i 
 

 / 
 
 ■ I 
 
 
 
 
 I 
 
 
 
 

 
 ‘■A 
 
 S ' ■ < 
 
 ^ ./ 
 
INSTITUTIONS 
 
 O F 
 
 M E D r C I N E. 
 
 PART I. 
 
 PHYSIOLOGY. 
 
 For the ufc of the Students in the Univerfity of Edinburgh. 
 
 By WILLIAM CULLEN, M.D. 
 
 Professor of the Practice of Physic, &c. &. 
 
 The third EDITION, Corrected, 
 
 EDINBURGH: 
 
 Printed for CHARLES ELLIOT, Edinbur,gh| 
 And T. C A D E L L, Strandy London. 
 
 
 M,DCC,LXXXV. 
 
Digitized by the Internet Archive 
 in 2016 
 
 https://archive.org/details/institutionsofme01cull 
 
ADVEERTISMENT. 
 
 T he following fheets were originally 
 printed as a text-book for the ufe of 
 Students of Medicine in the Univerlity of 
 Edinburgh; it of courfe contains only 
 Iketches of what was more fully delivered 
 in the Profeffor’s ledures. Though the 
 impreffion formerly made has been long 
 ago difpofed of, the Author did not choofe 
 to renew it till he fliould be at leifure to 
 give it to the public in a more complete 
 form. But Dr Gregory, who now teaches 
 this branch of medicine, thinking thofe 
 Iketches might be ufeful to his Students, 
 the author has confented to the republica- 
 tion : and it is now offered to the public 
 in a more corred manner. 
 
 •?.f;955 
 
Jiift publifhed Charles Elliot Edinburgh^ and 
 T. Cad ELL 'London^ 
 
 FIRST LINES 
 
 OF THE 
 
 PRACTICE OF PHYSIC. 
 
 By WILLIAM CULLEN, M.D. 
 
 Profeflbr of the Praftice of Phyfic in the Univcrfity of 
 Edinburgh ; Firft Phyfician to his Majefty for Scotland; 
 Fellow of the Royal College of Phyficians of Edinburgh, 
 of the Royal Societies of London, of Edinburgh, See. See. 
 
 NOW FIRST COMPLETED 
 
 In four volumes octavo. 
 
 Price 1 1 . 4s. in boards. 
 
 ALSO, 
 
 m CULLEN ON THE RECOVERY 
 
 OF 
 
 PERSONS DROWNED and SEEMINGLY DEAD. 
 Price One Shilling, 
 
INSTITUTIONS 
 
 O F 
 
 MEDICINE. 
 
 I. 
 
 M EDICINE is the art of preventing 
 and of curing difeafes, 
 
 IL 
 
 Before confidering the application of 
 this art to particular difeafes, certain ge- 
 neral doctrines are neceffary to be premi- 
 fed, which are called THE INSTITU- 
 TIONS OF MEDICINE. 
 
 Ill, 
 
( 6 ) 
 
 III. 
 
 The Inftitutions of Medicine are di- ‘ 
 vided into three parts. 
 
 The firfl treats of life and health. 
 
 The fecond delivers the general doc- 
 trine of difeafes. 
 
 The third delivers the general docclrine 
 concerning the means of preventing and 
 curing difeafes. 
 
 PART 
 
PART I. 
 PHYSIOLOGYe 
 IV. 
 
 T he dodlrlne which explains the con'^ 
 ditions of the body and of the mind 
 neceffary to life and health, is called PHY- 
 SIOLOGY, or the Do(51;rine of the Animal 
 Oeconomy. 
 
 The fun<5lions of the animal oeconomy 
 are many and various ; and fo complicated 
 with each other, that it is difficult to find 
 the moft proper order in which they may 
 be delivered. That, however, feems the 
 A 2 beft 
 
PHYSIOLOGY. 
 
 n 
 
 beft which confiders them as nearly as 
 may be according to the feries of caufes 
 and effects. 
 
 VI. 
 
 Upon this plan we fliall treat, 
 
 1. Of the folid matter, of which a 
 great part of every organ of the body con- 
 lifts. 
 
 2. Of the nervous fyftem, in which the 
 motions of the body for the moft part be- 
 gin, and upon which the motions pro- 
 duced in it chiefly depend. 
 
 3. Of the motion and circulation of the 
 blood, and of the feveral organs and 
 adlions employed in fupporting it. 
 
 4. Of the fundlions employed in fup- 
 porting and repairing the feveral folid 
 and fluid matters of the bodyj and, on 
 this occaflon, of the nature of the feveral 
 fluids themfelves. 
 
 5. Of 
 
 3 
 
PHYSIOLOGY. 
 
 9 
 
 5. Of the organs employed in receiving 
 and modifying the imprelTions of external 
 bodies neceflary to fenfation ; and of their 
 fever al fundiions. 
 
 6. Of the motions of the whole body, 
 or of its feveral parts which depend on 
 the adlion of mufcles, and not before ex- 
 plained. 
 
 7. Of the funflions peculiar to the 
 fexes, and of generation. 
 
 A 3 
 
 PHY- 
 
PHYSIOLOGY. 
 
 SECT. I. 
 
 \ 
 
 Of the Simple Solids. 
 
 VII. 
 
 The folid parts of the body feem to be 
 of two kinds ; one whofe properties are the 
 fame in the dead as in the living, and the 
 fame in the animate as in many inanimate 
 bodies ; the other, whofe properties appear 
 only in living bodies. In the laft, a peculiar 
 organization, or addition, is fuppofed to 
 take place; in oppofition to which, the 
 hfft are called the SIMPLE SOLIDS. Of 
 thefe only we fhall treat here ; and of the 
 others, which may be called VITAL SO- 
 LIDS, being the fundamental part of the 
 
 ner- 
 
PHYSIOLOGY. 
 
 1 1 
 
 nervous fyftem, we Ihall treat under that 
 title in the following fedion. 
 
 VIIL 
 
 The fimple folids are fuited to the pur- 
 pofes of the animal oeconomy by a certain 
 force of cohefion, joined with a certain 
 degree of flexibility and elafticity. 
 
 IX. 
 
 / 
 
 Thefe properties of the fimple folids, in 
 different parts of the body, in different 
 bodies, and on difierent occafions in the 
 fame body, are neceffarily in different de- 
 grees ; and this feems to depend upon the 
 difference of the mixture, aggregation, or 
 organization of the folid. 
 
 X. 
 
 The matter of the fimple folid every 
 
 where 
 
12 
 
 PHYSIOLOGY. 
 
 where, except in the bones, appears to be 
 an homogeneous aggregate ; and there is 
 no proper evidence of its being formed of 
 certain parts naturally difcrete and inco- 
 herent, which are cemented by others of a 
 different nature. 
 
 XI. 
 
 Of the flmple folid confidered as an ho- 
 mogeneous aggregate, the integrant part$ 
 are a mixt, which feems to be nearly of the 
 fame kind in all the different parts of the 
 human body, and perhaps in moft of the 
 parts of every animal : So far as we yet 
 know, the variety of it is very inconfider- 
 able. 
 
 XII. 
 
 This, which may be called the ANI- 
 MAL MIXT, is found, by chemical expe- 
 riment, 
 
PHYSIOLOGY. 
 
 13 
 
 riment, to be coo^derably different from 
 every kind of vegetable or foflll matter ; 
 but the fame experiments hardly teach us 
 any thing exadl or ufeful with refpedl to 
 the conftituent parts of this mixt. 
 
 XIII. 
 
 The only particular relative to this, 
 which we exadlly know, is, that the ani- 
 mal mixt is formed of water, and of fome 
 other matter concreting with it j that, on 
 different occafions, the flate of it is varied 
 by the proportion which the water bears 
 to the other concreting matter ; and that, 
 efpecially by a different proportion in this 
 refpecff, the fimple folid differs in its force 
 pf cohefion, flexibility, and elafticity (VIII.) 
 
 t ' 
 
 XIV. 
 
 The proportion of water to the oth'er 
 
 matter 
 
H 
 
 PHYSIOLOGY. 
 
 matter in the animal mlxt of different per- 
 fons feems to depend, in the firft place, 
 upon the nature of the original ftamina in 
 each; as the different ftate of the fimple fo- 
 lids, which appears early to diftinguifh ftx 
 and temperament, continues refpecftively 
 the fame through the whole of life, even 
 though the different perfons are under the 
 fame external circumftances. 
 
 XV. 
 
 But, in every particular perfon, that 
 proportion is conftantly changed by the 
 progrefs of life ; and this happens more or 
 lefs as other caufes concur. 
 
 XVI. 
 
 The caufes that can affedl the mixture of 
 the fimple folid, are either the flate of the 
 nutritious fluid conveyed by the ordinary 
 
 channels. 
 
PHYSIOLOGY. 
 
 15 
 
 channels, or fome matters from without in- 
 finuated into the folid. 
 
 XVII. 
 
 The Hate of the nutritious fluid may be 
 varied by the quantity and quality of the 
 aliment taken in, by the powers of concoc- 
 tion and aflimilation, by the circumftances 
 of application and^concretion, or by cer- 
 tain preternatural matters carried along 
 with it. 
 
 XVIII. 
 
 The external matter that may be ihfinu- 
 ated into the Ample folid is various, but 
 for the moft part is only aqueous moifture 
 in greater or lefs quantity, 
 
 XIX. 
 
 That thefe feveral caufes may affedl the 
 
 pro- 
 
i6 PHYSIOLOGY. 
 
 proportion of water in the fimple folid, and 
 thereby give a different ftate of it, is fuf- 
 ficiently obvious : That the fame caufes 
 may alfo affefl the other concreting mat- 
 ter, we can, in general, perceive to be pof- 
 fible ; but in what manner, or upon what 
 occasions they do fo, is not eafily dif- 
 cerned. 
 
 XX. 
 
 The properties of the fimple folid (VIII.) 
 may be alfo varied by its ftate of aggrega- 
 tion; and this again may be varied, i. By 
 the temperature of the atmofphere to which 
 the body is long expofed. 2. By the pref- 
 fure, external or internal, which is applied 
 to the folid. 3. By the degree of extenfion 
 of the folid beyond its natural ftate, which, 
 in every living body, is given more or lefs 
 to every part of the foft or flexible folids ; 
 
 and. 
 
PHYSIOLOGY. 
 
 17 
 
 and, lajily^ by the motion or reft to which 
 the folid is accuftomed. 
 
 XXL 
 
 / 
 
 The properties (VIII.) of the folid parts 
 are alfo varied by the ftate of their orga- 
 nization. This every where depends upon 
 an arrangement of fibres, the ftate of cel- 
 lular texture, or upon a texture of veftels ; 
 and therefore, to explain the different ftates 
 of organization, it will be enough to men- 
 tion the caufes of the differences which oc- 
 cur in thefe fundamental parts. 
 
 XXII. 
 
 Fibres may differ in fize by the feveral 
 caufes (XIV. — XXL) affedling the mix- 
 ture and aggregation of the matter of 
 which they are formed, and by thefe caufes 
 alone; but how far the organization of 
 
 any 
 
PHYSIOLOGY, 
 
 i8 
 
 any part depends upon an arrangement of 
 fibres, we cannot diftindlly perceive ; and, 
 if it does, we cannot perceive that the ftate 
 of fuch parts differs otherwife than by the 
 ftate of the cellular texture every where 
 interpofed between the fuppofed fibres. 
 
 XXIII. 
 
 The ftate of cellular texture is the moft 
 important circumftance in all organized 
 parts ; and it may be varied by many dif- 
 ferent caufes. 1. The texture may be more 
 denfe, and thereby firmer, as it has been 
 more prcffed by the actions of life or ex- 
 ternal force ; by which means efpecially it 
 is changed in the progrefs of life. 2. The 
 cellular texture may be increafed in bulk, 
 and rendered firmer by a new growth ta- 
 king place in it, as frequently happens in 
 membranes which are flowly and gradually 
 ftretched out. 3. The fame texture may 
 
 be- 
 
PHYSIOLOGY. 
 
 become weaker by fome part of it bein^ 
 eroded by acrid matters generated in the 
 body, or externally applied. 4. It is ana- 
 logous to this, that, when any part is fu- 
 flained by feveral layers of cellular texture 
 or membranes, fuch fupport is weakened 
 by one or more of thefe layers being cut 
 through ; and the fame weaknefs is indu- 
 ced when any external compreffion, which, 
 for fome time, had been applied, is taken 
 away. 5. The ftate of the cellular texture 
 is varied by the matter contained in its 
 cells ; which is fometimes a matter con- 
 creting into a folid mafs, and fometimes a 
 preternatural quantity of an aqueous ine- 
 laftic fluid. The bones formed in the firfl 
 manner may again become foft by the har- 
 dened matter’s being dilTolved and reab- 
 forbed. 6. When the mobility of parts on 
 one another depends upon the extent of 
 cellular texture connedling them, that mo- 
 bility is diminilhed or deflroyed by a great 
 I part 
 
20 
 
 PHYSIOLOGY. 
 
 part of the cellular texture being eroded or 
 cut away, and the remaining parts being 
 united together; fo that the parts are now 
 connedled by a fhorter portion of cellular 
 texture than before. 7. Parts naturally 
 feparate may lofe their mobility by being 
 joined together by a cellular texture formed 
 between them, as happens when any two 
 furfaces are for fome time kept clofely ap- 
 plied to each other. 
 
 XXIV. 
 
 In fo far as a folid part is formed by a 
 texture of vefTels; its properties (VIII.) 
 may be varied by the different dates of 
 thefe veffels ; which, i . may be more or 
 lefs full of fluids. 2. They may be chan- 
 ged into a folid mafs, by the fluid, con- 
 tained and ftagnating in them, concreting 
 into a folid. 3. They may be changed in- 
 to a folid, if the fluids that fliould pafs 
 
 through 
 
PHYSIOLOGY. 
 
 2 ! 
 
 through them are intercepted, and the ca- 
 vity is filled with a cellular texture j or, 
 4. They may be changed into a folid, if, 
 by collapfe or prefTure, the fides of the 
 vefTels are applied to each other and con- 
 crete together. 
 
 XXV. 
 
 The pathology of the fimple folids can- 
 not be properly feparated from their phy- 
 iiology ; and therefore many different ftates 
 of thefe folids, though fuch as are always 
 morbid are mentioned above. We think 
 it proper tof fubjoin here a fhort view of 
 the whole of that pathology. 
 
 XXVI. 
 
 The difeafes of the fimple folids are, 
 
 I. Thofe of the naturally foft parts. 
 
 I. Debility with flexibility. 
 
 Debile tenerum^ gradle^ Gauh. PathoL 
 
 161. I. 
 
 Debik 
 
22 
 
 PHYSIOLOGY. 
 
 Debile tabidum Gaub. ibid. 161.2. 
 
 2. Debility with fragility. 
 
 Debile fjfile Gaub. 161. 3. 
 
 3. Laxity. 
 
 Debile laxum^ jlaccidumt GauK 1 60. I . 
 
 4. Flaccidity. 
 
 Dehtle iners Gaub. 160. 2. 
 
 5. Rigidity diminifhing flexibility, 
 Rigidum tenax Gaub. 165. i. 
 
 6. Rigidity deftroying flexibility, 
 Rigidum durum 
 
 Il.^hofe of the naturally hard parts, 
 f. Flexibility. 
 
 Debile jiexile Gaub. 160. 3. 
 
 2. Fragility. 
 
 Fragile fpongic^um Gaub. 161. 4. 
 Fragile vitreum Gaub. 165. 3. 
 
 Of all thefe morbid afledlions, we fup- 
 pofe the remote and proximate caufes may 
 be underftood from what is delivered above 
 (XIV.— XXIV.) 
 
 Gaub. 165. 2. 
 
 SECT. 
 
PH.YSIOLOG Y. 
 
 SECT. IL 
 
 Of the Nervous System, 
 
 XXVIL 
 
 The nervous fyftem, as the organ of 
 fenfe and motion, is conneded with fo 
 fo many fundions of the animal oeconomy, 
 that the ftudy of it mull be of the utmoff 
 importance, and a fundamental part of 
 the ftudy of the whole oeconomy. 
 
 A general mew of the Nervous Syfem^ 
 XXVIIL 
 
 The nervous fyftem confifts of the me^ 
 dullary fubftance of the brain, cerebellumj 
 B 2 snedulla 
 
.•24 
 
 PHYSIOLOGY, 
 
 medulla 'oblongata, and fpinalis ; and of 
 the fame fub dance continued into the 
 nerves, by which it is diftributed to many 
 different parts of the body. 
 
 XXIX. 
 
 The whole of this fyflem feems to be 
 properly diflinguiflied into thefe four 
 parts, 
 
 I . The medullary fubftance contained in 
 the cranium and vertebral cavity; the 
 whole of which feems to confill; of di- 
 ftincfl fibres, but without the leveral fibres 
 being feparated from each other by any 
 evident enveloping membranes. 
 
 N. When we fpeak of functions, which are 
 or may be in commoa to every part of this 
 portion of the nervous fyftem, we fhall fpeak of 
 the whole under the title of the BRAIN; but, 
 when it is necefl'ary to diftinguifh the particular 
 parts, we fiiall take care to avoid ambiguity. 
 
PHYSIOLOGY, 
 
 ^5 
 
 2. Connedled with one part or other of 
 N° I. are, the NERVES, in which the 
 fame medullary fubflance is continued, 
 but here more evidently divided into 
 fibres, each of v/hich is feparated from 
 the others by an enveloping membrane 
 derived from the pia mater. 
 
 3. Parts of the extremities of certain 
 nerves (2.), in which the medullary fub- 
 ftance is divefted of the enveloping mem- 
 branes from the pia mater, and fo fituated 
 as to be expofed to the adlion of certain 
 external bodies, and perhaps fo framed as 
 to be affecled by the adlion of certain 
 bodies only ; Thefe we name the SEN- 
 TIENT EXTREMITIES of the nerves. 
 
 4. Certain extremities of the nerves (2.) 
 fo framed as to be capable of a peculiar 
 contractility : and, in confequence of their 
 fituation and attachments, to be, by their 
 contraction, capable of moving tpoft of the 
 folid and fiuid parts of the body. Thefe 
 
 B g we 
 
< 5.6 
 
 PHYSIOLOGY. 
 
 we name the MOVING EXTREMITIES 
 of the nerves : They are commonly named 
 MOVING or USCULAR FIBRES. 
 
 N. That the mufcular fibres are a continuation 
 of the medullary fubftance of the brain and nerves, 
 has not been fhown by the anatomifts, nor uni- 
 verfally admitted by the phyfiologifts; but we now 
 fuppofe it, and hope afterwards to render it fufii- 
 ciently prpbable. 
 
 Are the ganglions of the nerves to be 
 eonfidered as a part of the nervous fyftem 
 diftinguifhed by a peculiar fundtion ? 
 
 XXX. 
 
 Thefe feveral parts of the nervous fyflem 
 are every where the fame continuous me- 
 dullary fubftance which we fuppofe to be 
 the vital folid of animals, fo conftituted in 
 living animals, and in living fyftems only, 
 as to admit of motions being readily pro- 
 pagated from any one part to every other 
 
 part 
 
PHYSIOLOGY. 
 
 27 
 
 part of the nervous fyftem, fo long as the 
 continuity and natural living ftate of the 
 medullary fubftance remains. 
 
 N. It is obferved, that the compreffion of any 
 part of the medullary fubftance prevents the com- 
 munication of motion between the parts that lie 
 on different fides of the part compreffed ; and it is 
 '■probable, there are other caufes befides compref- 
 fion, which may alfo affedb the medullary fub- 
 ftance, fo as to interrupt in it the communication 
 of motion j but they are not difti£lly known. In 
 the mean time, we ufe the expreffion, that a nerve, 
 or other portion of the nervous fyftem, is freey to 
 denote its being free, not only from compreffion, 
 but from every other fuppofed caufe interrupting 
 the communication of motion. 
 
 The condition fitting the medullary fubftance 
 for having motion propagated in it, we fuppofe to be 
 the prefence of a certain fluid; which we therefore 
 name the nervous fluid, without meaning however 
 at prefent to determine any thing with regard to 
 its fource, nature, or mannner of a£Hng. 
 
 B 4 
 
 XXXI. 
 
28 
 
 PHYSIOLOGY. 
 
 XXXI. 
 
 In the living man, there is an immaterial 
 thinking fubftance, or MIND, conftantly 
 prefent ; and every phenomenon of think- 
 ing is to be confidered as an affeclion or fa- 
 culty of the mind alone. But this immaT 
 terial and thinking part of man is fo con- 
 nedled with the material and corporeal 
 part of him, and particularly with the 
 nervous fyftein, that motions excited in 
 this give cccalion to thought ; and thought, 
 however occafioned, gives occafion to new 
 motions in the nervous fyftem. This 
 mutual communication or Influence we 
 aflume with confidence as a facfl; ; But the 
 mode of it we do not underftanfl, nor pre- 
 tend to explain ; and therefore are not 
 bound to obviate the difSculties that at- 
 tend any of the fuppofltions which have 
 been made concerning it. 
 
 XXXIL 
 
PHYSIOLOGY. 
 
 29 
 
 XXXII. 
 
 The phenomena of the nervous fyftem. 
 occur commonly in the following order. 
 The impulfe of external bodies adls upon 
 the fentient extremities of the nerves ; 
 and this gives occafion to preception ojf 
 thought, which, as firft arifing in the 
 mind, we term SENSATION. This fen- 
 Iktion, according to its various modifica- 
 tion, gives occafion to VOLITION, or the ' 
 willing of certain ends to be obtained by 
 the motion of certain parts of the body ; 
 and this volition gives occafion to the con- 
 tradlion of mufcular fibres, by v/hich 
 the motion of the part required is proj 
 duced. 
 
 N. This is an example of the mofl; ordinary 
 cafe •, but we do not mean to fay it is the only 
 pafe of communication between the different 
 parts of the nervous fyftem. 
 
 XXXIII. 
 
30 
 
 PHYSIOLOGY. 
 
 XXXIII. 
 
 As the impulfe of bodies on the fen- 
 tient extremities of a nerve does not occa- 
 fion any fenfation, unlefs the nerve be- 
 tween the fentient extremity and the brain 
 be free (XXIX. 3.) ; and as in like man- 
 ner, volition does not produce any con- 
 tradlion of mufcles, unlefs the nerve be- 
 tween the brain and mufcle be alfo free ; 
 we conclude, from both thefe fadls, that 
 fenfation and volition, fo far as they are 
 connedled with corporeal motions, are 
 fun<fl;ions of the brain alone ; and we pre- 
 fume, that fenfation arifes only in con- 
 fequence of external impulfe producing 
 motion in the fentient extremities of the 
 nerves, and of that motion’s being thence 
 propagated along the nerves to the brain ; 
 and, in like manner, that the will opera- 
 ting in the brain only, by a morion begun 
 
 there, 
 
PHYSIOLOGY. 
 
 3 ^ 
 
 there, and propagated along the nerves, 
 produces the contradion of mufcles. 
 
 XXXIV. 
 
 From what is now faid, we perceive 
 more diftindly the different fundions of 
 the feveral parts of the nervous fyflem, as 
 diflinguifhed in (XXIX.), i. The fentient 
 extremities (XXIX. 3.) feem to be particu- 
 larly fitted to receive the impreffions of 
 external bodies ; and, according to the dif- 
 ference of thefe impreflions, and of the 
 condition of the fentient extremity itfelf, 
 to propagate along the nerves motions of 
 a determined kind, which, communicated 
 to the brain, give occafion to fenfation. 
 2. The bain (XXIX. i.) feems to be a 
 part fitted for, and fufceptible of, thofe 
 motions with which fenfation, and the 
 whole confequent operations of thought, 
 are connededj and thereby is fitted to 
 
 form 
 
32 PHYSIOLOGY. 
 
 form a communication between the mo- 
 tions excited in the fentient, and thofe in 
 confequence arifing in the moving extre- 
 mities of the nerves, which are often re- 
 mote and diftant from each other. 3. The 
 moving extremities (XXIX. 4.) are fo fra- 
 med as to be capable of contradtion, and 
 of having this contraction excited by 
 motion propagated from the brain, and 
 communicated to the contractile fibre. 
 4. The nerves, more ftrictly fo called 
 (XXlX. 2.), are to be confidered as a col- 
 leclion of medullary fibres, each invcloped 
 in its proper membrane, and thereby fo 
 feparated from every other, as hardly to 
 admit of any communication of motion 
 from any one to the others, and to admit 
 only of motion along the continuous me- 
 dullary fubftance of the fame fibre, from its 
 origin to the extremities, or contrariwife. 
 
 XXXV. 
 
PHYSIOLOGY. 
 
 35 
 
 XXXV. 
 
 From this view of the parts of the ner- 
 vous fyflem, of their feveral funflions and 
 communication with each other, it ap- 
 pears, that the beginning of motion in the 
 animal ceconomy is generally conne<fled 
 with fenfation ; and that the ultimate 
 effedls of fuch motion are chiefly adlions 
 depending immediately upon the contrac- 
 tion of moving fibres, between which and 
 the fentient extremities the communica- 
 tion is by means of the brain. Wherefore, 
 in ftudyipg the nervous fyflem, we judge 
 it proper to confider, i. Senfation, and 
 with that the general funflion of the fen- 
 tient extremities. 2. The acHon of the 
 moving fibres. 3. The fundlion of the 
 brain. In confidering thefe three heads, 
 the fundion of the nerves, more flridly fo 
 called, will of courfe be fufficiently ex- 
 plained. 
 
 2 
 
 CHAP. 
 
34 
 
 PHYSIOLOGY. 
 
 CHAP. I. 
 
 Of Sensation. 
 
 XXXVI. / 
 
 fenfations may be be confidered 
 as of two kinds : i. Thofe which 
 arife from the impulfe or impreffion of 
 external bodies, which we therefore name 
 SENSATIONS OF IMPRESSION. 
 2. Thofe which arife from the mind’s be- 
 ing confcious of its own adlion, and of 
 the motions it excites ; and thefe we name 
 SENSATIONS OF CONSCIOUSNESS. 
 
 Sen/atiom of Imprejfton. 
 
 XXXVII. 
 
 The fenfations of impreffion are very 
 X - various 5 
 
PHYSIOLOGY. 
 
 35 
 
 various ; but have been generally referred 
 to five heads or kinds, commonly called the 
 five fenfes ; that is, thofe of fight, hearing, 
 fmell, tafte, and touch. 
 
 XXXVIII. 
 
 The four firll of thefe are each of them 
 properly confidered, as forming one ge- 
 nus of fenfations : i . As the particular 
 
 fenfations comprehended under each head 
 (XXXVIII.), though very various, are, 
 however, perceived to have fome what com- 
 mon to all of them. 2. As thofe of the 
 fame genus all arife from impreflions made 
 upon one part of the body only, and that 
 of a peculiar organization. 3. As thofe of 
 the fame genus all arife from the adion of 
 external bodies of one kind only, or of 
 one and the fame quality, by means of 
 which they adl upon our organs. 
 
 XXXIX. 
 
3^ PHYSIOLOGY, 
 
 XXXIX. 
 
 No fuch chara<5ters concur in eftablilh” 
 mg one genus of the fenfations referred to 
 the fifth head of touch, which are various 
 in all thofe refpedls (XXXVIII.) ; and 
 phyfiologifts feem to have referred to 
 this head of touch every fenfation that 
 does not manifeitly belong to the other 
 four, and among the reft many of 
 the fenfations of confcioufnefs. It might 
 perhaps be ufeful to diftinguifli into ge- 
 nera, the feveral fenfations referred to 
 touch; but it is not neceffary to be done 
 here. 
 
 From certain fenfations referred to touch, 
 it appears, that not only the extremities 
 (XXIX. 3 .), but that every part of the 
 nervous fyftem (XXVIIL), is fentient with 
 refpe<ft to certain impreflions. 
 
 Sen^ 
 
PHYSIOLOGY. 
 
 37 
 
 Ssnfatmis of Confcioufnefs , 
 
 XL. 
 
 The fenfations of confcioufnefs may be 
 - i*eferred to the following heads : i . Thofe 
 of apperception, by which we are in ge- 
 neral confcious of thinking, of perceiving, 
 judging, and willing, and thereby of our 
 exiftence and identity. 2 . The fenfations 
 arifing from the particular date of thinking, 
 as perception, memory, and judgment, are 
 more or lefs clear, ready, or exa(51. 3 . The 
 
 fenfations arifing from the particular date 
 of volition, and its various modes. 4 . The 
 fenfations arifing from the general dace 
 of adlion, as vigorous or weak, eafy or 
 difiicult. 5 . The fenfations arifing from 
 particular adlions, or a confcioufnefs of 
 the adlions excited, and of the motion of 
 the different parts of the body. 6 . The 
 
 G fen- 
 
38 - PHYSIOLOGY. 
 
 fenfations arifing from the diminution or 
 abfence of impreffions. 
 
 Under each of thefe heads, a great num- 
 ber of particular fenfations are compre- 
 hended, but not neceffary to be farther 
 fpecified here. 
 
 La%vs or general Circimjlances of Senfation^ 
 XLL 
 
 Of the four firft genera (XXXVII.), the 
 fenfations ariiing give no indication of the 
 nature of the bodies ac5ling on our organs, 
 or of the mode of their adilion ; and when we 
 otherwife learn thefe circumftances, we can 
 perceive no necelTary connecStion between 
 them and the fenfations which they pro- 
 duce. But, from certain fenfations of 
 touch and confcioufnefs, we acquire the 
 notions of folid figure, of motion, im- 
 
 pulle. 
 
PHYSIOLOGY. 
 
 39 
 
 pulfe, impenetrability, and the communi- 
 cation of motion, and confider the fenfa- 
 tions as exa(5tly correfpondent to the cir- 
 cumftances of external bodies. At the 
 fame time, as we know of no other adtion 
 of bodies on each other but that of im- 
 pulfe; and as, in the cafe of the fenfations 
 of the four firft genera, we learn, that an 
 impulfe takes place, we have comprehend- 
 ed the whole under the title of Senfations of 
 Impreffion, and conlider all of them as per- 
 ceptions of impulfe. • 
 
 XLII. 
 
 To produce any fenfation of impreffion, 
 a certain force of impreffion is neceffary ; 
 and from a leffer force, no fenfation arifes. 
 The degree of force is likewife fo limited on 
 the Other hand, that, in a high degree, it 
 deftroys the organ ; and, in degrees ap- 
 proaching to this, a 'general fenfation of 
 C 2 
 
4 © 
 
 PHYSIOLOGY. 
 
 pain, rather than the fenfation of any par- 
 ticular objedl, is produced. 
 
 XLIir. 
 
 Within thefe limits, however, our fen- 
 fations are not exactly correfpondent to 
 the force of impreflion, nor do they make 
 any exadl ellimate of that force. Ufually 
 fenfation is relative to the change that is 
 produced in the nervous fyftemj and a fen- 
 fation«proves flrong or weak, only as it is 
 ftronger or weaker than that which had 
 immediately preceded it, or than that de- 
 gree of force to which the nerves had been 
 immediately before accuftomed. For this 
 reafon too the limits (XLII.) are very va- 
 riable. 
 
 XLIV. 
 
 DifFerent fonfatlons do not always impljr 
 
 2 
 
 a 
 
PHYSIOLOGY. 
 
 41 
 
 a different kind of adlipn in the bodies 
 producing them ; for fometimes diffe- 
 rent fenfations arife merely from a diffe- 
 rent degree of force in the fame kind of 
 adlion, as is manifeft in the cafe of heat 
 and cold. 
 
 ZLV. 
 
 To fenfation from imprefficn, a certain 
 duration of impreffion is neceffary. 
 
 XLVL 
 
 The mind’s refling for fome time upon 
 one fenfation, is called ATTENTION. 
 This, like the duration (XLV.), is necef- 
 fary to give an impreffion its full effedt. 
 
 XLVII. 
 
 The mind feems to be determined to 
 
 C 3 attention 
 
42 
 
 PHYSIOLOGY. 
 
 attention by the force of Impreflion ; by 
 the pleafure or pain arifing from it; by 
 the degree of emotion or paflion produced 
 by thefe ; and, laftly, by the emotion’s 
 being more or lefs related to the perfon 
 feeling. 
 
 XL VIII. 
 
 If the force and duration of impref- 
 lion, and the attention of mind, are all in. 
 the due degree, the fenfation often remains 
 for fome time after the impreflion or ac-= 
 tion of the external body has ceafed. 
 
 XLIX. 
 
 The mind admits of, or can attend to, 
 one fenfation only at one time. 
 
 L. 
 
.PHYSIOLOGY. 
 
 43 
 
 L. 
 
 I 
 
 Though the mind admits but of one fen- 
 fation at one time, feveral impreffions may 
 a6l at the fame time, if they be fuch as 
 can unite in producing a fingle fenfation ; 
 and fuch is the cafe of many of the im- 
 preffions which produce the particular 
 fenfations of the fame genus, as in thofe 
 efpecially of colour, found, fmell, and 
 talle, 
 
 LL 
 
 In each of thefe genera, many impref- 
 lions, which feparately produce particular 
 fpecies, can unite in producing a fingle 
 fenfation, which is always a neutral, or 
 one different from either of the feparate 
 fenfations. 
 
 LI!. 
 
44 
 
 PHYSIOLOGY. 
 
 LII. 
 
 This union of impreflions may take 
 pla(^e, either when the impreilions are ex- 
 a(5lly fynchronous, or when the one fuc- 
 ceeds the other before the fenfation of the 
 firft (XLVIII.) has ceafed. 
 
 LIII. 
 
 Though the motion excited in the fen- 
 tient extremities by impreflion remains 
 fome time, as in (XLVIII.), it muft be 
 fuppofed to become continually weaker, 
 till at length it ceafes altogether, and with 
 it the fenfation. 
 
 LIV. 
 
 The fame impreflion foon repeated, does 
 pot produce the fame ftrength of fenfation 
 as before.^ Hence, all new impreflions 
 
 are, 
 
PHYSIOLOGY. 
 
 45 
 
 are, ceteris parihus^ ftrongeft; and mode- 
 rate impreflions frequently repeated, pro- 
 duce no fenfation, unlefs their force is con- 
 fiderably increafed. 
 
 LV. 
 
 Acflions which at firft produced a fenfa- 
 tion of confcioufnefs, as accompanied with 
 volition, come, by repetition, to be per- 
 formed without any fenfation; or they 
 produce it only when they are performed 
 with uneafinefs, pain, or unufual force. 
 
 LVI. 
 
 Impreffions being given, their effedls in 
 producing fenfation are different in differ- 
 ent perfons, and in the fame perfon at dif- 
 ferent times. This muft arife from fome 
 difference in the date of the bodies adled 
 upon, which may perhaps be referred to 
 
 the 
 
46 
 
 PHYSIOLOGY. 
 
 the following heads : i . The ftate of the 
 common teguments, or other parts inter- 
 pofed between the impreffing body and the 
 medullary fubftance of the fentient extre- 
 mity. 2. The different ftate of the medul- 
 lary fubftance of the fentient extremities, 
 as given to it in the original ftamina. 
 
 3. The different ftate of tenfion in the me- 
 dullary fubftance of the fentient extremi- 
 ties, as given to it by the ftate of the 
 blood-veffels conftantly conne<fled with it. 
 
 4. The ftate of the fame medullary fub- 
 ftance, as affecded by heat or cold. 5. The 
 ftate of it as produced by former impref- 
 fions (XLIII. LIV.) 6. The ftate of the 
 nerves along which the motion is propa- 
 gated. 7. The ftate of the brain or fenfo- 
 rium. 8. The ftate of attention (XLVI. 
 and XLVII.) 
 
 Lvn. 
 
PHYSIOLOGY. 
 
 47 
 
 LVIL 
 
 Different parts of the body are fenfible, 
 and fenfibie only by means of nerves pre- 
 fent in them : but anatomy does not al- 
 ways determine certainly with regard to 
 the prefence or abfence of nerves ; and, 
 therefore, the fenfibility of feveral parts 
 can be determined by experiment only ; 
 which, however, is alfo fallacious. 
 
 LVIII. 
 
 Particular fenfations arife from impref- 
 lions on certain parts only : i . Becaufe the 
 fentient extremities in thefe parts are fo fi- 
 mated as to be expofed to the a(5lion of cer- 
 tain external bodies only. 2 . Becaufe the 
 fentient extremities are connected with an 
 organ that increafes the force of the exter- 
 •pal agent, or modifies its adion in the man- 
 
 ner 
 
48 
 
 PHYSIOLOGY. 
 
 ner neceflary to a determined impreffion. 
 3.Becaufe the fibres of certain fentient ex- 
 tremities are, by their fize or tenfion, fitted 
 to be adled upon by certain external bo- 
 dies only. 4. Becaufe certain fentient ex- 
 tremities are fo conflantly preferved in a 
 . certain date, as to render them more fen- 
 fible to a change. 
 
 Thefe circumftances determine the mode 
 of impulfe, but do not account for the fen- 
 fation arifing from it. 
 
 LIX. 
 
 Different fenfations are accompanied 
 with different judgements concerning the 
 bodies making impreflion, and the part of 
 the human body upon which it is made. 
 
 ^ Some fenfations are referred to bodies at a 
 diffance ; others, to external bodies in 
 contad ; and others to the feeling body it- 
 felf. 
 
 LX. 
 
PHYSIOLOGY. 
 
 49 
 
 LX. 
 
 When fenfations are referred to our own 
 bodies, it is in three feveral ways : They 
 are moft commonly referred to the part on - 
 which immediately the imprpffion is made; 
 and this, with regard to the external parts, 
 very accurately ; but, with regard to the 
 internal, much lefs fo : And commonly 
 the fenfations arifing from internal parts, 
 are referred to the incumbent external part, 
 with fame obfcure diftinftion between fu- 
 perficial and more deep. 2. Senfations are 
 fometimes referred, not to the part upon 
 which the impreHion is immediately made; 
 but to a diftant more fenfible part, to 
 which a motion is propagated from the 
 part impreffed. 3. As fenfations ufually 
 arife from impreflions made upon the ex- 
 tremities of the nerves, and are referred to 
 thefe; fo impreffions made on the nerves 
 
 in 
 
50 
 
 PHYSIOLOGY. 
 
 in their conrfe, are, fomedmes referred tc 
 the extremities from whence they had com- 
 monly arifen. 
 
 LXL 
 
 The fenfations of confcioufnefs (XL. 1.2.) 
 are referred to the encephalon. So are thofe 
 of XL. 3. if they are moderate; but, if 
 more vehement, they are often referred to 
 thofe parts in which their effefls are exert- 
 ed, as the heart and organs of refpiration. 
 The fenfations (XL. 4. and 5.) are feldom, 
 wdth accuracy, referred to particular parts, 
 but indiftindlly to a whole member. VVe 
 are not confcious of the addon of particu- 
 lar mufcles, except when their contradtioii 
 is fpafmodic. 
 
 LXIL 
 
 We are difpofed to combine our fenfa- 
 
 I 
 
 tions 
 
PHYSIOLOGY. 
 
 5 «- 
 
 tions as united in one objedl ; and thus 
 form what are called COMPLEX IDEAS. 
 
 LXIIL 
 
 We compare our feveral fenfations, and 
 from thence acquire new fenfations of 
 RELATION. 
 
 LXIV. 
 
 When fenfations formerly received are 
 again renewed by the fame objeds, it is, 
 for the moft part, with a confcioufnefs of 
 their having been formerly received; and 
 this faculty we call REMINISCENCE. 
 
 LXV. 
 
 Perceptions formerly received can be 
 renewed without the prefence or adion of 
 the objed which formerly gave occafion to 
 
 them 
 
PHYSIOLOGY. 
 
 them : and if this is attended with the con- 
 fcioufnefs of a difference between the vi- 
 vidity of the two perceptions, and particu- 
 larly of the abfence of the original objedls, 
 fuch a renewed perception is called ari 
 IDEA ; and the faculty by which this re- 
 newal is made, is called MEMORY. 
 
 LXVI. 
 
 Perceptions formerly received, can, 
 without the prefence of the original obje<5l, 
 be renewed alfo in fuch a manner, that the 
 mind does not perceive any difference be- 
 tween the original and the renewed per- 
 ception ; and therefore, fuch renewal is al- 
 ways attended with the perfuafion of the 
 prefence of the object. The faculty by 
 which fuch renewal is made, we call IMA- 
 GINATION, more Rridly. 
 
 LXVII. 
 
PHYSIOLOGY. 
 
 S3 
 
 LXVIL 
 
 Reminifcence depends upon the force 
 or frequent repetition of the former fenfa- 
 tion. 
 
 LXVIII. 
 
 Memory depends upon an aflbciation of 
 perceptions, which is formed hj their be-- 
 ing frequently repeated immediately after 
 each other ; by their being parts of the 
 fame complex idea ; and, by their having 
 relations marked. Memoi^ is generally 
 faithful cofuch alTociations; but it is more 
 or lefs fo in different perfons, according to 
 the number and importance of the rela- 
 tions marked ; according to the frequency 
 of the repetition of the fenfations, and the 
 marking of their relations ; and according 
 D 
 
 to 
 
54 
 
 PHYSIOLOGY. 
 
 to the difFerent dates of the brain, very 
 little known. 
 
 LXIX. 
 
 Imagination feems always to depend up- 
 on internal caufes, that is, upon caufes act- 
 ing in the brain. 
 
 LXX. 
 
 . Memory and imagination renew diftin(5l- 
 ly the ideas of feeing and hearing only. All 
 others are renewed imperfedtly, or not at all; 
 but all others may be affociated (LXVIII.) 
 with the fenfations or ideas of feeing and 
 hearing, fo that thefe become figns of die 
 othere. The memory, in renewing thefe 
 figns, fo far renews the idea belonging to 
 them, as to renew their feveral affociations 
 and relations ; to renew, in fome degree, 
 die pleafure or pain which formerly attend- 
 ed 
 
PHYSIOLOGY. 
 
 55 
 
 ed the fenfations themfelves; and particu- 
 larly to renew the emotions of mind, or 
 motions of the body, which the fenfations 
 formerly produced. 
 
 LXXL 
 
 ) 
 
 Moft of our fenfations, perhaps all of 
 them, are either pleafant or painful. 
 
 LXXIL 
 
 The words pleafmt and painful are com- 
 monly generic terms, each of them com- 
 prehending a great many fpecies, which 
 feem to require being alforted under feve- 
 ral different genera; Thus, in the iirft 
 place, our fenfations may be divided into 
 thofe we defire, and thofe we are averfe to. 
 Of thofe we defire, we may diftinguilh 
 thofe which arife from qualities we refer 
 to other bodies, from thofe we refer en- 
 D 2 tirely 
 
PHYSIOLOGY. 
 
 tirely to our own. The firft may be 
 named more ftricliv the AGREEABLE, the 
 laft the PLEASANT. In like manner, of 
 the fenfations we are averfe to, we may di- 
 fUnguifli the DISAGREEABLE and the 
 PAINFUL* But, farther, the lafl muft be 
 diftinguifhed from the fenfe of averfion, 
 which accompanies certain fenfations of 
 eonfeioufnefs, as the fenfe of debility, laffi- 
 tude, difficulty, 8cc. and particularly from 
 that which is referred obfcurely to internal 
 parts, and this we name ANXIETY. Thefe 
 fenfitions may be called the UNEASY ; 
 and every one diftinguiffies this kind from 
 that of the PAINFUL, more ftricUy fa 
 called.' Thefe laft feem to be always fen- 
 fations of impreffion, referred pretty accu- 
 rately to a particular part. 
 
 There is thus a foundation for eftabliffi- 
 ing different genera of the fenfations we 
 cleGre, and of thofe we are averfe to ; as 
 alfo, for greater precifion in the employ- 
 
 menc 
 
57 
 
 P II Y S I O L O G Y, 
 
 ment of terras : but tlie fixing the limits of 
 tliefe genera, and afforting the feveral fpe- 
 cies, may be (till difficult ; fo that we can- 
 not be certain of applying the terms every 
 where with ftri(5l propriety, 
 
 LXXIIL 
 
 The enumeration of the agreeable or dif- 
 agreeable, and even of the pleafant fenfa- 
 fadons, would not be of much ule here ; 
 and the enumeration of the uneafy and 
 painful, though much more interefting, 
 belongs to the pathology. However, we 
 think it proper to deliver here the few fol- 
 lowing propofitions. 
 
 LXXIV. 
 
 Senfation and a^lion, within certain li- 
 mits, are always defired ; a.nd the want of 
 lenfation, or imperfecl and indiftincfl: fen- 
 2 D 3 fations. 
 
5§ PHYSIOLOGY. 
 
 rations, are always uneafy. In a6lion of 
 every kind, the fenfations of debility and 
 difficulty are alfo uneafy. 
 
 LXXV. 
 
 In fenfations of impreffion, their being 
 pleafant or painful often depends on the 
 degree of force in the impreffion, allow- 
 ance being made for the fenfibility of the 
 fyftem'. 
 
 LXXVI. 
 
 As impreffions, by being repeated, pro- 
 duce weak fenfations (LIV.), impreffions, 
 at firft painful, may, by repetition, be 
 changed into pleafant, and the pleafant in- 
 to infipid and uneafy. Hence arifes, with 
 regard to moderate impreffions, the plea- 
 fure of novelty, the defire of variety, and 
 
 the 
 
PHYSIOLOGY. 
 
 S9 
 
 the dehre of increafing the force of pleafant 
 impreffions. 
 
 LXXVIL 
 
 There is a condition of impreffions, ren- 
 dering them objects of defire or averfion, 
 that cannot with certainty be referred to 
 their force. This condition we call the 
 quality of impreflions. 
 
 LXXVIII. 
 
 Impreffions are often rendered objedls of 
 defire or averfion, by combination, fuccef- 
 fion, and relation. 
 
 LXXIX. 
 
 No fenfations arife originally in the 
 mind, without a previous change in the 
 flate of the body. 
 
 D 4 
 
 LXXX. 
 
^ PHYSIOLOGY. 
 
 LXXX. 
 
 Certaia impreflions, and certain Hates of 
 the body^ like to thofe which produce the 
 fenfations of confcioufnefs, may both of 
 them adt upon the nervous fyftem, without 
 producing any fenfation. 
 
 SECT. 
 
PHYSIOLOGY. 6t 
 
 SECT. II. 
 
 CHAP. II. 
 
 Of the Action of moving Fibres. 
 
 ^HE moving fibres (XXIX, 4.), fb far 
 
 as yet known, are of one kind only, 
 and the fame every where, as in the moft 
 commonly known miifcles. Hence, the 
 terms moving and mufcular fibres are of 
 the fame import. 
 
 LXXXI. 
 
 LXXXil. 
 
62 
 
 PHYSIOLOGY. 
 
 /■ 
 
 LXXXII. 
 
 A mufcular fibre is fuppofed to have a 
 peculiar organization, different both from 
 that of the fimple folid fibre, and from that 
 of the medullary fibres in every other part 
 of the nervous fyftem ; but wherein that 
 peculiarity of organization confifls, is not 
 yet afeertained. 
 
 LXXXIII. 
 
 A mufcular fibre is endued with a eon- 
 tradlility, which is different from that of 
 the fimple folids, or of any inanimate 
 elaftics, efpecially in this, that the contrac- 
 tion of a mufcular fibre is excited by cau- 
 fes which do not affed thefe others. For, 
 the contradion of a mufcular fibre is ex- 
 cited by being extended ; and a contrac- 
 tion is produced, whilft the ftretching 
 
 power 
 
PHYSIOLOGY. 63 
 
 power continues to be applied. The fame 
 contracflion is alfo excited by various ap- 
 plications, whofe mode of a^lion we do 
 not perceive; but we know them to be 
 fuch as do not a£Fedl inanimate elaftics. 
 
 In refpefl of thefe caufes by which it 
 may be excited, the contradlility of mul- 
 cular fibres hath been called IRRITABI- 
 LITY. 
 
 LXXXIV. 
 
 The force of contradlion in mufcular 
 fibres is often much greater than that of 
 the caufes exciting it. 
 
 LXXXV. 
 
 The contra( 5 lility of mufcular fibres 
 fLXXXIII). appears efpecially in living 
 bodies, ceafes with life, or foon after, 
 and is probably never produced but with 
 
 life. 
 
64 
 
 PHYSIOLOGY. 
 
 life. Hence it is, that, by fome writers, 
 it Ls called the VITAL POWER; and the 
 nmfcular fibre endued with it, the LIVING 
 SOLID. Gaub. Patholog. 169, 170. 
 
 LXXXVI. 
 
 The contraaility (LXXXIII. LXXXIV. 
 and LXXXV ) hath been lupnofcd to be- 
 long to murcnlar fibres, independently of 
 their conneaion with the other parts of 
 the nervous fyftem ; and, upon that fup- 
 pofition, it hath been called the vis inftta. 
 
 Ihall call it the INHERENT POWER 5 
 Haller. PnVw. lln, 400. 
 
 LXXXVIL 
 
 The contraaion of mufcular fibres can 
 be excited by applications, made to other 
 parts of the nervous fyftem, as well as to 
 the mufcles; and, as the effeas of thofe 
 
 ap- 
 
 
PHYSIOLOGY* 
 
 65 
 
 applications made to the other parts of the 
 nervous fyftem can be prevented by liga- 
 tures made tipon the nerves between the 
 place of application and the mufcle to be 
 moved, it is concluded, that the contrac- 
 tion of mufcular fibres can be excited by 
 a power communicated to them by a mo- 
 tion propagated along the nerves. This 
 power is called the NERVOUS POWER. 
 
 LXXXVIir. 
 
 The nervous power (LXXXVII.) is 
 commonly determined to motion by the 
 will. This we fuppofe to adl in the brain 
 only (XXXIII.), and to depend upon fen- 
 fation, and other modifications of thought; 
 and this power, which is to be chiefly re- 
 ferred to the mind, and acis in the brain 
 only, we n^me the ANIMAL POWER. 
 
 
 LXXXIX. 
 
66 
 
 PHYSIOLOGY. 
 
 LXXXIX. 
 
 The facility with which the contra(5lion 
 of mufcular fibres can be excited, and the 
 force with which it can be performed, are 
 to be diftinguilhed. The firft we name 
 the MOBILITY, the laft the VIGOUR, of 
 mufcular fibres. Both have been con- 
 founded under the name of Irritability, 
 
 XC. 
 
 The mobility and vigour of mufcular 
 fibres (LXXXIX.) can both of them be 
 increafed or diminifhed by various means. 
 Whatever can excite the contradlion of 
 mufcular fibres is called a STIMULUS; 
 and, in general, the means of exciting con- 
 traction are cft.lled STIMULANT POW- 
 ERS. , The means dimlnifhing the mo- 
 bility and vigour of mufcular fibres are 
 called SEDATIVE POWERS. 
 
 I 
 
 xcr. 
 
1 
 
 PHYSIOLOGY. 67 
 
 XCI. 
 
 The inherent power (LXXXVI.) is fup- 
 pofed to be more vigorous, moveable, and 
 permanent, in certain mufcular fibres than 
 in others. 
 
 XCII. 
 
 The inherent power, or the contradlion 
 dependent upon it, can be excited by cer- 
 tain applications made either to the mus- 
 cles themfelves, or to the nerves connedled 
 with them ; and, in either cafe, the effedls 
 of fuch application are fo exadlly the fame 
 as to allow us to conclude, that the matter 
 in the nerves, and in the mufcular fibres, 
 is of the fame kind. 
 
 XCIII. 
 
 The mufcular fibres are fenfible to vari- 
 ous imprefhons, and are otherwife organs 
 
 of 
 
68 
 
 PHYSIOLOGY. 
 
 of the fenfations of confcloufnefs (XL. 4. 5.) 
 From this alfo, it is prefumed, that the 
 mufcular fibres confifi; of the fame matter 
 which is the fubjedl of fenfe in other parts 
 of the nervous fyftem (XXXIX.) 
 
 XCIV. 
 
 From XCIL XCIII. and other confi- 
 derations, we think it probable, that the 
 mufcular fibres are a continuation of the 
 medullary fubftance of the brain and 
 nerves, as alleged XXIX. 
 
 xcv. 
 
 Though the mufcular fibres confifi: of 
 the fame kind of matter as that in the 
 nerves, the latter fliow no contraclility^ 
 becaufe they have not the peculiar cr- 
 ganization (LXXXU.) of the former. 
 
 XG\T. 
 
I 
 
 PHYSIOLOGY, 
 
 XCVL 
 
 The nervous power (LXXXVII.), and 
 
 the inherent (LXXXVl.), may fubfift for 
 fome time without any Connedlion of the 
 nerves or mufcles with the brain; and 
 they fubhft alfo in entire bodies for fome 
 time after life has feemingly ceafed. Both 
 powers, however, are feemingly of equal 
 duration in thefe refpedls ; and neither 
 power feems to fubfift long but in entire 
 and living bodies. 
 
 probable, that the nervous and the inhe- 
 rent powers are fomewhat of the fame na- 
 ture ; and it is alfo probable, that, in en- 
 tire and living bodies, both the nervous 
 and inherent powers have a conftant de- 
 pendence upon the animal (LXXXVIII.) 
 
 XCVIL 
 
 From what is faid (XCIL— XCVL), it is 
 
 E 
 
 XCVIIL 
 
I 
 
 70 
 
 PHYSIOLOGY. 
 
 XCVIII. 
 
 The contracflion of mufcular fibres does 
 not depend immediately on the motion of 
 the blood, as it fubfifts in many animals 
 after ail motion of the blood has ceafed. 
 
 XCIX. 
 
 The contraction of mufcular fibres does 
 not depend on the inflation of veficles, or 
 other fuch analogous ftruClure, as the 
 ftiortening of the fibres in contraction is 
 often greater than take place in fuch ftruc- 
 ture» 
 
 C. 
 
 As the force of cohefion in the mufcular 
 fibres of living animals is much greater 
 than in thofe of dead ones, it is probable 
 from this and other confiderations, that 
 
 the 
 
PHYSIOLOGY. 
 
 7 ^ 
 
 the caufe of mufcular contradlion is an in- 
 creafe only of that fame power which gives 
 the contradility of the fimple folids, and of 
 other inanimate elaftics ; Haller, Prim, Lin. 
 407. 408. 
 
 If this is true, it will alfo explain why 
 the force of cohefion in mufcular fibres is 
 greater than that of the medullary fibres 
 in any other part of the nervous fyftem, 
 though both kinds of fibres (by XCIV.) 
 Gonfift of the fame kind of matter. 
 
 CL 
 
 In living and healthy animals, the muf- 
 cular fibres have a conftant tendency to 
 contradl j and this tendency we call their 
 TONE, or TONIC POWER. 
 
 CIL 
 
 The tonic power of mufcular fibres ne- 
 E 1 cefTarily 
 
72 
 
 PHYSIOLOGY. 
 
 cefTarily fuppofes their being conftantly in a 
 ftate of extenfion beyond their natural or 
 moft contracled date; and in this ftate they 
 are conftantly kept by the adlion of antago- 
 nift mufcles, by the weight of the parts 
 they fuflain, by fluids diftending the cavi- 
 ties they furround, and by their connedlion 
 with fuch diftended cavities, particularly 
 the blood-veffels. 
 
 cm. 
 
 As the diftention of mufcular fibres (by 
 LXXXIII.) proves a flimulus (XC.), we 
 conclude, that the tonic power in them 
 will cater 'is paribus be in proportion to the 
 degree of tenfion (CII.) 
 
 CIV. 
 
 If the inherent power (as in XCVII.) is 
 ill dependenence upon the nervous and 
 
 ani- 
 
PHYSIOLOGY. 
 
 73 
 
 animal powers, and thefe may be increafed 
 or diminiflied by various means, the tonic, 
 as a part^of the inherent power, muft in 
 fome rn'eafure be in proportion to the date 
 of the nervous and animal powers. 
 
 CV. 
 
 If the tonic power of any mufcular fibre 
 depends more upon its date of tendon 
 .(GUI.) than upon the date of the nervous 
 and animal powers (CIV.), fuch dbres will 
 be more affe<ded by changes iii the date 
 of tendon, than by changes in the date of 
 the nervous and animal powers; and on 
 the contrary, &c. 
 
 CVI. 
 
 The force of contradlion, or the vigour 
 of mufcular dbres, will be always as the 
 force of dimulus, and the vigour of the 
 E 3 ani- 
 
74 
 
 PHYSIOLOGY. 
 
 animal, nervous, and inherent powers 
 taken together. 
 
 evil. 
 
 The mobility of mufcular fibres 
 (LXXXIX.) feems often to be increafed 
 by caufes which weaken their vigour ; 
 and, therefore, it is induced by the dimi- 
 nution of tenfion, and by caufes weaken- 
 ing the animal, nervous, or inherent pow- 
 ers. 
 
 GVIII. 
 
 The ordinary contratflion of mufcular 
 fibres is difpofed fpontaneoufly to alter- 
 nate with a relaxation or extenfion of the 
 fame. 
 
 CIX. 
 
75 
 
 PHYSIOLOGY. 
 
 CIX. 
 
 In the ftraight mufcles and in the heart, « 
 the alternate contra(flions and relaxations 
 readly take place ; and that though a fti- 
 mulus is conflantly applied : but in muf- 
 cular fibres furrounding cavities, as in the 
 alimentary canal, bladder of urine, 8cc, 
 the alternate motions do not appear, unlefs 
 a portion of the fibres is cut out, and fepa- 
 rated from the reft. 
 
 CX. 
 
 From a difference in the ftate of a mufclc 
 contracted by inherent power, while the 
 member it fuftains is moved by external 
 force, and that of the fame mufcle con- 
 tracted by the power of the will, we per- 
 ceive that in the mufcles there may be 
 a ftate of relaxation without their exten- 
 fion. 
 
 CXL 
 
^6 PHYSIOLOGY. 
 
 GXI. 
 
 There is a ftate of the contraclion of 
 ttiufcles that is not difpofed fpontaneoufly 
 to alternate with relaxation, and in which 
 too the fibres do not eafily yield to extend- 
 ing powers applied: Such a ftate of con- 
 tra£lion is called a SPASM. 
 
 CXII. 
 
 When mufcles are excited to contradlon 
 by preternatural cafes, and are contradled 
 with unufual velocity and force, and efpe- 
 cially when the contra(5lions, alternating 
 with relaxation, are frequently and pre- 
 ternaturally repeated; fuch motions, are 
 called CONVULSIONS. 
 
 CXIII. 
 
 If the contraclipn of mufcles are exert- 
 ed 
 
PHYSIOLOGY. 
 
 11 
 
 ed with unufual force, and fuch contraC’- 
 tions are often repeated, they in a Ihort 
 time become uneafy and weaker ; and 
 though coutradlions are not exerted with 
 unufual rorce, if they are often repeated, 
 and for a long time, without an interval of 
 reft, they alfo become uneafy and weaker. 
 
 CXIV. 
 
 Within certain bounds, with refpedl to 
 force, frequency, and duration, the con- 
 tra(ftion of mufcles, by being repeated, is 
 
 performed with more facility and force. 
 
 \ 
 
 cxv. 
 
 Are not the contracftions of mufcles pro- 
 duced by the adlion of the animal power; 
 thofe which are more efpecially liable td 
 become uneafy and weak by frequent re- 
 petition ? 
 
 SECT, 
 
78 
 
 PHYSIOLOGY. 
 
 SECT. II. 
 
 CHAP. III. 
 
 Of the Functions of the Brain. 
 
 CXVI. 
 
 i-^ROM the efFedls of ligatures made up- 
 on the nerves, and of the deftrudion 
 of their continuity, it appears, that, in 
 their entire Rate, motions may be commu- 
 nicated from the brain to the other parts of 
 the nervous fyftem, and from the latter to 
 the former; and, from the fame experi- 
 ments it appears, that the brain (XXIX. i.) 
 
 IS 
 
PHYSIOLOGY. 
 
 79 
 
 is the organ of fenfation and volition, and 
 of the feveral intellectual operations inter- 
 vening between thefe : All which is con- 
 firmed by the effecfts of the organic aflPec- 
 tions of the brain upon the intelledtual fa- 
 culties. 
 
 f 
 
 CXVII. 
 
 The brain is thus the fenforium or cor- 
 poreal organ, more immediately connected 
 with the mind; and, fo far as a corporeal 
 organ is employed, all the operations of 
 thought arifing in confequence of fenfation 
 are operations of the brain, and are modi- 
 fied by its various condition ; Boerh. Inji. 
 Med. 581. Haller^ Prim. Lin. 570. Gaub. 
 Path. Med. 523. See afterwards (CXXII.) 
 
 CXVIll. 
 
 As certain imprefiions ac 5 t on the ner- 
 vous 
 
8o 
 
 PHYSIOLOGY. 
 
 vous fyftem, without producing any fen- 
 fation (LXXX.) ; and as, at the fame time, 
 there is hardly any communication be- 
 tween the different parts of the nervous 
 fyftem, but by the intervention of the 
 brain; it is from hence alfo probable, that 
 the brain, by its organization, is fitted to 
 propagate the motions arifing in one part, 
 to the other parts of the nervous fyftem ; 
 and, as thefe mechanical communications 
 produce different effecfts, according to the 
 different ftate of the brain itfelf, we, upon 
 the whole, conclude, that the brain is a_ 
 corporeal organ fufceptible of various con- 
 ditions, and thereby of conliderable in- 
 fluence in moft of the phenomena of the 
 nervous fyftem. 
 
 CXIX. 
 
 The acftlon of the brain, in moving the 
 feveral parts of the body, is excited by 
 
 various 
 
PHYSIOLOGY, 
 
 Ss 
 
 various caufes, or by the fame caufes in 
 different circumftances, 
 
 I. It is efpecially excited by the WILL, 
 diredling the motion of certain parts as 
 means to an end. 
 
 As the motion of certain parts is adap- 
 ted to various purpofes, we are confcious 
 of willing thefe purpofes, as they occafion^ 
 ally occur, and fo far alfo of the motion 
 of the parts concerned in ®them : but, 
 where the motion of the parts is connec- 
 ted with one fenfation, or a few only, the 
 motions required follow thefe fenfations 
 without our being confcious of fpecially 
 willing them; and, unlefs we have con- 
 tinued the pradlice of adapting the mo- 
 tions to different purpofes, we lofe the 
 power of doing fo, and the motions be- 
 come unavoidably connedled with thofe 
 fenfations which, for a long time, had a- 
 lone given occafion to them. In moft of 
 the inftances of what are called VOLUN- 
 TARY 
 
$2 PHYSIOLOGY. 
 
 TARY MOTIONS, we are confcious o£ 
 willing the end propofed, more than the 
 motions excited ; and, of the motions 
 produced, we are confcious chiefly of 
 thofe of a whole member, or ot cue gene- 
 ral effedl, and very little of the many par- 
 ticular motions that concur to produce it. 
 We are never confcious of the particular 
 mufcles employed. 
 
 2. The ac^on of the brain is excited by 
 the more general and vehement volitions 
 naT-ed EMOTIONS and PASSIONS. 
 Upon occafion of thefe, the confeioufnefs 
 of willing the particular motions produced 
 is always much lefs diftindl, and in many 
 cafes is not at all perceived. Of the laft 
 kind are, particularly, moft of the expref- 
 lions of the paiiions in the countenance 
 and geflure. 
 
 3. By the difpofition of human nature 
 to IMITATION. This imitation is fome- 
 times involuntary, often without confeiouf- 
 nefs ; 
 
PHYSIOLOGY. ^3 
 
 nefs; and the confcioufnefs which takes 
 place is often of the general purpofe only, 
 without that of the particular motions 
 produced ; or, at leaft, it is of thefe only 
 as a general effe(5l. 
 
 4. By APPETITES or delires, diredled 
 to certain external objecfls, and ariling from 
 fenfation, without any reafoning direcfling 
 to an end ; at leaft, without any other end 
 in the firft inftance but that of the grati- 
 fication of the defire. 
 
 5. By certain PROPENSITIES or defires 
 to remove an uneafy or painful fenfation, 
 in confequence of which motions are ex- 
 cited, which are not diredled to any exter- 
 nal objedt, but confined to the body itfelf. 
 
 Thefe motions are not forefeen ; nor are 
 we ever confcious of willing any thing, but 
 the general effedt. Of this kind, the chief 
 are the motions of fneezing, coughing, 
 fighing, hiccuping, vomiting, voiding 
 urine and feces, yawning, ftretching 
 
 [pan'- 
 
 2 
 
^4 PHYSIOLOGY. 
 
 i^pandkulatio)^ and thofe motions of reftleiT- 
 nefs and inquietude which pain and un- 
 eafinefs produce. Weeping and laughing 
 _are expreffions of emotion and pafTion. 
 
 In all thefe, as well as in the motions of 
 N° 4. fome volition is concerned, not only 
 as they can often be prevented by another 
 volition prefenting itfelf j but, befides, as 
 the feveral motions which occur in execu* 
 ting thefe ptopenfities are more or fewer, 
 and more or lefs forcible, according to the 
 vehemence of the propenfty or efibrt. 
 Very often the ftimulus to thefe propen- 
 hdes is irreliftible; and, unlefs the peculiar 
 ftimulus is prefent, the motions cannot be 
 produced by any volition. 
 
 6. By certain internal impreftions arifng 
 from the exercife of the functions of the 
 body itfelf, which produce no fenfation, 
 nor produce motions of which we are con- 
 fcious, except when exercifed in an unufual 
 manner. Such are the caufes of the mo- 
 I .. tions 
 
PHYSIOLOGY. 
 
 tions of the heart and arteries, of the or- 
 gans of refpiration, of the ftomach, inte- 
 ftlnes, and perhaps of many other parts. 
 With regard to moft of thefe rrtotions, it 
 may be fuppofed, that they are the me- 
 chanical effedls of their feveral caufes, a<5l- 
 ing upon the inherent power of mufcular 
 fibres (LXXXVb) ; but it is fufficiently 
 certain, that they alfo depend upon an 
 adtion of the brain ; and the efFedls of paf- 
 lions, as well as the effedls of deftroying 
 or comprelling the nerves of the organs 
 concerned, are proofs of it. 
 
 The motions mentioned in this article 
 are commonly fuppofed not to, be accom- 
 panied w’ith any volition of which we 
 are diftindlly confcious. This perhaps is 
 not ftridlly true with regard to moft of 
 them ; and, fo far as it is, it may be im- 
 puted to that repetition which deftroys 
 confcioufnefs (LV.) : But neither can we 
 entirely adopt this explanation ; as thefe 
 F motions 5 
 
86 
 
 PHYSIOLOGY. 
 
 motions, which are intended to follow one 
 ftimulus only, may be fuppofed to require 
 no exercife of volition, as in the cafe of the 
 heart, arteries, and alimentary canal, while 
 the adlion of refpiration, as adapted to va- 
 rious purpofes, continues to be a voluntary 
 motion. 
 
 7. By various occafional imprefllons of 
 external bodies, and by various occafional 
 Hates of the fyftem, or of its particular 
 parts which excite motions, not only in 
 the parts to which the impreflions are im- 
 mediately applied, but alfo in diftant parts, 
 on which they can operate only by the in- 
 tervention of the brain. Some of thefe 
 caufes operate with, others without, fenfa- 
 tioh or volition. 
 
 cxx. 
 
 In all, or any of thefe cafes in which the 
 aftion of the brain takes place, we do not 
 
 per- 
 
petceive the manner, that is, the mechani- 
 cal means, by which the feveral caufes 
 produce their efFedts ; and we perceive only 
 an inftitution of our Maker, eftablifhing 
 a connedion between the feveral caufes 
 and the motions which enfue. At the 
 fame time we, for the mofl part, perceive, 
 that the connexions eftablifhed are fuited 
 to the purpofe of the animal oeconomy ; 
 and particularly, to the purpofe of fupport'* 
 ing the fyftem in a certain condition for a 
 certain time, and of averting what might 
 hurt or deftroy it. This conftitution of 
 of the animal oeconomy we call NATURE j 
 and every where in the oeconomy we per- 
 ceive t;he vhes confervatrices and medicatri-^ 
 ces natures^ fo juftly celebrated in the fchools 
 of phylic. 
 
 eXXL 
 
 It is in confequence of this condltutlGn, 
 I F 2 thag 
 
88 PHYSIOLOGY. 
 
 that not only impulfe, and other caufes 
 which may be fuppofed to produce motion, 
 do accordingly excite motions in the ani- 
 mal oeconomy ; but that alfo many caufes 
 which feem to diminifh motion, do, how- 
 ever, produce an increafe of motion in ani- 
 mal bodies. Thus f^veral palTions which, 
 in their firft tendency, diminifh motion, 
 feveral propenfities arifing from debility 
 and difficulty of a^flion, the abfence of 
 iifual impreffions, evacuations, and other 
 caufes of relaxation, cold and narcotic 
 powers, are all of them caufes of confider- 
 able motions arifing in the animal fyftem. 
 
 CXXII. 
 
 As the mechanifin of the brain, fuited 
 to its feveral functions, is not at all per- 
 ceived (CXX.) ; and, at the fame time, as 
 very few of thefe fundtions are carried on 
 without fenfttion and volition, it muft ap- 
 
 pear 
 
PHYSIOLOGY. 
 
 89 
 
 pear from this, and many other confidera- 
 tions, that the mechanifm of the brain 
 (CXVII.) would not be fufficient for the 
 purpofe, without being united with a fen- 
 tient principle, or mind, that is conftantly 
 prefent in the living fyftem. But, at the 
 fame time, it is with little probability al- 
 leged, that the adminiftration of the cor- 
 poreal funtflions is entirely dire<fled by the 
 mind adting independently of the body, 
 and with intelligence perceiving the ten- 
 dency of impreffions, and exciting fuch 
 motions as may favour the beneficial, or 
 obviate the hurtful tendency of all caufes 
 ading upon the body. 
 
 We are certainly confcious of no fucli 
 adminiftration. Many impreffions have 
 their effedls without fenfation or volition. 
 In moft cafes where volition takes place, 
 it is very general, with little confcioufnefs 
 pf the motions excited, and none at all of 
 F 3 the 
 
90 
 
 PHYSIOLOGY. 
 
 the organs employed. The force of im- 
 preffion is every -where abfoiute; and it is 
 according to the force of impreflion, and 
 other mechanical conditions of the fyflem, 
 that the motions excited prove either fa- 
 lutary or pernicious. The general prin- 
 ciple, therefore, is ill-founded; it is not 
 neceffary, f vide Stahl. Praf. ad 'Junker. 
 Confp. Med.) ; it can be of no ufe, and may 
 be hurtful to the fyftem of phyfic. 
 
 CXXIII. 
 
 The adlion of the brain is often deter- 
 mined and regulated by cuftom and habit ; 
 that is, by laws eflabliflied by frequent and 
 uniform repetition. See above XLIII. LIV. 
 LV. LVL 5. LX. 3. LXVIll. and LXX. 
 for the effeds of cuftorn on fenfation, and 
 CXIV. for one effed of it on the adion 
 of moving fibres. It is now to be ob- 
 leped further, i. That cuftom determines 
 
 the 
 
PHYSIOLOGY. 
 
 91 
 
 the degree of tenfion (CII. Gill.) neceflary 
 to the a< 5 lion of mufcular fibres. 2. That 
 cuftom afibciates motions with fenfations, 
 which are not otherwife their caufes; fo 
 that the renewal of the fenfation, or of 
 its idea, renews alfo the motion. 3. That 
 cuftom afibciates difierent motions, fo that 
 they cannot be feparately performed, tho* 
 not originally, nor necefiarily connedled. 
 4. That cuftom determines the degree of 
 force and velocity with which motions can 
 be performed. 5. That cuftom deter- 
 mines the order of fucceifion in afibciated 
 motions, and the velocity with which they 
 fhall fucceed one another. 6. That cu-' 
 ftom eftablilhes the periodical return of 
 certain fenfations and motions, not origi- 
 nally necefiary to the cgconomy. 7. That 
 cuftom fixes an exabl period for the return 
 of certain fenfations and motions, which, 
 by the laws of the oeconomy, are difpofed 
 p return at intervals otherwife undeter- 
 F 4 ipined 
 
92 
 
 PHYSIOLOGY. 
 
 mined. Thefe laws, which may be efla- 
 blilhed by cuftom, are, many* of them, 
 
 I. with difficulty avoided ; they are often 
 rigidly fixed, have a confiderable influence 
 on the a6Uon of the brain, and govern the 
 revolutions of the animal fyftem. Thus, 
 any caufes producing a deviation from the 
 iifual degree of force and velocity (N’ 4.) 
 are apt to defiroy the meafure of it alto- 
 gether; and, in like manner, caufes pro- 
 ducing a deviation from the ufual order 
 and velocity in the fucceffion of motions 
 (N® 5.) are apt to deftroy the power of 
 the mind in following that order, or in 
 giving any meafure to the feveral motions 
 which fliould be performed; and perhaps, 
 in this way, the effeds of debility, of feve- 
 ral paffions, and of furprife, are in fome 
 ^fTkeafiire explained. 
 
 CXXIV, 
 
 / 
 
PHYSIOLOGY. 
 
 93 
 
 cxxiv. 
 
 The brain feems, by its conftitution, to 
 be difpofed to the alternate ftates of reft 
 and adivity ; as appears in the alternate 
 ftates of deep and waking, which conftant- 
 ly take place in every animal: but wherein 
 this conftitution confifts, it is difficult to 
 difcover. 
 
 cxxv. 
 
 The moft common opinion is, that the 
 brain is a decretory organ, which fecretes a 
 fluid neceffary to the fundlions of the ner- 
 vous fyftem; that this fluid is alternately 
 exhaufted and recruited, and thereby gives 
 occaflon to the alternate ftates of deep and 
 waking. But this fuppofltion is attended 
 with many difficulties, i . It is probable, 
 that the nevous fluid exifted in the animal 
 jsmbryo, before the adion of the heart, or 
 
 any 
 
94 
 
 PHYSIOLOGY. 
 
 any fecretory fundlion, could take place, 
 
 2. In animals which during thip winter 
 fuffer a temporary death, fuch as bats, 
 when by heat they are again reftored to 
 life, the vital power of the folids is re- 
 Itored before the fluidity of the blood. 
 
 3. The nervous fluid fubflfts in the 
 nerves and mufcular fibres long after 
 they are feparated from the brain, and 
 often when cut into many fmall parts. 
 
 4. Though it be true that the brain is a 
 fecretory organ, the fluid fecreted may be 
 deftined to another purpofe; and fo far as 
 we underfland that purpofe, the fluid fit for 
 it, muft be unfit for the purpofe of fenfe 
 and motion. 5. There is no appearance, 
 in any part of the nervous fyftem, of pro- 
 vifion made for an occafional accumulation 
 of the fecreted fluid; nor is there any evi- 
 dence of its adlually taking place. 6. The 
 phenomena of fleep and waking do not 
 correfpond with fuch a fuppofition ; as 
 
 fleep 
 
PHYSIOLOGY. 
 
 93 
 
 fleep often takes place when the fe- 
 creted fluid muft be copioufly prefent, 
 and waking can be protraded when the 
 fluid is exhaufted much beyond its ufual 
 meafure. 7. Both ftates are induced by 
 many caufes, which can hardly be flip- 
 pofed to ad: upon fecretion. 
 
 CXXVI. 
 
 A certain compreflion of the brain can 
 produce a ftate of the fyftem refembling 
 fleep ; but that ftate is in fome refpeds, 
 different from that of ordinary fleep : 
 And it does not by any means appear, 
 that natural and ordinary fleep depends 
 upon any compreffion of the brain, 
 
 CXXVII. 
 
 % 
 
 As it is therefore probable that fleep and 
 leaking do not depend upon a different 
 
 quantity ^ 
 
96 PHYSIOLOGY. 
 
 quantity of the matter of the nervous fluid 
 for the time prefent in the fyftem (CXXV.), 
 or upon any caufes interrupting its motion, 
 while the condition of the matter remains 
 the fame (CXXVI.); we are difpofed to 
 believe, that thofe ftates of fleep and wa- 
 king depend upon the nature of the ner- 
 vous fluid itfelf capable of becoming more 
 or lefs moveable ; that it is chiefly in the 
 brain fufceptible of thefe difierent condi- 
 tions ; and that, efpecially by its condition 
 there, it has its more general eflfe<fls on the 
 whole fyftem. 
 
 CXXVIII. 
 
 This may perhaps be confirmed by con- 
 fidering the remote caufes of fleep and wa- 
 king: And it appears, that cold, the ab- 
 fence of impreflions, attention to a fingic 
 fenfation, or to fenfations that have no 
 confequence in thought or a6lion ; the 
 
 finifhed 
 
PHYSIOLOGY. 
 
 97 
 
 finiflied gratification of all vehement de- 
 fires, fedative fenfations and impreflions, 
 evacuations, relaxation, and any violent, 
 frequent, or long continued exercife of 
 the animal power; are all of them, feve- 
 rally or together, caufes inducing deep. 
 
 CXXIX. 
 
 On the other hand, it appears, that a 
 certain degree of heat, all fenfations of im- 
 preffion, impreflions analogous to thofe 
 which produce fenfation, all fenfations 
 which lead to thought and adlion, and the 
 increafed impetus of the blood in the vef- 
 fels of the brain, are all of them caufes fa- 
 vouring or inducing a date of waking. 
 
 CXXX. 
 
 As mod of the caufes CXXVIIL are 
 evidently fuch as diminiih motion in the 
 I brain, 
 
98 PHYSIOLOGY. 
 
 brain, and tliore of CXXIX. arc fiicli as 
 increafe it; it is from thence probable, 
 that the nervous fluid in the brain is truly 
 capable of different Rates or degrees of mo- 
 bility, which we fliall call its Rates of EX- 
 CITEMENT and COLLAPSE ; but, with- 
 out intending, by thefe terms, to exprefs 
 or determine any thing with regard to the 
 nature of the nervous fluid, or wherein its 
 different Rates confiR. 
 
 cxxxr. 
 
 This fubjecR may be further illuRrated, by 
 obferving, that the excitement of the brain 
 appears to be in very different degrees on 
 different occafions. It feems to be greateR 
 in certain maniacs endued with uncom- 
 mon Rrength, refiRing the force of moR 
 impreffions, and with the utmoR difficulty 
 admitting fleep. 
 
 GXXXIL 
 
PH YS10L0,G Y. 
 
 99 
 
 . • CXXXII. 
 
 A lefler degree of excitement occurs in. 
 the ordinary ftate of waking men in health, 
 when the excitement is total with refpe6; 
 to the functions of the brain, but readily 
 admitting of deep. This excitement may 
 be conlidered as of two kinds ; either as it 
 refpedls the vigour, or as it refpedls the mo- 
 bility of the fyftem : And thefe different 
 dates of the brain are expreffedin the body, 
 by ftrength or debility, alacrity or fluggifh*- 
 nefs ; and in the mind, by courage or timi^ 
 dity, gaiety or fadnefs. 
 
 CXXXIII. 
 
 A degree of collapfe takes place in the 
 cafe of natural deep, when the collapfe 
 prevails fo far as to fufpend very entirely 
 the exercife of the animal fundlions j and, 
 though the exercife of the vital and natu- 
 ral 
 
lOO 
 
 PHYSIOLOGY. 
 
 ral continue, they are confiderably weak-=^ 
 ened. The partial collapfe that may take 
 place in the brain difcovers itfelf by the 
 delirium which appears in a ftate that of- 
 ten occurs as intermediate between deep 
 and waking ; and even in deep the collapfe 
 with refpedt to the animal fundlions, takes 
 place more lefs entirely ; whence the deep 
 is with or without dreaming, and the 
 dreaming is more or lefs adlive. 
 
 CXXXIV. 
 
 A dill greater degree of collapfe takes 
 place in the cafe of fyncope; in which 
 it is fo great, as to fufpend the exerclfe 
 of the vital fundlions concerned in the 
 circulation of the blood, notwithftand- 
 ing the force of habit in thefe, and their 
 being expofed to conftant flimuli. Here 
 the collapfe may be very confiderable ; but 
 there dill remains fome degree of excite- 
 
 ment 
 
PHYSIOLOGY. 
 
 lOI - 
 
 merit while the brain can be adled upon 
 by ftimuli, which ad. only on vital pow- 
 ers, and while its ufual excitement is ftill 
 recoverable by fuch ftimuli, 
 
 •y 
 
 cxxxv. 
 
 If the collapfe is ftill more complete and 
 irrecoverable, it is the ftate of DEATH, 
 
 CXXXVI, 
 
 From what is now faid of the excite- 
 ment and collapfe of the brain, it will ap- 
 pear, that we fuppofe LIFE, fo far as it is 
 corporeal, to confift in the excitement of 
 the nervous fyftem, and efpecially of the 
 brain, which unites the different parrs, and 
 forms them into a whole. But, as certain 
 other fundions of the body are neceflary to 
 the fupport of this excitement, we thence 
 G leanr. 
 
102 
 
 PHYSIOLOGY. 
 
 learn, that the caufes of death may be of 
 two kinds ; one that afls dire(5lly on the 
 nervous fyflem, deftroying its excitement; 
 and another that indirectly produces the 
 fame effedl, by deftroying the organs and 
 functions neceffary to its fupport. Of the 
 firft kind are chiefly the caufes of fleep 
 operating in a; higher degree; as cold, fe- 
 dative paflions, poifons, and all caufes of 
 very violent excitement. 
 
 cxxxvir. 
 
 $ 
 
 This fubjedl may receive ftill further 
 illuftration from confidering the flate of 
 the other parts of the nervous fyftem with 
 refpecft to excitement and collapfe. In the 
 nerves ftridtly fo called (XXIX. 2 .), we do 
 not know that the nervous fluid fuflers any 
 change, but what is exadlly correfpondent 
 to its flate in the brain and extremities ; 
 
 and 
 
PHYSIOLOGY. 
 
 103 
 
 and therefore the only difference of the 
 flate of the nerves to be taken notice of is 
 their being more or lefs free (XXX.) 
 
 CXXXVIII. 
 
 In the fentient extremities of the nerves 
 (XXIX. 3.), a difference of the ftate of the 
 nervous fluid arifes from the feveral caufes 
 mentioned (LVI. 2 . 3. 4. and 5.), which 
 give a different degree of fenfibility ; and 
 it is probable, that thefe different ftates of 
 the fentient extremities are analogous to 
 the different degrees of excitement in the 
 brain. 
 
 CXXXIX. 
 
 The moving extremities, or mufcular 
 fibres (XXIX. 4.), may alfo be in a differ- 
 ent condition with refpedl to excitement. 
 It is probable, that their conftitutiou is 
 G 2 fuch 
 
104 
 
 PHYSIOLOGY. 
 
 fuch as to admit of a higher degree of ex*^ 
 citement than any other portion of the 
 nervous fyftem ; and that upon this their 
 contracflility depends. But, whatever is in, 
 this, we perceive very clearly, that the con- 
 dition of mufcular fibres may be varied by 
 caufes affedling their tonic power (Cl.), or 
 their vigour and mobility (LXXXIX.), and 
 by the effedl of cuftom (CXIV.) ; and it 
 is probable, that the dates produced by 
 thefe caufes are analogous to the different 
 degrees of excitement in the brain (CXXX.) 
 and in the fentient extremities (CXXX VIII) ; 
 and thus the feveral parts of the nervous 
 fyflem (XXIX.), as they conlifl of the fame 
 kind of matter (XCIV.), are alfo fubjedt te 
 fimllar conditions. 
 
 CXL. 
 
 The beginning of motion in the nervous 
 fyftem is moft commonly accompanied 
 
 2 with 
 
PHYSIOLOGY. 
 
 ^^5 
 
 with fenfation, and the force of this in 
 producing its feveral effefls is more or lefs, 
 I. According to the force (XLII.), quality 
 (LXXVII.), and novelty (LIV.) of im- 
 preflion. 2. According to the fenfibility 
 of the fentient extremity and brain (LVI.) 
 3. According to the date of attention 
 (XLVII.) Thefe feveral caufes often con- 
 cur, frequently balance one another, and 
 are always to be confidered together. 
 
 CXLI. 
 
 The effedl of fenfation is commonly that 
 of exciting the a(5lion of the brain ; and 
 this is generally according to the; degree 
 of volition produced under the different 
 circumftances mentioned in CXIX. 
 
 CXLII. 
 
 The adtion of the brain excited by vo- 
 G 3 iidoii 
 
io6 PHYSIOLOGY. 
 
 lition or other caufes, is determined to par- 
 ticular parts of the body, moft conftantly 
 by the conne<5Iions eftablifhed in the fy- 
 Item (CXX.), but alfo occalionally, by ac- 
 quired habits, or the greater mobility of 
 certain parts. 
 
 CXLIII. 
 
 With regard to the connections efta- 
 blifhed in the fyftem (CXX. CXXI.) it is 
 to be obferved, as of great confequence in 
 pathology, that certain parts of the body 
 which have a common function and con- 
 ftitutipn, have thereby a peculiar relation 
 to the brain, fo as to be more liable to be 
 affe6ted by the different ftates of it, and in 
 their turn by the difference of their condi- 
 tion to affecft the brain. Such are, efpe- 
 cially, the organs of voluntary motion ; 
 the alimentary canal, and efpecially the 
 fbomach j the circulatory fyftem, and par- 
 ticularly 
 
PHYSIOLOGY. 
 
 107 
 
 tlcularly the extremities of the vefTels on. 
 the furface of the body j the uterine and 
 genital fyftem in females ; and fome others 
 to be mentioned in the pathology. 
 
 CXLlVo 
 
 The communications of motion between 
 the feveral parts of the nervous fyftem 
 which have been mentioned as inftances 
 of a particular fympathy between thefe 
 parts, are very feldom to be explained by 
 any contiguity or contadl, either in the 
 origin or courfe of the nerves of the com- 
 municating parts ; and more commonly 
 they may be explained, by fuppoling the 
 adlion of the impreflion to be general with 
 refpedl to the brain, and that the affeeftion 
 of particular parts depends upon the caufes 
 of determination (CXLII. CXLIII.) 
 
 When the adion of feveral parts, toge- 
 ther or fucceffively, are neceffary to pro- 
 G 4 duce 
 
io8 PHYSIOLOGY. 
 
 duce one effedl, thefe concur, though the 
 ftimulus exciting the adlion of the brain is 
 applied to one (ingle part only ; and com- 
 monly no other caufe of communication 
 can be afligned, but the feveral motions 
 being necelTary to the execution of the vo- 
 lition, propenlicy, &e. arifingfrom the fti- 
 mulus. 
 
 CXLV. 
 
 Thefe are the chief fads and laws rela- 
 tive to the nervous fyftem. The whole 
 might perhaps be illuftrated, and more 
 exadly afcertained, by^ a more particular 
 inquiry into the nature of the nervous 
 fluid j but we are not fo confident in our 
 opinion on this fubjed, or of the applica- 
 tion it will admit of, as to deliver it here. 
 
 SECT, 
 
PHYSIOLOGY. 109 
 
 SECT. IIL 
 
 pF THE Circulation of the Blood. 
 
 CXLVI. 
 
 HEclrculationoftheblood,byCixiX. 
 
 feems neceffary to the excitement of the 
 brain ; and for this, as well as other rea- 
 fons, it is, next after the fundlion of the 
 brain itfelf, the moft important of the ani- 
 mal ceconomy. 
 
 On this fubjedl, we fliall confider, i;The 
 purfe of the blood. 2 . The powers by 
 
 which 
 
I lo 
 
 PHYSIOLOGY. 
 
 which the blood is moved. 3. The laws 
 and general circumftances of the circula- 
 tion. 4. Refpiration as necelTary to it. 
 
 On all thefe fubjedls we fuppofe the 
 anatomy of the parts to be known. 
 
 SECT. III. 
 
 CHAP. I. 
 
 Of the Course of the Blood. 
 CXLVII. 
 
 "^^^y^OUNDS and hemorrhagies Ihow, 
 that, in living bodies, the blood 
 is in conftant motion, and flowing into 
 any one part from many others. 
 
 CXLVIII. 
 
PHYSIOLOGY. 
 
 Xli 
 
 CXLVIIL 
 
 In man, and other analogous animals, 
 which have once breathed for fome time, 
 the courfe of the blood is very conftantly 
 in the following manner. 
 
 From the left ventricle of the heart, the 
 blood paffes into the trunk of the aorta, 
 and fucceffively into the following velTels 
 and cavities, viz. the branches of the aorta, 
 the branches of the vena cava, the trunk of 
 the fame, the right auricle of the heart, the 
 right ventricle of the fame, the pulmo- 
 nary artery, the pulmonary veins, the left 
 auricle of the heart; from which laft it 
 paffes into the left ventricle of the heart to 
 return again into the fame courfe as be- 
 fore. 
 
 From all this, it appears, that in the ar- 
 teries the ufual courfe of the blood is from 
 the heart towards the extreme branches *of 
 
 i. • 
 
 thefe ; 
 
112 PHYSIOLOGY. 
 
 thefe ; and that, in the veins, the courfe is 
 in the contrary direction, from the extreme 
 branches towards the heart. 
 
 CXLIX. 
 
 The courfe of the blood through the 
 cavities of the heart, as above defcribed, is 
 not in a continued flream, but alternately 
 interrupted and free during the contrac- 
 tion and dilatation of thofe cavities, which 
 alternately happen. Thus, while the left 
 ventricle of the heart is in a flate of con- 
 traction, the blood pafTes out of it into the 
 aorta ; but at the fame time no blood 
 pafles into it from the left auricle, which 
 is then dilated and filled by the blood flow- 
 ing into it from the pulmonary vein. It 
 is only when the ventricle is emptied by 
 contraction, and confequencly relaxed, that 
 the blood pafTes into it from the auricle, 
 jarged by the contractions of the auricle 
 
 and 
 
PHYSIOLOGY. 1 15 
 
 and adjoining finus venofus, which fucceed 
 immediately to that of the ventricle. Du- 
 ring this contradlion of the auricle and 
 filling of the ventricle, no blood pafles 
 from the ventricle into the aorta, nor til! 
 a contradlion of the ventricle fucceeds in 
 confequence of its being filled. The fame 
 circumftances take place with regard to 
 the right ventricle and auricle of the heart, 
 and precifely at the fame times ; for it ap- 
 pears, that the two ventricles of the heart 
 are contradled and relaxed at the fame 
 time ; and, in like manner, the two au- 
 ricles. 
 
 CL. 
 
 That the courfe of the blood, as de- 
 fcribed CXLVIll. and CXLIX. is its 
 ufual and conftant courfe, appears from 
 infpedlion of the heart in living animals ; 
 from the fituation of the valves of the 
 
 heart ; 
 
PHYSIOLOGY. 
 
 114 
 
 heart ; from the fituation of the valves at 
 the orifices of the aorta and pulmonary- 
 artery ; from the fituation of the valves of 
 the veins ; from the effects of ligatures 
 made upon the arteries and veins ; from 
 the eflfedls of hasmorrhagies of the veins ; 
 from obfervations with the microfcope; 
 and, laftly, from experiments of infufion 
 and transfufion in living, and of injedion 
 in dead animals. 
 
 CLI. 
 
 This courfe of the blood is, however, 
 in fome parts, and upon fome occafions, a 
 little changed, i. In the penis, and fome 
 other parts, the blood does not pafs from 
 the extreme arteries immediately into con- 
 tinuous veins; but is poured out into an 
 intermediate cellular texture, from which 
 it -is afterwards received by the extreme 
 veins. 2. In the fmall branches of the 
 2 aorta, 
 
PHYSIOLOGY. 
 
 aorta, the blood does not move conftantly 
 in a dire( 5 lion from the heart towards the 
 extremities ; but, in certain portions of 
 them, is fometimes retrograde tothatcourfe. 
 In this, it is favoured by the anaftomofes 
 frequent between the fmall veiTels, which, 
 at the fame time, prevent this deviation 
 from being confiderable or durable, while 
 the adion of the heart continues. 
 
 CLII. 
 
 In the courfe of the venous blood, there 
 is alfo fOme variety, i . In the fmall veins, 
 the blood is liable to have a motion retro- 
 grade to its ufual diredlion (CXLVIII.), as 
 in the arteries (CLI. 2.) * 2. The blood, 
 returning to the heart from moft parts of 
 the body, paffes fucceflively from fmaller 
 into larger veins, by a feries of pretty re- 
 gularly increafing vefTels, till they form 
 the vena cava entering the heart. But 
 
 this 
 
ii6 PHYSIOLOGY. 
 
 this is varied in the abdomen, where the 
 veins carrying the blood returning from 
 every vifcus included in that cavity, except 
 the kidneys and genitals, unite in forming 
 the vena portarum, whereby they undergo 
 a peculiar diftribution. 3. The veins re- 
 turning the blood from the extreme arte- 
 ries in the brain, do not carry it to the 
 heart by a feries of regularly increaling 
 velfels, but by the interpolition of finufes, 
 into which the fmall veins ilTuing from 
 the cortical part of the brain immediately 
 pour their blood. 4. As the courfe of the 
 blood through the veflels of the lungs is 
 not at all times equally free, and particu- 
 larly'’ is confiderably interrupted at the 
 end of exfpiration ; fo at the fame time it 
 is alfo interrupted at entering the right 
 ventricle of the heart; and this often occa- 
 fions fome regurgitation, or retrograde mo- 
 tion, in both the afcending and defcending 
 
 cava. 
 
 CLIir. 
 
PHYSIOLOGY. 
 
 CLIIL 
 
 The whole of the fluids carried in the 
 aorta to its extreme branches, are not re-* 
 turned again by the continuous veins to 
 the heart, as, by fecretory vefTels, a part of 
 them is conftantly carried out of the courfc 
 above defcribed. Some of thefe fecreted 
 fluids are thrown entirely out of the body,, 
 and others are poured into certain cavities, 
 for various purpofes of the oeconomy ; 
 and fome of thefe are again returned into 
 the courfe of the circulation. Of thefe 
 laft, there is a peculiar fluid which, from 
 the extremities of the arteries, is poured 
 out in a liquid form, or exhaled in that of 
 vapour, into perhaps every cavity and va- 
 cuity of the body. This, after having 
 ferved the purpofe of the eflPufion, feems 
 intended to be regularly returned again 
 into the courfe of the circulation ; and ac- 
 5 H cordingly, 
 
ii8 PHYSIOLOGY. 
 
 cordingly, in all the feveral cavities into 
 which it had been efTufed, there are abfor- 
 bents which again take it in. Thefe do 
 not carry the fluid immediately into the 
 veins ; but uniting, form the veflels called 
 LYMPPIATIGS, which, in their courfe, 
 pafs through the conglobate glands, and at 
 length terminate either in the receptacle of 
 the chyle, in the thoracic diufl, or in the 
 left fubclavian vein ; and, in this way, re- 
 turn the abforbed fluid into the courfe of 
 the oi'dinary circulation. 
 
 CLIV. 
 
 There are abforbent veflels, not only in 
 all the feveral cavities, but alfo on the ex- 
 ternal furface of the body, by which many 
 extraneous matters may be introduced in- 
 to it. , ' 
 
 CLV. 
 
PHYSIOLOGY. 
 
 319 
 
 CLV. 
 
 Moil of the fluids fecreted from the cir“ 
 culatiug mafs, and poured into cavities^ 
 may be abforbed from thefe, and returned 
 again by the lymphatics, as in CLIII. to the 
 courfe of the circulation. But the fame 
 fecreted fluids feem often to be returned 
 alfo into the courfe of the circulation by a 
 regurgitation, of retrograde motion, in the 
 excretory and fecretory veflels. 
 
 H a 
 
 SECT. 
 
t2Q 
 
 PHYSIOLOGY* 
 
 SECT. Ill 
 
 CHAP. II. 
 
 Of the Powers moving the Blood. 
 
 HE chief power by which the blood 
 
 is moved, and the circulation carried 
 on, is the adlion of the heart, or its repeat- 
 ed contracflions occurring, as mentioned 
 CXLIX. For this purpofe, the heart is a 
 mufcular part ; the adlion of which may 
 depend upon an inherent power flimulated 
 by the dilatation of its cavities ; but this 
 
 CL VI, 
 
 in 
 
PHYSIOLOGY. 
 
 I2I 
 
 inherent power requires the conftant fup-^ 
 port of the nervous and animal powers, 
 and is often aduated by thefe. 
 
 CL VII. 
 
 The contradlion and relaxation of the 
 heart, or, as thefe are called, its fyftole and 
 diaftole, are neceffarily alternate by the 
 general law (GVIII.) affecting all mufcles, 
 and by the ftimulus from the influx of 
 venous blood being alternately applied and 
 removed. 
 
 CLVIIL 
 
 If we may be allowed to eftimace the 
 vigour of mufcles by the number of their 
 fibres, we muft fuppofe the force of the 
 heart to be very confiderable ; but it is 
 very difficult to obtain any exadl eflimate 
 of its abfolute force. It is perhaps the re^ 
 H 3 lative 
 
322 
 
 PHYSIOLOGY. 
 
 lative force only that we are concerned ta 
 know. 
 
 CLIX. 
 
 Do the arteries, by their contra6lion, 
 contribute to promote die motion of the 
 blood begun by the heart ? They cannot 
 do it by the elafticity of the fimple folid 
 in their flrudlure ; and can do it only by 
 their being endued with a mufcular 
 power, whereby they may, in their con- 
 traction, communicate to the moving 
 blood more than was loft of the force of 
 the heart in dilating them. That they 
 are indued with a mufcular contractility 
 (LXXXiL), is probable from the appear- 
 ance of the mufcular fibres in their ftruc- 
 turcj from their irritability appearing in 
 the experiments of Verfchuir ; from their 
 becoming flaccid on tying the nerves be- 
 longing to them 5 from the motion of the 
 
 blood 
 
PHYSIOLOGY. 123 
 
 blood being fupported, when the force of 
 the heart is confiderably weakened ; from 
 the motion of the blood becoming languid, 
 when the adUon of the arteries is dedroy- 
 ed } from the velocity of the blood in the 
 extreme arteries being greater than was to 
 be expeded from the velocity of the blood 
 ilTuing out of the heart, .expofed to fo 
 many caufes of retardation as confcantly 
 occur ; and, laflly, from the velocity and 
 impetus of the blood in different parts of 
 the body, and <it different times, being 
 unequal, v/hile the addon of the heart 
 continues the fame. 
 
 It is probable, that the mufcular fibres 
 of the arteries become more irritable as 
 the arteries are more dlflant from the 
 heart. 
 
 H 4 
 
 CLX, 
 
S24 
 
 PHYSIOLOGY. 
 
 CLX. 
 
 The tone and adlion of the arteries, as 
 a mufcular part, may be increafed by fti- 
 muli immediately applied to them, or by 
 the increafed force of the nervous and ani- 
 mal powers with regard to them; and 
 they may be diminifhed by fedative powers 
 applied, or by weakening the nervous and 
 , animal powers. 
 
 CLXI. 
 
 There does not appear to be any ofcilla- 
 mry motion in the extreme arteries inde- 
 pendent of the a(ftion of the heart. 
 
 CLXII. 
 
 There does not appear to be any opera- 
 tion of capillary attradHon in the extreme 
 arteries ; nor does th,ere feem to be any 
 
 occalion 
 
PHYSIOLOGY. 
 
 ^25 
 
 occafion for fuch a power in any part of 
 the arterial fyftem. 
 
 CLXIII. 
 
 The power of derivation (Vis derivationis 
 111. Halleri 174.) in the fanguiferous fy-^ 
 ftem, feems to be no other than that which 
 arifes from the fulnefs of contractile velTels. 
 
 CLXIV. 
 
 The motion of the blood in the arteries 
 of any particular part is promoted by the 
 aClion of adjoining mufcles. ' 
 
 CLXV. 
 
 / 
 
 The blood in the vena cava, and its 
 branches,' is moved by the aClion of the 
 heart and of the arteries. Thefe powers 
 are affifted by the aClion of mufcles, which, 
 
 in 
 
126 PHYSIOLOGY. 
 
 in their contra£lion, prefs the veins lying 
 between their feveral fibres ; and alfo, by 
 the fwelling of their whole mafs, prefs the 
 adjoining veins. Thefe veins are commonly 
 provided with valves, which determine the 
 effect of all prefTure upon them, to be the 
 motion of the blood towards the heart. 
 
 The great trunks, both of the vena cava 
 and pulmonary vein, are provided with 
 mufcular fibres, and manifeftly endued 
 with mufcular contradility. 
 
 CLXVI. 
 
 In the abforbent veffels, the fluids are 
 probably taken in by a capillary attrac- 
 tion. 
 
 CLXvir. 
 
 In the lymphatic veffels, provided wdth 
 numerous valves, which neceffarily deter- 
 
 mine 
 
PHYSIOLOGY. 
 
 127 
 
 mine the motion of the contained fluid to 
 be towards the heart, the fluid is mo- 
 ved by the preflure of the neighbouring 
 mufcles and arteries. But further, as 
 the lymphatics are remarkably irritable, 
 it is probable that the fluid in them is 
 moved by a periftaltic motion begun by 
 the adlion of their abforbent extremities. 
 
 \ 
 
 CLXVIII. 
 
 The motion of the blood through the 
 veflels of the lungs, depends upon refpira- 
 tion, to be confldered hereafter. 
 
 SECT 
 
£28 
 
 PHYSIOLOGY, 
 
 SECT. Ill, 
 
 [CHAP. m. 
 
 Of the Laws of the Circulation, 
 
 CLXIX. 
 
 ^^HE velocity of the blood paffing out 
 of the left ventricle of the heart into 
 the aorta, may be eftimated from knowing 
 the quantity of blood paffing out at each 
 fyftole, the area of the orifice of the aorta, 
 and the time occupied by the fyftole; but 
 none of thefe circumilances are exadlly 
 afcertained. 
 
 CLXX. 
 
t 
 
 PHYSIOLOGY. 129 
 CLXX. 
 
 As the blood moves onwards through 
 the arteries, the velocity (CLXIX.) fuffers 
 a confiderable retardation from feveral 
 caufes. I. From the capacity of the ar- 
 teries being enlarged as they are more di- 
 ftant from the heart. 2. From the fre- 
 quent flexures of the arteries. 3. From 
 the angles which the branches make with 
 the trunks from which they arife. 4. From 
 anaftomofes. 5. From the vifcidity of the 
 blood. 6. From the fri(5lion of adhefion, 
 7. From the weight and rigidity of the 
 parts furrounding the arteries. 
 
 CLXXL 
 
 The velocity (CLXIX.) and the caufea 
 of retardation (CLXX.) being given, the 
 velocity of die blood in the arteries will 
 
 be 
 
PHYSIOLOGY. 
 
 130 
 
 be as the frequency of the fyftole of the 
 heart. 
 
 CLXXII. 
 
 The frequency of the fyftoie of the heart 
 ■will be more or lefs, i . As the blood in the 
 veins is more or lefs quickly returned to 
 either ventricle of the heart. 2. As the 
 ventricles of the heart are more or lefs en- 
 tirely evacuated at each fyftole. 3. As the 
 mufcular fibres of the heart are more or 
 lefs moveable. 4. As the adion of the 
 nervous and animal powers are more or 
 lefs increafed with refped to the heart. 
 
 CLXxiir. 
 
 As the arteries of a healthy body are al- 
 ways full, the blood thrown out of the ven- 
 tricles into the arteries during the fyftole 
 of the heart, can only find a place there 
 
PHYSIOLOGY. 
 
 132 
 
 by pufliing on the blood with the velocity 
 (GLXIX.), or by dilating the arteries; but 
 as the refiftances CLXX. prevent the 
 blood from flowing with the velocity 
 CLXIX. the blood thrown out’ of the 
 heart inufl;, in fome meafure, dilate the ar-> 
 terles, and thereby form what is called the 
 PULSE. 
 
 CLXXIV. 
 
 It appears, that, in the arteries to a cer« 
 tain length, the blood moves fafter during 
 the fyftole than during the diaftole of the 
 heart; but as the reflftances and caufes of 
 retardation become greater in every por- 
 tion of the arteries as it is more diftant 
 from the heart, fo the acceleration of the 
 blood during the fyftole of the heart mull 
 be greater in any portion of the arteries 
 nearer the heart than in the next adjoin- 
 ing that is more diftant ; and fo far as this 
 
 takes 
 
K32 
 
 PHYSIOLOGY. 
 
 takes place, a dilatation of the arteries 
 will happen, even from a fmall quantity 
 of blood thrown out of the ventricles. 
 
 CLXXV. 
 
 As the refiftances to the blood’s motion 
 in the blood-velTels increafe with the di- 
 ftance from the heart, there may be a part 
 of the fanguiferous fyftem in which the 
 motion of the blood will not be accele- 
 rated during the fyftole of the heart, and 
 in which, therefore, no pulfe can be dif- 
 cerned. This happens in the extreme 
 branches of the aorta; and no pulfe is ever 
 obferved in the extreme branches of the 
 vena cava. 
 
 CLXXVI. 
 
 The velocity and impetus of the blood 
 
 in 
 
PHYSIOLOGY. 133 
 
 « 
 
 in the whole fyftem of blood- vefTels will 
 always be as the adlion of the heart and 
 arteries taken together. 
 
 CLXXVIL 
 
 The velocity and impetus of the blood, 
 in any particular part of the fyftem, will 
 be, I. As the part is more or lefs diftant 
 from the heart. 2. As the cireumftances 
 (CLXX.) take place more or lefs in the part. 
 
 3. As the gravity of the blood concurs 
 with, or oppofes its motion in the part. 
 
 4. As caufes increafing or diminifhing 
 the adlion of the arteries of the part are 
 applied or removed. 
 
 CLxxvm. 
 
 The quantity ^of blood diftributed to 
 any particular part of the fanguiferous fy- 
 ftem, will be greater or lefs according to 
 I the 
 
134 
 
 PHYSIOLOGY. 
 
 the velocity and impetus of the blood in 
 the part, by CLXXVII. ; and according 
 to the refiftances in other parts being in- 
 creafed or diminilbed by conftridlion, 
 compreffion, ligature, pofition, relaxation, 
 or aperture. 
 
 CLXXIX. 
 
 The flexibility and contratflility of the 
 blood-veflels render the effects of all in- 
 creafe or diminution of refiftance in any 
 particular part moft confiderable in the 
 neareft, and very little fo in the more re- 
 mote, veflels of the fyftem. By this we are 
 to judge of the celebrated dodtrines of de- 
 rivation and revulfion. 
 
 % 
 
 CLXXX. 
 
 The quantity of blood diftributed to the 
 
 dif- 
 
PHYSIOLOGY. 
 
 different parts of the fyftem, is in different 
 proportion at different periods of life. 
 
 1. The capacity and force of the hearty 
 in proportion to the" fyftem of veffels, is 
 greater at the beginning of life than at any 
 after period. Till the body arrives at its 
 full growth, the capacity of the veffels in- 
 creafes in greater proportion than that of 
 the heart ; but from that period, the ca- 
 pacity of the veffels is conftantly dimi- 
 nifhing, while that of the heart fuffers 
 little change. 
 
 2. A greater quantity of blood is con- 
 tained in the arteries, in proportion to 
 that which is contained in the veins, at 
 the beginning of life, than at any after 
 period. From the time that the body has 
 arrived at its full growth, the quantity of 
 blood contained in the veins, in propor- 
 tion to that which is contained in the ar- 
 teries, is conftantly increafing. 
 
 3. The veffels of the head receive a 
 
 I a greater 
 
136 PHYSIOLOGY. 
 
 greater’quantity of blood, in proportion to 
 the reft of the fyftem, at the beginning of 
 life than at iny after period. 
 
 4. Any general increafe of the a( 5 Uon of 
 the heart and arteries determines the blood 
 more copioufly to the extreme arteries on 
 the furface of the body, than to thofe of 
 the internal parts. 
 
 5. The equilibrium of the fanguiferous 
 fyftem, with regard to the diftribution of 
 the blood, may be changed by various 
 caufes (CLXXVII. CLXXVIIL); and thefe 
 caufes continuing to operate for fome time, 
 induce a habit which renders the changed 
 diftribution neceflary to the health of the 
 fyftem. 
 
 6. The lymphatic fyftem is fuller in 
 young perfons than in old. 
 
 SECT. 
 
PHYSIOLOGY. 137 
 
 SECT. III. 
 
 CHAP. IV. 
 
 Of Respiration. 
 
 CLXXXI. 
 
 J^Espiration confifts of the mo- 
 tion of infpiration, or the admiffion 
 of air into the lungs ; and of exfpiration, 
 or the expulfion of air from the fame j .al- 
 ternately happening. 
 
 CLXXXII. 
 
 Refpiration takes place in man, and 
 I 3 other 
 
138 PHYSIOLOGY. 
 
 other analogous animals, foon after the 
 infant is taken from the uterus of the mo- 
 ther, and is expofed to the air. After it 
 has taken place for a little time, it is ever 
 after neceffary to the continuance of life, 
 as it is abfolutely neceffary to the continue 
 ance of the circulation of the blood. ’ 
 
 CLxxxiir. 
 
 The lungs are a hollow fpongy mafs, 
 capable of confining air, and readily di- 
 latable by it. By the wind- pipe, they are 
 open to the atinofphere; and they are fo 
 fituated in the thorax, that the air muft 
 enter into them, if the cavities of the tho- 
 rax, in which they are placed, are enlarged. 
 For^ as there is no air in thefe cavities, and 
 the external air cannot enter into them, 
 the enlargement of the thorax muft form 
 a vacuum around the lungs, which the ex- 
 |.grnal heavy and elaftic air will fupply by 
 
 enter*!^ 
 
PHYSIOLOGY. 
 
 139 
 
 entering into and dilating the kings, while 
 thefe do not allow the air to pafs through 
 them into the cavities of the thorax. 
 
 CLXXXIV. 
 
 Infpiration therefore depends upon the 
 enlargement of the capacity of the thorax; 
 and this is performed chiefly by the con- 
 tradlion of the diaphragm. This in its 
 relaxed ftate is fufpended by the mediafti- 
 num, and its middle tendinous part is rai- 
 fed high in the thorax ; wherefore, as this 
 middle part, by the contraction of the 
 mufcular, is moved downwards, the tho- 
 rax is thereby confiderably enlarged. 
 
 CLXXXV. 
 
 The capacity of the thorax is alfo en- 
 larged by the motion of the ribs upwards, 
 whereby the curvatures of oppoflte ribs 
 I 4 . are 
 
140 PHYSIOLOGY. 
 
 are fet at a greater diftance from eack 
 other ; and, by the fame motion, the fter- 
 num is moved outwards, and fet at a great- 
 er diftance from the vertebras of the back. 
 The motion of the ribs upwards is caufed 
 by the contra<ftion of both layers of inter- 
 coftal mufcles. That the mufcles called 
 
 internal intercoftals concur with the exter- 
 
 » 
 
 nal in railing the ribs, appears from the 
 lituation of thofe mufcles, from the greater 
 mobility of the inferior ribs, from the in- 
 fpecftion of thofe mufcles in living ani- 
 mals, and from experiments imitating their 
 adlion. In more violent and laborious in- 
 fpirations, the railing of the ribs is aflifted 
 by many mufcles attached to the ribs, and 
 ariling from the clavicle, humerus, fca- 
 pula, and vertebrae of the neck or back. 
 
 CLXXXVL 
 
PHYSIOLOGY. 
 
 141 
 
 CLXXXVI. 
 
 By the enlargement of the thorax, a di« 
 Jatation of the lungs is produced,' in pro- 
 portion to the bulk of air entering into 
 them; but the dilatation may often be 
 greater by the air that enters into the 
 lungs being heated and ratified ; and the 
 greateft diftenfion of the lungs is obtained 
 by a conflridlion of the glottis confining 
 the air that has already entered into the 
 lungs? • 
 
 CLXXXVIL 
 
 As infpiration, or the admiflion of 
 air into the lungs, depends upon the en- 
 largement of the thorax, the diminution 
 of it muft expel the air, or produce exlpi- 
 ration. The capacity of the thorax is d»^ 
 piiniflied, while the mufcles dilating it are 
 fpontaneoufly relaxed, by the elafticity of 
 
 the 
 
142 PHYSIOLOGY. 
 
 the ligaments connecling the ribs with the 
 vertebras, and by the elafticity of the carti- 
 lages and ligaments connedling the ribs 
 with the fternum ; both which |)owers, 
 commonly affifted by the weight of the 
 ribs themfelves, bring the ribs and fter- 
 num into the pofttion they were in before 
 infpiration. At the fame time, the elafti- 
 city of the mediaftinum draws the dia- 
 phragm upwards ; and the contraflion of 
 the abdominal mufcles both preffes the 
 diaphragm upwards, and pulls die ribs 
 downwards ; and, in the laft, they are af- 
 lifted by the fterno-coftal and infra-coftal 
 mufcles. While thefe powers concur in 
 diminiftiing the capacity of the thorax, 
 the expulfion of the air from the lungs is 
 affifted by the elafticity of the lungs them- 
 felves, and bv the contraction of the muf- 
 cular fibres of the bronchias. 
 
 CLXXXVIII. 
 
PHYSIOLOGY. 
 
 143 
 
 CLXXXVIII. 
 
 Thefe are the ordinary powers of exfpi- 
 ration, which, depending upon the re- 
 action of elaftic parts, is performed flowly, 
 and with little force: but when it is ne- 
 ceffary to perform it with more velocity 
 and force, fome other and very powerful 
 mufcles, as the quadratus lumborum, fa- 
 crolumbalis, and longiffimus dorfi, concur 
 in pulling down the ribs ; and, at the fame 
 time, the abdominal mufcles, a(5luated by 
 the animal power, are contra6;ed with 
 greater velocity and force than in fpon- 
 taneous exfpiration. 
 
 CLXXXIX. 
 
 The fituation of the blood-velTeis of the 
 lungs is fuch, that, in the contra6led ftate 
 of this vifcus, thefe vefTels muft be much 
 folded and ftraitened j and it appears, that, 
 
 in 
 
144 
 
 PHYSIOLOGY. 
 
 in the foetus, where they are conftantly in a 
 contradled ftate, their capacity is not fuffi- 
 cient to tranfmit, in the time required, the 
 whole of the blood returning to the heart 
 by the vena cava ; but, after refpiration 
 has been repeated for fome time by the 
 dilatation of the lungs to a certain degree 
 in infpiration, their blood- veffels are un- 
 folded, lengthened, and enlarged, fo as to 
 be capable of tranfmitting the whole blood 
 of the cava. 
 
 cxc. 
 
 In the infant who has breathed for 
 fome time, the whole blood of the vena 
 cava pafTes into the right ventricle of the 
 heart, and from thence enters into the vef- 
 fels of the lungs ; but, in the contra<5led 
 ftate of the lungs which occurs at the end 
 of exfpiration, the blood cannot be proper- 
 
 ly 
 
PHYSIOLOGY. 14s 
 
 ly tranfmitted ; and, for that purpofe, an 
 infpiration becomes abfolutely neceflary. 
 
 CXCI. 
 
 It is, however, under a certain degree 
 of infpiration only, that the blood is freely 
 tranfmitted through the veffels of the 
 lungs; for, if the infpiration is full, and 
 continued, fo that the lungs are thereby 
 much diftended, we find that this ftate alfo 
 interrupts the free pafiage of the blood, 
 and renders exfpiration neceflary. 
 
 Exfpiration becomes alfo neceflary ; be- 
 caufe, perhaps, the air long retained in the 
 lungs lofes a part of its elafticity, and be- 
 comes thereby unfit to keep the lungs di- 
 ftended; but, more certainly, and more 
 efpecially, becaufe, in an animal that has 
 breathed for fome time, there is a noxious 
 vapour,, conftantly arifing from the lungs, 
 which, if not diflblved by the air, and 
 
 carried 
 
f 
 
 146 PHYSIOLOGY. 
 
 / 
 
 carried out of the lungs, proves pernicious 
 to life. 
 
 CXCII. 
 
 From what has been faid, it appears, 
 that the alternate motions of infpiration 
 and exfpiration are neceffary to the circu- 
 lation of the blood, and otherwife alfo to 
 the health of the body ; and it appears alfo, 
 that the more frequent the alternate mo- 
 tions of refpiration are, the more quickly 
 is the blood tranfmitted from the right to 
 the left ventricle of the heart. 
 
 CXGIII. 
 
 We can now perceive alfo the caufes 
 exciting thefe alternate motions ; and we 
 find no occafion for fuppofing them to 
 arife from any caufes alternately inter- 
 rupting the motions of the nervous fluid. 
 
 l 
 
 or 
 
PHYSI’OLOGY. ' 147 
 
 or of the blood, into the mufcles concerned 
 in thefe fundlions. 
 
 Infpiration, or the adlion of the mufcles 
 producing it, is excited, in all cafes of 
 general effort, to remove pain and uneafi- 
 nefs ; and it is, perhaps, a propenfity, of 
 this kind that gives the firft beginning to 
 refpiration in the new-born infant, ex- 
 pofed to feveral new and uneafy impref- 
 fions. 
 
 For the continuance of refpiration, in- 
 fpiration is efpecially excited by the fenfe 
 of uneaflnefs that attends any difficulty in 
 the paffage of the blood through the vef- 
 fels of the lungs ; but this uneaflnefs arifes, 
 in fome meafure, at the end of every ex- 
 fpiration, and is much increafed by any 
 continuance of this ftate. 
 
 CXCIV. 
 
 Exfpirauon, in fome meafure, neceffarily 
 
 fuc- 
 
148 PHYSIOLOGY. 
 
 fucceeds infpiration, by the fpontaneous 
 laxation of the infpiratory mufcles (CVIII.), 
 while the elafticity of the membranes, li- 
 gaments, and cartilages ftretched in infpi- 
 ration, brings back the ribs and dia- 
 phragm into their former fituations ; and 
 the fame effe(fls are alfo produced by the 
 adlion of the abdominal mufcles, and of 
 the mufcular fibres of the bronchia ; both 
 of which are ftretched, and thereby ex- 
 cited in infpiration. 
 
 In the cafe of ordinary infpiration, thefe 
 caufes are fufiicient to produce a fponta- 
 neous exfpiration. But as it appears that 
 a violent and long continued infpiration 
 interrupts the paftage of the blood through 
 the lungs, this creates an uneafinefs, and a 
 propenfity, which muft produce a relaxa- 
 tion of the infpiratory, and excite a con- 
 tradlion of the exfpiratory, mufcles. 
 
 It is further to be fuppofed, that, in ani- 
 mals which have breathed for fome time, 
 I cuftom 
 
PHYSIOLOGY. 
 
 149 
 
 ciiftom has alTociated the feveral motions 
 concerned both in infpiration and exfpira- 
 tion ; fo that an irritation applied to any 
 part of them neceffarily excites the whole; 
 and it may alfo be fuppofed, that habit 
 determines thefe motions regularly to fuc-- 
 ceed one another. 
 
 cxcv. 
 
 In this manner (CXCIII. CXCIV.) re- 
 fpiration is continued for the general pur- 
 pofes of the animal oeconomy; but the 
 feveral motions of which it confifts are alfo 
 occalionally excited and varioully modi- 
 fied by the will, intending particular ef- 
 fedls to be produced by thefe motions. 
 They are alfo excited, and variouily modi- 
 fied, by certain emotions and paflions, and 
 give particular expreffions of thefe. They 
 are often excited alfo by imitations ; and 
 they are particularly excited by propenfi- 
 K ties 
 
PHYSIOLOGY. 
 
 , ^ 5 ° 
 
 ties to remove pain and uneafinefs, which 
 operate more frequently on refpiration 
 than upon any other fundlion. 
 
 CXCVL 
 
 The confideration of the effedls of refpi- 
 ration on the animal fluids is delayed till 
 the nature of thefe fluids fhall have been 
 more generally confidered. 
 
 SECT. 
 
PHYSIOLOGY. 15 ^: 
 
 SECT. IV. 
 
 Of the Natural FuNcTiONSi 
 
 CXCVIL 
 
 animal body from a fmall be- 
 ginning, grows to a confiderable 
 lize; and at the fame time, from the period 
 of the birth, during the whole of after lifcj 
 the body fuffers, by various means, a daily 
 and confiderable wafte. 
 
 K 2 
 
 CXGVIII. 
 
152 
 
 PHYSIOLOGY. 
 
 CXCVIII. 
 
 The increafe of bulk, therefore, mnft be 
 
 I 
 
 acquired, and the daily wafle fupplied, by 
 matters taken into the body, the molt part 
 of which, from the prefumed purpofe of 
 them, we name ALIMENTS. 
 
 CXCIX. 
 
 A great part of thefe aliments, as taken 
 into the body, are of a different nature 
 from the matter of the body itfelf; or at 
 leaft, are in fuch a flate as not to be fit for 
 being immediately applied to the purpofes 
 of it : they mufl, therefore, be changed, 
 and fitted to the purpofes of the occonomy, 
 by powers within the body itfelf. 
 
 cc. 
 
PHYSIOLOGY. 
 
 153 
 
 CC. 
 
 The converfion, or affimilation, of the 
 aliments to the nature of the foHds and 
 fluids of the animal body ; the farther 
 changes of thefe fluids for various pur- 
 pofes, by fecretion; and the application 
 of fome part of them in nutrition, or in 
 increafing the growth of the body ; make 
 what are called the NATURAL FUNG= 
 TIONS. 
 
 3 
 
 SECT 
 
i54 PHYSIOLOGY. 
 
 SECT. IV. 
 
 CHAP. I. 
 Of Digestion. 
 
 CCI. 
 
 f^HE term digeJHon is commonly em- 
 ployed to fignify the fundlion of the 
 flomach alone in changing the aliments ; 
 ' but, in this chapter, we are to confider all 
 the changes of thefe as they occur fuccef- 
 fively in the different flakes through which 
 the matters pafs. 
 
 CCIL 
 
PHYSIOLOGY. 
 
 
 ccir. 
 
 Animals are determined to take in ali- 
 ment, by the appetites of hunger and 
 third. 
 
 CCIIL 
 
 Hunger is an appetite depending upon 
 a fenfation referred to the ftomach, and 
 arifing from a particular date of it. 
 
 This date feems to be in fome refpecd 
 the degree of emptinefs, but more efpe- 
 cially the date of contraction in the muf- 
 cular fibres which emptinefs gives occafion 
 to. This date of contraction may alfo be 
 excited by certain dimulants applied; but 
 more commonly it depends upon, and is 
 correfpondent to, the date of inanition, 
 and therefore of contraction, in the veTels 
 of the Ikin emitting the matter of perlpira- 
 tion. 
 
 K 4 
 
 CCIV. 
 
S56 physiology. 
 
 CGIV. 
 
 Thirfl is an appetite for liquids, ■which 
 depends upon a fenfation chiefly referred 
 to the internal fauces, and ariling from 
 the drynefs or heat of thefe parts; from 
 acrimony applied to them, or exifting in 
 the fluids poured out there ; from the pu- 
 trefcency or vifcidity of the contents of 
 the flomach ; and from all increafed eva» 
 cuations. 
 
 ccv. 
 
 Thefe appetites determine men to take 
 in a great variety of folid and liquid mat- 
 ters,, direded by inftinclive likings and 
 difgufts ; in fome inftances correded by 
 e:sperience. 
 
 CCVL 
 
PHYSIOLOGY. 
 
 157 
 
 CCVI. 
 
 Of the matters chofen, It appears that 
 fome of them are fuited to fupply the 
 matters of the fluids or folids of the body-j 
 and therefore properly named Aliment; 
 while others of them are fuited only to 
 improve the relifh of aliment, or to ob- 
 viate fome deviations ready to happen in 
 the bufinefs of digeftion ; and thefe we 
 name CONDIMENTS, 
 
 GCVII. 
 
 The proper alimentary matters are ani- 
 mal or vegetat^le only. 
 
 CCVIII. 
 
 The animal aliments feem to be fb near- 
 ly of the fame nature with the matter of 
 
 the 
 
3jS PHYSIOLOGY. 
 
 the body itfelf, that, to be rendered fit for 
 the pnrpofes of the geconomy, they feem 
 to require no other change but that of be- 
 ing rendered fluid. 
 
 CCIX. 
 
 But the vegetable aliment is very diffe- 
 rent from the matter of the animal fluids . 
 or folids, and muft therefore be changed 
 into the nature of thefe by the powers 
 CXCIX; and as many animals are nou- 
 rifhed by vegetable aliment alone, and as 
 perhaps all animal matters may be ulti- 
 mately traced to a vegetable origin, it will 
 appear, that, to account for the produ(5lion 
 of animal matters, it is efpecially, and in 
 the firft place, neceffary to lliow how vege- 
 table matter may be converted into ani- 
 mal. 
 
 ccx. 
 
PHYSIOLOGY. 
 
 559 
 
 ccx. 
 
 If we coniider the many different odours, 
 taftes, and colours, which are to be ob° 
 ferved in different vegetables, we fhould be 
 ready to think that vegetable matter is of 
 very' great variety: but we know that 
 the matter diftinguifhed by its fenfible 
 qualities makes but a fmall part of the 
 whole of any vegetable j and that, befides 
 the matter peculiar to each, there is in 
 moft, perhaps in all vegetables, a large pro- 
 portion of common matter, which we pre- 
 fume to be the matter adapted, and that 
 very univerfally, to the aliment of ani- 
 mals. 
 
 CCXl. 
 
 It is this common matter of vegetables, 
 therefore, that we are to confider here ; 
 
 and 
 
i6o PHYSIOLOGY. 
 
 and we think it may be confidered as of 
 three kinds only ; that is, oily, faccharine, 
 and what feems to be a combination of 
 thcfe two. 
 
 CGXII. 
 
 The oily matter of vegetables, which 
 makes part of the aliment of animals, is 
 without any fenfible odour or tafte; and is 
 not only very nearly the fame in the many 
 different vegetables from which we take it, 
 but is alfo in all of thefe fo nearly a-kin 
 to the oil which appears in animals, that 
 it is not neceffary to fuppofe any confider- 
 able change to be made upon the vegetable 
 oil on its being taken into the bodies of 
 animals. 
 
 ccxiir. 
 
 It is the faccharine matter, and efpecially 
 
 this 
 
PHYSIOLOGY. i6i 
 
 this when blended with oily, matter in dif- 
 ferent proportion, that makes the greateft 
 part of the common matter of vegetables, 
 and is the chief part of the vegetable 
 aliment of animals. It is this, therefore, 
 that we have efpecially to conf der here ; 
 and, as it lies in vegetables, it is different 
 from the moft part of animal matters in 
 the following refpetts. 
 
 It is readily fufceptible of a vinous and 
 acetous fermentation, and fpontaneoufly 
 enters into the one or the other of thefe ; 
 and, without undergoing more or lefs of 
 thefe, it perhaps never enters into a piitre- 
 faiElive fermentation. 
 
 The fame matter treated by diftillation, 
 without addition, gives out always, in the 
 firft part of the diftillation, an acid, and 
 only afterwards a volatile alkali in ftnall 
 proportion. 
 
 The fame vegetable matter, treated by 
 calcination, leaves aflies, which contain a 
 
 fixed 
 
PHYSIOLOGY. 
 
 162 
 
 fixed alkali, and an- earth that is or may bs 
 converted into a quick-lime. 
 
 CCXIV. 
 
 In all thefe refpeds, the common matter 
 of animals is confiderably different. 
 
 This enters fpontaneoufly into a putre- 
 fa(5live fermentation, and that without: 
 palling through the vinous or acetous : 
 At leaft, thefe are not to be diftincily per- 
 ceived. 
 
 The fame animal matter, treated by di- 
 ffillation, gives out always, in the firft part 
 of the diftillation, a volatile alkali in large 
 proportion, and only afterwards by a great 
 force of fire it gives out an acid. 
 
 Animal matters, treated by calcination, 
 leave allies, in which no alkali is to be 
 found ; and the earth is not calcareous, nor 
 convertible into a quick- lime, by any 
 means vet known. 
 
 4 
 
 1 
 
 ccxv. 
 
PHYSIOLOGIC. 
 
 5 % 
 
 ccxv. 
 
 Thefe differences are fufficlently mark- 
 ed ; but it is proper to obferve here, that 
 the vegetable matter we treat of, by un- 
 dergoing a putrefactive .fermentation, is 
 changed fo, as to acquire very exaCUy moft 
 of thofe characters of animal matter we 
 have juft now mentioned. 
 
 CCXVI. 
 
 The aliment being thus confidered, wc 
 proceed to confider the changes it under- 
 goes after being taken into the animal 
 body; but, firft, of the courfe it paffes 
 through, and of the motions it is fubjeCled 
 to in its progrefs. 
 
 GCXVIL 
 
i 64 physiology. 
 
 CCXVII. 
 
 The aliment is taken into the mouth 3 
 and there the more folid parts of it are 
 commonly fubjeded to a triture, or what 
 is called manducation. At the fame time 
 a quantity of faliva, and of the other 
 fluids of the mouth, with fome portion of 
 our drink, is intimately mixed with it, 
 whereby the whole is reduced to a foft 
 pulpy mafs. In this date, by the action 
 of deglutition, it pafles through the fauces 
 into the cefophagus, by which it is con- 
 veyed into the ftomach. 
 
 CCXVIIL 
 
 Here the aliment is detained for fome 
 time, fubjecfted to a conftant agitation and 
 fome preflure, both by die contraclions 
 of the different parts of the ftomach it- 
 1 felf, 
 
PHYSIOLOGY. 165 
 
 felf, and by the alternate preiTure of the 
 diaphragm and abdominal mufcles. Af- 
 ter fbme lime, however, fifft the more 
 fluid parts, and at length the moft minute 
 parts of the folid matter are pulhed thro’ 
 the pylorus ‘into the duodenum. 
 
 CCXIX. 
 
 The matters received from the ftomach 
 into the duodenum pafs on from thence 
 fuccelTively through the feveral parts of 
 the inteftinal canal ; and, in the whole of 
 the courfe, the matters are ftill fubjedled to 
 the alternate preflTure of the diaphragm 
 and abdominal mufcles, and to the con- 
 tractions of the inteftines themfelves. 
 
 ccxx. 
 
 Through the whole courfe of the inte- 
 nines, but efpecially in thofe named the 
 I h fmall^ 
 
i66 PHYSIOLOGY. 
 
 fmall^ the more fluid part of the content*, 
 and particularly- the peculiar fluid we 
 name chyle^ is taken into the veflels named 
 laBeah. ' Thefe, from imperceptible be- 
 ginnings on the internal furface of the in- 
 teftines, unite into larger veflels laid in the 
 mefentery, and convey the chyle, and 
 what accompanies it, firft into the conglo- 
 bate glands of the mefentery, and from 
 thence to the receptaciiliim chyli^ as it is 
 called. From this the chyle pafles by the 
 thoracic du<5t into the left fubclavian vein. 
 In one or other part of this courfe of the 
 chyle, the veflels carrying it are joined by 
 lymphatics, returning the lymph from al- 
 moft every part of the body. 
 
 CCXXI. 
 
 The matters contained in the inteflinal 
 canal, not taken into the ladfeals, are moved 
 onwards in the courfe of the inteftines, be- 
 coming 
 
PHYSIOLOGY. 
 
 167 
 
 coming by degrees of a thicker confiftence, 
 efpecialiy in the colon, where their motion 
 is confiderably retarded; but, at length, 
 they are moved onwards to the extremity 
 of the re( 5 lum, where their weight, bulk, 
 and acrimony excite motions which throw 
 them entirely out of the body. 
 
 CCXXII. 
 
 This is the courfe of the" alimentary 
 matters, fo far as tney can be confidered as 
 any ways in a feparate ftate. Of the mo- 
 tions of the feveral organs concerned in 
 this courfe, we pafs over thofe of mandu- 
 cation, deglutition, or others depending on 
 the aclion of mufcles, the funcflions of 
 which are readily underftood from a 
 knowledge of their htuadon; and we are 
 here to confider only the motions of the 
 alimentary canal itfelf. 
 
 L 2 
 
 2 
 
 CCXXIII. 
 
i68 PHYSIOLOGY. 
 
 CCXXIII. 
 
 The motions in the cefophagus depend 
 upon the adlion of its mnfcular fibres, which 
 are chiefly thofe forming a chain and cir- 
 larly furrounding it. This tube, by the 
 morfel of food pufhed into it by the a<5lion 
 of deglutition, is neceflTarily dilated, and 
 its circular fibres are thereby excited to a 
 contradlion. But as thefe fibres are fuc- 
 ceflively dilated, fo are they alfo contracfled, 
 and pufli on their contents through the fe- 
 veral portions of the tube, alternately and 
 fucceflively dilated and contracted, giving 
 the appearance of a vermicular motion, 
 and what is commonly called periftaltic. 
 This motion may be propagated either up- 
 wards or downwards.; and the direction of 
 it is in the one or the other way, as the 
 motion happens to begin at the upper or 
 lower extremitv. 
 
 CC.XXIV. 
 
PHYSIOLOGY. 
 
 169 
 
 CCXXIV. 
 
 The motion of the flomach is not fo 
 fimple. Its mufcular fibres are in like 
 manner irritable by dilatation, and its 
 circular fibres muft therefore be in fome 
 meafure fubjedled to a fucceflive dilatation 
 and contradlion. But, though the direc-. 
 tion of fuch motions is from the left to 
 the right, this does not immediately pufii 
 the contents of the ftomach into the in- 
 teftines. It feems to be the purpofe of 
 the ceconomy, to detain the aliment for 
 fome time in the ftomach; and therefore, 
 any confiderable dilatation of the circular 
 fibres, efpecially that v^hich occurs in a 
 full ftomach, feems to have the eftedl of 
 exciting the longitudinal fibres to a con- 
 traction, which draws the two orifices of 
 the ftomach nearer to one another. By 
 this the pylorus is raifed up and rendered 
 L 3 lefs 
 
170 PHYSIOLOGY. 
 
 lefs eafily paflable; and probably, at the 
 fame time, the peculiar band of circular 
 fibres which furround the pylorus, are 
 more firmly contradled, and render it lefs 
 pervious. Vide CXLiV. From hence 
 it is, that the diredlion of the perifialtic 
 motion of the flomach is fdinecimes from 
 the left to the right, and fometimes alfo 
 the contrary way. It is, how^ever, moft 
 conftantly in the firft manner ; becaufe it 
 is commonly begun from the oefophagus, 
 and becaufe, when it is inverted, the re- 
 finances on the left from the blind fac 
 of the fiomach, from the higher fituation 
 of the cardia, and from the conftridlion 
 of this by the diaphragm in infpiration, 
 are commonly more confiderable than the 
 refinance at the pylorus. The contents of 
 the fiomach, therefore, are at length pufii- 
 ed through the pylorus ; in the firfi place, 
 the more fluid contents, as thefe occupy 
 the antrum pylori, while the more folid, 
 
 having 
 
H y S I O L o G Y. 
 
 171 
 
 having their air loofened by fermentation, 
 are rendered fpecifically lighter, and float 
 nearer the upper orifice. But at length, 
 as the ffcomach is in any meafure emptied, 
 the pylorus is lefs raifed, is more relaxed, 
 and allows matter to pafs more eafily; 
 and, at the fame time, the empty ftomach 
 eontradled more, is efpecially towards the 
 right extremity contracted to fuch a degree, 
 as to embrace the fmalleft. folid matters, * 
 now fallen down into it, and to pu£h them 
 through the pylorus. 
 
 This is an idea of the ordinary motions 
 of the ftomach ; but they are, upon fome 
 occafions, fubjeft: to other modifications, 
 as in eructation, rumination, and vomit- 
 ing, which, however, as morbid, we re° 
 ierve to be confidered in the pathology. 
 
 ccxxv. 
 
 The motions of the inteftines will be 
 
 readily 
 
 y 
 
PHYSIOLOGY. 
 
 172 
 
 readily \mderftood, from what has been 
 faid of thofe of the oefophagus. Any por- 
 tion of the inteflinal canal being dilated, 
 will in confequence be contradled, and 
 will urge on its contents in the fame di- 
 redlion in which the motion was begun. 
 But as the force here is gentle ; and as, in 
 the long courfe of the canal, there occur 
 many flexures, dififerent pofitions, and 
 occafional irritations ; it is obvious, that 
 refiftances and ftronger contractions may 
 frequently occur here, to change the di- 
 rection of the motion : accordingly we 
 find it frequently changed, and direCled 
 from below upwards, in fo far that the 
 contents of the inteflines frequently pafs 
 into the ftomach. But the motions of the 
 inteftines are, however, moft conftantly 
 directed from above downwards, both be- 
 caufe they are commonly begun from the 
 ftomach, and becaufc, when inverlions do 
 occur, there is commonly flill fo much 
 
 refift“ 
 
k 
 
 PHYSIOLOGY. 173 
 
 refiftance at the pylorus, and more efpC“ 
 daily at the valve of the colon, as to turn 
 the diredion again into its proper courfe. 
 In the colon, from its pofition, ftrudure, 
 and the confiftence of its contents, the 
 progrefs of thefe is more flow and difficult; 
 and it is therefore here affifted by the lon- 
 gitudinal fibres peculiarly difpofed, fo as 
 by their contradion to contribute more to 
 the dilatation of every fucceeding portion 
 of the inteftine. 
 
 CCXXVI. 
 
 The chyle is taken into the ladeals, and 
 moved onwards in thefe in the fame man- 
 . ner (CLXVII.) as the lymph is in the feve- 
 ral lymphatics in other parts of the body, 
 to which the ladeals are in ftrudure and 
 fituation exadly fimilar. 
 
 CCXXVII, 
 
# 
 
 174 PHYSIOLOGY, 
 CCXXVII. 
 
 The courfe of the alimentary matters, 
 and the motions by which they are carried 
 on, being now explained, we return to 
 confider the feveral changes which the ali- 
 ment undergoes in this courfe. 
 
 ccxxvni. 
 
 In the mouth, if the aliment taken in be 
 of a folid confiftence, it is here, as we have 
 faid, fubjedled to a triture ; and if our 
 food is of a foft and moifl kind, we are 
 inftindfively direfled to take in along with 
 it fome dry matter, as bread, that the 
 whole may be fubjedled more certainly to 
 a complete manducation. By this our ali- 
 ment is not only more minutely broken 
 down, but is alfo intimately mixed, with 
 the liquids at the fame time taken in, with 
 
 the 
 
PHYSIOLOGY. 
 
 *75 
 
 the faliva and other fluids of the mouth, 
 and with a quantity of air intangled by 
 thefe vifcid fluids. 
 
 CGXXIX. 
 
 In this divided and moiftened ftate, the 
 aliment is taken down into the ftomach, 
 where it is farther diffolved, the vegetable 
 matter of it begins to be changed to the . 
 nature of animal, and the oily parts of the 
 whole begin to be united with the watery. 
 But thefe changes by folution, aflimila- 
 tion, and mixture, require ^o be feparately 
 confidered. 
 
 / 
 
 ccxxx. 
 
 The folution here, as in other cafes, may 
 be afllfted by the mechanical divifion of 
 the folid matter, by the agitation of the 
 diflblving mafs, and by the application of 
 
 heat, 
 
176 PHYSIOLOGY. 
 
 heat; and, with thefe affiftances, the folu- 
 tionmuft be performed by the application 
 of a proper menftruum. 
 
 CCXXXI. 
 
 The divifion of the folid is fometimes 
 aflifted by a previous cookery, and com- 
 monly by the manducation we have men- 
 tioned ; but the human ftomach does not 
 feem by any mechanical powers to contri- 
 bute to this. It gives only a moderate 
 agitation, which, in any cafe, contributes 
 little to mechaaical divifion. 
 
 CCXXXII. 
 
 The degree of heat applied here, being 
 that of the common temperature of the 
 human body, may aflifl the folution ; but 
 it is of no confiderable power, and no af- 
 fiftance is got from any cloferiefs of the 
 
 vefTel 
 
PHYSIOLOGY. 177 
 
 veflel which occurs here. Upon the whole, 
 the afliftances applied here are not conli- 
 derable, and the fpeedy fblution that takes 
 place muft be chiefly owing to the power 
 of the menftruum. 
 
 CGXXXIII. 
 
 The menftruum that appears here, is a 
 compound of the liquid matters taken in, 
 of the faliva, and of the gaftric liquors : 
 but in all, or any of thefe, we do not 
 readily perceive any confiderable folvent 
 power ; nor, by any artifice, in employing 
 thefe out of the body, can we imitate the 
 folutions performed in the ftomach. 
 
 CCXXXIV. 
 
 However, from what happens in the 
 ftomachs of certain animals, there is ground 
 to prefume, that indeed in every one there 
 
 is 
 
178 PHYSIOLOGY. 
 
 is a peculiar folvent. But whether this be 
 a menftruum dividing the folid into inte- 
 grant parts, and thereby reducing it to a 
 fluid ftate, or if the folvent here be a pe- 
 culiar fermentative power, refolving mat- 
 ters more or lefs into conftituent parts, is 
 not clearly perceived. 
 
 ccxxxv. 
 
 The latter is the moft probable, as the 
 circumftances of fermentation very con- 
 fhantly appear ; and as the deviations 
 which at any time appear in the courfe of 
 digeftion, appear always to be an excefs 
 of fermentation, either acefcent or putre- 
 facSlive. 
 
 CCXXXVh 
 
 The bufinefs feems to us to proceed in 
 this manner. The fluids of the ftomach 
 
 have 
 
PHYSIOLOGY. 
 
 179 
 
 Lave the power of fuddenly aftd powerfaliy 
 loofening the fixed air of the alimentary 
 matters, which is the firfl ftep towards pu*- 
 trefadlion, and that which moft efFe<5lually 
 breaks down the texture, and perhaps the 
 mixture of bodies. But we now know, 
 that putrefcent bodies are very powerful 
 in exciting an acefcent fermentation in 
 vegetable fubftances, which the human 
 ftomach is hardly ever without j and that 
 this acefcency therefore, in the next place, 
 very conftantly fucceeds, and an acid is 
 produced in the flomach. This acidity 
 makes the effects of the putrefadlion dif- 
 appear ; and the acidity in its turn difap- 
 pears alfd, probably by its being abforbed 
 by, or united with, the putrefcent and oily 
 matters here prefent ; and it is in this 
 manner that we fuppofe that the animal 
 fluid is produced, and daily renewed by 
 the combination of a freih portion of acid 
 with putrefcent fluids previoufly- exifting 
 
 in 
 
iSo PHYSIOLOGY. 
 
 in the body; The daily produdlion of 
 acid in the human ftomach, and its readily 
 difappearing again, without Ihowing any 
 morbid efFedls, renders our do<5lrine fuf- 
 ficiently probable. 
 
 CCXXXVII. 
 
 This is the allimilation of vegetables that 
 I fuppofe to take place, and is begun in the 
 ftomach, but is not completed there : for 
 we obferve, that the long retention of the 
 alimentary matters in the ftomach, whe- 
 ther from the infolubility of the matter, 
 or from an obftru(5lion of the pylorus, 
 produces a greater degree of acidity ; and, 
 in general, the acidity which commonly 
 prevails ip the ftomach does not difappear 
 but in the after courfe of the aliment. 
 
 CCXXXVIII. 
 
PHYSIOLOGY. 
 
 i8i 
 
 CCXXXVIII. 
 
 It is efpecially the bile, added to the 
 matters which have pafTed from the fto” 
 mach into the duodenum, that is fitted to 
 cover the acidity which appeared in the 
 ftomach. It is probable alfo, that the pan- 
 creatic and inteftinal liquors contribute to 
 the fame effedl ; and it is perhaps for the 
 fame purpofe, that the lymph is conftantly 
 added to the chyle in its courfe. But, af- 
 ter all, we muft reft in the general idea, 
 and own that we do not know exacftly how 
 this matter proceeds, nor what the feveral 
 fluids, added to the aliment in the differ- 
 ent parts of its courfe, truly contribute to 
 the changes of it. 
 
 CCXXXIX. 
 
 It is probable, however, that, by the 
 M 
 
 mix- 
 
i 82 
 
 PHYSIOLOGY. 
 
 mixture mentioned, the peculiar fluid 
 which we name the chyle is produced ; for, 
 though it is certain that a variety of fluid 
 matters may enter the ladleals and accom- 
 pany the chyle there, it is ftill probable, 
 that there is a peculiar fluid produced by 
 the aftions of the ftomach and inteflines, 
 and fuch as becomes the principal ingre- 
 dient in the animal fluids afterwards form- 
 edj that is ftridly intitled to that apellation. 
 This chyle does not appear in the ftomach j 
 but firft in the duodenum, and more co- 
 pioufly ftill in the jejunum and firft part of 
 the ileum. It appears Indeed in the v/hole 
 of the ileum, coecum, and colon, but in 
 the laft lefs copioufly: all which fliows, 
 that a particular mixture is neceflary to it; 
 and at the fame time that it is not made at 
 once, but fucceflively in the courfe of the 
 inteftines. 
 
 CCXL. 
 
PHYSIOLOGY. 
 
 183 
 
 CCXL. 
 
 It remains to fpeak of the mixture of 
 the oily with the watery parts of the ali- 
 ment. This we cannot w’^ell explain; but 
 it is of confequence to obferve here, that 
 fuch a mixture is adlually made. It is 
 evident that a large quantity of oil in a 
 feparate ftate is taken in as a part of our 
 aliment, but at the fame time no oil com- 
 monly appears in a feparate ftate in the 
 mafs of blood ; it mu ft therefore be united 
 with the other parts of the mafs in the 
 way of mixture. Hitherto the phyfiolo- 
 gifts have hardly mentioned any other 
 means for this union of oil but the appli- 
 cation of vifeid fluids ; but thefe can oc- 
 calion only a diffufion, and fome means of 
 mixture muft neceflfarily be fuppofed. 
 What thefe however are, we do not cer- 
 tainly know. They do not produce their 
 effed in the firft paflfages ; foi* in the chyle, 
 M 2 till 
 
.184 PHYSIOLOGY. 
 
 till it enters the fubclavian vein, the oil 
 appears to be only in a diffufed ftate, and 
 probably the perfect mixture is only made 
 in the palTage through the lungs. 
 
 CCXLI. 
 
 It may be proper here to take notice of 
 another matter which conftantly enters 
 into the mixture of animal fluids. This 
 is air, which, by different means, can be 
 extracted in confiderable quantity from 
 every kind of animal matter. What is 
 properly the origin of this, when and 
 %vhere it is infinuated into the animal 
 fluids, and by what means it is either fix- 
 ed in thefe or loofened from them, are all 
 queflions not yet refolved ; but perhaps 
 
 neceffarv to be refolved, before we can 
 ^ * 
 
 fpeak with any confidence of the changes 
 which the animal fluids undergo in diffe- 
 rent parts of the fyftem. We can obferve, 
 ’ in 
 
PHYSIOLOGY. 
 
 185 
 
 In the mean time, that a quantity of air 
 is always prefent in the chyle in a very 
 loofe ftate ; that it becomes more fixed in the 
 mafs of blood after this has paffed through 
 the lungs; and that again, in the different 
 fecreted fluids, the air appears to be in fome 
 of them ftill fixed, and in others much more 
 loofe; and it is probable, that all this has a 
 particular relation to the produ6lion and 
 properties of the different fluids of ani*- 
 fnals. 
 
 GCXLIL 
 
 We have now followed the courfe of the 
 aliments, fo far as we can confider them 
 as any ways in a feparate ftate ; but we 
 do not perceive, that, in any part of this 
 courfe, the proper animal fluids are en- 
 tirely formed; And it is very jnftly fup- 
 pofed, that the proper mixture or affimi- 
 'ladon is not finifhed till the chyle, mixed 
 M 3 with 
 
 / 
 
i86 
 
 PHYSIOLOGY. 
 
 ■with the mafs of blood, has undergone 
 the acHon of the lungs, through the 
 vefTels of which it muft almoft imme- 
 diately pafs, after entering the fubclavian 
 vein, and feemlngly before it is applied 
 to any of the purpofes of the animal eco- 
 nomy. 
 
 CCXLIII. 
 
 What change the fluids undergo in paf-. 
 flng through the lungs, or by what means 
 the fuppofed changes are produced, after 
 all that has been faid, feems ftill to be very 
 little known. 
 
 The mechanical powers of preflure, 
 commonly fpoken of, do not in fadl take 
 place, nor are their fuppofed effedls any 
 ways confiftent with found philofophy; 
 and, on the other hand, it is very probable, 
 that the changes produced are the efie<5ls 
 either of chemical reparation or mixture. 
 
 What 
 
PHYSIOLOGY. 
 
 187 
 
 What has been fuppofed to be perform- 
 ed in this way by an 'abforption of air, or 
 of a particular matter from it, is very un- 
 certain in fadi, and has led to a flill more 
 uncertain reafoning. 
 
 It is now certain, that a quantity of 
 mephitic air, and perhaps fome other mat- 
 ters are conftandy exhaling from the lungs 
 of living animals, and are carried off by 
 the atmofpherical air alternately entering 
 and iffuing from the lungs. This is a 
 pretty certain evidence that fom'e change 
 of mixture is going on in the fluids paf- 
 fing through the lungs; but from what 
 particular portion of the fluids the mephi- 
 tic air proceeds, or what is the effedt of its 
 feparation, we know not: And indeed, as 
 we have faid before, what are the effedls 
 of the adlion of the lungs upon the flate 
 of the fluids, we are very uncertain. Up- 
 on the whole, we flill know but little of 
 M 4 the 
 
i88 PHYSIOLOGY. 
 
 the produdlion, or formation, of the ani- 
 mal fluids; and therefore from the con- 
 fideration of their formation, we have 
 learned little of their nature ; but we muft 
 now try to difcover what we can of it, by 
 examining thefe fluids as they are found 
 already formed in the blood-veflels. 
 
 SECT, 
 
PHYSIOLOGY. 
 
 189 
 
 SECT. IV. 
 
 CHAP. II. 
 
 Of Animal Blood. 
 
 CCXLIV. 
 
 *^HE red fluid pafling from the lungs 
 to the left ventricle of the heart, and 
 thence by the aorta and its branches to 
 every part of the body, may be confidered 
 as a mafs containing, either formally or 
 ynaterially, every part of the animal fluids ; 
 
 and 
 
190 PHYSIOLOGY. 
 
 and may therefore be called the cominon 
 mafs of blood. This term, hov/ever, muft 
 be ftriclly confined to the circulating 
 fluids while they retain their red colour ; 
 for when they lofe this, it is always in 
 confequence of fome feparation of parts. 
 The fame red fluid, indeed, as it is found in 
 the veins, has alfo fulTered fome feparation 
 of parts; but as the blood in the veins 
 is never entirely deprived of the whole 
 of any matter that was prefent in the 
 arteries, fo we think the venous blood may 
 flill be confidered as a part of the common 
 mafs. 
 
 CCXLV. 
 
 This mafs of blood we find to be an 
 heterogeneous aggregate; and it will be 
 proper to inquire into the feveral parts of 
 this before we employ any chemical trials 
 
 for 
 
PHYSIOLOGY. 
 
 191 
 
 for difcovering the mixture of the whole, 
 or of its parts. 
 
 . ' CCXLVI. 
 
 We difcover the parts of this aggregate 
 chiefly by the fpontaneons feparation of 
 them, which takes place upon their being 
 drawn out of the veflels of a living ani- 
 mal. 
 
 CCXLVII. 
 
 The feparation commonly proceeds in 
 this manner. Immediately after the blood 
 is drawn out, it exhales a fenflble vapour, 
 and, after fome time, it is found by that 
 exhalation to have loft a part of its weight, 
 more or lefs, according to the degree of 
 heat it is expofed to, according to the ex- 
 tent of furface by which it is expofed to 
 the air, and probably alfo according to 
 
 dif- 
 
PHYSIOLOGY. 
 
 192 
 
 difFerent conditions of the blood itfelf. 
 The matter thus exhaling may be called 
 the halitus or vapour of the blood. 
 
 ccxLviir. 
 
 Soon after the blood has been drawn out 
 of the veflels, it lofes its fluidity, and the 
 whole of it concretes into one foft gelati- 
 nous mafs : but after fome time, there oozes 
 out from this mafs a thin fluid ; and as the 
 reparation of this proceeds, the mafs con- 
 tracts into a fmaller bulk, and, in propor- 
 tion, becomes more denfe. 
 
 CXLIX. 
 
 This is the feparation which almoft 
 always takes place, and has at all times 
 been obferved by phyficians. The fluid 
 part is called ferum; and the thicker con- 
 
 fiftent 
 
PHYSIOLOGY. 
 
 m 
 
 fiftent part has been called cruor, but 
 more properly the crajfamentiim. 
 
 CCL. 
 
 Both parts feeem homogeneous and flm- 
 ple, but are not. , For, if the cralTamen- 
 tum taken from the ferum be laid up- 
 on a linen cloth, and water is poured upon 
 it, the water walhes off a red coloured part, 
 and carries it through the pores of the 
 cloth; and there remains a whitifh, con- 
 fiftent, but foft and tough mafs, not to be 
 further diminifhed or feparated into parts 
 by any ablution. 
 
 A like experiment fhows always a like 
 matter prefent in the mafs of blood ; and 
 upon feveral occafions, both while the 
 blood remains within the veffels, whether 
 of the living or dead body, and when it is 
 drawn out of the veffels of the living, this 
 matter fpontaneoufly feparates from the 
 2 other 
 
194 
 
 PHYSIOLOGY. 
 
 other parts of the blood. It is therefore a 
 part conftantly prefent in the blood. It is 
 what Gaubius, after Malpighi, calls the 
 Jibra fangiiinis. Mr Senac names it the 
 coagulable lymph) and we fhall fpeak of it 
 under the title of the gluten of the blood. 
 When it appears upon the furface of the 
 blood drawn out of the veffels of living 
 animals, it is called the hiflammatory cruf. 
 
 CCLI. 
 
 When the blood is viewed with a mi- 
 crofcope, whether as moving in the veffels 
 of a living animal, or when out of the 
 veffels remaining ftill fluid, there are cer- 
 tain parts of it which appear of a round 
 figure, and alfo of a red colour, while the 
 reft is almoft colourlefs. The parts thus 
 diftinguifhable ’by their figure are called 
 the red globules ; and it appears that the 
 red colour of the whole mafs depends up- 
 
 2 on 
 
PHYSIOLOGY. 
 
 195 
 
 on the prefence of th-efe only. It is chiefly 
 thefe parts which are waflied ofF from the 
 craflainentum in the experiment above- 
 mentioned; and we now conclude, that, 
 befldes the globules, the gluten, and a por- 
 tion of ferum that happens to be entangled 
 in the pores of the concreting mafs, there 
 is no other matter evident in the crafla- 
 mentum, 
 
 CLII. 
 
 The ferum is a tranfparent fluid of very 
 little colour, and feemingly Ample ; but if 
 it be expofed to a heat of 156 degrees of 
 Fahrenheit’s thermometer, it concretes in- 
 to a firm and almoft tranfparent gelly ; 
 and, if this be cut into minute pieces, there 
 exudes from it a thin colourlefs fluid of a 
 faline tafte. In proportion as this fluid 
 is more carefully feparated, the coagu- 
 lated part becomes infipid, and in all its 
 
 pro- 
 
196 PHYSIOLOGY. 
 
 properties refembles the gluten feparated 
 from the craffamentum. From hence we 
 are ready to conclude, that the ferum, as 
 obtained by fpontaneous reparation, con- 
 lifts of a portion of gluten diftolved in a 
 faline fluid, which we name the SEPtO” 
 SITY. 
 
 CCLIII. 
 
 From the whole that has been faid from 
 CCXLIV. to CCLII. it appears that there are 
 three diftincft portions and kinds of matter 
 in the common mafs of blood; that is, red 
 globules, gluten, and ferofity. What other 
 matters may alfo be there, we lhall con- 
 fider afterwards; but, in the mean time, 
 lhall fay a little more of each of the parts 
 we have already mentioned. 
 
 CCLIV. 
 
PHYSIOLOGY; 
 
 197 
 
 CCLIV. 
 
 The red globules have been confidered 
 as an oily matter, and from thence their 
 difhindt and globular appearance has been 
 accounted for; but there is no dire(5l proof 
 of their oily nature, and their ready union 
 with and dilFufibility in water renders it 
 very improbable. As being microfcopical 
 objedls only, they have been reprefented 
 by different perfons very differently* Some 
 have thought them fpherical bodies, but 
 divifible into fix parts, each of which in 
 their feparate date were alfo fpherical ; 
 but other perfons have not obferved them 
 to be thus divifible. To many obfervers, 
 they have appeared as perfectly fpherical, 
 while others judge them to be oblate fphe- 
 roids or lenticular. To fome they have 
 appeared as annular; and, to others, as 
 containing a hollow veficle. All this, with 
 feveral other circumftances relating to 
 N them. 
 
198 PHYSIOLOGY. 
 
 them, very varioufly reprefented, fliow 
 fome •uncertainty in microfcopical obfer- 
 vations ; and it leaves me, who am not 
 converfant in fuch obfervations, altogether 
 uncertain with refpedl to the precife na- 
 ture of this part of the blood. The che- 
 mical hiftory of it is equally precarious ; 
 and therefore, what has been hitherto faid 
 of the producHon, and changes happening 
 to thefe red globules, we choofe to leave 
 untouched. We (hall afterwards fay fome- 
 thing with refpecfl to their general ufe in 
 the animal fyftem ; and now we fbali at- 
 tempt to explain the caufe of fome chan- 
 ges, which in certain circumflances appear 
 in the colour of the whole mafs of blood. 
 
 CCLV. 
 
 We fuppofe that the red globules, when 
 viewed iingly, have very little colour; 
 and that it is only when a certain number 
 
 2 of 
 
PHYSIOLOGY. 
 
 199 
 
 of them are laid upon one another, that 
 the colour appears of a bright red : but 
 this alfo hath its limits ; fo that, when the 
 number of globules laid on one another 
 is coniiderable, the colour becomes of a 
 darker red. Upon this fuppofition, the co- 
 lour of the mafs of blood will be brighter 
 or darker as the colouring part is more or 
 lefs diffufed among the other parts of the 
 mafs; and we think this appears to be 
 truly the cafe, from every circumftance 
 that attends the changes which have been 
 at any time obferved in the colour of the 
 blood. 
 
 CCLVI. 
 
 The gluten of the blood, from its re- 
 femblance on the one hand to the albu~- 
 men ovi^ and on the other to the matter 
 of the folids of animal bodies, we conhder 
 as the principal part of animal fluids, as 
 N 2 that 
 
200 
 
 PHYSIOLOGY. 
 
 that which is immediately formed of the 
 aliment taken in, and as that which is 
 employed in increafing the growth of the 
 folids, or in repairing their wafte, 
 
 CCLVII. 
 
 9 
 
 But it is well known, that the animal 
 fluids in general, and particularly the 
 gluten, is prone to putrefadlion; and that, 
 even in the living body, if frefli aliment be 
 not conftantly taken in, and alfo if certain 
 excretions which carry off putrefcent mat- 
 ter be not conftantly fupported, a confider- 
 able putrefadlion certainly takes place. 
 From hence we are led to think, that fome 
 approach to putrefacftion conftantly takes 
 place, even in the moft healthy bodies ; 
 and that it appears efpecially in an evo- 
 lution of faline matter ; and that this, ta- 
 ken up by the water conftantly prefent, 
 forms the feroftty. We fuppofe it is this 
 
 which 
 
PHYSIOLOGY. 
 
 201 
 
 which affords the vapour of the blood, 
 (CCXLVII.) and that it is the ferohty dif- 
 folving a portion of the gluten which forms 
 the feriim that appears upon fpontaneous 
 reparation (CCXLVII.) 
 
 CCLVIII. 
 
 The faline matters impregnating the fe- 
 rolity, if we may judge from the analyfis 
 of urine, are of various kinds ; but par- 
 ticularly, there is prefent an ammoniacal 
 fait, now well known under the name of 
 the ejfential fait of urine^ which, if not ori- 
 ginally formed, is at leafl moft copioufly 
 evolved in animal fluids. 
 
 CCLIX. 
 
 . Thefe are our conjeclu res concerning the 
 parts of animal blood ; and it remains to 
 fay in what proportion each of them is 
 • N 3 pre- 
 
202 
 
 PHYSIOLOGY. 
 
 prefent in it. This will perhaps be always 
 difficult ; and in the mean time we can 
 perceive, that many eftimates formerly 
 made could not be exa6t, as the feveral 
 parts were not properly known ; and, 
 while judging chledy from the appearances 
 upon fpontaneous feparatlon, phyficians 
 were not aw^are how much thefe are affect- 
 ed by the circumitances of extravafation, 
 and by thofe in which the blood is placed 
 after being drawn out. There are not 
 yet indeed experiments made to afcertain, 
 with any exadlnefs, the pnoportion of the 
 feveral parts mentioned: but it is probable, 
 that tlie red globules make a fmall part of 
 the whole ; that the gluten, if we con- 
 fider both what is in the craffamenmm and 
 in the ferum, is in much larger propor- 
 tion ; but that the watery portion is the 
 largeft of all, and at the fame time that 
 this has always a confiderable quantity of 
 faline matter diffolved in it. 
 
 CCLX. 
 
CCLX. 
 
 We would next put the quefdon, By 
 what means the parts of this heterogeneous 
 mafs are kept fo equably diffufed among 
 one another, and the fluidity of the whole 
 fo conftantly preferved ? This we fuppofe 
 to be done chiefly by motion and heat, 
 and by the parts difpofed to concrete be- 
 ing kept from the contaft of any matters 
 to which they might adhere more firmly 
 than they do to the other parts of the blood. 
 The diffufed parts we fuppofe to be prefent 
 only in thofe veflels in which a confider- 
 able degree of agitation is conftantly kept 
 up ; and we fuppofe alfo, that the heat al- 
 ways here prefent, both diminilhes the co- 
 hefion of the gluten, and increafes the fol- 
 vent power of the ferofity. Experiments 
 made with neutral falts feem to confirm 
 the latter ; and it is alfo probable, that 
 N 4 the 
 
PHYSIOLOGY. 
 
 a 04 
 
 the fame folvent power may be increafed 
 by a quantity of air that is conftantly in- 
 termixed with the mafs of blood while it 
 remains in the veffels, and is under a con- 
 ftant agitation. It is fuppofed, that an at- 
 tention to thefe feveral circumftances will 
 explain moll of the cafes of fpontaneous 
 feparation that occur either in the living 
 or dead body, within the veffels or with- 
 out them ; but the detail would be too 
 Jong for this place. 
 
 CCLXL 
 
 We fhall add here a few words on the 
 ufe of this lingular compofition of animal 
 blood which we have been conlidering. 
 
 It appears evidently, from many circum- 
 ilances of the animal oeconomy, that its 
 fiinclions require a fyftem of velfels con- 
 Jlantly filled, and even dillended ; but as, 
 at the fame tim^j thefe velfels mull be 
 
 open 
 
PHYSIOLOGY. 
 
 205 
 
 open by a multitude of their extremities, 
 if all the fluids were fuch as could pafs by 
 thefe extremities, the fyftem could not be 
 kept filled for a few minutes. It is necef- 
 fary, therefore, that the fluids fhould be 
 partly of fuch a fize as that they cannot 
 pafs through the fmaller vefTels, and partly 
 in a difiufed ftate only, which has com- 
 monly the fame efFeft. Hence it is, that 
 the red globules, under the ordinary im- 
 petus of the heart and arteries, are ftricftly 
 confined to certain vefTels ; and it is pro- 
 bable, that, in the like circumftances, the 
 diffufed gluten does not go much farther. 
 This ferves to keep the larger vefTels of the 
 fyflem conftantly filled. But, on the other 
 hand, the Terofity being fufiiciently fluid, 
 might be fuppofed to run off by the many 
 outlets open to it, and thereby to leave the 
 fluids in the larger vefTels of a confiflence 
 unfit to circulate. This, however, Teems 
 alfo to be obviated by the vifcidity of the 
 
 grofier 
 
2o6 
 
 PHYSIOLOGY. 
 
 grolTer parts of the blood, fufficient always 
 to entangle fo much of the more fluid, as 
 may be neceflary to preferve the due flui- 
 dity of the whole, 
 
 CCLXII. 
 
 I 
 
 The heat of the human body, fupported 
 by powers within itfelf, is probably the 
 effedl of the motion of the blood, and 
 might have been treated of when we were 
 confidering that fubje(5l. But as^ many 
 perfons fuppofe it to depend in fome mea- 
 fure on the nature of the fluids, we have 
 refcrved it for this place ; and here per- 
 haps to fay only, that the queftion con- 
 cerning the caufe of animal heat is not yet 
 folved. 
 
 CCLXIir. 
 
 The opinion of animal-heat’s being the 
 
 effecft 
 
PHYSIOLOGY. 207 
 
 effe<5l of mixture, is to be little regarded ; 
 as the matters fuppofed to be mixed, the- 
 place ill which the mixture is made, and 
 the other circumftances relating to it, are 
 equally hypothetical, and the whole is ill 
 fupported by any analogy. 
 
 CCLXIV. 
 
 Pvlore fpecioufly is animal-heat fuppofed 
 to be the effect of putrefaction, towards 
 which there is certainly fome approach in 
 animal bodies ; but the opinion is ftill 
 very doubtful. Yox^jirji^ the effeCt of any 
 degree of putrefaction in producing heat is 
 not well afcertained. Secondly^ It is not 
 fupported by any analogy, that putrefac- 
 tion, in the degree to which only it pro- 
 ceeds in living bodies, is capable of pro- 
 ducing the heat appearing there. And, 
 lajily^ Whatever is the degree to which 
 putrefaction proceeds in living bodies, it 
 
 does 
 
2o8 
 
 PHYSIOLOGY. 
 
 does not appear that there is any increafe 
 •of heat correfpondent to the increafe of 
 putrefaction, and rather the contrary. 
 
 CCLXV. 
 
 The fuppofitions either of mixture, or of 
 putrefaction, as the caufe of animal-heat, 
 are both of them rejected by this, that the 
 generation of heat in animal bodies is ma- 
 nifeftly dependent on another caufe, that 
 is, the motion of the blood. For the power 
 of generating heat in any animal is not 
 perfect till the motion of the blood in it is 
 fully eftablifhed; and, when the genera- 
 ting power is eftablifhed, we perceive the 
 heat to be increafed or diminifhed as va- 
 rious caufes increafe or diminifti the mo- 
 tion of the blood. In dying animals, the 
 heat grows lefs as the motion of the blood 
 grows lefs ; and when at death this ceafes 
 altogether, the heat ceafes alfo, commonly, 
 
 at 
 
PHYSIOLOGY. 
 
 209 
 
 at leaft, as foon after death as we can fup- 
 pofe a body of the fame bulk to lofe the 
 heat it had acquired. 
 
 CCLXVI. 
 
 This conne6lion between the heat and 
 motion of the blood feems in general to 
 be well proved ; and, though it may be 
 difficult to reconcile certain appearances to 
 it, we would fo far admit of the fuppoh- 
 tion, as to inquire, in the next place, into 
 the manner in which the motion of the 
 blood may generate heat. 
 
 CCLXVII. 
 
 On this fubjefl, the moft common opi- 
 nion is, that the heat is produced by the 
 attrition of the particles of the blood upon 
 one another, or of thefe on the internal 
 furface of the veffels in which they move. 
 
 But 
 
210 
 
 PHYSIOLOGY. 
 
 But we cannot find any analogy to fupport 
 either the one or the other fuppofition. 
 
 The attempt made to fupport the latter 
 fuppofition, by endeavouring to fhow, that 
 upon this the equality of heat in the dif- 
 ferent parts of the fame body is well ex- 
 plained, deferves little regard, as it is 
 founded on doubtful principles and mifta' 
 ken fadls. 
 
 CCLXVIII. 
 
 The equality of heat in the different 
 parts of the fame body feems to require 
 the generating power to be very generally 
 diffufed over the whole ; but it does not 
 feem to require its being precifely equal in 
 every part, as the interpofitlon of pretty 
 large veffels in every part of the body, and 
 the fpeedy communication of the fluids 
 from any pne part to every other, will fuf- 
 ficiently account for the equality of heat, 
 
 though 
 
PHYSIOLOGY. 
 
 2IS 
 
 though the generating power Ihould be in 
 forae meafure confined to certain parts 
 only. 
 
 However, we take no notice of the fup- 
 pofitions which have been made of the ge- 
 nerating powers being confined to certain 
 fmall portions of the fyftem only. Thefe 
 fuppofitions give no relief in the general 
 theory, and they are not fupported by any 
 particular evidence. The breathing ani- 
 mals are the warmeft ; but that they are 
 warmer becaufe they breathe, is not more 
 probable than that they breathe becaufe they 
 are warmer. 
 
 CCLXIX. 
 
 With refpefl to this theory, which de- 
 duces animal-heat from the motion of the 
 blood, we muft ovvui, that it is attended 
 with feveral difficulties. It will be difficult 
 to fhow, in fo many animals of different 
 
 age, 
 
212 
 
 PHYSIOLOGY. 
 
 age, fize, and temperament, in which the 
 degree of heat is nearly the fame, that the 
 motion of the blood, in all its circumftan** 
 ces, is alfo exactly the fame; or to fhow, 
 in the different animals in which the de- 
 gree of heat is confiderably different, that 
 the motion of the blood in each is CQrref- 
 pondent to the difference of heat. May it 
 not be fnppofed, that there is fome cir- 
 cumftance in the vital principle of animals 
 which is in common to thofe of the fame 
 clafs, and of like oeconomy, and which de- 
 termines the effedl of motion upon the 
 vital principle to be the fame, though the 
 motion a<5ling upon it may be in different 
 circumftances ? 
 
 CCLXX. 
 
 In all we have hitherto faid of animal 
 fluids, we have confidered the common 
 mafs of blood as confiding of three parts 
 
 I 
 
 or 
 
PHYSIOLOGY. 
 
 113 
 
 or three kinds of matter only ; but many 
 more have been fuppofed to be prefent 
 in it, and we fhall inquire upon what 
 ground. 
 
 It is common to fuppofe, that the ali- 
 ment or the chyle formed of it is not per- 
 fecflly affimilated in paffing once only thro’ 
 the lungs; but that, for fome time after 
 fuch paffage, it continues to circulate with 
 the blood under the fame form and of the 
 fame qualities which it had when it firft 
 entered the fubclavian, and particularly in 
 this flate to furnifli the milk which is fe- 
 creted in the breads of females. There is, 
 however, no proper evidence of the chyle’s 
 ever appearing in the blood-veffels, and the 
 appearances of it alleged can be otherwife 
 accounted for. The arguments for the 
 fame opinion, which are drawn from the 
 confideration of the fecretion of milk, are 
 embarraffed with many difficulties. 
 
 O 
 
 CCLXXI. 
 
i 
 
 114 PHYSIOLOGY, 
 
 CCLXXI. 
 
 It is probable, that the animal fluid 
 (CCLV. CCLVI.) is in a conftant progrefs, 
 and hardly for a moment ftationary, or 
 therefore uniformly the fame over the whole 
 of the common mafs. Some part of it is 
 that which was lafl formed, ahd therefore 
 the neareft to the vegetable matter from 
 which chiefly it was produced ; while ano- 
 ther part of it is that which has remained 
 longed in the body, and is therefore the 
 neared to putrefadlion. Between thefe two 
 there may be feveral intermediate dates, 
 which, however, like the neared fhades of 
 the fame colour, are not didinguifliable by 
 our fenfes or experiments. 
 
 CCLXXIL 
 
 B elides the difference of matter arlling 
 
 from 
 
PHYSIOLOGY, 
 
 ii5 
 
 from the progrefs of the animal fluids 
 there have been other matters fuppofed 
 prefent in the common mafs, and as com- 
 monly conftituent parts of it. Such are a 
 mucous matter, like to the mucous matter 
 • of vegetables ; and a gelatinous matter, 
 like to that which is extracted by decodlion 
 from the folid parts of animals. But 
 there is no evidence of either being for- 
 mally prefent in the mafs of blood, and 
 the fuppofition is founded on miftaken fadts 
 and falfe reafonings. 
 
 CCLXXIII. 
 
 But it is proper to be obferved here, that 
 many extraneous matters may, by differ- 
 ent ways, be introduced into the blood- 
 veffels ; and that many of the fecreted 
 fluids, fometimes very different from any 
 thing that exifled before in the mafs of 
 blood, may, by abforption or regurgita- 
 ^ O 2 tion, 
 
2i6 physiology. 
 
 tion, be again taken into the blood- veflels : 
 But, with regard to all of thefe, whether 
 extraneous matters, or thofe produced in 
 the body itfelf, it is probable that hardly 
 any of them enter into the mixture of the 
 animal fluid, and that they are only dif-- 
 fufed in the ferofity till they can be again 
 thrown out of the blood-veflels by the 
 readieft outlets. The oil of the adipofe 
 membrane is frequently, and perhaps ne- 
 ceflarily reabforbed, and' feems to be, be- 
 lides the lymph, the only reabforbed matter 
 which enters again into the mixture of the 
 animal fluid. 
 
 SECT. 
 
PHYSIOLOGY. 
 
 217 
 
 SECT. IV. 
 
 CHAP. III. 
 
 Of Secretion. 
 
 CCLXXV. 
 
 ^^FTER thus confidering the parts of 
 the mafs contained in the red veflels, 
 we muft next conlider the feveral fluids 
 which appear in the other parts of the 
 of the body. 
 
 O 3 
 
 CCLXXVL 
 
CCLXXVI. 
 
 All of thefe we fuppofe to be derived 
 from the common mafs, as they appear in 
 velTels continuous with thofe of the com- 
 mon mafs, and as their appearance ceafes 
 when the communication of the veflels 
 containing them, with the fanguiferous 
 velTels, is any how interrupted. 
 
 CCLXXVIL 
 
 The fluids thus derived from the com- 
 mon mafs Teem to be produced in confe- 
 quence of a certain flrudlure, with per- 
 haps fome other condition in the extreme 
 veflels through which the fluids pafs ; and 
 a part having fuch a ftrudure, is called a 
 gla?id or fecretory organ^ the function of 
 which, from the moft obviovs notion of 
 the manner of it, is called fecretmi. 
 
 CCLXXVIIL 
 
PHYSIOLOGY. 
 
 219 
 
 CCLXXVIII. 
 
 The ftru( 5 lure of the organ, and the 
 manner of its fundlion, feem to me for 
 the mofi: part unknown; at leaft, what 
 we know or fuppofe with regard to the 
 ftrudlure hardly in any cafe applies to the 
 explanation of the function. 
 
 CCLXXIX. 
 
 If it any how appeared that the feveral fe-=> 
 creted fluids were all of them previoufly exifl- 
 ent in the fame forms in the mafs of blood, 
 it would not perhaps be difiicult to explain 
 what might be ftridtly called a fecretion. 
 But fuch previous exiftence does not appear; 
 for, except the matter of exhalation into the 
 feveral cavities of the body, and the matter 
 of urine and of perfpiration, we find no 
 O 4 proper 
 
220 
 
 PHYSIOLOGY. 
 
 proper evidence of any other fecreted fluids 
 prefent in the mafs of blood. We cannot 
 find there, either milk, mucus, or oil, and 
 much lefs the appearance of many other 
 fluids which are only' found after they 
 have pafled through certain organs. 
 
 CCLXXX. 
 
 This being the cafe, the conflderations 
 of the phyfiologifts with regard to the ve- 
 locity of the blood, and other circumftan- 
 ces favouring the feparation of the parts of 
 a fluid which are only diffufed among one 
 another, deferve no attention. The effecls 
 of different apertures may go fome length ; 
 but we can perceive their particular appli- 
 cation only in the few cafes of a Ample 
 feparation. In moft others, there appears 
 to be a change of mixture ; but we per- 
 ceive neither the precife changes that are 
 made, nor the the caufe of them. 
 
 CCLXXXL 
 
PHYSIOLOGY. 
 
 221 
 
 CCLXXXI. 
 
 Till we can difcover thefe more clearly, 
 we may in the mean time obferve, that 
 the adlion of the veffels of the fecretory or- 
 gan has a confiderable fhare in deter- 
 mining both the quantity and quality of 
 the fecreted fluid, and that both very often 
 are very little affedled by the general ftate 
 of the circulation, or by the different con- 
 ditions of the mafs of blood. 
 
 t 
 
 CLXXXII. 
 
 It would feem that no other fecretion 
 but thofe of perfpiration and fweat are 
 manifeftly increafed by the increafed ac- 
 tion of the heart and arteries (CLXXXL), 
 and that mcfl; of the other fecretions are 
 increafed only by flimulants applied to 
 their organs. Thefe flimulants may be 
 
 either 
 
222 
 
 PHYSIOLOGY. 
 
 either fuch as are immediately applied ex- 
 ternally or internally to the excretory, or 
 perhaps to the fecretory vefTels; or they 
 may be fuch as are applied to the fenfo- 
 rium, or to diftant [parts of the nervous 
 fyftem, which by the laws of the animal 
 ceconomy have a connedlion with the or- 
 gans of fecretion. Thefe ftimulants, at 
 the fame time that they a6l in either of 
 thefe ways on the fecretory organs, for the 
 mod part have no fenfible effect on the 
 general (late of the circulation of the 
 blood. 
 
 CCLXXXIII, 
 
 With refpedl to the influence of the 
 condition of the common mafs of blood 
 upon the feveral fecretions, we prefume 
 that the (late of the quantity of the fluids 
 in general will affe6l the quantity of every 
 
 fecretion j but the effedls of the quantity 
 
 of 
 
PHYSIOLOGY. 
 
 223 
 
 of the whole mafs are very remarkable 
 only, with refpe<5l to the fecretions of per- 
 Ipiration, urine, and milk. 
 
 The qualities of the common mafs may 
 alfo be prefumed to afFe<fl the feveral fe- 
 cretions : but the effedl of thefe qualities 
 appears moft remarkable in the fame fecre- 
 tions of perfpiration, urine, and milk; 
 and, even in thefe, the effeft feems to de- 
 pend upon the proportion of water more 
 than upon that of any other matter in the 
 common mafs. With refpecft to the other 
 fecretions, we cannot perceive that any of 
 them are increafed by a particular matter 
 pn^fent in the mafs of blood, except it be 
 fuch a matter as ftimulates the fecretory 
 organ. 
 
 CCLXXXIV. 
 
 The feveral fecretions are frequently 
 pbferved to affedl each other mutually, fo 
 
 that 
 
224 PHYSIOLOGY. 
 
 that the increafe of one dimlnifhes ano- 
 ther, and vice verfa. This feems to de- 
 pend either upon a change of determination 
 in the courfe of the blood (CLXXVIII.), 
 or upon a change in the ftate of fluidity 
 of the common mafs, or perhaps upon a 
 connection eftablifhed between the difle- 
 rent organs of fecretion as parts of the 
 nervous fyflem ; and, except it be in the 
 cafe of perfpiration and urine, we cannot 
 perceive that the effect of the flate of one 
 fecretion upon that of another depends 
 upon an increafe or diminution of any 
 particular matter in the mafs of blood, 
 
 CCLXXXV. 
 
 After mentioning thefe generalities with 
 refpect to fecretion, we fliould, perhaps, 
 proceed in the next place to confider the 
 application of them to the particular fe- 
 cretions, and alfo to confider more parti- 
 cularly 
 
PHYSIOLOGY. 
 
 225 
 
 cularly the feveral fecreted fluids ; but we 
 omit both thefe fubjedts, as we prefume 
 the former will be obvious from what is 
 already faid; and with refpecfl to the latter, 
 we have not yet a fufficient number of ex- 
 periments to proceed any length in it. 
 
 SECT. IV. 
 CHAP. IV. 
 
 Of Nutrition. 
 
 CCLXXXVI. 
 
 '^JNder this title we might conlider how 
 the matter both of the fluids and fo- 
 lids of the body is fupplied: but, after 
 what we have formerly faid of the taking 
 
 m 
 
226 
 
 PHYSIOLOGY. 
 
 in and aillmiladon of the aliment, we have 
 nothing now to add with refpedl to the 
 fluids ; and we therefore confine ourfelves 
 here to confider in what manner the folid 
 parts obtain their increafe of matter and 
 growth, or have their occafional wafle re- 
 paired. 
 
 CCLXXXVII. 
 
 There is no doubt that the folids are 
 formed of the fluid prepared from the ali- 
 ment in the manner we have faid; but it 
 is required how to fay what portion of the 
 fluids is employed in nourifhing the folids, 
 by what channels the nourifliment is con- 
 veyed to them, and, being applied there, 
 how from fluid it becomes folid. 
 
 CCLXXXVIIL 
 
 With regard to the firfl queftion, we 
 have no doubt in aflerting, that in ovi- 
 X parous 
 
PHYSIOLOGY. 227 
 
 parous animals, it is the albumen ovi that 
 is employed in nourifhing the chick; and 
 we prefume that it is an analogous fluid 
 which is employed in nourifliing the bird, 
 during the whole time of its growth. We 
 think the analogy may be fafely applied 
 with refpe(5l to all animals, the folid matter 
 of which is of the fame kind with that of 
 the oviparous. 
 
 CCLXXXIX. 
 
 This analogous fluid we take to be the 
 gluten of the blood, properly diluted and 
 freed from any adhering faline matter. 
 
 ccxc. 
 
 To determine in what manner this nil- 
 tritious fluid is applied to the nourifhment 
 of the folids, it is necelfary to confider 
 what are the Ample fundamental folids, 
 of which all the others are formed. 
 
 ccxcr. 
 
228 
 
 PHYSIOLOGY. 
 
 CCXCI. 
 
 It feems to be the opinion of the greater 
 part of modern anatomifts, that the folid 
 parts confift entirely of a cellular texture, 
 of various denfity in the different parts; 
 and indeed, the flrudlure of the greateft 
 part of the folids is evidently of this kind. 
 But at the fame time, it is alfo true, that 
 a fibrous ftrudlure is to be obferved almofl 
 every where in the body. It appears in 
 the medullary fubflance of the brain and 
 nerves, in the mufcles and tendons, in the 
 arteries, in the excretories of the glands, 
 in the lymphatic veffels, in the alimentary 
 canal, in the uterus and bladder of urine, 
 in the ligaments, in moft membranes ; 
 and it is to be feen in thofe membranes 
 which are afterwards changed into bones, 
 efpecially whilft this change is going on. 
 
 2 
 
 CCXCII. 
 
PHYSIOLOGY. 
 
 229 
 
 CCXCII. 
 
 From this view of the univerfality of a 
 fibrous ftrudure in animal bodies we are 
 difpofed to believe, that thefe fibres are the 
 fundamental part of animal folids; that 
 they are the primordial ftaminal part of 
 animal bodies ; 'and that the cellular texture 
 is, for the moft part, an accretion formed 
 upon thefe fibres. 
 
 The confideration of the ftru(flure and 
 growth of vegetables feems to illuftrate 
 and confirm this opinion. 
 
 CCXCIII. 
 
 At the fame time, from the fibrous parts 
 (CCXCI.) being evidently, in moft inftan- 
 ces, parts of the nervous fyftem, and 
 from the gradual formation of the foe- 
 tus in which the nervous fyftem is firft 
 formed, we think it probable, that the 
 P whole 
 
230 PHYSIOLOGY. 
 
 whole of the fibres in the different parts 
 of the body are a continuation of the 
 nerves ; and this again will lead to the 
 conclufion, that the nourifliment of the 
 foft and homogeneous folid every where 
 is conveyed to it by the nerves. 
 
 CCXCIV, 
 
 This fiippofes alfo, what is otherwife 
 probable, that the cortical part of the 
 brain, or common origin of the nerves, is 
 a fecretory organ, in which the gluten of 
 the blood being freed from all faline mat- 
 ter before adhering to it, becomes fit for 
 the nourifliment of the folids, and being 
 poured in a fufficiently diluted flate upon 
 the organ of the nerves, it is filtrated along 
 the fibres of thefe, and is thus conveyed 
 to every ftaminal fibre of the fyflem. We 
 fuppofe, at the fame time, that the me- 
 dullary, or what may be called the folid 
 
 matter 
 
PHYSIOLOGY. 
 
 231 
 
 matter of the nerves, is in the living body 
 conftantly accompanied with a fubcle ela- 
 flic fluid, which fits them for being the 
 organs of fenfe and motion, and which 
 probably is alfo the means by which the 
 nutritious fluid is carried on in the fub- 
 fiance of the nerves, from their origin to 
 their extremities. 
 
 In what manner the nutritious fluid, 
 thus carried to the feveral parts, is there 
 applied, fo as to increafe the length of the 
 nervous fibre itfelf, or to form a cel- 
 lular texture upon its furface, and in 
 what manner from fluid it becomes folid, 
 we cannot explain ; nor can thefe particu- 
 lars be explained upon any other fuppofi"- 
 tion that has been formed with relpedl to 
 nutrition. 
 
 ccxcv. 
 
 It Is probable, that, for a certain time, 
 P 2 at 
 
232 PHYSIOLOGY. 
 
 at its firfl beginning, the growth of ani- 
 mal bodies proceeds in the fame manner 
 as that of v’^egetables: but it is evident, that, 
 at a certain period, in the growth of ani- 
 mals, a different ceconomy takes place; 
 and that, afterwards, the growth feems to 
 depend upon an extenfion of the arteries 
 in length and widenefs by the blood pro- 
 pelled into them by the powers CLVI, 
 CLIX. It may be fuppofed, that this ex- 
 tenfion of the arteries is applied to every 
 fibre of the body, and that by the exten- 
 fion of thefe it gives an opportunity to the 
 application and accretion of nutritious 
 matter ; to the growth therefore of the 
 fibre itfelf, and to the growth of cellular 
 texture on its furface. Perhaps the fame 
 extenfion of the arterial fyflem gives oc- 
 cafion to the fecretion of fluids, which 
 poured into the cellular texture already 
 formed, according to the difpofition of 
 thefe fluids to concrete more or lefs firmly, 
 
 gives 
 
PHYSIOLOGY. 233 
 
 gives the clifFereiit degrees of denhty and 
 hardnefs which appears in different parts 
 of the body. 
 
 CCXCVL 
 
 By this extenfion of the arterial fyfleiBj, 
 the feveral parts of the body are gra- 
 dually evolved, fome of them fooner, 
 others later, as by the conftitution of the 
 original ftamina, or after occurrences, 
 they are feverally put into the conditions 
 CLXXVII. CLXXVill. by which they are 
 more or lefs expofed to the impetus of 
 the blood, and fitted to receive a greater 
 quantity of it. But as the parts by thefe 
 caufes firft evolved wdll increafe the moO; 
 in the denfity of their folid parts, they will 
 therefore more and more refift their fur- 
 ther growth ; and by the fame ref fiance, 
 will determine the blood with more force, 
 and in greater quantity, into the parts not 
 
 P 3 then 
 
PHYSIOLOGY. 
 
 c 
 
 234 
 
 then fo far evolved. Hence the whole 
 fyltern will be at length evolved, and every 
 part of the folids will, in refpedl of deniity 
 and refinance, be in balance with every 
 other, and with the forces to which they 
 are feverally expofed. 
 
 CCXCVII. 
 
 The extenfion of the arteries (CCXCV.) 
 depends upon the reiiftances which occur 
 to the free tranfmiilion of the blood 
 through them, as in CLXX. ; and fur- 
 ther, from a reliftance in the veins. For, 
 as a conliderable portion of the blood (by 
 CCLXL) does not commonly pafs into 
 the fmaller branches of the arteries, but 
 mull pafs very entirely into the veins; fo 
 thefe, by their capacity conftantly dimi- 
 nifliing as they approach nearer to the 
 heart, and by their coats being of a denfity 
 and firmnefs fufficient to prevent further 
 
 dila- 
 
PHYSIOLOGY. 
 
 235 
 
 dilatation, confiderably refill the free paf- 
 Page of the blood frorn the arteries into 
 ihemo 
 
 CCXCVIIL 
 
 While thefe refiflances continue, the ar- 
 tefies, and with them almofl every fibre 
 of the body, mud be extended at every 
 fyftole of the heart ; and with this exten- 
 fion, the growth of every part will pro-* 
 ceed : but, as every part, by its receiving 
 an addition of folid matter, becomes more 
 denfe and rigid ; fo it is lefs eafily extend- 
 ed, and perhaps lefs readily receives an ac- 
 cretion of new matter, than before. Hence 
 it is, that the more the body grows, it ad- 
 mits of any additional growth more flowly ; 
 and unlefs the extending powers increafe 
 in the fame proportion with the increafing 
 denfity of the folids, there muft be a period 
 which thefe two powers will balance each 
 
 otherj 
 
236 PHYSIOLOGY. 
 
 other, and the growth will proceed no far- 
 ther. But, as it is evident, that the bulk 
 and weight of the heart, and probably there- 
 fore its force, does not increafe with the 
 increaling bulk of the body, and that the 
 adlion of the heart is the principal ex- 
 tending power in the fyftem; it is alfo 
 plain, that the extending power does not 
 increafe in the fame proportion with the 
 increafing denlity of the folids ; and there- 
 fore, that thefe two powers will, at a cer- 
 tain period, come to balance each ocher. 
 
 • 
 
 CCXCIX. 
 
 But not only is the force of the heart 
 thus conftantly diminilhing, with refpecl 
 to the refinance of the arteries ; but the 
 force of the heart, though it were flill fub- 
 fifting, has, from other caufes, lefs effedd 
 in extending the arteries. The blood is 
 more confined to the arteries, and extends 
 
 them 
 
PHYSIOLOGY. 237 
 
 them further in proportion to the relift- 
 ance in the veins, as in CCXGVII. ; and 
 this refiftance in the veins, and the exten- 
 fion of the arteries depending upon it, will 
 be more or lefs, according to the refpec- 
 tive denhty of thefe two fets of vellels. 
 But it appears from the experiments of 
 Sir Clifton Wintringham, that the denlity 
 and firmnefs of the veins with refpedl to 
 their correfpondent arteries, is much greater 
 in young animals than in old; and thence 
 it appears, that during the growth of ani- 
 mals, the arteries are acquiring an increafe 
 of denlity in a greater proportion than the 
 veins are at the fame time ; and therefore, 
 that the reliltance in the veins with refped: 
 to the arteries, mull be conllantly dimi- 
 nifhing ; that the veins will therefore re- 
 ceive a greater proportion of blood ; that 
 in the fame proportion the arteries will be 
 lefs extended; and, laJUy^ that the dim!- 
 nifhed reliftance in the veins, concurring 
 
 2 with 
 
23S PHYSIOLOGY. 
 
 with the diminifhed force of the heart, 
 will the fooner bring the increahng rigidity 
 of the arteries, and therefore of every 
 fibre of the body, to be in balance with the 
 extending powers ; at lead, fo far as to pre- 
 vent their producing any further growth. 
 
 ccc. 
 
 This account of the change of the re- 
 fiftances in the arteries and veins, with 
 refpecd to one another, is agreeable to 
 phenomena, which fliow that the arteries 
 are larger, and contain more blood in pro- 
 portion to the veins in young animals, 
 than in old ; that arterial hemorrhagies, 
 occur moft frequently in young perfons ; 
 and that congeftions in the veins, with 
 hemorrhagies, or hydropic effufions de- 
 pending upon them, occur moft frequently 
 ill old age. 
 
 CCCI. 
 
PHYSIOLOGY. 
 
 239 
 
 cccr. 
 
 It is probable, that the refiftance both 
 of arteries and veins goes on increafing, 
 while the force of the heart is not increa- 
 fing at the fame time : but it appears alfo, 
 that, from the diminilhed force of the 
 heart, and the compreflion which the 
 fmaller veflels are conftantly expofed to 
 from the diftention of the larger, the ac- 
 tion of the mufcles and other caufes, the 
 number of fmall veflels, and therefore the 
 capacity of the whole fyftem, is conftant- 
 ly diminilhing fo much, that the heart 
 may ftill for fome time be fuflicient for 
 the circulation of the blood. But, while 
 the refiftances in the veflels are conftantly 
 increafing, the irritability of the moving 
 fibres, and the energy of the brain, are 
 at the fame time conftantly diminilhing; 
 and therefore the power of the heart muft 
 
 I 
 
 at 
 
240 
 
 PHYSIOLOGY. 
 
 at length become unequal to its talk, the 
 circulation muft ceafe, and death enfue. 
 
 CCClI. 
 
 The unavoidable death of old perfons is 
 thus in part accounted for; but it is, how- 
 ever, ftill probable, that the fame event 
 proceeds chiefly from the decay and total 
 extindtion of the excitement or vital power 
 (CXXXVI.) of the nervous fyftem, and 
 that from caufes very independent of 
 the circulation of the blood, and ariling in 
 the nervous fyftem itfelf in confequence 
 of the progrefs of life. This feems to be 
 proved by the decay of fenfe, memory, in- 
 telledl, and irritability, which conflantly 
 takes place, as life advances beyond ^ cer- 
 tain period. 
 
 FINIS. 
 
\ 
 
 
 
 
 ■ t 
 
 f* ■ 
 
 
 ;t>k^- 
 
 WA». 
 
 i ‘ V. /» 
 
 
 
 .■y 
 
 V 
 
 


 .*■ 
 
 f 
 
 •'!^^'^ .-• V’ 
 
 . 4 ^ 
 
 * 1 , 
 
 ■s' ' 
 
 
 
 ' ( 
 
 / 
 
 jf 
 
i 
 
 
 
 
 
 
 "-'r' 
 
 fcV^T .f * v'.'’^ ■•■' 
 
 -V 
 
 • 5 ^ i -*• 
 
 ' / 
 
 4 
 
 
 ,.«■ 
 
 y