required the ribs to be moveable la
terally, as well as backwards, in order to yield to the force
thus exerted. The broad convex surfaces on which they
play give them, in this respect, an advantage which the or
dinary mode of articulation would not have afforded. The
spinous processes in this tribe of serpents are short and wide
ly separated, so as to allow of flexion in every direction. In
the rattle-snake, on the other hand, their length and oblique
position are such as to limit the upward Dending of the spinal
column, although, in other respects, its motion is not restrict
ed. The vertebras at the end of the tail are furnished with
broad transverse processes for the attachment of the first
joints of the rattle.
But of whatever variety of flexions we may suppose the
lengthened body of a serpent to be capable, it will, at first
view, be difficult to conceive how these simple actions can
be rendered subservient to the purposes of progression on
land: and yet experience teaches us that few animals ad
vance with more celerity on the surface of the ground, or
dart upon their prey with greater promptitude and precision.
vol. i. — 40

314 THE MECHANICAL FUNCTIONS.
\They raise themselves without difficulty to the tops of the
highest trees, and escape to their hiding-places with a quick
ness which eludes observation and baffles the efforts of their
pursuers. The solution of this enigma is to be sought for partly in
the structure of the skin, which, in almost every species, is
covered with numerous scales; and partly in the peculiar
conformation ofthe ribs. The edges ofthe scales form rough
projections, which are directed backwards, so as to catch the
surfaces of the bodies to which they are applied, and to pre
vent any retrograde motion. In some species, the integu
ment is formed into annular plates, reminding us ofthe struc
tures so prevalent among worms and myriapode animals.
Each scale is connected with a particular set of muscular
fibres, capable of raising or depressing it, so that, in this way,
it is converted into a kind of toe; and thus the body rests
upon the ground by numerous fixed points of support.
This support is farther strengthened by the connexion of
the ribs with the abdominal scuta, or the scales on the under
side of the body. The mode in which the ribs become aux
iliary instruments of progressive motion was first noticed by
Sir Joseph Banks.* Whilst he was watching the movements
of a Coluber of unusual size, which was exhibited in London,
and was moving briskly along the carpet, he thought he saw
the ribs come forward in succession, like the feet of a cater
pillar. Sir E verard Home, to whom Sir Joseph Banks pointed
out this circumstance, verified the fact by applying his hand
below the serpent, and he then distinctly felt the ends of the
ribs moving upon the palm, as the animal passed over it.
The mode in which the ribs are articulated with the spine is
peculiar, arid has evidently been employed with reference to
this particular function of the ribs, which here stand in place
of the anterior and posterior extremities, possessed by most
vertebrated animals, and characterizing the type of their os
seous fabric. In the ordinary structure, the head of each rib
has a convex surface, which plays either on the body of a
* Philos, Trans, for 1812, p. 163.

PROGRESSIVE MOTION IN SERPENTS. 315
single vertebra with which it is connected, or upon the two
bodies of adjacent vertebras ; but in serpents the extremity
of the head of the rib has two slightly concave articular sur
faces, which play on a convex protuberance of the vertebra.
This structure is attended with the advantage of preventing
the ribs from interfering with the motions of the vertebras
upon one another. At their lower ends the ribs of one side
have no connexion with those of the other, nor are they
joined to any bone analogous to a sternum ; for, except in the
Ophiosaurus and the Blind-worm (Anguis fragilis,) there is no
vestige either of a sternum or scapula, in any animal of this
class. Each rib terminates in a slender cartilage, tapering
to a point, which rests, for its whole strength, upon the upper
surface of one of the scuta, or broad scales on the lower side
of the body. These scuta, which are thus connected with
the ends of the ribs, and which are moved by means of short
muscles, may be compared to hoofs, while the ribs themselves
may be considered as performing the office of legs. The
ribs move in pairs ; and the scutum under each pair, being
carried along with it in all its motions, and laying hold of the
ground by its projecting edge, becomes a fixed point for the
advance of the body. This motion, Sir E. Home observes,
is beautifully seen'when a snake is climbing over an angle to
get upon a flat surface. When the animal is moving on a
plane, it alters its shape from a circular or oval form, to one
that approaches to a triangle, of which the surface applied
to the ground forms the base. Five sets of muscles are pro
vided for the purpose of giving to the ribs the motions back
wards and forwards, by which, as levers, they effect this
species of progression. These muscles are disposed in regular
layers; some passing over one or two ribs to be attached to
the succeeding rib. In all snakes the ribs are continued
backwards much beyond the region occupied by the lungs ;
and although the anterior set are subservient to respiration,
as well as to progressive motion, it is evident, that all those
posterior to the lungs must be employed solely for the latter
of these purposes.

316 THE MECHANICAL FUNCTIONS.
It is easy to understand how the serpent can slowly ad
vance, by this creeping, or vermicular motion, consisting in
reality of a succession of very short steps. But its progress
is accelerated by the curvatures into which it throws its
body; the fore part being fixed, and the hind part brought
near to it; then, by a reverse process, the hind part being
fixed, and the head projected forwards. By an alternation of
these movements, assisted by the actions of the ribs, serpents
are enabled to glide onwards with considerable rapidity,
and without attracting observation. But where greater ex
pedition is necessary, they employ a more hurried kind of
pace, although one which exposes them more to immediate
view. The body, instead of being bent from side to side, is
raised in one great arch, of which the two extremities alone
touch the ground; and these being alternately employed as
points of support, are made successively to approach and to
separate from each other, the body being propelled by bring
ing it from a curved to a straight line.
There is yet a third kind of motion, which serpents oc
casionally resort to, when springing upon their prey, or
when desirous of making a sudden escape from danger..
They coil themselves into a spiral, by contracting all the'
muscles on one side of the body, and then, suddenly throw
ing into violent action all the muscles on the opposite side,
the whole body is propelled, as if by the release and un
winding of a powerful spring, with an impulse which raises
it to some height from the ground, and projects it to a con
siderable distance.
Thus these animals, to which nature has denied all ex
ternal members, are yet capable, by the substitution of a dif
ferent kind of mechanism, still constructed from the elements
belonging to the primitive type of vertebrated animals, of
silently gliding along the surface of the earth, of creeping
up trees, of striding rapidly across the plain, and of exe
cuting leaps with a vigour and agility which astonish the
beholder, and which, in ages of ignorance and superstition,
were easily ascribed to supernatural agency.

SAURIAN REPTILES. 317

§ 4. Sauria.
The conformation of those parts of the frame which are
subservient to progressive motion becomes more perfect in
the class of Saurian reptiles, which includes all the Lizard
tribes. Several links of connexion with the preceding, class
may still be noticed, marking the progress of development,
as we follow the ascending series of animals. Rudiments
of the bones of the extremities, and, also, of the sternum,
make their appearance very visibly in the Ophiosaurus,
and in the blind worm, (Anguis fragilis.) The Siren la-
certina has two diminutive fore feet, placed close to the
head. The Lacerta lumbricoides of Linnssus, or the Bipes
canaliculars of Lacepede, which is found in Mexico, and'
of which a specimen is preserved in the collection at Paris,
has a pair of very short feet, also placed near the head, and
divided into four toes, with the rudiment of a fifth. The
Lacerta bipes (Linn.) or Sheltopusic of Pallas, has, on the
other hand, a pair of hind feet only, but extremely small, to
gether with rudiments of a scapula and clavicle concealed
under the skin. Next in order must be placed the Chal-
cides, or Snake-lizard, (Fig. 210,) and the Lacerta seps, ani
mals frequently met with in the South of France, and which
have four minute feet, totally inefficient for' the support of
the body, and only remotely useful in contributing to its
progressive undulations.
Ascending from these, we may form a series of reptiles,
in which the development of the limbs becomes more and
more extended, till we arrive at Crocodiles, in which they
attain a considerable degree of perfection. As a consequence
of this greater development of the skeleton, we find the
trunk divisible into separate regions. We now, for the first
time, meet with a distinct neck, separating the head from the'
thorax, which is itself distinguishable from the abdomen;
and a distinct sacrum is interposed between the lumbar and
the caudal vertebras.

318 THE MECHANICAL FUNCTIONS.
A farther approach to the higher classes, is observable in
the number of cervical vertebrae, which is almost constantly
seven; as we shall find it to be in the mammalia. The arti
culations of the vertebrae are similar to those of serpents, in
asmuch as they consist of ball and socket joints. In that of
the occipital bone with the first vertebra of the neck, we find
that nature again reverts to the simpler form of a single con
dyle projecting from the body of the occipital bone, instead
of lateral condyles proceeding from its leaves, as we noticed
was the structure in the batrachia. The caudal vertebras
are always numerous, and the tail is compressed vertically,
which is the form most favourable for progression in water.
They are remarkable, also, for having inferior spinous pro
cesses attached to the bodies by cartilages ; a structure ana-
logbus to that which we have seen in fishes.
The number of ribs differs in different, species of Sauria:
they are always articulated to the extremities of the trans
verse processes of the vertebras, of which they appear to be
continuations. Processes of this description also occur in
the neck, attached to the transverse processes ofthe cervical
vertebrae; and these have been regarded as cervical ribs.
Their presence are impediments' to the flexions of the neck:
whence arises the difficulty which the crocodile appears to
have in bending the neck, while turning round upon the ani
mal he is pursuing. In the thorax, the ribs are connected
with a broad sternum; but there are other ribs, both before
and behind, which have no such termination, and therefore
bear the name offdlse ribs.
The pelvis consists chiefly of the iliac bones, which, as
in the batrachia, pass backwards to form the articular cavity
for the thigh bone. Two small and slender bones extend
forwards from the pubic bones, on the under side of the
body, apparently for the purpose of supporting the abdomi
nal viscera.* The bones of the extremities are very perfectly
formed, approaching in their shape and arrangement very
* They appear to be analogous to the marsupial bones peculiar to a family
of mammalia.

FEET OF THE GECKO.

319

nearly to the corresponding parts of the skeleton of the
higher orders of quadrupeds. The toes are usually provided
with membranes spread between them, to assist in swim
ming. The form of the tail, which is generally compressed
vertically, like that of fishes, though perhaps not to an equal
degree, is another indication of their being formed for an
aquatic life; for where the tail has this shape, we always find
that the chief muscular power is bestowed upon it as an in
strument of aquatic progression, producing, by its lateral
flexions, a horizontal movement of the body. Crocodiles
and alligators, for instance, which have this conformation,
are comparatively weak when on land, and as soon as they
have seized their prey, their efforts are always directed to
drag it into the water; knowing that when in their own ele
ment they can readily master its struggles, and de'stroy it by
drowning. In the Gecko tribe, we find a particular mechanism pro
vided for effecting the adhesion of the feet to the objects to
which they are applied. It is somewhat analogous to that
employed in the case* of the house-fly, already mentioned.
Each foot has five toes; all, except the thumb, terminated
by a sharp curved claw. On the under surface of each toe
(represented in Fig. 211) there are as many as sixteen trans-
2\2 211 verse slits, leading to the same
number of cavities, or sacs ;
these open forwards, and their
J>k B_HN»*9Sl external edge is serrated, ap
pearing like the teeth of a small-
toothed comb. A section of
the foot, showing these cavi
ties, is seen in Fig. 212. All
these parts, together with the
cavities are covered or lined
with cuticle. Below them are
large muscles which draw
down the claw;, and from the
tendons of these muscles arise two sets of smaller muscles,

320 THE MECHANICAL FUNCTIONS.
situated so as to be put upon the stretch, when the former are
in action. By the contractions of these muscles, the orifices
of the cavities, or sacs to which they belong, are opened,
and the serrated edges applied accurately to the surfaces
with which the feet are in contact. Sir Everard Home, in
his account of this structure, compares it to the sucking disk
of the Remora.* By its means the animal is enabled to walk
securely upon the smoothest surfaces, even in opposition to
the tendency of gravity. It can run very quickly along the
walls or ceiling of a building, in situations where it cannot
be supported by the feet, but must depend altogether upon
the suspension derived from a succession of rapid and mo
mentary adhesions.
Although the Sauria are better formed for progressive
motion than any of the other orders of reptiles, yet the
greater shortness and oblique position of their limbs, com
pared with those of mammiferous quadrupeds, obliges them
in general to rest the weight of the trunk of the body oh the
ground, when they are not actually moving. None of these
reptiles have any other kind of pace than that of walking,
or jumping; being incapable of performing either a trot or
a gallop, in consequence of the obliquity of the plane in
which their limbs move. The Chameleon walks with great
slowness and apparent difficulty ; and we have seen that, in
consequence of the structure of the bones of its neck, the
Crocodile, though capable of swift motion in a straight line,
is unable to turn itself round quickly. The general type of
these reptiles, having reference to an amphibious life, has
not attained that exclusive adaptation to a terrestrial exist
ence, which we find in the higher orders of the Mammalia.
But before proceeding to consider these, we have to notice
a singular group of animals, whose conformation appears to
be exceedingly anomalous, and as if it interrupted the regu
larity ofthe ascending series, of which it seems to be a col
lateral ramification. * Philosophical Transactions for 1816, p. 151, and 323.

CHELONIAN REPTILES. 321

§ 5. Chelonia.
The order of Chelonian Reptiles, which comprises all
the tribes of Tortoises and Turtles, appears to constitute an
exception lo the general laws of conformation, which pre
vail among Vertebrated Animals: for instead of presenting
a skeleton wholly internal, the trunk of the body is found
to be enclosed on every side in a bony case, which leaves
openings only for the head, the tail, and the fore and hind
extremities. That portion of this osseous expansion which
covers the back is termed the Carapace; and the flat plate
which defends the lower part of the body is termed the
plastron. It is a form of structure which reminds us of the
defence provided for animals very low in the scale of or
ganization, such as the echinus, the Crustacea, and the "bi
valve mollusca. Yet the substance which forms these strong
bucklers, both above and below, is a real osseous structure,
developed in the same manner as other bones, subject to all
the changes, and having all the properties of these struc
tures. The great purpose which nature seems to have had
in view in the formation of the Chelonia is security ; and for
the attainment of this object she has constructed a vaulted
and impenetrable roof, capable of resisting enormous pres
sures from without, and proof against any ordinary mea
sures of assault. It is to the animal a strong, castle, into
which he can retire on the least alarm, and defy the efforts
of his enemies'to dislodge or annoy him.
These considerations supply us with a key to many of
those apparent anomalies, which cannot fail to strike us in
viewing the dispositions of the parts of the skeleton (Fig.
213,) and the remarkable inversion they appear to have un
dergone, when compared with the usual arrangement. We
find, however, on a more attentive examination, that all the
bones composing the skeleton in other vertebrated animals
exist also in the tortoise; and that the bony case which en
velops all the other parts is really formed by an extension
vol. i. — 41

322 THE MECHANICAL FUNCTIONS.
of the spinous processes of the vertebras and ribs on the one
side, and of the usual pieces which compose the sternum on
the other. The upper and lower plates thus formed are
united at their edges by expansions of the sternocostal ap
pendices, which become ossified. Thus, no new element
has been created; but advantage has been taken of those al
ready existing in the general type of the vertebrata, to mo

dify their forms, by giving them different degrees of rela
tive development, and converting them, by these trans
formations, into a mechanism of a very different kind, and
subservient to other objects than those to which they are
usually applied. It is scarcely possible to have stronger
proofs, if such were wanting, of the unity of plan which has
regulated the formation of all animal structures, than those
afforded by the skeleton of the tortoise.
The first step taken to secure the relative immoBility of
the trunk, is to unite in one rigid bony column all its verte-

CHELONIAN REPTILES. , • 323
bras, and to allow of motion only in those of the neck, and
of the tail. The former, accordingly, are all anchylosed to
gether, leaving, indeed, traces of their original forms as se
parate vertebras, but exhibiting no sutures at the place of
junction. The canal for the spinal marrow is preserved, as
usual, above the bodies of these coalesced vertebras, and is
formed by their united leaves; the arches being completed
by the spinous processes. But these processes do not ter
minate in a crest as usual; they are farther expanded in a
lateral direction, forming flat pieces along the back, which
are united to one another by sutures, and which are also
joined to the expanded ribs, so as to form the continuous
plane surface of the carapace. The transverse processes of
the vertebras are well marked, but, though firmly united to
the ribs, do not give rise to them; for the ribs, which are
flattened and expanded, so as to touch one another along
their whole length, are inserted below, between the bodies
of every two adjoining vertebras ; while above, they are
united by suture with the plates of the spinous processes.
This change in the situation of the ribs is the consequence
of the change in their office. When designed to be very
moveable, we find them attached either to the extremities of
the transverse processes, or to the articular surfaces of a sin
gle vertebra; but where solidity and security are to be pro
vided, they are always inserted between the bodies of two
vertebras. This we shall find to be the case also in birds,
where the bones of the thorax are required to be immovea
ble. It is remarkable, indeed, that a great number of the
peculiarities which distinguish the conformation of the che
lonia from that of other reptiles, indicate an approach to the
structure of birds ; as if nature had intended this small group
of animals to be an intermediate link of gradation to that
new and important type of animals destined for a very dif
ferent mode of existence.
The sterno-costal appendages which connect the ribs to
the sternum, are, in most animals, cartilaginous; thoughoc-
casfenally we find them partially ossified. In the tortoise,

324 THE MECHANICAL FUNCTIONS.
however, their ossification is not only complete, but has
been expanded laterally, so as to form a continuous surface
with the extremities of the ribs and with the edges of the
plastron, and completely to fill up the vacancy between
them; constituting a dense and solid wall, which entirely
closes the sides of the general bony case. So strong is the
tendency to ossification in all these pieces, that the sutures
at first formed between them are often, in process of time,
obliterated; and the bony fibres are continuous throughout a
great extent of surface.
The most remarkable metamorphosis in the osseous sys
tem of this new type is that which occurs in the sternum.
So expanded are all its parts, that it is difficultto recognise
this bone under the disguised form in which it constitutes
the plastron, or broad plate, which, as we have seen, covers
the whole of the underside of the body. Yet, by a careful
examination of its structure, both in the young animal, and
also in the adult, when the sutures are not obliterated, we
may easily recognise the nine elements of the sternum;
namely, the one in the middle and fore part, and the four
pairs of lateral pieces; each having been formed from its re
spective centre of ossification. In form and relative propor
tion, indeed, they are widely different from the same parts
as they are presented in the skeletons of other animals; yet
in number and in relative situations they preserve that con
stancy and uniformity so characteristic of the beautiful har
mony which pervades all animal structures.
It is to be noticed, also, that as the plates, which form
this investing case, are bony structures, they could not
with any safety have been exposed to the action of the at
mosphere. Hence we find them covered throughout with
a thin horny plate, originally a production of the integu
ment. It is this substance which is commonly known by
the name of tortoise shell.*

* It should be observed, that the divisions of these plates, which appear
'externally, bear no relation to the sutures which separate the subjacent bones,

CHELONIAN REPTILES. 325
The immobility of the trunk is compensated, as far as re
gards the safety of the head, by the great flexibility of the
neck which is composed of seven vertebras, unencumbered
by processes, and capable of taking a double curvature like
the letter S, when the head is to be retracted within the ca
rapace. These vertebras are joined by the ball and socket
articulation common to all the existing species of reptiles.*
The articulation of the head with the neck is effected in the
same manner; but it is interesting to remark that the occi
pital condyle, which is situated at the lower margin of the
great aperture, though presenting a single convex surface,
¦ 515 p__— ^____p yet has that surface evidently di
vided into three parts ; the two up
per portions being lateral, and the
lower portion in the middle. These
three articular surfaces are seen im
mediately below the central aper
ture, f, in Fig. 215, which exhi
bits the skull of the Testudo mydas, viewed from behind.
Although closely approximated, a faint line of demarcation,
which divides their surface, indicates an incipient tendency
to separate; we shall find that, in the farther steps of deve
lopment which occur in the higher classes, this separation
actually takes place by the obliteration of the lower articu
lar surface, and the transfer of the two lateral surfaces to the
condyloid processes arising from the development of the
leaves of the occipital bone.
The singular conformation of the bones of the head, in the
turtle, affords fresh evidence in support of. the theory that
these bones were originally vertebras. The brain of this ani
mal is exceedingly small; and yet the skull, when viewed
from above, presents an appearance of great breadth, as if it
enclosed a cavity of large dimensions. But if we look upon
so that it is not possible to draw inferences respecting the form of the latter
from the mere inspection of the external shell.
* The expression of this fact is thus qualified, because it does not apply to
many fossil or extinct species, such as the Ichthyosaurus.

326 THE MECHANICAL FUNCTIONS.
it from behind, as is shown in Fig. 215, we soon discover
that the real cavity in which the brain is lodged, and to
which the aperture at f leads, is very small, only just admit
ting the end of the finger, and that the broad plates of bone,
p, p, which form the upper surface of the skull, have no re
lation to this cavity, and are merely extended over the tem
poral muscles, which are of very large size, occupying the
whole of the spaces s, s ; which spaces are completely sur
rounded by these bones. It would appear that the same ten
dency to lateral expansion, which exists in the spinous pro
cesses of the dorsal vertebrae, prevails, also, among those
which contribute to form the skull. The parietal bones,
which represent the spinous processes of the second cranial
vertebra, after having performed their primary office of pro
tecting the hemispheres of the brain by closing dver them,
still proceed in their development, forming first a crest on
the upper part of the real cranium, and then separating to
the right and left, and expanding horizontally into the upper
roof (p, p,) already mentioned, for the protection of the tem
poral muscles. This great breadth of the head in the turtle'
gives the animal an aspect of superior intelligence, to which
character, from the really diminutive size of its brain, it is,
in no respect, entitled. As the turtle is unable to withdraw
its head within the carapace, such extraordinary protection
appears to have been necessary ; for it is not met with in the
tortoise, which has a carapace sufficiently capacious to give
shelter to the head whenever occasion may require.*
This arrangement of the expanded spinous processes and
ribs in the Chelonia gives rise to a singular inversion in the po
sition of the scapula ; for it is here placed on the inside of the
ribs and sternum, that is, between the carapace and plastron.t
* The analogy of the spine of the occipital bone with that of a vertebra
is farther shown by this bone extending backwards to a considerable length,
exactly in the manner of the spinous processes of the cervical vertebrse in
other animals.
¦j- The anomalous situation of these bones, and the strangely disguised
forms which their several parts assume, render it very difficult to recognise

CHELONIAN REPTILES. 327
The humerus is remarkably curved, especially in the tortoise,
where it has the form nearly of a semi-circle. The radius
and ulna are distinct from each other; the carpus and pha
langes are short and stunted, forming a compressed kind of
hand. The pelvis, like the scapula and clavicle,, is enclosed with
in the bony shell which protects the trunk. The sacrum is
moveable upon the last dorsal vertebra ; and the coccygeal
vertebras are continued from it, forming a short tail. The
femur is short and powerful, and somewhat bent, but less so
than the humerus; and the rest ofthe bones of the hind ex
tremity are similar to those of the fore leg.* All the feet
are joined obliquely to the limbs which support them, giving
the animal an apparent awkwardness of gait, as if it were
obliged to walk upon club feet. The impulse which they
give being lateral and oblique, renders them more efficacious
for progression in the water than on land ; this circumstance,
in conjunction with the constitutional torpor of the animal,
sufficiently accounts for the excessive, and, indeed, prover
bial tardiness of its movements.
Security appears still to be the object aimed at in the me
chanism of all the other parts of the skeleton. The articu
lations at the shoulders and the hips are such as facilitate
the complete retraction of the limbs within the carapace.
After the head has been drawn in by the double, or, serpen
tine flexion of the neck, the kpees are brought together, and
the whole limb withdrawn within the shell, the fore legs
folding completely over the head, so as to cover and protect
it most effectually. For this purpose, the carpus and meta
carpus are exceedingly flattened, and approximate to the fin-
in the skeleton the several pieces which corresppnd to the normal type of
the scapula, acromion, coracoid bone, and clavicle; and anatomists are not
yet agreed as to the proper designations which are applicable to these bones
in the Chelonia.
* The cylindrical bones of the tortoise are solid throughout, and have no
cavity for containing marrow, as in the more highly developed bones of the
mammalia. This is seen in the section pf the femur, Fig. 214.

328 THE MECHANICAL FUNCTIONS.
like form which we shall presently see exemplified in the
cetaceous tribes. The phalanges are also large and length
ened, forming a kind of oval hand, or rather paddle, the
functions of which it is well calculated to perform. The
curvature ofthe humerus is of great advantage to the tor
toise in assisting it to turn itself, when, by any accident, it
has been laid on its back. ,
Considerable differences may be noticed in the structure
of the several species of Chelonia, according to the diversity
of their habits. Tortoises which live on land, require more
complete protection by means of their shell than turtles, or
Emydes, which dwell in the water: hence the convex
ity of their carapace, the solidity of its ossification, its im
moveable connexion with the plastron, and the complete
shelter it affords to the head and limbs. Turtles, on the
other hand, receiving support from the element in which
they reside, require less provision to be made for these ob
jects. Their carapace is smaller, has a more flattened form,
and cannot afford protection to the head and limbs. These
latter organs are proportionally larger, present a greater de
velopment of the radius and ulna, and are compressed into
a flat expanded surface. Previously to the retraction of the
head and limbs within the shell, the air is expelled from the
large cavities of the lungs, by the vigorous actions ofthe ab
dominal muscles, which exist in these animals as well as in
all the vertebrata, although here they are covered by the
bones, and compress the lungs by pushing the abdominal
viscera against them. This sudden expulsion of air is the
cause of the long continued hissing sound which the tortoise
emits while preparing to retreat into its strong hold.
The ribs, though they at first assume the form of broad
plates immoveably united to the spine, when they have pro
ceeded a certain distance, separate from each other, and re
sume their usual form; the intervening spaces between two
adjacent ribs being here filled up by membrane. The plas
tron is united with the carapace by membrane, likewise; and
the sternum, instead of forming one broad plate of bone, has

CHELONIAN REPTILES. 329
the intervals between its imperfectly developed elements
also membranous. All this renders the whole shell less
compact, more flexible, and weaker; but the movements of
the animal are quicker and more energetic.
These characteristic differences between the aquatic Che
lonia and those that live on land are still more strongly
marked in the genus Trionyx, or soft tortoise; which is des
titute of scales, and in which many of the pieces that are
bony in the tortoise are replaced by simple cartilage or
membrane. The enormous weight of the shell of the turtle would be
a serious impediment to the motion of this animal in the
water, were there not some provision made for diminishing
the specific gravity in the body. This purpose is answered
by the great capacity of the lungs, which, when inflated
with air, nearly fill the thorax, and give great buoyancy to
the whole mass. Thus, wherever there exists a supposed
inconvenience, dependent on the fulfilment of one condition,
we are certain to meet with a compensation in the structure
of some other part, and in the mode of executing some
other function. An express provision for giving buoyancy
has been made in the construction of the shell of a species
of tortoise inhabiting the coasts of the Seychelle Islands.
The under surface of the shell, instead of being gently con
cave, as in land tortoises, has a deep circular concavity in
the centre, above four inches in depth, which, when the
animal goes into the water, retains a large volume of air,
buoying up the whole mass while it remains in that element.*
The greater size of turtles, when compared with tortoises,
is a farther instance of the superior facility with which or
ganic, growth proceeds in aquatic than in land animals
formed on the same model of construction.
* Home's Lectures, vi. 37.
vol. I. — 42

( 330 )

CHAPTER IX.
MAMMALIA.
§ 1. Mammalia in general.
The singular animals, so remarkable for their anomalous
shapes, their torpid vitality, and their amphibious constitu
tion, which have lately occupied our attention, appear placed
by nature as forms of transition, in the passage from those
vertebrated animals which dwell in the water, to those
which inhabit the land. The class of Mammifera, or Mam
malia, comprehends all the animals which possess a spinal
column, breathe air by means of lungs, and are also warm
blooded, and viviparous, conditions which render it neces
sary that they should possess organs, called mamma, en
dowed with the power of preparing milk for the nourish
ment of their young; a peculiarity from which the name of
the class is derived. But they are not exclusively land
animals ; for among the mammalia must be ranked several
amphibious and aquatic tribes, such as the seal, the walrus,
the porpus, the dolphin, the narwhal, the cachalot, and the
whale; animals which, however widely they differ in their
habits and external conformation from terrestrial quad
rupeds, possess, in common with the latter, all the essential
characters of internal structure and of functions above enu
merated. These characters belong also to the human spe
cies, which must consequently, in its zoological relations, be
ranked as a genus of the class mammalia. So numerous,
indeed, are the analogies which connect the natural families
of this class with our own race, that we must ever feel a
deep interest in the accurate investigation of their compara
tive anatomy and physiology ; and it has been found, accord
ingly, that the progress which has, of late years, been made

MAMMALIA. 381
in this bran6h of science has materially enlarged our know
ledge of the structure, the functions, and the physical histo
ry of man; subjects with which our welfare has obviously
the closest and most intimate relation.
The principle of analogy which prevails so generally in
the inferior departments of the animal creation, may be also
traced in the class mammalia; for we always find its influ
ence more conspicuous in proportion as the objects compre
hended in the natural series of beings are more numerous.
and more diversified. Scarcely any of the great natural as
semblages of animals exhibit more variety in their habits
and modes of existence, than the one we are now examining.
Each race' has its peculiar destination, with regard to the
kind of food by which it is nourished, and the means by
which that food is obtained. The carnivorous tribes wage
war with the larger animals, whom they either spring upon
unawares, or openly pursue and overpower, displaying the
savage energies of their nature, in practising all the arts of
ferocious and sanguinary destruction. Others, intent on
meaner prey, resort to divers stratagems for its possession ;
some are designed to feed chiefly on the mollusca, and others
swallow insects only. The numerous tribes which are
formed to subsist on vegetable food exhibit, in like manner,
a great diversity of construcuon, adapted to the particular
nature of that subsistence, whether it be herbage, or the
leaves of trees, or fruits, or seeds, or the coarse fibres of
wood and bark. While all are gifted with powers to ob
tain the nourishment they require, those that have not been
armed with weapons of attack, are still provided with in
struments of defence, or with means of flight. Each has its
respective sphere of operation ; and to each its appropriate
soil, habitation, climate, and element have been assigned.
It is easy to. conceive that all these various circumstances
must lead to great diversities in the apparatus for mastica
tion and for digestion, in the organization of the senses, in
the construction of the instruments of locomotion and of pre
hension, and in the general form of the body to which*1 these

332 THE MECHANICAL FUNCTIONS.
various parts are to be adapted. Yet, amidst all these varia
tions, we may perceive the same laws of analogy connecting
the whole into one series, and assimilating all these multi
form structures to one common standard. The same organ,
however modified in its shape and size, however stinted in
one, or developed in another, is ever found in its appropriate
place, and retains the same connexions with adjacent organs,
whether we seek it in the carnivorous or the herbivorous
quadruped, in the inhabitant of the land or of the water, of
the frigid or of the torrid zone ; or in animals of the most di
minutive or most colossal statures.
As an example, we may take the vertebrae of the neck.
It is a universal law, that this part of the spinal column
shall, in every animal of the class mammalia, consist of
neither more nor less than seven vertebras. Whatever be
the length or shortness ofthe neck, whether it be compressed
into a small space, as in the elephant and the mole, whether
it be lengthened to allow the head to reach the ground, as
in the horse and the ox, or whether it be excessively pro
longed to allow the animal to reach the tops of the trees, as
in the cameleopard, still this same constant number is pre
served in the vertebras which it contains. When the neck
is long, each individual vertebra must necessarily be length
ened in the same proportion. Thus, in the Cameleopard,
the vertebras of thp neck consist of seven very long tubes,
joined together endwise, with scarcely any development of
spinous iprocesses, lest they should impede the bending of
the neck. The greatest contrast to this structure is met
with in the Dolphin, and other Cetacea, which present exter
nally no appearance whatever of a neck, but whose skeleton
exhibits cervical vertebras, closely compressed together, and
exceedingly thin, and m,ost of theni united together;* every
bone, thus formed, however, retains the marks of having
originajly consisted of separate vertebrae; and still, in this
* In the cachalot, the whole of these seven vertebrae are usually anchy-
losed into one bone.

CETACEA. 333
extreme case, the number of primary pieces is constantly
seven.* <
§ 2. Cetacea.
Remarkable exemplifications of the law of uniformity of
organic structure are furnished by the family of the Ceta
cea, which includes the whale, the cachalot, the dolphin, and
the porpus, and exhibits the most elementary forms of the
type of the mammalia, of which they represent the early,
or rudimental stage of development. Here, as before, we
have to seek these first elements among the inhabitants of
the water; for whenever, in our progress through the ani
mal kingdom, we enter upon a new division, aquatic tribes
are always found to compose the lowest links of the ascend
ing chain. Here, also, we observe organic development
proceeding with more rapidity, and raising structures of
greater dimensions in aquatic than in terrestrial animals.
The order Cetacea comprises by far the largest animals
which inhabit the globe. Whatever may have been the
magnitude of those huge monsters which once moved in
• The Bradypus tridadylus, or three-toed sloth,- was, till very lately,
thought to constitute a notable exception to this law, being described as
having nine, instead of seven, cervical vertebrae. It is now found, however,
that the last two of these vertebrae, which appeared to be supernumerary,
ought properly to be classed among the dorsal vertebrae, of which they possess
the distinctive characters, not only from the form and size of their transverse
processes, but also from their having small bony appendices, articulated with
them by a regular joint at their extremities, and corresponding exactly, both
in shape and situation, to the ribs, of which they may, in fact, be considered
as rudiments. These small bones have been observed, both by Meckel and
by Cuvier, attached to the ninth vertebra: and Mr. T. Bell has recently not
only confirmed the observations of these anatomists, but has farther discovered
that similar rudimental ribs are attached also to the eighth vertebra. (See
Philosophical Magazine, third series, iii. 376.) The Bradypus torquatus,
which has been said to possess eight cervical vertebrae, will, perhaps, on closer
examination, be hereafter found not to deviate, any more than the three-toed
sloth, from the normal type, as regards the number of these vertebra. In
stances have occurred of supernumerary cervical processes, or ribs in the hu
man skeleton. (See Edinburgh Medical and Surgical Journal, xl. 304.)

334 THE MECHANICAL FUNCTIONS.
the bosom of the primeval ocean, or stalked with gigantic
strides across antediluvian plains, and whose scattered re
mains bear fearful testimony of the convulsions of a former
world, certain it is that, at the present day, the whales of
the northern seas are the most- colossal of the living animal
structures existing on the surface of this planet.
A cursory survey of the organization of the tribes belong
ing to this semi-amphibious family, will impress us with the
resemblance they bear to fishes; for they present the same
oval outline of the body, the same compact form of the
trunk, which is united with the head without an intervening
neck; the same fin-like shape of the external instruments of
motion, and the same enormous expansion and prolongation
of the tail, which is here also, as in fishes, the chief agent in
progression. With all this agreement in external charac
ters, their internal economy is conducted upon a totally dif
ferent plan; for although constantly inhabiting the ocean,
their vital organs are so constructed as to admit of their
breathing only the air of the atmosphere, and the conse
quences which flow from this difference are of great import
ance. The necessity of aerial respiration compels them to
rise, at short intervals, to the surface of the water; and this
air, with which they fill their lungs in respiration, gives
their bodies the buoyant force which is required to facilitate
their ascent, and supersedes the necessity of a swimming
bladder, an organ which is so useful to fishes.
With the intent of diminishing still farther their specific
gravity, nature has provided that a large quantity of oily
fluid shall be collected under the skin, a provision which an
swers, also, the purpose of preserving the vital warmth of
the body. A great accumulation of this lighter substance is
formed on the upper part of the head, apparently with a view
to facilitate the elevation to the surface of the spiral, or ori
fice of the nostrils, which is placed there.* ,
Another peculiarity of conformation, in which the cetacea
* The substance called Spermaceti is lodged in cells, formed of a cartilagi
nous substance, situated on the upper part of the head of the Cachalot.

CETACEA. 335
differ from fishes, and which has also an obvious relation to
their peculiar mode of breathing, is in the form of the tail,
which, instead of being compressed laterally, and inflected
from side to side, as in fishes, is flattened horizontally, and
strikes the water in a vertical direction, thereby giving the
body a powerful impulsion, either towards the surface, when
the animal is constrained to rise, or downwards, when, by
diving, it hastens to escape from danger.
All the essential and permanent parts of the skeleton of
vertebrated animals, that is, the spinal column, and its im
mediate dependencies, the skull, the caudal prolongation,
and the ribs, are found in that of the Cetacea. The thorax
is carried very much forwards, especially in the whale, and
the neck is so short as to be scarcely recognisable ; for the
object of the conformation is here, as in that of fishes, to
allow free scope for the movements of the tail, and ample
space for the lodgement of its muscles. For the purpose of
giving greater power and more extensive attachment to these
muscles, the transverse processes of the dorsal and lumbar
vertebrae are expanded both in length and breadth, and, be
ing situated horizontally, offer no impediment to the vertical
flexure of the spine. For the same reason, the ribs are con
tinued in a line with the transverse processes, and articu
lated with their extremities, thus giving still farther breadth
to the trunk.
As there is a total absence of hinder extremities, so there
is no enlargement of any of the vertebras corresponding to a
sacrum, and the caudal vertebrae are uninterrupted continua
tions of those of the trunk. They develope, however, parts
which are met with only among fishes and reptiles, namely,
arches composed of inferior leaves* and spinous processes,
enclosing and giving protection to a large artery. Although
the bones of the legs do not exist, yet there are found, in the
hinder and lower part of the trunk, concealed in the flesh,
and quite detached from the spine, two small bones, appa-
* These leaves being formed of cartilage, are generally lost when the
bones are macerated for the purpose of preparing the skeleton.

336

THE MECHANICAL FUNCTIONS.

rently corresponding to pelvic bones, for the presence of
which no more probable reason can be assigned than the
tendency to preserve an analogy with the more developed
structures of the same type. ><'
A similar adherence to the law
of uniformity in the plan of con
struction of all the animals belong
ing to the same class, is strikingly
shown in the conformation of the
bones of the anterior extremities of
the cetacea; for, although they pre
sent, externally, no resemblance to
the leg and foot of a quadruped, being
fashioned into fin-like members, with
a flat, oval surface, for striking the
water, yet, when the bones are stripped
of the thick integument which covers
them, and conceals their real form,
we find them, (as may be seen in Fig.
216) exhibiting the same divisions into
carpal and metacarpal bones, and pha
langes of fingers, as exist in the most
highly developed organization, not
merely of a quadruped, but also of a
monkey, and even of man.

§ 3. Amphibia. I
In the small tribe denominated by Cuvier Amphibia, and
consisting of the Phoca, or Seal, and the Trichecus, or
Walrus, we perceive that an advance is made towards a
fuller development ofthe limbs; these animals having a dis
tinct neck and pelvis, and both hind and fore extremities.
In the seal, the hind legs are drawn out posteriorly to a con
siderable length, and placed parallel to each other : when
united and alternately raised and depressed, they perform

AMPHIBIA. 337
the same office as the tail of the cetacea, and propel the ani
mal forwards: but when employed separately, they are more
qualified to act as oars. The walrus has feet still more de
veloped, and distinctly divided into toes, which are disposed
so as to strike backwards against the water.

§ 4. Mammiferous Quadrupeds in general.
From the imperfectly developed aquatic and amphibious
tribes, we gradually ascend to the more finished structures
of mammiferous quadrupeds, which are expressly fitted for
progression on land. In these the powers of development,
not being expended in the mere effort of giving expansion
to the several textures, and of swelling the bulk of the frame,
somet^nes to inordinate dimensions, are employed rather in
reducing the elements ofthe organization into compact forms,
and in concentrating their energies, so as ultimately to at
tain the extent of power and harmony of action, which are
displayed in the higher orders of warm-blooded^quadrupeds.
It is to these favoured tribes that we must look for exam
ples of the most complete development of the skeleton, and
the most advantageous disposition of mechanic force. We
have seen that reptiles, from the comparative shortness of
their limbs, and the torpidity of their muscular powers, are
but ill adapted for rapid progression. All the more per
fectly formed quadrupeds of the class mammalia, having the
trunk of the body raised high upon the limbs, possess great
range of motion, and can traverse with fewer steps a given
space. ^
The offiee of the limbs, as far as they are concerned in
progressive motion, is two-fold. They have, first, to sus
tain the weight of the body, which they must do by acting
in opposition to the force of gravity; and they must, second-
• ly, give the body an impulse forwards. Let us consider
more particularly the relations which the structures bear to
each of these two functions.
vol. i. — 43

338

THE MECHANICAL FUNCTIONS.

The limbs of quadrupeds constitute four columns of sup-
port to the trunk, which is placed horizontally above them;
but the whole weight of the body, together with that of the
head and neck, does not bear equally upon them ; the fore
extremities almost always sustain the greater part of that
weight, both because the fore part of the trunk is itself hea
vier than the hind part, and because it is loaded with the
additional weight of the head and neck. Hence, in the usual
attitude of standing, the pieces of which the fore limbs are
composed are required to be placed more in a straight line
than those of the hinder limb ; for the power of a column to
support a weight is the greater in proportion as it approaches
to the perpendicular position. The hind limbs are composed
of exactly the same number of divisions; but the separate
portions are usually longer than those of the fore extremity,
and consequently if they had been disposed vertically in a
straight line, they would have elevated the hinder part of
the trunk to too great a height compared with the fore part.
This is obviated by their forming alternate angles with one
another. As the pelvis connects the spine with the joint of
the hip, and even extends farther backwards, the thigh bone
must necessarily be brought forwards; then the tibia and
fibula, which compose the bones of the leg, must be carried
backwards to their junction with the bones of the foot; and,
again, the foot must be turned forwards' in its whole length
from the heel to the extremities of the toes. On comparing
the positions of the corresponding divisions of the anterior
and posterior extremities, we observe that they incline, when
bent, in opposite directions; for in the former we find, in fol
lowing the series of bones frpm the spine, that the scapula
proteedsferwards, the humerus backwards; the radius and
ulna again forwards, and the fore foot backwards, positions
which are exactly the reverse of the corresponding bones of
the hind limb. (See Fig. 218, page 350.)
The weight of the body, in consequence of this alternate
direction ofthe angles at the successive joints, must always
tend, while the quadruped is on its legs, to bend each limb:

MAMMIFEROUS QUADRUPEDS. 339
a tendency which is required to be counteracted by the ac
tions of the muscles which are situated on the external side
of each of those angles. These muscles are the extensors of
the joints; that is, the muscles which tend to bring their
parts into a straight line. It is, in fact, by this muscular ac
tion, much more than by simple rigidity, that the limb sup
ports the superincumbent weight of the body. It is evi
dent that greater muscular force is necessary for this purpose
when the joints are bent, than when they are already ex
tended; and the portions ofthe fore legs being naturally in
this condition, require less power than those of the hinder
legs to retain them in their proper relative positions.
The most complete instance of a verticaUarrangement of
the bones of the extremities is seen in the Elephant; where,
in order to sustain the enormous weight of the body, the
limbs are shaped into four massive columns, of which the
several bones are disposed nearly in perpendicular lines. By
this means, the body is supported With scarcely any muscu- L
lar effort, and the attitude of standing is, in this animal, a
state' of such complete repose, that it often sleeps in that po
sition. The elephant which was kept some years ago at the
Menagerie at Paris, although much enfeebled by a lingering
disorder, was never seen to lie down till the day on which
he died. When he was in the la* stage of debility, what
seemed to give him most distress was the effort requisite fo
support his head: and, in order to relieve the muscles ofthe
neck, which Were strained in that exertion, he was in the
habit of extending his trunk perpendicularly to the ground',
by contracting all the muscular fibres which run transverse
ly in that organ, and of thus forming a vertical prop for the
head. But in almost all other quadrupeds, the mere act of
standing, though a state of comparative rest, implies, for the
, reasons already given, a degree of muscular exertion ; and
they can enjoy complete repose only by letting the body re
cline upon the ground.
The conformation of the hind extremities, which, as we
have seen,is not so well, calculated' for the.. simple support of

340 THE MECHANICAL FUNCTIONS.
the trunk, is, on the other hand, better adapted to give it
those impulses which are to effect its progressive movements.
The nature of those movements, and the order in which they
succeed each other, are different according to the peculiar
mode of progression which the animal practises, the degree,
of speed it is desirous of exerting, and the particular end it
has in view. The paces of a quadruped usually distin
guished, are the walk, the trot, the gallop, the amble, and
the bound.
In slow walking, only one foot is raised from the ground
at the same moment, so that three points of support always
exist for sustaining the weight of the body. If the centre
of gravity be situated, as it generally is, nearly over the mid
dle of the quadrangular base formed by the feet, while they
rest upon the ground, the first effort to advance which the
quadruped makes, propels the centre of gravity forwards.
This it accomplishes by pressing one of its hind legs against
the ground, which leg, being thus fixed by the resistance it
there meets with, becomes the fulcrum of the first move
ments. The extensor muscles of the limb are now exerted
in giving the body an impulse forwards. As soon as this
impulse has been given, the muscles which had been in ac
tion are relaxed, and the leg is raised from the ground,
brought forwards, and laid down close to the fore foot of the
same side. This fore foot. is next raised and advanced, and
then the same succession of actions takes place with the
hind and the fore foot of the other side.
An attentive examination of the conditions of these suc
cessive positions will shew'that, amidst all the changes which
take place in the points of support* the stability of the body
is constantly preserved. It is an elementary proposition in
mechanics that all that is necessary for ensuring the sup
port of a body on any given base, is that the vertical line
drawn from the centre of gravity shall fall within that base.
When the animal is standing, the feet form a quadrilateral
base, and the centre of gravity is in a vertical line passing
either through the centre ofthe base, or, as, for the reasons

PROGRESSIVE MOTION OF QUADRUPEDS. 341
already mentioned, more frequently happens, through a point
a little in front of the exact centre. At the time when the
hind foot which began the action is raised from the ground,
the centre of gravity, having been by that action impelled
forwards, still remains above the base formed by the other
three feet, and which is now reduced to a triangle. That
hind foot being set down, while the corresponding fore foot
is raised, a new triangular base is formed by the same hind
foot together with the two of the other side, which have
not yet been raised. The centre of gravity is still situated
above this new triangle, and the body is consequently still
supported on these three feet. The fore foot may now be
advanced without endangering the stability of the body ; and
by the time this foot is set down, and has thereby formed a
new quadrilateral basis with the other feet, the centre of gra
vity has arrived above the centre of this new base. But at
this moment the centre of gravity is again urged forwards
by the other hind foot, which now comes into action, and
repeats on the other side the same succession of actions,
which are attended with the same consequences as before.
Thus, during its whole progress, the animal is never for an
instant in danger of falling; for whichever ofthe feet may be
raised from the ground, the other three feet are always so
placed as to form a stable base of support.
In quick walking it often happens that quadrupeds raise-
their fore foot on either side a little before the hind foot
comes to the ground. This is shown by the impression
made by the latter being on the same spot, or even rather
in advance of the impression, made by the former. But
the time during which the body is thus supported only by
two feet is so short as not sensibly to influence the results.
In consequence of the obliquity of the alternate impulses
given to the centre of gravity by the successive actions of
both the hind legs, a slight degree of undulation is occa
sioned; but these undulations are only lateral. A trot may
be considered as a succession of short leaps made by each
set of feet taken diagonally; that is, by the right fore foot,
and the left hind foot; or, vice versa, the one set being raised

342 THE MECHANICAL FUNCTIONS.
together a short time before the others have reached the
ground: so that during that minute interval of time all the
feet are in the air at the same moment ; and during the re
maining portion of the time, the body is resting, upon the
two feet placed diagonally with regard to each other. The
undulations are here chiefly vertical, instead of lateral, as
they are in the walking pace.
A gallop is a continued succession of longer leaps made
by the two hind feet in conjunction. In this case, the cen
tre of gravity is lifted higher from the ground, and is pro
jected in a wide arch, and with great velocity.
In the amble, both the legs on one side are raised toge
ther; so that the impulsions given are directed much more
laterally than in any other pace, and the body is thrown into
a strong undulatory motion from side to side.
Another kind of pace is the bound, which is often prac
tised by deer, and is performed by striking the ground with
all the legs at the same moment. It consists, therefore, like
the gallop, of a series of leaps; but their direction is more
uniformly upwards, from the concurrence of all the legs in
the same action.
Nature has purposely endowed different tribes of animals
with very different capacities to execute progressive move
ments, by the variations she has introduced into the com
parative lengths of the several parts of the trunk, and the
size and mobility of the extremities. Of all the large ani
mals, the Lion has been constructed with the finest propor
tions for conferring both strength and activity. The mass
of his body is supported more by the fore than by the hind
extremities. In walking, the lion takes long strides, and ex
hibits strongly the lateral undulations of the trunk.
Quadrupeds having a very long, or a very massive body,
or whose limbs are short, and nearly of equal height, are, in
capable of advancing by a gallop, or at least cannot sustain
this pace without a painful effort, and never but for a short
time. The Tiger, which has a longer body than the lion,
gallops with less facility; and runs chiefly by an accelera
tion of its walking pace. It excels principally in the vigour

PROGRESSIVE MOTION OF QUADRUPEDS. 343
and extent of its bounds; for which it is admirably qualified
by the prodigious power of its muscles, enabling it to spring
forwards upon its victim with an impetus which nothing
can resist.
The speed with which a quadruped is capable of advancing
depends more on the disposition of the muscles and the ex
tent of the articulations, and more especially on the power
of the extensors of the hind extremities, than on the form
of the body. Great length and muscularity in the hind legs
are generally attended with considerable power of leaping.
This is exemplified in the Jerboa and the Kanguroo, ani
mals, which, from the disproportionate shortness of their
fore legs, are totally incapacitated from walking; and for
the same reason, they cannot run with any degree of swift
ness. It is only in climbing up a steep acclivity that the
jerboa is enabled to employ all its limbs: in a descent, on
the contrary, it uses only its fore legs, the hinder being
dragged after them. But, when pursued, these animals are
capable, for a long continuance, of taking leaps of nine feet
distance, and of repeating these leaps so quickly, that the
Cossacks, though mounted on the swiftest horses, are unable
to overtake them.
The Kanguroo, in almost all his movements, brings into
action his powerful tail, which is furnished with very strong
muscles, and may be considered as constituting a fifth limb.
It is of great assistance to the animal in taking leaps, and
during its repose, contributes, together with the hind feet,
to support the weight of the body, as on a tripod, and to
leave at liberty the flbre legs, which may then be employed
as arms.
The Hare and the Rabbit furnish other instances of an
extraordinary length of the hinder legs depriving the ani
mal of the power of walking, and obliging it to move for
wards only by a succession of leaps. The hare may be said,
indeed, to walk with its fore legs only, while it hops or gal
lops with the hinder : but this disadvantage is amply com
pensated by its amazing swiftness when running at full speed.

344 v THE MECHANICAL FUNCTIONS.
Animals, like the hare, in which, from the great length
of the hinder limbs, the posterior half of the body is higher
than the anterior, run much better up an acclivity than on
level ground. In a descent, on the contrary, they are
obliged to pursue an oblique and zig-zag course, otherwise
they would be in danger of oversetting, as happens occa
sionally to the Agouti and the Guinea pig, when these ani
mals attempt to run downhill.
The Sloth, which is formed for clinging with great tena
city to the boughs of trees, presents a remarkable contrast
to the animals we have just noticed; its fore legs being much
longer than the hinder, and its movements being proverbi
ally slow. The peculiar modifications of its muscular powers
are probably consequences of the singular mode in which,
as I shall afterwards have occasion to notice, its arteries are
distributed. The Cameleopard, likewise, has the fore legs much longer
than the hinder. The object of this conformation was pro
bably to elevate the anterior part of the spine, so as to raise
the head as much as possible, and also to give a considera
ble inclination to the whole column, for the purpose of dis
tributing more equally the weight of the head and of the
very long neck upon all the legs ; for the length of the neck
is fully equal to that of the trunk. It is evident that if the
body had been placed in the usual horizontal position, the
anterior extremities would have had to support the whole
of the enormous ^weight of this neck and head. This pecu
liarity of structure, however, introduces considerable modi
fications in the mode of progression ^f the animal. The
ordinary pace of the cameleopard is the amble ; but it has
also a slower walking pace, and occasionally a gallop. In
the amble, its' undulation is so considerable as to give it the
appearance of being lame. A similar kind of limping gait,
arising from the same cause, namely, the disproportionate
elevation of the fore part of the spine, has been observed in
the Hyena.

RUMINANT QUADRUPEDS. 345

§ 5. Ruminantia.
In following the series of Mammalia in the order which
best exhibits their successive stages of development, I shall
commence with those whose digestive apparatus is formed
to extract nourishment exclusively from the vegetable king
dom. The first assemblage that presents itself to our notice
is the remarkable family of Ruminants, which feed princi
pally on herbage. Wherever the earth is closed with ve
getation, it requires neither skill nor exertion on their part
to seek and to devour the rich repast which is profusely
spread under their feet. To remove from one pasture to
another, to browse, and to repose, constitute the peaceful
employments of their lives, and satisfy the chief conditions
of their existence. To these purposes the whole conforma
tion, of their skeleton, and especially of those parts which
constitute the limbs, is adapted. The anterior extremities
having only to support the weight of the fore part of the
trunk, and to assist in progressive motion, have a less com
plicated arrangement of joints, and exhibit many of those
consolidations of the bones, which tend to simplify the
structure^ and contribute to its strength.
But though never incited by the calls of appetite to en
gage in sanguinary warfare, they are yet liable to the as
saults of many ferocious and well armed adversaries, and ate
often unprovided with any adequate means of defence ; their
only resource, therefore, is to avoid the dangers of the en
counter by a rapid and precipitate flight. To confer this
power appears to have been the object airhed at by nature
in every part of the conformation of these animals. It is
among the ruminant tribes that the fleetest of quadrupeds
are to be found, such as the -gazelle, the antelope, and the
deer, animals which exhibit the highest perfection of struc
ture belonging to this type. We may observe that the
parts composing the hind legs are longer, and inclined to
vol. i.— -44

346

THE MECHANICAL FUNCTIONS.

one another at angles more acute in these animals than in
other tribes of mammalia, so that they are always ready for
instantly commencing their flight, and springing forwards
on the slightest notice of danger. (See Fig. 218, page 350.)
As it was necessary, from the situation of theirfood, that
their heads should reach the ground in grazing, we find that
the neck has been much elongated, that the muscles which
raise the head have been enlarged and strengthened, and that
the spinous processes of the back and neck have been much
expanded, in-order to allow of sufficient surface for the at
tachments of these muscles. The effort requisite to raise,
and even support the head, is very considerable; as will ap
pear when we reflect that its weight acts by means of an ex
tremely long lever; for such is the mechanical officeieS'the
elongated neck. But, in order to economize the muscular
power, an elastic ligament is employed to sustain the weight
ofthe head. This, which is termed the ligamentum nucha,
and is represented at n, in Fig. 217,' is formed of a great
number of bands which connect the hinder part ofthe cra
nium, at the ridge of the occipital bone, and all the spinous
processes of the neck, with those of the back, the separate
slips from each being successively joined together, and com
posing a ligament of great length, and power. It differs, in

its structure, from ordinary ligaments, being highly elastic,
so that it yields to the extension of the neck when the animal

RUMINANT QUADRUPEDS. 347
lowers its head, and gives considerable assistance to the mus
cles in raising it. In the deer and the ox, which toss their
heads with force, and especially in the males, which are
armed with antlers or horns, the muscles performing these
motions are remarkably strong, and the spinous processes of
the back particularly prominent. In the loins, on the con
trary, we find ihe transverse processes more enlarged, for
the purpose of giving a powerful mechanical purchase to the
muscles which are inserted into them.
The chest of ruminant quadrupeds is corrtpressed laterally,
in order to allow room for the unrestrained motions of the
ianterior extremity; and the sternum projects so as to resem
ble the keel of a ship. The bones of the anterior extremity
are not joined to the rest bf the skeleton by means of any
bone corresponding to a clavicle ; but they are connected
with the spine and ribs only by ligaments and muscles; so
that the fore part of the trunk is, in fact, suspended between
the limbs by its muscular attachments alone. This is not
the case with the hind extremities ; for their bones commence
with the pelvis, which proceeds backwards from the sacrum,
but with a considerable inclination downwards, and has a
deep hemispherical cavity for the lodgement of the round
head of the thigh bone. The lengthened forms of the iliac
bones, and, also, of the scapula, provide for the application
of muscles of considerable length, which are, consequently,
capable of communicating to the parts they move a greater
velocity than could have been effected by muscles of equal
strength, but with shorter fibres.
Both the humerus in front, and the femur behind, are so
short as to appear, on a superficial view, to form part of the
trunk, being entirely enveloped and concealed by the large
muscles connecting them with the body. The heads of the
two humeri, in consequence of the absence of the clavicle,
are brought very near each other, so as to occupy a situation
as nearly as possible underneath the weight which the limb
has to support.
The radius and ulna, which are the two bones of the fore

348 THE MECHANICAL FUNCTIONS.
arm, although completely separate at an early period of
growth, soon unite to form but one bone. This union be
gins at their lower end, and proceeds upwards to within a
short distance from the top, where a separation may still be
observed in the processes which project from that end, form
ing for some way down a distinct suture. This union of
the two bones must, of course, preclude all rotatory motion;
but it is calculated to give the joint great security : and this ap-:
peaf s to have been the main object in the conformation of the
whole limb. The same process of consolidation takes place
in the hind leg, between the tibia--and the fibula, which are
so completely united, as to afford scarcely any trace of their
having been originally separate.
The carpus and the tarsus are both of very limited extent,
and consist of. a smaller number of pieces than usually oc
cur in these joints. The consolidation of parts is most con
spicuous in the succeeding division of the limb, namely,
that constituting the metacarpus in the anterior, and the
metatarsus in the hind extremity. In either case we find it
consisting not of five bones, as in the more highly organized
carnivorous mammalia, but of a single bone only, termed
the cannon bone. In the early periods of ossification, how
ever, they each consisted of two slender bones, lying close
and parallel to each other ; but afterwards united by an os-
sific deposition, which fills up the interval between them,
and leaves behind no trace of suture.* In proportion as the
young animal acquires strength, the union of these two bones
becomes still more intimate by the-absorption of the parti
tion which separated their cavities ; so that ultimately they~
constitute but one cylinder, with a single central cavity,
which is occupied by marrow.
The cannon bone is much elongated, both in the fore and
hind extremity ; so that the carpus and tarsus, which are the
commencements of the real feet, are raised considerably
* The observations which establish this fact are detailed by G. St. Hilaire,
in apaper in the "Memoires du Museum," x. 173.

RUMINANT QUADRUPEDS. 349
above the ground. It is a common mistake, arising from
the height of these joints, and the names they bear in ordi
nary language, to consider them as the knees of the animal.
The slightest inspection ofthe skeleton will be sufficient to
show that what is called the knee in the fore leg is properly
the wrist; and in the hind leg, the part so misnamed is re
ally the heel. Thus, the foot, especially in the posterior ex
tremity, is of great length; a structure which is evidently-
intended to give greater velocity to the actions of the mus
cles, while it at the same time ensures the utmost steadiness
and security of motion-
At the lower extremity of the cannon bone there are two.
articular surfaces, indicating the originally separate ends of
its two component bones. They are for the articulation of
the two following bones, which are also very long, and which
correspond in situation to the first phalanges of the fingers
and toes. These are followed by a second and third set of
phalanges; the last of which terminate in hoofs. All rumi
nant quadrupeds have thus a double hoof; a character which
is peculiar to this family.
Thus, then, has Nature moulded the organs of progressive
motion in this remark able tribe of animals to accommodate
them to the peculiar conditions of their existence, while she
has still preserved their relations to the primitive type of
the class to which they belong. Thus has she bestowed .
upon them the slender and elegant forms, so pleasing to the
eye, which characterize the fleetest racer, and has provided
for the agile, yet firm and secure movements which they are
to exercise in various ways in eluding the observation, and
escaping from the pursuit of their stronger and more saga-?
cious foes. This purpose they effect, a,t one time by rapid
flight across extensive tracts of country; at another, by re
tirement into unfrequented forests, or mountains of difficult
access, crossing their rugged surfaces in all directions, clam-.
bering their precipitous acclivities, and fearlessly bounding
oyer intervening abysses, from point to point, till the place
pf safety is attained on some rocky eminence. From this

350

THE MECHANICAL FUNCTIONS.

secure station the Alpine chamois looks down upon its pur
suers, and defies their farther efforts at capture or molestation.

The astonishing feats of agility practised by this animal, and
by which the most experienced hunters are perpetually baf-

RUMINANT QUADRUPEDS. ' 351
fled in their attempts, to approach it, sufficiently attest the
perfection of its organization in reference to all these ob
jects. The chamois has often been seen to leap down a per
pendicular precipice of twenty or thirty feet in height,
without sustaining the slightest injury. How the ligaments
that bind the joints can resist the violent strains and concus- -
sions they must be exposed to in these quick and jarring
efforts, is truly wonderful. ,
While Nature has provided these animals with the means
of safety from their more formidable enemies, she has not
left them altogether without defence against their more equal
rivals in the field. It is on the head that she has implanted
those powerful arms which are sometimes wielded with dead
ly effect in their mutual combats. Even when not furnished
with horns, the animal instinctively strikes with its fore
head where the frontal bone has been expanded and forti
fied apparently with a view to this mode of attack. Thus,
the ram butts with its head without reference to the horns,
which are coiled so as to be turned away from the object to
be struck. In the deer and the ox tribes, however, the horns
are formidable weapons of offence: and it will be interest
ing to inquire into the nature of these organs, and the phe
nomena of their production.
The antlers of the male 'stag are osseous structures, sup
ported on short and solid tubercles of the frontal bone: af
ter remaining nearly a year, they are cast off, and soon re
placed fay a newly formed antler, which is of larger size than '
the one which was lost. Previously to the formation of
this structure, those branches of the artery, termed the ca
rotid, which supply blood to the frontal bone, are observed
very rapidly to dilate and to throb with, unusual force > and
all the blood vessels of the skin of the part where the antler
is to arise, soon become distended with blood; an effect
which is accompanied by general heat and redness, like a
part in a state of high inflammation.* Presently the skin is
1 * These phenomena are connected with periodical changes in the consti,
tution relating to the reproductive functions.

352 THE MECHANICAL FUNCTIONS.
elevated by the growth of a tubercle from the subjacent
bone: this tubercle is at first a cartilage, and after it has at
tained a certain size, becomes ossified, and grows like other
osseous structures, first shooting into the form of a length
ened cylinder, and then dividing into branches. It is fol
lowed in its elongation by the skin, which during the whole
time that the antler is growing is extended over it in every
part, forming what is called, from the delicate investment of
hair, its velvet xoat. The blood vessels of the proper mem
brane of the antler, or periosteum, Still continuing to sup
ply it with the materials required for its growth and conso
lidation, deposite so great an abundance of bony matter, that
its enlargement is exceedingly rapid. The whole antler,
which often weighs nearly thirty pounds, has been known
to be completely formed in ten weeks from the time of its
first appearance. There is no other instance in the animal
kingdom of so rapid a growth; which is the more remarka
ble from its occurring in a small part of the system, and
in a bony structure. -¦
After the antler has attained its full size, a deposition of
osseous substance still continous at its base, around the
trunks of the arteries which are proceeding along the invest
ing membrane of the bone for the purpose of conveying
nourishment. The accumulation of this substance raises a
ring called the burr, round that part of the antler; and by
encroaching on the arteries themselves, it gradually dimi
nishes their capacity of conveying blood, and they at length
become entirely obliterated. The bone, no lohger receiving
a superabundant nourishment, ceases to grow ; the integu
ments which covered it, decay, and becoming dry and shri
velled, are torn by rubbing against trees, and peel off in long
shreds, leaving the antler exposed, which, by the continued
effects of the same kind of friction> soon acquires a polished
surface. During many months, the antler being sufficiently nourished
by its own interior vessels, continues in a living state, and
preserves its connexion with the system. But, at length, the

RUMINANT QUADRUPEDS. 353
arteries, whether from the effect ofthe progressive deposition
of osseous matter, or from some change in the balance of the ,v
vital powers, shrink, and become, by degrees, obliterated.
The antler dies in consequence, and, although it continues to
adhere to the skull, it is only as a foreign body, and it is not
long destined to remain thus attached ; for the absorbent ves
sels are now actively employed in scooping out a groove of
separation between the living and the decayed substance, at
the place where the base of the antler is contiguous to the
frontal bone. As soon as this has proceeded to a sufficient
depth, the adhesion ceases, and the slightest concussion occa
sions the fall ofthe whole structure. After the separation of
the antler, the eminence ofthe frontal bone, o.n which it stood,
is left rough and uneven, like that of a fractured part : but
the surrounding integuments soon close over, and cover it
completely; until the period arrives when it is to be replaced
by a new antler, which exhibits the same succession of phe
nomena,- in its growth and decay, as its predecessor, only
that its development is usually carried farther, the new stem
being both thicker and longer, and the branches wider and
more numerous. The antler of each successive year has,
consequently, a different form from that of the preceding;
and, when the animal has attained a certain age the extre
mities1 of the branches present broad expansions of bone,
which the antler's of an earlier growth had never exhibited.
The short bony processes which extend in a perpendicular
direction on the head ofthe cameleopard, are analogous, in
some ofthe circumstances of their formation, to the antlers of
the deer, being of an osseous nature, and continuous with the
frontal bone :-but, in other respects, they are very different ;
for, instead of being annually shed, they remain through life,
and continue to be covered with the integuments, which re
tain, at the extremities, a tuft of hair. The development of
these processes, in the young animal, takes place in the same
manner as that of an antler, but it reaches only to a certain
point, upon attaining which, the growth is arrested, and never
proceeds farther. The arteries cease to deposite superabun-
vol. i. — 45

354 THE MECHANICAL FUNCTIONS.
dant nourishment, but continue to maintain an exact equili
brium between the expenditure and the supply ; so that the
horns ofthe cameleopard are never shed, and remain perma
nent bony structures. ,
A farther modification of this process occurs in the con
struction of the horns of the ox and of the sheep; for in
these the bony processes arising from the frontal bones are
invested with a covering composed of horn, the nature of
which is totally different from bone. Two tubercles may
be seen in the young calf, proceeding from the bones of the
forehead : the skin covering these tubercles, unlike that
which precedes the antlers of the deer, is unusually thick
and hard. As the skull expands, this portion of integu
ment becomes more and more callous, till it is converted,
by the action of the subjacent vessels, into a solid, hard,
elastic, and insensible fibrous substance, fitted to give effec
tual protection to the subjacent bony layers which are form
ing underneath it. The highly vascular membrane, from
which these new structures chiefly arise, appears to have
different powers of production at its two surfaces ; for while
the inner surface is forming the osseous portion of the horn,
and supplying the phosphate of lime required for the con
struction of its plates and fibres, the exterior surface is add
ing successive layers of horny substance to the inner side
of those portions which had been before deposited. These
two operations, which offer a remarkable contrast, both as
to the mode of their performance, and as to the nature of
the resulting products, are carried on at the same time, and
by the same organ, but on different sides. The bony basis
of the horn is an organic structure, which continues to be
nourished by vessels forming part of the general system :
the horn is a mere excretion, which appears to be destitute of
vessels, and is, consequently, removed from the influence of
the living powers. Thus the growth of horn is somewhat
analagous to that of shell; for the layers which compose it
are deposited in succession ; each new layer is agglutinated
to the inner surface ofthe preceding; and each has the shape

RUMINANT QUADRUPEDS.

355

of a hollow cone, occupying the part towards the apex of
the former cone, and extending farther towards the' base.
Hence a longitudinal section of the whole presents the ap
pearance represented in the annexed figures (218*,) where
a is the section of the horn of an Ox, and b, a similar sec
tion of the horn of an Antelope. C is a magnified view of
the extremity of the latter, together with a portion of the
bone d, which occupies the axis of the horn.
In this process of the formation of horn, as happens in
that of shells, there sometimes occur irregularities, or peri
odical intermissions and increase of action in the secreting
organs, giving rise to transverse grooves or ridges. These
may be ^een in the horns of the goat, in which the fibres
are short, and laid one over another with the same regula
rity as the^iles of a house. The tendency in these horns

to assume a spiral form is explicable on the same principles
as those which regulate the growth of turbinated shells.

356 THE MECHANICAL FUNCTIONS.
The horns of the ox and of the antelope tribes are formed
of longer and more continuous fibres, which are closely
compacted together, and exhibit very distinctly the series
of hollow cones of which they are composed.
The horns of the Rhinoceros, both of the one and the two
horned species, grow from the integument covering the
nose, to which they adhere without having any connexion
with the subjacent bones. They have a pyramidal shape,
and are composed of parallel fibres, resembling hairs, agglu
tinated together into a solid mass by a material which acts
as a cement. This fibrous structure is most distinctly seen
at the base of the horn, where the ends of the fibres project,
like those of a brush, from the surface. When these horns
are sawn transversely, and examined with a magnifying
.glass, a great number of orifices are seen, marking the
empty spaces that intervene between the hairs; and if the
section be made in a longitudinal direction, the same spaces
give rise to the appearance of parallel grooves. These horns
are not deciduous, like those of the stag; but continue to, ad
here to the skin, and to grow from the root, in proportion
as they are worn at the extremity.

§ 6. Splipeda, <
The Splipeda form a natural family of quadrupeds, in
cluding the Horse, the Ass, the Quagga, the Zebra, &c.
which are very nearly allied in their conformation to the
ruminant tribe. To combine fleetness with strength has
been the obvious design , of nature in the construction of
these animals. We find, accordingly, that the consolidation
of the bones of the foot is carried still farther than in the
ruminant tribe; for, in place of the two parallel phalanges,
which are, in the latter, articulated with the cannon bone,
there is here only a single metatarsal bone. The three pha
langes, of which that single finger consists, bear the names of
the pastern, the coronet, and the coffin bone; and the hoof, of

MAMMALIA SOLIPEDA. 357
course, is single, likewise ; there is, also, a small bone, con
nected with the last, and called the shuttle bone. To the
cannon bone are joined, behind, and on the side, two much
shorter and very slender bones, which are rudiments ofthe
other metacarpal bones. They have been termed the sty
loid, or splint bones; and are generally united by ossifica
tion with the cannon bone. The scapula of the horse is
very narrow, and placed very nearly in a straight line with
, the humerus ; which latter bone is very short, and scarcely '
descends below the line of the chest. The thigh-bone is also
unusually short. The muscles, which extend the joint, and
throw the thigh backwards, in kicking, are particularly pow
erful. This is the natural defensive action of the horse: and
its force is increased by a particular process with which'the
bone is furnished, and which has the form of a strong curved
spine, situated on the outside, and opposite to the lesser tro
chanter,* giving .to the muscles the advantage of a long le
ver. The cervical vertebras have only short spinous pro
cesses, that they might not interfere with the motions of the
neck. In the vertebras of the back, on the other hand, these
processes are remarkably long, especially at the part where
the shoulder rests; their projection constituting what is
called the Withers.

§ 7. Pachydermata.
From the horse we pass, by a natural transition, to the
Pachydermata, a small group of animals interesting by their
peculiarities, and by their being remnants of a very extensive
tribe, which formerly inhabited the earth, but have now al
most entirely disappeared. Although they feed upon grass,
they do not ruminate, nor are they cloven-footed. They are,
for the most part, huge and unwieldy animals, with thick in
teguments, rendered tough by a large mass of condensed cel
lular substance, which forms the chief defensive armour of
* This process has been termed the processus recurvatus femoris.

358 THE MECHANICAL FUNCTIONS.
those that are destitute of either tusk, proboscis, or nasal
horn. The most remarkable genus of this family is the Elephant,
the colossal giant of quadrupeds. The many peculiarities
which are observable in the conformation of this animal
have all an obvious relation to the circumstances of its con
dition. Formed for feeding on a great variety of vegetable
substances, and more especially on the tender shoots of trees,
fruits, and grains, as well as on herbage, and succulent roots,
its organs of mastication are powerful, and its teeth of great
size. The whole of this apparatus requires an immense de
velopment of bone to render it efficient; so that the head,
with its huge tusks and grinders, is of enormous weight.
Had this ponderous head been suspended at the end of a
neck of such length as to admi& of its being carried to the
ground, as is the case in grazing animals, it would have de
stroyed the balance of the body, and would have required
greater force to raise and retain it in a horizontal position
than could have been given by any degree of muscular
"power. Nature has accordingly abandoned this form of
structure, and has at once curtailed the neck, bringing the
head close to the trunk of the body, and supporting it by
means of short, but powerful muscles, which are not im
planted in any particular point of the skull, as they are in
other quadrupeds, where the occipital bone forms a crest or
ridge for that purpose; but the general surface ofthe cranium
has been enlarged by an immense expansion given to its inte
rior cellular structure, and, thus, the muscles are attached to
a considerable extent of bone, instead of being affixed to a
single process, which would have incurred great risk of being
broken off by their action. These large cells are constructed
with a view to combine strength with lightness;' the plates
which form their sides being disposed in a radiated manner
towards the circumference, and arranged with great regulari
ty; and the cells themselves, instead of containing marrow, are
filled with air, by means of communications with the Eusta
chian tubes, which open into the nostrils: thus, a great extent
of surface is given to the skull, without any addition to its

MAMMALIA PACHYDERMATA. 359
weight. The ligamentum nuchas also comes in aid- of the
muscular power, being here of vast size and strength.
The head being limited in its range of motion by its ap
proximation to the trunk, the mouth cannot be applied di
rectly to seize the food: and some means were, therefore, to
be provided for bringing the food to the mouth. For this
purpose, a new organ, the proboscis, lias been constructed: it
consists of a cylinder, perfectly flexible, and of- a length suf:
ficient to reach the ground, when the elephant is standing.
The animal has the power of moving it in all possible direc
tions, by means of a prodigious number of muscular fibres,
which are collected in small bands,some passing transverse
ly, and radiating from the interior towards the circum
ference, others situated more obliquely, and a third set run
ning longitudinally, and forming an exterior layer; but they
are all variously interlaced together so as to compose a very
complicated arrangement. The extremity of the proboscis,
which is endowed with great sensibility, is furnished with an
appendix, resembling a finger, most of the functions of which,
indeed, it is- capable of performing.
For the formation of this admirable member it has not
been necessary to deviate from the ordinary laws of deve
lopment by the creation of a new organ; thesame end being
accomplished .by the extension of a structure already be
longing to the type of mammiferous animals. In several of
the pachydermata the nostrils are already considerably ad
vanced, so as to form a moveable snout: this is observable
in a certain degree in the Hog; it is still more. remarkably
seen in the Tapir, which has a snout so lengthened and so
moveable as very much to resemble, though on a small
scale, the proboscis of the elephant. This latter organ, then,
may be considered as merely an elongation of the nostrils,
which have been drawn out to suit a special purpose, very
different from the function to which that part is usually sub
servient.* * A defective development ofthe bones ofthe nasal cavity, while the na
tural growth ofthe soft parts has continued, has often, in the case ofthe hu-

360 THE MECHANICAL FUNCTIONS.
While fleetness and elasticity are the results of the me
chanical conformation ofthe horse, solidity and strength are
the objects chiefly aimed at in the construction of the pachy
dermata. The limbs have a great weight to sustain, in con
sequence of the huge size of the body; and hence the seve
ral bones which compose the pillars for its support are ar
ranged nearly in vertical lines. The joints of the elbow and
knee are placed low from the body; the ulna in the forelegs,
and the fibula in the hinder, are fully developed, and are
distinct from the radius and the tibia. The number of the
toes, instead of being reduced to one, as* in the horse, or to
two, as in ruminants, is here increased to five: though, in
consequence of their being very short, and of the skin which
covers and surrounds them being very thick, they hardly
appear externally, and are distinctly recognised only in the
skeleton. It would carry me far beyond the limits of the present
work, were I to engage in a detailed examination of all the
varieties of forms and structures which occur in the mecha
nism of the different tribes of mammalia, in reference to the
purposes they are intended to serve, and to the peculiar cir
cumstances of the animal to which they belongs I must ne
cessarily pass over a multitude of instances of express adap
tation, which are suited only to particular cases, and are,
consequently, of minor importance as regards the general
plans of organization. In the sort of bird's-eye view that
I am taking of the endless modifications of structure which
have been executed in conformity with those plans, 1 am
only able particularly to notice such as are most remarkable.
§ 8. Rodentia.
As the tribes of mammalia we have hitherto examined,
employ the anterior extremities for the purposes of progress-
man fcstus, given rise to a monstrosity very much resembling the trunk of the
tapir or of the elephant. (See Geoffrey St. Hilaire.)

MAMMALIA R0DE1S.TIA. 361
sion only, they are destitute of a clavicle. In most of those
which follow, and where a greater development of the limb
confers more extensive and more varied powers of motion,
applicable to a greater range of objects, this bone is found.
In the greater number, however, it is merely in a rudimental
state; that is, developed only to a certain extent, one portion
being1 bony, and the rest cartilaginous; as if the ossification
had' been arrested at an early stage. These imperfect cla
vicles are too short to connect the scapula with the sternum ;
the rest of the space being eked out by cartilage, and by li
gaments: but, still, they are of great use in affording points
of attachment to the muscles of the limb, and giving them the
advantage of acting by a rigid lever. The carnivorous tribes,
which make considerable use of their fore paws in striking
and' seizing their prey, have clavicles of this description.
Those quadrupeds which have to execute still more complex
actions with their fore feet, have perfect clavicles, extending
from- the' shoulder to the chest, and connecting the bones of
the anterior extremity with the general frame-work of the '
skeleton. Thisis the case in a large proportion ofthe family
of Rodentia, such as the Squirrel, which employs its paws
for holding objects; and the Beaver, which likewise makes
great use of its fore feet, which might almost be termed
Hands, in building its habitation;* Animals that dwell m
trees, and require to grasp with force the branches in moving
along, them, such as the Sloth, have also distinct clavicles.
Animals which rake or dig the ground, as the Mole, the
the Ant-eater, and- the Hedge-hog, are all provided with these
$ones, which, by keeping the shoulders at the same constant
distance from the trunk, and affording a firm axis for the
rotatory motions of the limb, materially assist them in the
performance of these actions.
* The beaver presents a singular modification in the structure of the tail,
Which is expanded into a flattened oval disk, covered by a skin beset with ¦
scales; and which is used by the animal as a paddle for supporting itself oil-
the water; or for quickly diving to the bottom. There does. not appear to-
be any truth in the opinion commonly entertained, that the Beaver employs''
its toil as a trowel for plastering the mud walls of its dwelling.
VOL. 1. — 46

362 THK MECHANICAL FUNCTIONS..

§ 9. Insectivora.
In the tribe of Insectivorous quadrupeds we meet with
several races which present singular conformations. In none
are these anomalies more remarkable than in the mole, an
animal which nature has formed for subterranean residence,
and whose limbs are constructed with a view to the rapid
excavation of passages under ground. The hands of the
mole, for its fore paws almost deserve that appellation, are
turned upwards and backwards for scooping the soil, while
the feet are employed to throw it out with great quickness.
These mining operations are aided by the motions of the
head, which is lifted with great power, so as to loosen the
ground above, and overcome the resistances that may be op
posed to the progress of the animal. That no impediment
might be offered to these motions of the head, the spinous
processes of the cervical vertebras have not been suffered to
extend upwards. Large muscles are provided for bending the
head backwards upon the neck ; and they are assisted by a
cervical ligament of great strength, which is generally in
part ossified. The muscles of the fore extremities are also
of extraordinary power. The scapula is a long and slender
bone, mOre resembling a humerus in its shape than an ordi
nary scapula: the humerus, on the contrary, is thick, and
square, and the clavicle is short and broad. The radius and
the ulna are distinct from each other; the hand is very large
and expanded; the palms being turned outwards and back
wards, and its lower margin being fashioned into a sharp
cutting edge. The carpal bones and the phalanges of the
fingers are very much compressed ; but they are furnished
with large nails, which compose'more than half the hands;
and they are expressly constructed for digging, being long,
broad, and sharp at the extremities. The sternum has a
large middle crest, and is prolonged at its extremity into a
sharp process, having the figure of a ploughshare, thus af-

INSECTIVOROUS MAMMALIA. 303
fording an extensive surface of attachment for the large pec
toral muscles, from which the limb derives its principal
force. The head terminates in front by a pointed nose,
which is armed at its' extremity with a small bone, intended
to assist in penetrating through the ground.
While all this attention has been paid to the 'development
of the anterior part of the body to which these instruments
specially contrived for burrowing are affixed, the hinder
part is comparatively feeble, and appears stinted in its
growth, and curtailed of its fair proportions: The pelvis is
expeedingly diminutive, being reduced to a slender sacrum;
and it is thrown far back from the abdomen, to which it
could give no effectual protection. Hence the animal,- when
above ground, walks very awkwardly, and is unable to ad
vance but by an irregular and vacillating pace.*
We have seen that there is a tribe of fishes armed ex
ternally with sharp spines, which they are capable of erect
ing when in danger of attack. The Pordupine and the Hedge
hog, which belong to the family of insectivorous quadrupeds,
are furnished with a similar kind of defensive armour.
For the purpose of erecting these bristles, when the animal
is irritated or alarmed, there is provided a peculiar set of
muscular bands, which forms part of the usual subcutaneous
layer, termed the panniculus carnosus. In the hedgehog
these muscles are very complicated, and give the animal
the power of rolling itself into a ball. A minute descrip
tion of these muscles has been given by Cuvier, who found
that the whole body is enveloped in a large muscular bag,
or mantle, lying immediately under the integuments ; and
capable, by the contraction of different portions of its fibres,
of carrying the skin over a great extent of surface. In the -
usual state of the animal, this broad muscle appears on the
* The only quadrupeds which resemble the mole in the perfect adaptation
of their structure to the purposes of burrowing, are the Wombat and the
Koala, which are among the many extraordinary animals inhabiting the con
tinent; of Australia. Their hind legs are constructed in :. manner very much
.resembling the human fore-arm. (See Home, Lectures, &c. i. 134.)

264 THE MECHANICAL FUNCTIONS.
back (as represented in Fig. 219,) contracted into a thick
oval disk, of which the fibres are much accumulated at the
circumference. From the edges of this disk there pass

down auxiliary muscles towards the lower -parts ofthe body;
the action of which muscles tends to draw the skin down
wards, and to coil it over the head and paws, in the manner
shown in Fig. 220, like the closing of the mouth of a great
bag. § 10. Carnivora.
The type of the Mammalia may be considered as having
attained its full development in the carnivorous tribes, which
comprehend the larger beasts of prey. As their food is ani
mal, they require a less complicated apparatus for digestion
than herbivorous quadrupeds, possess greater activity and
strength, and enjoy a greater range of sensitive and intel
lectual faculties. In accordance with these conditions, we
may notice the greater expansion of their brain, the'su-r
perior acuteness of their senses, and their, enormous mus
cular power. The trunk of the body is lighter <than that
bf vegetable feeders, especially in the abdominal region,
and is compressed laterally: the spine is more pliant and
elastic,* the limbs have greater freedom of motion, the ex
tremities are more subdivided, and they are armed with
* The suppleness of the spine might at once be inferred, on the simple in
spection of the skeleton, from tlie circumstance that the vertebrae of the
neck and loins have a comparatively small development of their spinous pro
cesses.

CARNIVOROUS MAMMALIA.

365

formidable weapons of offence and destruction. Great me
chanical power was required for raising the head, not only
on account of the force to be exerted in tearing flesh,. but
also that these animals might be enabled to carry away their
prey in their mouths. Hence we find that in the Lion, of
which the skeleton is represented in its relations to the out
line ofthe body, in Fig. 221, the first vertebra ofthe neck,
or atlas, has very widely expanded transverse processes,
while the second vertebra has a largely developed spinous
process, fpr supplying levers for the muscles which have to
perform these and other actions in which the head is con
cerned. The whole of the remaining part of the skeleton of these
animals is constructed with reference to their predatory na
ture. The sudden springs with which they pounce upon
their prey must impart to the whole osseous frame the most
violent concussion. The first stroke with whieh they attempt
the destruction of their victims is given with the fore leg; so
that, had the limb been rigidly connected with the ster
num, by means of an entire clavicle, its motions would have

been too limited, and danger of fracture would have been in
curred. The scapula is broad, and the humerus of great

366 THE MECHANICAL FUNCTIONS.
length, compared with the^ame bones^in ruminants; and the
latter has, besides, a large surface for its articulation with
the former of these bones, thus allowing of a great range of
motion : the radius and ulna are perfectly distinct, and play
extensively on each other.
The fore feet rest on the ground by means of the second
¦of the three joints of which each toe is composed. The last
-phalanges are raised at right angles to the former, for the
purpose of supporting the claws in an erect position. It has
been considered of such importance to preserve these formi
dable instruments constantly sharp, and in a condition fitted
for immediate use, that an express contrivance has been re
sorted to for this purpose. It consists in a sheath, within
which the claws, when not employed, are kept retracted, by
means of an elastic ligament, which constantly tends to with
draw them within the sheath: and they are, at the same time,
so connected with the tendons of the flexor muscles of the
toes, that the moment these muscles are thrown into action,
which is the case when the animal aims a stroke with its,
paw, the claws are instantly drawn out, and combine in in
flicting the severest lacerations.*
Connected with the superior strength of the hind, extre
mities, we find the pelvis extending farther backwards, and
more in a perpendicular line with the femur. This latter
bone is longer and more slender than in the horse, but it is
more compact in its form, and its processes are more strong
ly developed: the fibula is a separate bone from the tibia.
The muscles, in general, are more divided into portions, and
are thus capable of greater diversity of action, at the same
time that they have greater power than those of herbivorous
quadrupeds. The articular surfaces are of greater extent,
and are lubricated with a more copious supply of synovia;
their ligaments are more delicate and more numerous; and the
* There exists, concealed in the tuft of hair, at the extremity ofthe lion's
tail, a small conical and slightly curved claw, which is attached to the skin
only, and not to the last caudal vertebra: its use is probably to increase the
«ffeot of blows given with the tail.

MjAMMALIA QUADRUMANA. 367
joints in general, adapted to a greater variety of movements.
All these provisions are evidently directed to confer great
freedom and facility of motion, and to enlarge the sphere of
action of the body generally, as well as of the limbs.

• § 11. Quadrumana.
We may trace in the series of quadrupeds which have
come under pur review, a gradual increase in the develop
ment of the hind feet; beginning from the horse, which is
single-hoofed, or solipede; next to which rank, the cloven-
footed ruminants, a tribe which includes the camel, whose
foot is widely expanded, for the purpose of treading secure
ly on sand; then come the Rhinoceros, which has three
hoofed toes; the Hippopotamus, -which has four; and the
Elephant, which has five. To these succeed another series,
where nails, or c]aws, are substituted for hoofs; as is the case
with all the Carnivora, which, standing on the extremities
of their toes, have been termed Digitigrades. Then follow
the Plantigrade quadrupeds, such as the bear, the badger,
the hedgehog and the mole, which rest with the whole foot
on the ground, and are, in consequence, able to make great
use of their fore paws. . These conduct us to the family of
the Quadrumana, comprehending the Monkey and the Le
mur tribes, which are characterized by having the inner toe
quite distinct from the others, like the human thumb, and
which appear, therefore, as if they had four hands.
The Quadrumana present the nearest approximation to
the human structure; they are naturally inhabitants of the
forest, and their conformation is adapted to the actions of
climbing upon trees, of grasping the branches, and of spring
ing from the one to the other, with precision and agility. It
is here that they are at home; it is here that they gather the
food which is most suited to their nature; it is here that they en
gage in successful combats with serpents and other enemies;
-retaining their positions in perfect security on the moving^

368 THE MECHANICAL FUNCTIONS.
branches, or sportively swinging by their extremities in the
air. Both the feet and the hands are formed for this species
of prehension; and many are farther provided with a strong
ly prehensile tail, which is an instrument, admirably adapted
to all these purposes. Hence, the attitude most natural to
these animals is- neither the horizontal one of quadrupeds,
nor the erect posture of man, but an intermediate or semi-
erect position.
This view ofthe living habits of the quadrumana will af
ford the key to most of the peculiarities of structure they
present to our observation. The head, being no longer sus
pended at the end of a horizontal, or recurved neck, is, in
the usual attitude of the animal, supported chiefly by the
cervical vertebras. The greater development of the brain^
and, more especially, of its posterior lobes, creates a neces
sity for an extension ofthe occipital bone in that direction;
a portion of the weight to be sustained by the atlas, is, ac
cordingly, thrown behind the centre of motion, which is at
its articulation with the latter bone; and this weight tends,
therefore, to balance that of the anterior part of the head :
hence, there is no need of the strong cervical ligament, which
is so universally met with in quadruped's", and, although this
ligament exists in the monkey, it is very slender, and of no
very great extent.
Great mobility has been conferred on the spine by the
form of its articulations; and the caudal vertebras are gene
rally greatly multiplied to form a tail of considerable length,
which, in the AleleS) or spider monkey of America, is
moved by powerful muscles, and is an organ of great flexi
bility and strength. Monkeys possess a distinct clavicle, a
lengthened humerus and femur, a radius and ulna movea
ble upon each other, and a hand nearly approaching to the
human construction. But the thumb is less developed, and
its muscles are much weaker than in man.
The bones of the pelvis, as well as those of the leg, are
elongated, for the purpose of giving greater length to the mus
cles which are to move their several parts; by this means,

THE HUMAN FRAME. 369
although the force with which they act may be somewhat
lessened, yet the velocity of the motion they produce is in
creased in the same proportion.. The fibula is here a bdne
of more importance than in quadrupeds ; for it performs a
motion of rotation round the tibia, analogous to that of the
radius upon the ulna, giving a great extent of action to the
foot, and converting the leg into an arm, as we have already
seen that the foot itself is transformed into a hand. A small
inclination is given to the articulation ofthe tarsus with these
last mentioned bones, which imparts a degree of twist to the
feet, throwing the sole inwards, and causing the monkey
while walking to rest chiefly on its outer edge. This seem
ing defect gives a slight appearance of awkwardness to the
gait: it is not, however, to be viewed as an imperfection, for
it is evidently designed to assist the animal in climbing trees,
which is its most usual action, the oblique position of the
foot enabling it most effectually to lay hold of the branches.
Monkeys are evidently no,t formed to excel in swiftness ; for
the heel, in these animals, presents.no large projection, as in
other orders of mammalia; nor are the muscles which are
inserted into the heel particularly powerful : they hardly,
indeed, can be said to compose a calf as in the human leg.
§ 12. Man.
The series of structures modelled on the characteristic
type of the Mammalia, after having exhibited the successive
development of all, its elements, attains the highest perfec
tion in the human fabric; for even independently of those
prerogatives of intellect and of sensibility, by which Man
is so far exalted above the level of the brute creation, both
his physical structure and his physiological constitution place
him incontestably at the summit of the scale of terrestrial
beings. Considered zoologically, indeed, the human species
must rank among the Mammalia, and it even makes a near ap
proach to the Quadrumana; yet there exists many peculiari
ties of structure, which entitle Man to be placed in a sepa-
vol. 1.— 47

370 THE MECHANICAL FUNCTIONS.
rate order, where disclaiming any close alliance with inferior
creatures, he proudly stands alone, towering far above them
all. It is not, however, on a pre-eminence in any single phy
sical quality or function that this title to superiority can be
founded; for in each of these endowments man is excelled
in turn by particular races of the lower animals; but the
chief perfection of his frame consists in its general adapta
tion to an incomparably greater variety of objects, and an infi
nitely more expanded sphere of action. As the beauty of an
edifice depends not on the elaborate finishing of any one por
tion, but results from the general suitableness of the whole
to the purposes for which it was constructed, so the excellence
of the human fabric is to.be estimated by the exquisite pro*
portion and harmony subsisting among all its parts, and per-1
vading the whole system of its functions. The design of
its structureand economy embraces widely different, and far
higher aims than those contemplated in the organization of
any of the inferior animals. Destined to possess an intel->
lectual, a social, and a moral existence, Man has had every
part of his organization modified with an express relation ta
these great objects of his formation. This wiA best appear
when we come to examine the organs which are subservi
ent to the sensitive and active faculties; but even here,
where our views must, for the present, be limited to the
mechanical circumstances of his structure,- the proofs are
sufficiently numerous to warrant this general conclusion.
Man presents the only instance among the mammalia of
a conformation by which the erect posture can be perma
nently maintained, and in which the office of supporting the
trunk of the body is consigned exclusively to the lower ex
tremities. To this intention the form and arrangement of
all the parts of the osseous fabric, and the position and adj
justments of the organs of sense have a well marked refer
ence.* The lower limbs are qualified to be the efficient
* In most quadrupeds, as we have seen, the thorax is deep in the direc-
tien from the sternum to the spine, but is compressed laterally, for the evi'-

THE HUM/AN FRAME. 371
instruments of progression by their greater length and mus
cularity, compared with the generality of quadrupeds. The
only exceptions to this rule occur in those mammalia which
are constructed expressly for leaping, such as the Kanguroo
and Jerboa, where, however, the hind legs are employed
almost solely for that mode of progression. The Quadru
mana, which come nearer to the human form than any of
the other tribes, have the lower limbs comparatively weak.
In almost all other quadrupeds the disproportion is still
greater, the thigh being short, and almost concealed by the
muscles of the trunk, and the remainder of the limb being
slender, and not surrounded by any considerable mass of
muscles. The articular surfaces of the knee joint are broader, and
admit of greater extent of motion in man than in quadru
peds: hence the leg can be brought into- the same line with
the thigh, and form with it a straight and firm column of
support to the trunk ; and the long neck of the thigh bone
allows of more complete rotation. The widely spread basin
of the pelvis effectually sustains the weight of the digestive
organs, and they rest more particularly upon the broad ex
pansion ofthe iliac bones: in quadrupeds, these bones, having
no such weight to support, are much narrower.
The base on which the whole body is supported in the
erect position is constituted by the toes, and by the heel,
the bone of which projects backwards at right angles to the
leg. Between these points the sole of the foot has a conca
vity in two directions, the one longitudinal, the other trans
verse, constituting a double arch. This construction, be
sides conferring strength and elasticity, provides room for
the convenient passage of the tendons of the toes, which
proceed downwards from the larger muscles of the leg, and
dent purpose of bringing the fore limbs nearer to each other, that they
might more effectually support the anterior part of the trunk. In Man, on
the contrary, the thorax is flattened anteriorly, and extends more in width
than in depth; thus throwing out the shoulders, and allowing an extensive
range of motion to the arms.

372 THE MECHANICAL FUNCTIONS.
also for the lodgement of smaller muscles affixed to each in
dividual joint, and for the protection of the various nerves
and blood vessels distributed to all these parts. The con
cavity of the foot adapts it also to retain a firmer hold of the
inequalities of the ground on which we tread. The muscles
which raise the heel, and which compose the calf of the
leg, are of great size and strength, and derive a considerable
increase of power from the projection of the bone of the
heel, into which their united tendons are inserted. In all
these respects the human structure possesses decided advan
tages over that of the monkey, with reference to the specific
objects of its formation.
It is impossible to doubt that nature intended man to as
sume the erect attitude, when we advert to the mode in
which the head is placed on the spinal column. The enor
mous development of the brain, and of the bones which in
vest it, increases so considerably the weight of that part of
the head, which is situated behind its articulation with the
vertebras of the neck, that the balance of the whole is much
more equal than it is in the monkey, where the weight of
the fore part greatly preponderates. The muscles which
bend the head back upon the neck, and retain it in its na
tural position, are therefore not required to be so powerful
as they must be in quadrupeds, especially in those which
graze, and in which the mouth and eyes must frequently
be directed downwards, for the purpose of. procuring food.
In man this attitude would, if continued, be extremely fa
tiguing, from the weakness of those muscles, and the absence
of that strong ligament which sustains the weight of the
head in the ordinary horizontal attitude of quadrupeds.
" Pronaque cum spectant animalia cactera terrain,
Os homini sublime dedit, cselumque tueri
Jussit, et erectos ad sidera tollere vultus." — Ovid.
The space comprehended by the two feet is extremely
narrow, when compared with the extended base on which
the quadruped is supported. Hence, the stability of the body

THE HUMAN FRAME. 373
must be considerably less. The statue of an.elephant, placed
upon a level surface, would stand without danger of overset
ting; but the statue of a man, resting on the feet, in the usual
attitude of standing, would be thrown down by a very small
impulse. It is evident, indeed, that in the living body, if
the centre of gravity were at any moment to pass beyond ^
the base, no muscular effort which could then be made, would
avail, to prevent the body from falling. But the actions of
the muscles are continually exerted to prevent the yielding
of the joints under the weight of the body, which tends to
bend them. In quadrupeds, less exertion is requisite for
that purpose; and standing is in them, as we have seen, a
posture of comparative repose fin man it requires nearly as
great an expenditure of muscular power as the act of walk
ing. Soldiers, on parade, experience more fatigue by re
maining in the attitude of standing, than they would by
marching, during an equal time. Strictly speaking, indeed,
it is impossible for even the strongest man to remain on his
legs, in precisely the same position, for any considerable
length of time. The muscles in action soon become fatigued,
and require to be relieved by varying the points of support,
so as to bring other muscles into play. Hence, the weight
of the body is transferred alternately from one foot to the
other. The a ction of standing consists, in fact, of a series of
small and imperceptible motions, by which the centre of
gravity is perpetually shifted from one part of the base to
another; the tendency to fall to any one side being quickly
counteracted by an insensible movement in a contrary direc
tion. Long habit has rendered us unconscious of these ex
ertions, which we are, nevertheless, continually making;
but a child learning to walk finds it difficult to accomplish
them successfully. It is one among those arts which he has
to acquire, and%hich costs him, in the apprenticeship, many
painful efforts, and many discouraging falls. But whenever
nature is the teacher, the scholar makes rapid progress in
learning; and no sooner have the muscles acquired the ne
cessary strength, than the child becomes an adept in ba-

374 THE MECHANICAL FUNCTIONS.
lancing its body in various attitudes, and, in a very short
time, is unconscious that these actions require exertion.
In walking, the first effort that is made consists in trans
ferring the whole weight of the body upon one foot, with a
view to fix it on the ground; and, then, the other foot, being
at liberty, is brought forwards. By this action, the centre
of gravity is made to advance, till it passes beyond the base
of the foot: in this situation, the body, being unsupported,
falls through a certain space, and would continue its descent,
were it not that it is received on the other foot, which, by
this time, has been set upon the ground. This falling of the
body would, if not immediately checked, become very sen
sible; as happens when, on walking inattentively, the foot
we had advanced comes down to a lower level than we were
prepared for; in which case, the body, having acquired a
certain velocity by its greater descent, receives a sudden
shock when that velocity is checked, and thus a disagreea
ble jar is given to the whole frame.
While the weight of the body is thus transferred, alter
nately, from one foot to the other, the centre of gravity not
only rises and falls, so as to describe, at every step, a small
arch, but also vibrates from side to side, so that the series of
curves it describes, are somewhat complicated in their form.
This undulation of the body, from one foot to the other,
would scarcely ever be performed with perfect equality on
both sides, if we trusted wholly to the sensations communi
cated by the muscles, and if we were not guided by the sense
of sight, or some other substitute. Thus, a person blind
folded cannot walk far in a straight line ; for, even on a le
vel plane, he will incline unconsciously either to the right
or to the left.
In all quadrupeds, and even also in the quadrumana, the
fore extremities more or less contribute to the support and
progression of the body : it is only in man that they are
wholly exempted from these offices, and are at liberty to be
applied to other purposes, and employed as instruments of
prehension and of touch. In the power of executing an in-

THE HUMAN FRAME. 375
finite variety of movements and of actions, requiring either
strength, delicacy, or precision, the human arm and hand,
considered in their mechanism alone, are structures of unri
valled excellence; and, when viewed in relation to the in
tellectual energies to which they are subservient, plainly re
veal to us the divine source, from which have emanated this
exquisite workmanship, and these admirable adjustments, so
fitted to excite in our breasts the deepest veneration, and to
fill us with never-ceasing wonder.
To specify all the details of express contrivance in the
mechanical conformation of the hand would alone fill a se
parate treatise: but I must refrain from pursuing this inte
resting subject, as, fortunately, the task has devolved upon
one far more able than myself to do it justice.

( 376 )

CHAPTER X.
VERTEBRATA CAPABLE OF FLYING.
§ 1. Vertebrata without Feather s, formed for flying.
Few problems in mechanic art present greater practical
difficulties than that of raising from the ground, and of sus
taining and moving rapidly through the air an animal body,
composed as it must be of many ponderous organs which are
requisite for the performance of the higher functions of life :
yet Nature has achieved all this, not only in endless tribes
of the more diminutive invertebrate animals, but also in the
more solid and massive organizations which are modelled on
the vertebrate type. These objects have been accomplished;
in all cases, without the employment of any other than the
ordinary elements of those organizations; modified, indeed,
to suit the particular purpose in view, but yet essentially the
same, regulated by the same laws of development which
prevail throughout the whole animal system. The adapta
tion of these elements to the construction of an apparatus of
so refined a nature as that which is required for flying, im
plies the deepest foresight, the most extensive plan, and the
most artificial combinatibn of means. The foundations for
these peculiar forms of mechanism are laid in the primeval
constitution of the embryo ; and a long and curious series of
preparatory changes must take place before the completion
of the finishedstructures. Of this we have already had a
remarkable example in the metamorphoses of insects, which
exhibit, in their last stage of development, the highest (de
gree of perfection compatible with the articulate type.
Birds, in like manner, present, us with the highest refine-

POWER OF FLYING. 377
ment of mechanical conformation winch can be attained by
the development of a vertebrated structure.
The power of flying is derived altogether from the resist
ance which the air opposes to bodies moving through it, or
acting upon it by mechanical impulse. In the ordinary
movements of our own bodies, this resistance is scarcely
sensible, and hardly ever attracts notice ; but it increases in
proportion to the surface which acts upon the air, and still
more according to the velocity of the moving body; for the
increase is not merely in the simple ratio of the velocity,
but as its square, or perhaps, even a higher power. In order
that an animal may be able to fly, therefore, two principal
conditions are required : there must, first, be a considerable
extent of surface in the wings, or instruments which act
upon the air; and there must, secondly, be sufficient mus
cular power to give these instruments a very great velocity.
Both these advantages are found combined in the anterior
extremities of birds, and no animals belonging to any other
class possess them in the same perfection. No quadruped,
except the bat, has sufficient muscular power in its limbs,
however aided by an expansion of surface, to strike the air
with the force requisite for flight. No refinement of me
chanic ingenuity has ever placed the Daedalian art of flying
within the reach of human power; for even if the lightest
possible wings could be so artificially adapted to the body as
to receive the full force of the actions of the limbs, however
these actions might be combined, they would fall very far
short of the exertion necessary for raising the body from
the ground.
Examples, however, occur in every one of the classes of
vertebrated animals, where an approach is made to this fa
culty. In the Exocetus, or flying-fish, the peetoral fins have
been enormously expanded, evidently for the purpose of en
abling the animal to leap out ofthe water, and support itself
for a short interval in the air; but its utmost efforts are inade
quate to sustain it beyond a few moments in that element,
vol. r.- — 48

378 THE MECHANICAL FUNCTIONS.
and it can never rise to more than five or six feet above the
surface ofthe water**
A species of lizard, called the Draco Volans, has a singu
larly constructed apparatus, which appears like two wings,
affixed to the sides of the back, and quite independent of
either the fore or the hind extremities. By the aid of these
moveable flaps, the animal is able to descend from the tops
of trees, or flutter lightly from branch to branch; but this is
the utmost that it can accomplish by means of these imper
fect organs. The construction of these anomalous mem
bers is highly curious in a physiological point of view ; as
showing how Nature, in effecting a new purpose, is in
clined to resort to the modification of structures already
established as constituent parts of the frame, in preference
to creating new organs, or such as have no prototype in the
model of its formation. Frequent proofs of this law, indeed,
are afforded by the comparative examination ofthe anatomy
of the organs of progressive motion. The ribs, in particular,
are often the subject of,, these conversions to uses very dif
ferent from their ordinary function, which is that of assist
ing in respiration. Thus, we have seen that in the Tortoise
they are expanded to form the carapace, uniting with corre
sponding dilatations of the sternum, and sterno-costal appen
dages, in composing a general osseous incasement to the
body. In Serpents, again, the ribs are employed as organs
of progressive motion ; performing the functions of legs, and
having affixed to their extremities the abdominal scuta, by
way of feet. The cervical ribs of the Cobra de Capello, or
hooded snake of the East Indies, are employed for the me
chanical purpose of supporting an expansion of the skin of
the neck, which forms a kind of hood, capable of being
raised or depressed at the pleasure of the animal.* These
ribs are entirely unconnected with the respiration ofthe ser
pent. In the Draco volans, which was to be furnished with in-
* Phil. Trans, for 1804, p. 346.

FLYING LIZARD.

379

struments for assisting it in its distant leaps through the air,
it is again the ribs which are resorted to for furnishing the
basis of such an apparatus. On each side of the dorsal ver
tebrae, as is seen in the skeleton of this animal (Fig. 222,)
the eight posterior ribs on each side, instead of having the
usual curvature inwards, and instead of being continued
round to encircle the body, are extended outwards and elon
gated, and are covered with a thin cuticle, derived from the
common integuments. The ordinary muscles which move
the ribs still remain, but with greatly increased power, and
serve to flap these strangely formed wings at the pleasure of
the animal, during its short aerial excursions.

222

Among the mammalia,. We meet with a few species which
have a broad membrane, formed of a duplicature ofthe skin,
extended like a cloak from the fore to the hind extremities,
and enabling the animal to flutter in the air, and to break its
fall during its- descent from the branches of trees. Struc
tures of this kind are possessed by the Sciurus volans, or

380

THE MECHANICAL FUNCTIONS.

flying squirrel, and also by some other species of the same
genus. They are seen on a still larger scale in the Lemur
volans, or Galeopiihecus. The resistance which these broad
expansions of skin oppose to the air, when the limbs are
spread out, enables the animal to descend in perfect safety
through that medium from very considerable heights; but
these appendages to the body are mere parachutes, not wings,
and none of the animals which possess them can, by their
means, and with the utmost efforts which their muscles are
capable of exerting, ever rise from the ground, or even sus
pend themselves for a moment in the air.
The only quadruped that can properly be said to be en
dowed with the power of flying is the Bat. In this animal
the portions of the skeleton (f, Fig. 223) which correspond

to the phalanges of the fingers are extended to an enormous
length, and the pectoral muscles, which move the anterior
extremities, are of extraordinary size and power. In the
larger species, each wing is at least two feet in length. The
fine membrane, which is spread between these lengthened
fingers, has its origin in the sides of the neck, and reaches
all along the body to the extremities of the hinder legs,
which it encloses in its folds. Thus, not only is the sur
face, by which it acts upon the air, sufficiently' extensive,
but the muscular power, by which its motions are effecled,
is adequate to give it those quick and sudden impulses which
are requisite for flying; and thus, although its structure is
totally different from that of birds, it yet performs fully the

BAT. 381
office of a real wing. The bat flies with perfect ease, even
while carrying along with it one or two of its young : it is
not, however, fitted for very long flights.
The conformation of the skeleton is adapted to this new
and important function. , The chest is broad and capacious
to admit of free respiration while the animal is flying, and
to afford ample space for the attachment of the large mus
cles which have become necessary. The scapulas (s). are
large, and of a singular form, and they are kept at a consi
derable distance asunder by the expanded chesl : their cora
coid processes are also large, and extend in the direction of
the sternum. The clavicles (c) are of enormous size and
length, being larger than either the scapula or the sternum,
and remarkably curved in their shape. The sternum is
much developed, extending laterally, and having a project
ing crest along the middle of its lower surface. The hu
merus (h) is strong, but short ; apparently in order to avoid
the danger of its being snapped asunder by the violent ac
tions of the pectoral muscles, had it been longer. As the
leading object of the structure is to give power to the wing,
there was no necessity for the rotatory motion of the bones
of the fore-arm ; and accordingly we find them consolidated
into one (r;) or rather no part of the ulna is developed, ex
cept the process of the olecranon, or elbow, which has be
come soldered to the radius.
These advantages in the construction of the fore extremi
ties are obtained at the expense of the hinder, which are too-
feeble to support the weight of the body in the upright posi
tion required for walking, in consequence ofthe centre of gra
vity being between the wings. On a level plane, indeed, the
bat can advance only by a kind of crawling or hopping mo
tion. The whole anterior half of the trunk is much more
fully developed than the posterior half, which appears as if
its growth had been arrested. The pelvis (p.). is of diminutive
size, compared with the rest of the skeleton : the pubic bones
are lengthened backwards, and are joined merely at a small
point. The whole posterior limb is short, the femur (f) com-

382 THE MECHANICAL FUNCTIONS.
paratively long, and the fibula is a very slender bone, yet
quite distinct from the tibia (t.) The slight degree of mo
tion which is thus allowed between them is useful to the ani
mal, in enabling the feet to lay hold of cornices or other pro
jecting parts of the roofs of buildings, on which the animal
fastens itself, and hangs with the head' downwards. It is
probably with the intention of facilitating this action that the
toes are turned completely backwards; and that they are of
a curved shape, and generally armed with sharp claws. A
bony appendix (a) projects outwards from the heel, for the
purpose of supporting the hinder prolongation of the mem
brane, which often extends between the hind feet, and is far
ther sustained by the tail, in those species which have the
spine prolonged to form one.
Bats are also provided with another instrument for sus
pending themselves to projecting objects, formed by the
thumb (b,) which is, apparently for this express purpose,
detached from the fingers that support the wing, and is ter
minated by a strong claw, which projects, even when the
wings are folded, and is useful in progression, by serving as
a point of support. § 2. Birds.
It is in birds alone that we find the most perfect adapta
tion of structure to the purposes of rapid and extensive
flight: in them the frame of the skeleton, the figure, position,
and structure of the wings, the size of the muscles, the pecu
liar nature of their irritability, and even the outward form
of the body, have all a direct and beautiful relation to the
properties of the element in which nature has intended them
to move. In their formation, a new, and in as far as relates
to the organs of progressive motion, a more developed type
is adopted; still preserving a conformity with the general
plan of the vertebral organization, and with the general
laws of its development.
The skeleton of birds has the same constituent parts as
that of other vertebrated classes: the bones of the anterior

BIRDS. 383
/ extremity, though destined exclusively to support the wing,
retain the same divisions, and are composed of the usual
elements ; and the general form of the body is that best cal
culated to glide through the air with the least resistance.
As birds swallow their food entire, there is no necessity for
any part of the bulky apparatus of hard and solid teeth, large
muscles and heavy jaws which are required by most quad
rupeds : hence the head admits of being greatly reduced in
its dimensions; and the form of the beak, which is drawn to
a point, and cuts the opposing air, tends to facilitate the pro
gress of the bird in its flight.
In the conformation of the body, also, every circumstance
which could contribute to give it lightness has been sedulous
ly provided. The general size of birds is considerably small
er than quadrupeds of corresponding habits. No where has
Nature attempted to endow a huge ponderous animal, like
the fabled Pegasus, with the power of flight. Great con
densation has been given to the osseous substance,* in order
that the greatest degree of strength might be procured with
the same weight of solid materials; and the mechanical ad
vantage derived from their being disposed in the circum
ference rather than in central masses, has been obtained to
the utmost extent. The horny material, of which the stems
of the feathers are constructed, are, in like manner, formed
into hollow cylinders, which, compared with their weight,
are exceedingly strong. A similar shape has been given to
the cylindrical bones, which are fashioned into tubes with
¦dense but thin sides: most ofthe other bones have likewise
been made hollow, and instead of their cavities being filled
with marrow, they contain only air.f Thus, the whole ske
leton is rendered remarkably light: that, for instance, of the
* Ossification not only proceeds more rapidly, but is also carried to a
greater extent in this class of animals than in any other; as a proof of which,
the tendons, especially those of the muscles of the legs, are frequently ossi
fied. | In the bat there is no provision of this kind for lightening the bones;
and we find them containing marrow, as in other mammalia, and not air.

384 THE MECHANICAL FUNCTIONS.
Pelicanus onocrotalus, or white Pelican, which is five feet in
length, was found by the Parisian Academicians to weigh only
twenty-three ounces, while the entire bird weighed nearly
twenty-five pounds. The cavities in the bones communicate
with large air cells, which are distributed in various parts of
the body, and which contribute still farther to diminish its spe
cific gravity; and by means of canals which open into the
air passages of the lungs, this air finds a ready outlet when
it becomes rarefied by the ascent of the bird into the higher
regions of the atmosphere.*
The conditions in which birds are placed with regard to
the density of the surrounding medium, as well as their mode
of progression, are so opposite to those of fishes, that we
should expect to find great corresponding differences in
their conformation. These two classes of vertebrata, accord
ingly, are remarkably contrasted with respect to the struc
ture of their skeletons. In fishes we have seen that the chest
and all the viscera are carried as far forwards as possible;
the respiratory organs and the centre of circulation being
close to the head, the neck having disappeared, and the
trunk being continued into the lengthened tail, in which the
chief bulk of the muscles are situated. In birds, on the con-
* This air, being contained in the interior of the body, which preserves
a very elevated temperature, must be constantly in a state of greater rarefac
tion than the cooler external air; a condition which must contribute in some
slight degree to render the whole body lighter than it would otherwise have
been. It appears to me, however, that considerably greater importance has
been attached to this circumstance than it really possesses. Many have gone
so far as to represent the condition of a bird as approaching to that of a bal
loon filled with a lighter gas than atmospheric air; and have been lavish in
their expressions of admiration at the beauty of a contrivance which thus
converted a living structure into an aerostatic machine. A litte sober con.
sideration will suffice to show that the amount of tlie supposed advantages
resulting to the bird from the diminution of weight, occasioned by the dif
ference of temperature between the air included in its body and the exter
nal atmosphere, is perfectly insignificant. Any one who will take the trou
ble to calculate the real diminution of weight arising from this cause, under
the most favourable circumstances, will find that, even in the case of the
largest bird, it can neyer amount to more than a few grains.

BIRDS.

385

trary, the ribs, and the viscera which they protect, are
placed as far back along the spinal column as possible; and
a long and flexible neck extends from the trunk to the head,
which is thus carried considerably forwards. These cir
cumstances are very apparent in the skeleton ofthe swan, re
presented in Fig. 224. In a fish, progressive motion is

effected principally by the movements ofthe tail, which im
pels the body alternately from side to side: in a bird, the
only instruments of motion are the wings, which are affixed
to the, fore part of the trunk, and are moved by muscles situ
ated in that region. In the fish, the spine is flexible near-
vol. i. — 49

386 THE MECHANICAL FUNCTIONS.
ly throughout its whole extent; in the bird, it is rigid and
immoveable in the trunk, and is capable of extensive motion
only in the neck.
In order that the body may be exactly balanced while the
bird is flying, its centre of gravity must be brought precisely
under the lme connecting the articulations of the wings with
the trunk ; for it is at these points that the resistance of the
air causes it to be supported by the wings. When the bird
is. resting on its legs, the centre of gravity must, in like
manner, be brought immediately over the b.ase of support
formed by the toes : it becomes necessary, therefore, to
provide nUans for shifting the centre of gravity from one
place to another, according to circumstances, and to adjust
its position with considerable nicety; otherwise there would
be danger of the equilibrium being destroyed, and the body
oversetting. The principal means of effecting these adjust
ments consist in the motions ofthe head and neck, which last
is, for that purpose, rendered exceedingly long and flexible.
The pumb,er of cervical vertebrae is generally very consi
derable ; in the mammalia, as we have seen, there are always
seven, but in mgny birds there are more than twice that
number. In the swan (Fig. 224,) there are twenty-three,
and they are joined together by articulations, generally al
lowing free motion in all directions; that is, laterally, as well
as forwards and backwards. This unusual degree of mobi
lity is conferred by a peculiar mechanism, which is not met
with in the other clg.ss.es of vertebrated animals. A cartilage
is interposed between each of the vertebras, to the surfaces of
which these cartilages are curiously adapted, being enclosed
between folds' of the membrane lining the joint; so that each
joint is in reality double, consisting of two cavities, with an
intervening cartilage.*
It is to be observed, however, that in consequence of the
positions of the oblique processes, the upper vertebrae of

* See Mr. H. Earle's paper on this subject in the Philosophical transac
tions for 1823, p. 277.

BIRDS. 387
the neck bend with more facility forwards than backwards;
while those in the lower half of the neck bend more readily
backwards: hence, in a state of repose, the neck naturally as
sumes a double curvature, like that of the letter S, as is well
seen in the graceful form of the swan's neck. By extend
ing the neck in a straight line, the bird can, while flying,
carry forwards the centre of gravity, so as to bring it under
the wings ; and when resting on its feet, or floating on the
water, it can transfer that.centre backwards, so as to bring it
towards the middle of the body, by merely bending back the
neck into the curved form which has just been described; and
thus the equilibrium is, under all circumstances, preserved by
movements remarkable for their elegance and grace.*
Another advantage arising from the length and mobility
of the neck is, that it facilitates the application of the head
to every part of the surface of the body. Birds require this
power in order that they may be enabled to adjust their
plumage, whenever it has, by any accident, become ruffled.
In aquatic birds, it is necessary that every feather should be
constantly anointed with an oily secretion, which preserves
it from being wetted, and which is copiously provided for
that purpose by glands situated near the tail. The flexibili
ty of the neck alone would have been insufficient for enabling
the bird to bring its bill in contact with every feather, in
order to distribute this fluid equally over them; and there is,
accordingly, a farther provision made for the accomplish
ment of this object in the mode of articulation of the head
with the neck. We haVe seen that, in fishes, and in most
reptiles, this articulation consists of a ball and socket joint;
a rounded tubercle of the occipital bone being received into
a hemispherical depression in the first vertebra of the neck.
In the mammalia the plan is changed, and there are two ar-
* The great mobility of the neck enables the bird to employ its beak as
an organ of prehension for taking its food; an object which was the more ne
cessary in consequence ofthe conversion ofthe fore extremities into wings,
of which the structure is incompatible with any prehensile power, such as is
often possessed by the anterior extremity of a quadruped-.

388 THE MECHANICAL FUNCTIONS.
ticular surfaces, one on each side ofthe spinal canal, formed
on processes corresponding to the leaves of the first cranial
vertebra, and assimilating it more to a hinge joint. In birds,
however, where, as we have just seen, the most extensive
lateral motions are required, the plan of the ball and socket
joint is again resorted to ; and the occipital bone is made to
turn upon the atlas by a single pivot. So great is the free
dom of motion in this joint, that the bird can readily turn-
its head completely back upon its neck, on either side.
As spinous or transverse processes of any length would
have interfered with the flexions of the-neck, we find scarce
ly a trace of these processes in the cervical vertebras of birds.
But another, and a still more important consideration was to
be attended to in the construction of this part of the spine.
It must be recollected that the spinal marrow passes down
along the canal formed by the arches of the vertebrae, and
that any pressure applied to its tender substance would in
stantly paralyze the whole body, and speedilyput an end to
life. Some extraordinary provision was therefore required
to be made, in order to guard against the possibility of this
accident occurring during the many violent contortions into
which the column is liable to be thrown. This is accom
plished in the simplest and most effectual manner, by en
larging the diameter ofthe canal at the upper and lower part
of each vertebra, while, at the
middle, it remains of the usual
size, so that the shape of the ca
vity, as is well seen in Fig. 225,.
which shows a vertical section
ef one of the cervical vertebrae
of the ostrich, resembles that of
an hour glass.* Thus, a wide
space is left at the junction of
each successive vertebra, allow
ing of very considerable flexion,
* For the specimen from which this engraving was made,- 1 am indebted
(to the kindness of Mr. Owen.

BIRDS. 389
without reducing the diameter of the canal beyond that of
the narrow portion, and, therefore, without producing com
pression ofthe spinal marrow. Mr. Earle found* that ver
tebras united in this manner may be bent backwards to a
right angle, and laterally to half a right angle, without inju
ry to the enclosed nervous substance. The design of this
structure is farther evident from its not existing in the dor
sal and lumbar portions ofthe spine, which admit of no mo
tion whatever, and where there is no variation in the diame
ter of the spinal canal.
A plan entirely different is followed in the vertebrae of
the back and loins. For the purpose of ensuring the proper
actions of the wings, the great object here is to prevent mo
tion, and to give all possible strength arid security; and ac
cordingly the whole of this portion of the spine, together
With the sacrum, is consolidated into one piece. All the
processes are largely developed, and pass obliquely from
one vertebra to the next, mutually locking them together;
and, in order most effectually to preclude the possibility of
any flexion, the spinous processes, and sometimes even the
bodies of the dorsal vertebras' are immoveably soldered to
gether by ossific matter, so as to form one continuous bone.
The sacrum (v, Fig. 224) consists ofthe union of a great
number of vertebras, as many as twenty being anchylosed
together for this purpose; so that they form a bone of great
length. Tho coccygeal vertebrae (<z) are also numerous, but
are compressed into a small space, and enjoy great latitude
of motion, being subservient to the movements of the tail.
The ribs are numerous, and of considerable' strength: they
send out processes, which are directed backwards, passing;
over the next rib; before they terminate, and giving very ef
fectual support to the walls of the chest. The ribs are con
tinued along the abdomen, and afford protection to the vis
cera in that cavity; and some arise even from the sacrum,.
and from the iliac bones. Those which are in front are
* In the paper already quoted, p. 278.

390 THE MECHANICAL FUNCTIONS.
united to the sternum (s) by means of sternal appendices,
which are ossified, and appear as the continuations of the
ribs, or as if the ribs were jointed in the middle.
The sternum is of enormous size, extending over a con
siderable part of the abdomen, and having a large perpendi
cular crest descending, like the keel of a ship, from its lower
surface. The object of this great development is to furnish
extensive attachment to the large pectoral muscles employed
to move the wings, and which, taken together, are generally
heavier than the rest of the body. Considered with refe
rence to all the other muscles, and to the weight of the body
itself, these pectoral muscles are of enormous strength. The
flap of a swan's wing is capable of breaking a man's leg; and
a similar blow from an eagle has been known to be instantly
fatal. The bat is the only instance, among the mammalia,
where the sternum presents this peculiar carinaled, or keel
like shape ; and the purpose is evidently the same as in the
bird.* The scapula is generally a small and slender bone. The
coracoid bone (k) is largely developed, and assumes much of
the apppearance of a clavicle.! But the real clavicles (c) are
united below, where they join the forepart of the sternum,
appearing as one bone, which, from its forked shape, has
been denominated the furcular bone. In the fowl it is
commonly known by the name of the merry-thought. This
bone, placed at the origin of the wings, and stretching from
one to the other, is of great importance as constituting a firm
basis for their support, and for securing their steadiness of
action ; and being, at the same time, very elastic, it tends to

* Notwithstanding the great modification which the sternum has received
in the bird, when compared with its form in the tortoise and the quadruped,
we may still trace the same nine elements entering into its composition,
though developed in very different proportions.
¦j- Many have considered this bone as being the clavicle, and have regarded
the furcular bone as a new bone, or supplementary clavicle; but all the ana
logies of position and of development are in favour ofthe views stated in the
text.

BIRDS. 391
restore them to their proper situations, after they have been
disturbed by any violent impulse.
The wing of a bird does not, at first view, present much
analogy with the fore extremity of a quadruped; but on a
closer/examination we find it to contain all the principal
bones of the latter, though somewhat altered in shape, and
still more changed in their functions. Yet still the same
unity of plan, and perfect harmony of execution may be,
discerned in the mechanism of this refined instrument of a
higher mode of progression.
The head of the humerus (h) has a compressed form ; and
in order to obtain great extent of motion, it is made to play
by a very small cylindrical surface upon the scapula; thus
admitting of the complete descent of the wing, unobstructed
by any opposing process, but at the same time limiting its
motion to one plane. It is connected below, by broad at
tachments, to the radius and ulna, forming with them a hinge
joint. These latter bones are separate, and of great length,
but so firmly united together by ligament as scarcely to have
any motion on one another. The carpus (w,) consists of
two bones only, the one articulated with the radius, the
other with the ulna. They move together as one piece ;
but, contrary to what takes place in quadrupeds, the move
ments are made from side to side, instead of their consisting
of flexion and extension; this variation from the usual struc
ture being for the purpose of folding down the joints of the
wing, and bringing them close to the body. The metacar
pus (m) consists originally of two bones, which soon become
united into one at the upper part. On the radial side it has
a process, derived perhaps from a third metacarpal bone,
which is anchylosed at a still earlier period of ossification;
and to this process a small pointed bone1 is connected, cor
responding to a rudimental thumb (t.) There are generally
two fingers, of which the first exhibits traces of having been
originally two*" bones : the inner finger consists of two or
three long phalanges, and the outer one of a single phalanx:
there js sometimes also a rudimental bone corresponding to

392 THE MECHANICAL FUNCTIONS.
a little finger. The degree of development of these bones
varies in different tribes of birds.
Feathers are attached to all these divisions of the limb,
namely, to the humerus, the fore arm, the hand, and occa
sionally to the single phalanx of the thumb. The structure
Of feathers is calculated in 'an eminent degree to combine
the qualities of lightness and of strength, which we else
where rarely find united. The horny materials of which
the stem of the quill is made are tough, pliant, and elastic;
and, as we have already seen, are disposed in the most ad
vantageous manner for resisting flexion by being formed
into a hollow cylinder. But the vane of the feather is still
more artificially constructed; being cpmposed of a number
of flat threads, or filaments, so arranged as to oppose a much
greater resistance to a force striking perpendicularly against
their surface, than to one which is directed laterally; that
is, in the plane of the stem. They derive this power of re
sistance from their flattened shape, which allows them to
bend less easily in the direction of their flat surfaces than
in any other; in the same way that a slip of card cannot
easily be bent by a force acting in its own plane, though it
easily yields to one at right angles to it. Now it is exactly
in the direction in which they do not bend that the fila
ments of the feather have to encounter the resistance and
impulse of the air. It is here that strength is wanted, and
it is here that strength has been bestowed.
On examining the assemblage of these laminated filaments
still more minutely, we find that they appear to adhere to
one another. As we cannot perceive that they are united
by any glutinous matter, it is evident that their connexion
must be effected by some mechanism invisible to the un
assisted eye. By the aid of the microscope, the mystery is
unravelled, and we discover the presence of a number of
minute fibrils, arranged along the margin of the laminae, and
fitted to catch upon and clasp one another, Whenever the la
minae are brought within a certain distance. The fibrils of
a feather from the wing of a goose are represented magnified

FEATHERS OF BIRDS. 393'
at a, a, b, b, Fig. 226, as they arise from the two sides of
the edges of each lamina: they are exceedingly numerous,
above a thousand being contained in the space of an- inch;-

and they are of two kinds, each kind having a different form'
and curvature. Those marked a, a, which arise from the
side next to the extremity of the feather are branched or
tufted, and bend downwards, while those marked b, b, pro
ceeding from the other side Of the lamina, or that nearest
the root of the feather, are shorter and firmer, and do not
divide into branches, but are hooked at the extremities, and
are directed upwards. When the two laminae are brought
close to one another, the long, curved fibrils of the one be
ing carried over the' short and straight fibrils of the other,-
both sets become' entangled together; their crooked ehds
fastening into one another, just as the latch of a door falls'
into the' cavity of the catch which is fixed in the door-post
to receive it. The way in which this takes place will be
readily perceived by making a section of the Vane of a fea
ther across the laminae, and' examining, with a good micro
scope, their cut edges, while they are gently separated from
one another. The appearance they then present is exhibited
in- Fig. 227, which shows distinctly the fornv direction, and
relative positions of each' set of fibrils, and the manner in
which they lay hold of one another. This mechanism is re
peated over every part ofthe feather, and constitutes a close-
vol. i. — SO

394 THE MECHANICAL FUNCTIONS.
ly reticulated surface of great extent, admirably calculated
to prevent the passage of the air through it, and to create,
by its motion, that degree of resistance which it is intended
the wing should encounter.* In feathers not intended for
.flight, as in those of the ostrich, the fibrils are altogether
wanting: in those of the peacock's tail, the fibrils, though
large, have not the construction which fits them for clasp
ing those of the contiguous lamina; and in other instances
they do so very imperfectly.
A construction so refined and artificial as the one I have
been describing, and so perfectly adapted, to the mechanical
object which it is to answer, cannot be contemplated without
the deepest feeling of admiration, and without the most eager
curiosity to gain an insight into the elaborate processes,
which, we cannot doubt, are employed by nature in the for
mation of a fabric so highly finished, and displaying such
minute and curious workmanship. It is only very recently
that we have been admitted to a close inspection of the com
plicated machinery, which is put in action in this branch of
what may be called organic architecture; and certainly none
is more fitted to call forth our profoundest wonder at the
comprehensiveness of the vast scheme of divine providence,
which extends its care equally to the perfect construction of
the minutest and apparently most insignificant portions of
the organized frame, whether it be the down of a thistle, the
scales of a moth, or the fibrils of a feather, as well as to
the completion of the larger and more important organs of
vitality. Every bird, on quitting the egg, is found to be covered
on all parts except the under side, with a kind of down, con
sisting of minute filaments, collected in tufts, and resem-
* A very clear account ofthe mechanism described in the text is given by
Paley, in the 12th chapter of his " Natural Theology." Many of the mi
nuter details I have supplied from my own observations with the microscope.
The branched form of the upper fibrils* and the reticulated structure ofthe
laminae themselves, when viewed with a high magnifying power, are parti-
oularly beautiful microscopic objects.

1 FEATHERS OF BIRDS. 395
bling in their arrangement the fibres of a camel-hair pencil.
Each tuft contains about ten or twelve filaments, growing
from the upper ends of bulbous roots implanted in the skin>
and which are the rudiments of the organs that afterwards
form the feathers, of which this down, serving the purpose
of a first garment, hastily spread over the young bird, is but
the precursor; for the tufts generally soon fall off and disap
pear, except in the rapacious tribes, as the eagle and the vul
ture, where they remain attached to the feathers for a con
siderable time.
While this temporary protection is given to the integu
ment, extensive preparations are making underneath for fur
nishing a more effective raiment, adapted to the future wants
of the bird. The apparatus by which the feathers are to be
formed is gradually constructing ; and its rudiments are re
ceiving the necessary supply of nutrient juices, and of ves
sels for their circulation, together with their usual comple
ment of nerves and absorbents. When first visible, this
organ has the form of a very minute cone, attached by a fila
ment proceeding from its base to one of the papillae of the
skin, and establishing its connexion with the living system.
In the course of a few days, this cone has become elongated
into a cylinder, with a pointed extremity, while its base is
united to the skin by a more distinct bond of connexion,
formed by the enlarged vessels, which are supplying it with
nourishment. It is in the interior of this cylinder that all
the parts of the feather are constructed ; their earliest rudi
ments being formed at the upper part, or apex of this organ;
and the materials of the several parts of the feather being
successively deposited and fashioned into their proper shapes
in different plaees: for while the first laminas are construct
ing in one portion of the cylinder, the next are only just be
ginning to be formed in another; and while the outer cover
ing of the stem is growing from one membrane, the interior
spongy tissue is deposited in other places, in various stages
of softness or consolidation: so that the whole composes a
isystem of operations, which may be said to resemble in its

396

THE MECHANICAL FUNCTIONS.

complication at least, although on a microscopic scale, an
extensive manufactory. Hence will be readily understood
how great must be the difficulty of tracing all the steps of
these multifarious processes, which are carried on in so
small a space : and this difficulty is much increased from the
circumstance that the organ in which they take place is it
self only developed as the work proceeds, its different parts
being produced successively in proportion as they are want
ed, and their form and structureundergoing frequent varia
tion in the course of their development.

The most elaborate and apparently accurate researches on
this intricate subject, are those lately undertaken by M.
Frederick Guvier, from whose memoirs* I have selected
the following abridged statement of the principal results of
his observations. It will be necessary, in order to obtain a
* Memoires du Museum, xiii. 327; and Annales des Sciences Naturelles,
ix. 113.

FEATHERS OF BIRDS. 397
clear idea of the several steps of the process to be described,
to advert to the structure of a feather in its finished state.
For this purpose we need only examine a common feather,
such as that represented in Fig. 228, where s is the posterior
surface of the solid stem, which, it will be perceived, is
divided into two parts by a longitudinal groove* and from
either side of which proceed a series of laminae, composing,
with their fibrils, what is termed the vane of the feather
(v.) The lines from which these laminae arise, approach
one another at the lower part of the stem, till they meet at
a point, where the longitudinal groove terminates, and where
there is a small orifice (o,) leading to the interior of the quil}.
From this part the transparent tubular portion of the quill
(t) commences; and at its lower extremity (l) there exists
a second, or lower orifice.
The entire organ which forms the feather, and which may
be termed its matrix, is represented in Fig. 229, when it
has attained the cylindric form already described; of which
a is the apex, or conical part, that rises above the cuticle,
and b the base, by which it is attached to the corium, or true
skin. A white line is seen running longitudinally the whole
length of the cylinder, and another, exactly similar to it, is
met with on the opposite side : the one corresponds in situ
ation to the front, and the other to the back of the stem of
the future feather. On laying open the matrix longitudi
nally, as is shown in Fig. 230, it is found to be composed
of a sheath or capsule, and of a central pulpy mass, termed
the bulb. The capsule consists of several membranous lay
ers (c, e, s, i,) which are more consolidated near the apex,
and become gradually softer and more delicate, as we trace
them towards the base of the matrix, where their formation
is only beginning to take place.
The laminas and their fibrils, the assemblage of which
constitutes the vane of the feather, are the parts which are
first formed; and their construction is effected in the space
between the outer capsule (c,) and the central bulb (b,) in a
mode which is exceedingly remarkable, and different from

398 THE MECHANICAL functions.
that of the formation of any other organic product with
which we are acquainted. Instead of growing from a base,
like hairs, and other productions of the integuments, by suc
cessive depositions of layers, the materials which are to
-i compose the laminae are cast in moulds, where they harden
and acquye the exact shape of the recipient cavities. The
next object of our curiosity, then, is to learn the way in
which these moulds are constructed; and on careful exami
nation they appear to be formed by two striated membranes,
the exterior one (e) enveloping the other (i,) or interior
membrane. These membranes are separated by a series of
partitions, which commence at the edges of the longitudinal
white band, seen in Fig. 229, and wind obliquely upwards
till they reach the opposite longitudinal band already de
scribed, where they join a longitudinal partition which oc
cupies a line answering to that posterior band. Thus they
leave between them narrow spaces, which constitute so
many compartments for the deposition, as in a mould, of the
material of each lamina. The course'of these channels, and
their junction at the back of the matrix is seen at s, Fig.
230. It is exceedingly probable, though from the minute
ness of the parts it is scarcely possible to obtain ocular de
monstration of the fact, that the fibrils of the laminas are
constructed in a similar manner, by being moulded in still
more minute compartments, formed by transverse mem
branous partitions.
The proper office of the bulb, after it has supplied the
materials for the formation of the laminae, is to construct
the stem of the feather, and unite the laminas to its sides.
For this purpose the anterior portion of the bulb deposites
on its surface a plate of horny substance, while another plate
is formed by the posterior part in the interior of the bulb.
Thus the bulb becomes divided into two portions, one ante
rior and the other posterior. The former of these, after
having finished the external plate, proceeds to form the
spongy substance, which is to connect the two plates, and
the posterior portion of the bulb embraces the inner plate,

FEATHERS OF BIRDS. 399
and gradually folds it inwards till its sides meet at the mid
dle groove along the back of the stem. The anterior part
of the bulb, during the process of filling up the stem, exhi
bits a series of conical-shaped membranes, as is- seen ire
the section, Fig. 231 ; the points of the cones being directed'
upwards, and their intervals being occupied by the spongy
substance in different stages of consolidation, and more per
fected in proportion as they are situated nearer the apex of
the stem.
While the construction of the feather, in its different
stages, is thus advancing from below, those parts which
are completely'forrhed, are rising above the surface of the ,
skin, still enveloped in the capsule which originally protected
them, but the upper portions of which, from the action- of
the air, and the obliteration of the vessels that nourished
them, now decaying, shrivel and fall off in shreds, allowing
the successive portions of the feather ta come forth, and the
laminas to unfold themselves as they rise and' assume their
proper shapes. This successive evolution proceeds until the
principle parts of the stem and of the vane are completed p
and then a different kind of action takes place. The poste
rior part of the bulb now contracts itself, and bringing the*--
edges of that surface of the stem closer together, at length
unites them at the superior orifice (o,)Fig. 228; where the
laminae, which follow these lines, also terminate. Having;
thus performed' the office assigned to it, it ceases tp be '
nourished, and is incapable any longer of depositing a horny
covering to the feather : all that remains of its substance is a
thin membrane which adheres to thej outside of the tubular
part or barrel of the quill, and which must be scraped1 off
before the latter can be used as a pen. The tubular part is
the product of the anterior part of the bulb, which now
ceases to deposite the spongy substance, but forms a transpa
rent horny material over the whole of its external surface?
but as it retires towards the root, it leaves a succession of
very thin pellucid membranes, in the form of cones, which,
when dried, form what is termed the pith of the quill. The

400 THE MECHANICAL FUNCTIONS.
last remnant of the bulb is seen in the slender ligament
which passes through the lower orifice, and preserves the
attachment of the feather to the skin. In process of time,
this also decays, and the whole feather is cast off, prepara
tory to the formation of another, which, in due season, is to
replace it. All the feathers, are in general, moulted annual
ly, or even at shorter periods ; and the same complicated
process is again begun and completed by a new matrix pro
duced for the occasion1, every time' a new feather is to be
formed. It is impossible, on reviewing these curious facts, not to
be struck with the admirable art and foresight which are
implied in all this long and complicated series of operations.
While the bird was thus nourished by the fluids of the egg,
the ground had already been prepared for its future plumage,
and for the formation of instruments of flight. A tempora
ry investment of down is in readiness to shelter the tender
chicken from the rude impressions of the air, and an
apparatus is preparing for the construction of the most re
fined instruments- for clothing and for motion : first, the scaf
folding, as it may be called, is erected, by the help of which
each portion is built up in succession, and in proper order.
Nature's next care is to construct the vane, which is the part
of the feather most essential to its office ; and then to form
the shaft to which the vane is to be affixed, and from which
it receives its support : lastly, she forms the barrel of the
quill, which is prolonged for the purpose of converting it
into a lever of sufficient length for the mechanical office it
has to perform* In proportion as each structure is finished,
she negleets not to remove the scaffolding which had been
set up as a temporary structure ; the membranes, with all
their partitions, are carried away, the vascular pulp of the
bulb is absorbed, and its place supplied by air, thus securing
the utmost lightness, without any diminution of strength.
Is it possible for any rational mind, after meditating upon
these facts, to arrive at the persuasion that they are all the
mere results of chance ?

WINO OF BIRDS. 401
Several circumstances remain to be noticed respecting the
structure and actions of the wings of birds. If we attend to
the mode of their articulation with the scapula, we find it
producing a motion oblique with regard to the axis of the
body, so that the stroke which they give to the air is directed
both downwards and backwards ; and the bird, while moving
forwards, is at the same time supported in opposition to the
force of gravity. The different portions of the wing are
likewise so disposed as to be contracted and folded together
when the wing is drawn up, but fully expanded when it de
scends in order to strike the air. It is obvious that, without
this provision, a great part of the motion acquired by the
resistance of the air against the wing in its descent would
have been lost by a counteracting resistance during its as
cent. The disposition of the great feathers is such that they
strike the air with their flat sides, but present only their
edges in rising; what is called feathering the oar in rowing
is a similar operation, performed with the same intention,
and deriving its name from this resemblance.
As the inclination of the wing is chiefly backwards, the
greatest part of the effect produced by its action is to move
the body forwards. Birds of prey have a great obliquity
of wing, and are consequently better formed for horizontal
progressive motion, which is what they chiefly practise in
pursuing their prey, than for a rapid perpendicular ascent.
Those birds, on the contrary, which rise to great heights in
a direction nearly vertical, such as the Quail and the Lark*
have the wings so disposed as to strike directly downwards,
without any obliquity whatsoever. For the same reason,
birds rise better against the wind, which, acting upon the
oblique surface presented by the wings during their flexion,
contributes to the ascent of the body, on the same principle
that a kite is carried up into the air when retained in an
oblique position. This circumstance is particularly observa
ble in the ascent of birds of prey, whose wings have a great
obliquity, and, when fully expanded, present a very large
extent of surface.
vol. i. — 51

402 THE MECHANICAL FUNCTIONS.
The actions of the tail, which operates as a rudder, are
useful chiefly in directing the flight. When the tail is short,
this office is supplied by the legs, which are in that case
generally very long; and being raised high and extended
backwards in a straight line, are of considerable assistance
in the steerage of the animal. In many birds, as in the
wood-pecker, the tail is much employed as a support to the
body in climbing trees. The caudal vertebras are often nu
merous, but are short and compressed together; they are re
markable for the great development of their transverse pro
cesses, and for having spinous processes both on their lower
and upper sides. The last vertebra, instead of being cylin
drical, has abroad carinated spine for the insertion of large
feathers. Birds could not, of course, be always on the wing ; for a
great expenditure of muscular power is constantly going on
while they support themselves in the air. Occasional rest
is necessary to them as well as to other animals, and means
are accordingly provided by nature for their mechanical
support and progressive motion while on land.
The anterior extremities having been exclusively appro
priated to flight, and constructed with reference to the pro
perties of the atmosphere, the offices of sustaining and of
moving the body along the ground must be intrusted wholly
to the hind limbs. The centre of gravity, before sustained
by the wings, must now be brought over the new basis of
support formed by the feet; or rather, as it is placed far for
wards, the feet must be considerably advanced so as to be
brought underneath that centre. But as the bones of the
posterior extremity have their origin from the remote part
of the pelvis, which is elongated backwards, at a considera
ble distance from the wings, it became necessary to lengthen
some of their parts, and to bend their joints at very acute
angles. We accordingly find that while nature, in ihe for
mation of the limb, has preserved an accordance with the
vertebrated type, both as to the number of pieces which
compose it, and as to their relative situations, she has devi-

FEET OF BIRDS. 403
ated from the model of quadrupeds in giving much greater
length to the division corresponding to the foot. At the
same time that the foot is brought forwards, the toes are
lengthened, and made to spread out so as to enclose a wide
base, over which the centre of gravity is situated. The ex
tent of this base is so considerable that a bird can, in general,
support itself with ease upon a single foot, without danger of
being overset by the unavoidable vacillations of its body.
The femur is short compared with the tibia, which is ge
nerally large, especially in the order of Gralla, or wading
birds: the fibula is exceedingly slender, and always united,
at its lower part, with the tibia; and there is a total defici
ency of tarsal bones, except in the Ostrich, where rudiments
of them may be traced. Already we have seen, in ruminant
quadrupeds, that these bones have dwindled to a very small
size, but here they have wholly disappeared. The long
bone which succeeds to the tibia, though considered by some
anatomists as the tarsus, is, properly, the metatarsal bone,
and in the Grallas is of great length. At its lower end it has
three articulations, shaped like pulleys, for the attachment of
the three toes: there is, besides, in almost all birds, a small
rudiment of another metatarsal bone, on which is situated
the fourth toe. The number of bones which compose each
respective toe appears to be regulated by a uniform law.
The innermost toe, which may be compared to a thumb, con-
' sists invariably of two bones: that which is next to it in the
order of sequence has always three; that which follows has
four; and the outermost toe has five bones : the claws in eve
ry case being affixed to the last joints, which have, therefore,
been termed the ungual bones. This remarkable numerical
relation, among the several bones of the toes, exists quite in
dependently of their length.
There is one whole order of birds which are particularly
fitted for climbing and perching upon trees, having the two
middle toes parallel to each other, and the inner and outer
toes turned back, so as to be opposed to them in their action.
They are thus enabled to grasp objects with the greatest fa-

404 THE MECHANICAL FUNCTIONS.
cility ; having, in fact, two thumbs, which are opposable to
the two fingers. They have been termed Scansores, or Zy
godactyly Almost all other birds have three toes before,
and one behind.
From this enumeration, it would appear as if Nature, in
modifying the type of vertebrated animals to suit the pur
poses required in the bird, had purposely omitted one of the
toes which are usually five in number. But instances occur
of birds, in which we may trace the rudiment of a fifth toe
high upon the metatarsus, and upon its inner side. The
spur of the cock may be regarded as having this origin.
What confirms this view of the subject, is, that in those
birds which have only three toes, namely, in the Emu, the
' Cassowary, and the Rhea, it is again the inner toe. which
disappears, leaving only the three outer toes, namely, those
which have, respectively, three, four, and five phalanges.
The Ostrich has only two toes, one having four, and the
other five phalanges; here, again, it is the innermost ofthe
three former, that is, the one having three phalanges, which
has been suppressed.*
A bird is capable of shifting the position of the centre of
gravity of its body according as circumstances require it,
simply by advancing or drawing back its head. While fly
ing, the neck is stretched forwards to the utmost, in order
to bring the centre of gravity immediately under the origin
of the wings, by which the body is then suspended. When-
birds stand upon their feet, they carry the head back as
far as possible; so as to balance the body on the base of sup
port. When preparing to sleep, they bring the centre of
gravity still lower, by turning the head round and placing
it under the wing. These motions of the head are again re
sorted to when the bird walks ; and the centre of gravity is
thus transferred alternately from one foot to the other: hence
• The last bone of the outer toe of the ostrich is very small, and being
usually lost in preparing the skeleton, has been overlooked by naturalists ;
but Dr. Grant has ascertained, by the careful dissection of a recent specimen,
the existence of this fifth phalanx.

FEET OF BIRDS.

405

in walking, the head of a bird is in constant motion; whilst
the duck and other birds, whose legs are very short, have a
waddling gait. It may be observed that the more perfect
ly predaceous birds are not the best formed for walking; be
cause, where they use their feet for that purpose, their talons,
which are required to be kept sharp for seizing and tearing
their prey, would be blunted ; and accordingly the eagle,
when moving along the ground, supports itself partly by the
motion of its wings.
In roosting', birds often support themselves upon their
perch by means of one leg only, the other being folded close
to the body. They even maintain this attitude with greater
ease and security than if they rested upon both feet. The
true explanation of this curious fact was long ago given by
Borelli. On tracing the tendons (t, t Fig. 233) of the mus
cles (m, m) which bend the claws, and enable them to grasp
an object, we find them passing over the outer angles of each
of the intervening joints, so that whenever these joints are
bent,'as shown in Fig. 234, those tendons are put upon- the

stretch, and mechanically, or without any action of the mus*
cles, tend to close the foot. When the bird is on its perch,
this effect is produced by the mere weight of the body,-
which of course, tends to bend all the joints of the limb on
which it rests ; so that the greater that weight, the greater is

406 THE MECHANICAL FUNCTIONS.
the force with which the toes grasp the perch. All this
takes place without muscular effort or volition on the part
of the bird. It remains in this position with more security
on one foot than it would have done by resting upon both;-
because, in the latter case, the -weight ofthe body being di
vided between them, does not stretch the tendons sufficient
ly. In this position, the bird not only sleeps in perfect se
curity, but resists the impulse of the wind and the shaking
of the bough.
The great length of the toes of birds enables them to stand
steadily on one leg; and in this attitude many employ the
other foot as a hand ; especially parrots, whose head is too
heavy to be readily brought to the ground. Some birds,
which frequent the banks of rivers, are in the practice of
holding a stone in one foot, while they rest upon the other :
this contributes to increase their stability in two ways ; first,
it adds to the weight of the body, which is the force that
stretches the tendons, and causes them to grasp the bough;
and secondly, it also lowers the centre of gravity.
The stork, and some other birds belonging to the same
order, which sleep standing on one foot, have a curious me
chanical contrivance for locking the joint ofthe tarsus, and
preserving the leg in a state of extension without any mus
cular effort. The mechanism is such as to withstand the ef
fect of the ordinary oscillations of the body, when the bird
is reposing; but it is easily unlocked by a voluntary muscu
lar exertion, when the limb is to be bent for progression.
On these occasions the ball of the metatarsal bone is driven
with some force into the socket of the tibia.*
I must content myself with this general View of the me
chanism of birds ; as it would exceed the limits within which
I must confine myself, to enter more fully into the peculi
arities which distinguish the different orders and families.
* This mechanism is noticed by Dr. Macartney, in the Transactions of the
Hoyal.lrish Academy, vol. xiii. p. 20, and is more fully described in Eees's
Cyclopedia, Art. Bihd. He observes thatjDOth Cuvier and Dumeiil have
committed an error in referring this peculiarity of structure to the knee in
stead ofthe tarsal joint.

FEET OF BIRDS. 407
Some of the more remarkable deviations from what may be
considered as the standard conformation, may, however, for
a moment arrest our attention.
The Ostrich, of all birds, presents the greatest number of
exceptions to the general rules which appear to regulate
the conformation of birds, and in many of its peculiarities
of structure it makes some approach to that which charac
terizes the quadruped. Though this bird is provided with
wings, it was evidently never intended that they should be
used for the purposes of flight. Hence the chief muscular
power has been bestowed on the legs, which are remarka
bly thick and strong, and well fitted for rapid progression.
The sternum is flat, and does not present the keel-like pro
jection which is so remarkable in that of all other birds;
The clavicles do not reach the sternum, nor even meet at
the anterior part ofthe chest to form the furcular bone: for
as the wings are not employed in flying, the usual office of
that bone is not wanted. The form of the pelvis is differ
ent from the ordinary structure; for the pubic bones, which
in all other birds are separated by an interval, here unite as
they do in quadrupeds.
The feathers are unprovided with that elaborate apparatus
of crotchets and fibres, which are universally met with in
birds that fly. The filaments of the ostrich's feathers, in
consequence of having none of these fibrils, hang loose and
detached from one another, forming the fine hair or down,
which, however ornamental as an article of dress, must be
viewed, when considered physiologically, as a species of de
generacy in the structure of feathers.
The Penguin, in like manner, has a wing, which is, by its
shortness, totally unfitted for raising the body in the air: it
has, indeed, received a very different destination, being
'formed for swimming. In external form, it resembles the
anterior extremity of the turtle; but, still, we find it con
structed on the model of the wings of birds; as if nature had
bound herself, by a law, not to depart from the standard of
organization, although the purpose of the structure is alto
gether changed. As penguins are intended for a maritime

408 THE MECHANICAL FUNCTIONS.
life, all their extremities are formed for swimming.. Their
legs are exceedingly short, and placed far backwards; so that
these birds are compelled, when resting on their feet on the
shore, to raise their bodies in a perpendicular attitude, in
order to place the centre of gravity immediately above the
base of support; a posture which gives them a strange and
grotesque appearance.
I have already alluded to the lengthened legs and feet of
the waders, the utility of which to birds frequenting marshy
places, and shallow waters, is very obvious. Their legs are
not covered with feathers, which would have been injured
by continual exposure* to wet. But birds of a truly aquatic
nature have their toes webbed, that is, united by a mem
brane, a mechanism which qualifies them to act as oars, and,
indeed, gives them a great advantage over all artificial oars,
that have been constructed by human ingenuity; for, as soon •
as the expanded foot has impelled the water behind it, the
toes collapse, and, while it is drawn forward, it presents a
very small surface to the opposing water. Their plumage
is so constructed as to prevent the water from penetrating
through it; and for the purpose of preserving it in this con
dition, these birds are provided with an oily fluid, which
they carefully spread over the whole surface of their bodies.
The Swan, and many other water-fowls, employ their wings
as sails, and are carried forwards on the water with consi
derable velocity, by the mere impulse ofthe wind.
Birds excel all other vertebrated animals in the energy of
their muscular powers. The promptitude, the force, and
the activity they display in all their movements, and the un
wearied vigour with which they persevere, for hours and
days, in the violent exertions required for flight, far exceed
those of any quadrupeds, and imply a higher degree of irri
tability, dependent, probably, on the great extent of their
respiratory functions, than is possessed by any other class of
animals.

END OF VOL. I.

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