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 Arboreal life and the evolution of the h 
 
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Head of an Adult Male Mandrill. 
 
ARBOREAL LIFE 
 
 EVOLUTION OF THE HUMAN EYE 
 
 A Revised Publication op the 
 
 BOWMAN LECTURE 
 
 Delivered before the Ophthalmological Society of the 
 United Kingdom in May, 1921 
 
 BY 
 
 E. TREACHER COLLINS, F.R.C.S. 
 
 CONSULTING SURGEON, ROYAL LONDON OPHTHALMIC HOSPITAL (mOORFIELDs) ; 
 CONSULTING OPHTHALMIC SURGEON, CHARING CROSS HOSPITAL 
 
 WITH COLORED FRONTISPIECE AND 25 ILLUSTRATIONS 
 IN THE TEXT 
 
 LEA & FEBIGER 
 
 PHILADELPHIA AND NEW YORK 
 1922 
 
COPYBIGHT 
 
 LEA & FEBIGER 
 1922 
 
 hunted iN u. s. a. 
 
PREFACE. 
 
 It is customary for the rising generation to regard the 
 Victorian Era as characterized by the wearing of preposterous 
 clothing, and by living in rooms full of ugly furniture, with 
 decorations devoid of taste. However these things may 
 have been, there can be no doubt that it was an era which 
 produced a number of scientific men of outstanding ability, 
 whose like we shall not look upon again. 
 
 One of these great Victorian scientists was Sir William 
 Bowman, who, early in his career, earned for himself a world- 
 wide reputation by the number and accuracy of his investi- 
 gations into the minute anatomy of the various tissues of 
 the body. Some of his most important observations were 
 in connection with the minute anatomy of the eye; and 
 though originally a general surgeon, so great a repute did 
 he gain as an ophthalmologist that he was gradually forced 
 to restrict his practice to that branch of his profession. 
 
 During the middle part of the Nineteenth Century, Bow- 
 man was the most' prominent ophthalmologist in England, 
 as von Graefe was in Germany and Donders in Holland. 
 These three distinguished men came together for the first 
 time at the Moor fields Hospital in the summer of 1851, the 
 year of the Great International Exhibition in London. 
 Linked together by the pursuit of knowledge in a branch 
 of their profession, which was at that time growing in a 
 most exciting fashion, they became the firmest of friends 
 
IV PREFACE 
 
 for the remainder of their lives. In the year they first met, 
 Helmholtz had invented the ophthalmoscope, whereby, as von 
 Graefe said, a new world became opened up for exploration. 
 In the same year Dr. A. Cramer, working under Donders' 
 guidance, by observing alterations in curvature in the anterior 
 surface of the lens, discovered the seat of accommodative 
 power, while Bowman, but a few years previously, had 
 demonstrated the muscular character of what had up to 
 that time been regarded as the ciliary ligament. 
 
 So great was Donders' regard for Bowman that he dedi- 
 cated his epoch-making book to him in the following glowing 
 terms : 
 
 " To William Bowman, F.R.S., whose merits in the advance- 
 ment of physiology and ophthalmology are equally recog- 
 nized and honored in every country, this work on the 
 Anomalies of Refraction and Accommodation is in testimony 
 of the warmest friendship and of esteem inscribed by the 
 author." 
 
 When, in 1880, the Ophthalmological Society of the United 
 Kingdom was instituted, Bowman was naturally elected 
 its first president. He held the post for three years and 
 manifested the greatest interest in the society's progress and 
 prosperity. In 1883, the council of the society passed the 
 following resolution: "That in recognition of Mr. Bowman's 
 distinguished scientific position in ophthalmology and other 
 branches of medicine, and in commemoration of his valuable 
 services to the Ophthalmological Society, of which he was 
 the first president, the council shall each year, or periodi- 
 cally, nominate some person to deliver a lecture before the 
 society, to be called The Bowman Lecture, which shall 
 consist of a critical resume of recent advances in ophthal- 
 mology or in such subject or subjects as the council shall 
 select, or of any original investigation, and shall be delivered 
 
PREFACE V 
 
 at a special meeting of the society held for the purpose, at 
 which no other business shall be transacted." 
 
 No better method could have been adopted for paying 
 tribute to the character and attainments of Sir William 
 Bowman than the institution of such a lectureship. Great 
 as were his own original contributions to ophthalmology, 
 he probably did still greater work for it by the stimulating 
 and kindly help he rendered to others. It seems, therefore, 
 most appropriate that his name should continue to be associ- 
 ated with original work, not only that of his own country- 
 men, but also that of distinguished ophthalmologists from 
 other parts of the world. 
 
 The Bowman Lecture which is here reprinted from Vol. 
 XLI of the Transactions of the Ophthalmological Society 
 is the twenty-second that has been given. The names of 
 those who have delivered the preceding twenty-one are 
 as follows: Sir Jonathan Hutchinson, F.R.S.; J. Huglings 
 Jackson, M.D., LL.D., F.R.S.; Prof. Zehender (Rostock); 
 Henry Power; Sir Henry Swanzy; Prof. Hansen Grut 
 (Copenhagen); Priestley Smith; R. Marcus Gunn; Prof. 
 E. Fuchs (Vienna); Sir F. W. Mott, K.B.E., M.D., F.R.S.; 
 Prof. Sattler (Leipzig); Edward Nettleship, F.R.S.; E. 
 Landolt, M.D. (Paris); Sir George Berry, M.D., LL.D.; 
 v. Morax, M.D. (Paris). 
 
 The writer of this lecture cannot claim to have been a 
 / pupil of Bowman's, only a pupil of several of his pupils. 
 From their lips and example he has, however, learned to 
 reverence the work of their master and to try and follow 
 humbly in his footsteps. 
 
 In the Transactions the lecture appeared under the title 
 of " Changes in the Visual Organs Correlated with the Adop- 
 tion of Arboreal Life and the Assumption of the Erect Pos- 
 ture." In the reproduction of it in book form a shorter title 
 
VI PREFACE 
 
 has been thought desirable. The author wishes to express his 
 indebtedness to the council of the Ophthalmological Society 
 for its permission to reproduce some of the illustrations, 
 and to his friend Dr. William Campbell Posey, at whose 
 instigation and with whose kindly aid the lecture has been 
 reproduced in its present form. E. T. C. 
 
 London, 1922 
 
CONTENTS. 
 
 Introduction ... .17 
 
 CHAPTER I. 
 The Field of Vision . . 22 
 
 CHAPTER II. 
 Light-sense 42 
 
 CHAPTER III. 
 
 FOBM-SENSE ..... .51 
 
 CHAPTER IV. 
 Accommodation and Convergence . . 57 
 
 CHAPTER V. 
 Color-sense . . . . 83 
 
 CHAPTER VI. 
 The Protective Mechanisms of the Eyeball . . 94 
 
LIST OF ILLUSTRATIONS. 
 
 Frontispiece (Colored). Head of Adult Male Mandril. 
 
 Figure 1. Skull of a Hornless Sheep . 23 
 
 Figure 2. Skull of a Dog . . . . 23 
 Figure 3. Diagram Showing Relative Size of the Cornea to the 
 
 Eyeball in Different Mammals .... . . 25 
 
 Figure 4. Sections through the Iris of a Horse ... 29 
 Figure 5. Front of a Horse's Eye from which the Cornea has 
 
 been Removed to Expose the Iris with the "Corpus Nigrum" 30 
 
 Figure 6. Sperm Whale . .33 
 
 Figure 7. Diagram to Show Hypothetical Semi-decussation of 
 
 Macular Nerve Fibers at the Chiasma .37 
 
 Figure 8. Headlight Fish. ^Ethoprora Lucida . . 47 
 
 Figure 9. Ciliary Body of the Echidna . .... 58 
 
 Figure 10. Ciliary Body of the Great Ant-eater .... 59 
 
 Figure 11. Ciliary Body of the Rabbit .... . . 60 
 
 Figure 12. Ciliary Body of the Wallaby . 60 
 
 Figure 13. Ciliary Body of the Ox ... . . . 61 
 
 Figure 14. Ciliary Body of the Cat . 61 
 
 Figure 15. Ciliary Body of the Rhesus Monkey 62 
 
 Figure 16. Ciliary Body of the Chimpanzee . . 63 
 
 Figure 17. Ciliary Body of the Human Eye ... 63 
 
 Figure 18. Varieties of the Ciliary Muscle in the Human Eye . 68 
 Figure 19. Diagram to Show the Relative Size of the Lens in 
 
 Different Species of Mammals . 71 
 
 Figure 20. Diagram to Show the Size of the Lens of the Human 
 
 Lens at Different Times in Fetal Life . . . . . 73 
 Figure 21. Diagram to Show Relation of Ciliary Body to the 
 
 Side of the Lens at Different Times in Fetal Life ... 75 
 Figure 22. Eye of an Antelope Showing the Retractor Bulbi or 
 
 Choanoid Muscle .... . 96 
 
 Figure 23. Section Through Gegenbauer's Muscle from the Inner 
 
 Surface of the Cornea of a Sheep 96 
 
 Figure 24. Drawings Showing the Arrangement of Parts at the 
 
 Inner Canthus in Different Mammals 104 
 
 Figure 25. Section Showing Lining Membrane of the Suborbital 
 
 Pit of a Sheep 105 
 
EVOLUTION OF THE HUMAN EYE. 
 
 INTRODUCTION. 
 
 The influence of the environment in the production of 
 changes in the visual organs is a large and interesting sub- 
 ject—so large that it is only possible to attempt dealing 
 with a comparatively small part of it in this lecture. The 
 part most full of interest to this society must necessarily 
 be that which concerns the higher mammals. A study of 
 the morphology of their visual organs shows that there is a 
 very close resemblance between those of man and monkeys, 
 and some very wide differences between those of monkeys 
 and the lower mammals. The chief cause of these marked 
 changes is, I propose to show, to be found in the alteration 
 in environment produced by the adoption of arboreal life. 
 
 Dr. Wood Jones 1 in 1915 delivered a lecture at the Royal 
 College of Surgeons "On the Influence of the Arboreal 
 Habit in the Evolution of the Reproductive System," and 
 I cannot do better than quote here the following passage 
 from his opening remarks: 
 
 "No doubt the assumption of the upright posture has 
 done much in putting the finishing touches upon the evolution 
 of the human stock, but we must be careful to give it no 
 more than its real value. Long before the fashion of walk- 
 ing upright had been even foreshadowed in the crouching 
 gait of the half-finished product, a potent factor was 
 2 
 
18 EVOLUTION OF THE HUMAN EYE 
 
 at work producing those changes which ultimately per- 
 mitted, and culminated in the erect carriage. This factor 
 which paved the way, and to which most of the general 
 changes of human and primate bodily conformation are 
 due, was undoubtedly the arboreal habit, the habit of climb- 
 ing and living up trees, instead of dwelling upon the ground 
 beneath. We are too apt to lay too much stress upon man's 
 descent from the branches. It was not coming down the 
 tree that was so important in the evolution of the human 
 stock; it was the climbing up, the first step toward the 
 freedom of the branches, that counted in the progress of 
 the mammalian stock from which man has sprung." 
 
 In this first lecture Dr. Wood Jones confined his obser- 
 vations to the evolution of the reproductive system, but 
 at its close remarked "that every system and every part of 
 man 's body tell the same story." In three lectures delivered 
 the following year he extended the subject, and dealt with 
 the influence of the arboreal habit in the general evolution 
 of man. It is my purpose in this lecture to discuss its 
 influence in the evolution of the visual organs. 
 
 The life of mammals in their struggle for existence may 
 be compared to a prolonged game of hide-and-seek. Some, 
 like the Rodentia, have found safety by burrowing in the 
 ground; some, like the Cetacea, by adopting aquatic life; 
 some, like the Quadrumana, by climbing up into the trees; 
 and others, like the Ungulata, by living in herds and acquir- 
 ing protective coloration. The adoption of life in herds and 
 protective coloration is a game at which two can play— the 
 attackers and the attacked. Some of the Carnivora hunt 
 in packs, and most of them have acquired colors and markings 
 which serve as effective camouflage in the districts which 
 they inhabit. For this great game of hide-and-seek the 
 sense-organs have become developed to different degrees 
 
INTRODUCTION 19 
 
 in the various classes of mammals in accordance with their 
 environments. 
 
 The herbivorous terrestrial mammals, whose safety and 
 survival depend upon their swiftness of flight from danger, 
 such as some of the Rodentia and the Ungulata, require 
 sight, smell and hearing so developed as to enable them to 
 detect quickly the approach of danger over a wide area. 
 They grasp their food directly with their mouths, finding 
 that which is suitable more by smell and by the touch of 
 their acutely sensitive snouts than by sight. Their visual 
 organs must, therefore, be adapted for the widest possible 
 range of circumferential vision, sufficiently acute over a wide 
 area to enable them rapidly to detect any moving object 
 —such vision as is aptly termed "-panoramic vision," the 
 highest perfection of which may be described as vision which 
 enables an animal to see in its entire circumference at one 
 and the same time. They also require, to protect them- 
 selves from the evils which prowl by night as well as by 
 day, a capacity for seeing in dim as well as in bright light. 
 
 The terrestrial carnivorous animals, with their powerful 
 development of tooth and claw, require for their sustenance 
 and survival smell and sight, which will enable them to 
 track their prey and pounce accurately upon it when within 
 a suitable distance. They grasp their food with their mouths, 
 steadying it with their front paws while tearing it to pieces 
 with their teeth. Their visual organs must, therefore, be 
 so adapted that they can concentrate the sight of both eyes 
 on their victims at the distance from which they spring, 
 and follow quickly any movements such victims may make 
 in their endeavors to escape. For these purposes binocular 
 vision is more essential than panoramic vision. As the 
 animals upon which these Carnivora prey are comparatively 
 large, accurate vision for fine detail is not requisite. Good 
 
20 EVOLUTION OF THE HUMAN EYE 
 
 vision in dim lights, rather superior to that of their victims, 
 is essential for success in their pursuits. 
 
 Mammals like the Simiae, who have found safety from 
 carnivorous foes by life in the trees, have in learning to 
 climb acquired the capacity of using their forelimbs for pre- 
 hensile purposes. They require sight which will enable 
 them to swing and spring with accuracy from bough to 
 bough. Assuming the semierect posture, they grasp their 
 food with their hands and convey it by them to their mouths. 
 Their visual organs have, therefore, to be adapted for con- 
 siderable accuracy in the judgment of distances, varying 
 considerably in degree; hence not only binocular vision but 
 stereoscopic vision is required, together with accurately 
 associated powers of accommodation and convergence. 
 As the food of these arboreal mammals consists of fruits 
 and insects, which they pick up with their fingers, they re- 
 quire color-sense and a high degree of acuity of vision for small 
 objects. Safe in their arboreal resorts, away from the dangers 
 which prowl by night in the land below, vision in dim lights 
 is not so essential for their safety as it is for some other 
 classes of mammals. 
 
 Man, having descended from the trees and become a 
 hunter and also exposed to the attacks of carnivorous foes, 
 has succeeded in maintaining his existence by the assumption 
 of the erect posture, which has entirely freed his forelimbs 
 for the use of weapons of offence and defence. The manu- 
 facture and use of such weapons has necessitated increased 
 precision in the judgment of distances and of concentration 
 in the thoughtful recognition of detail. The elevation of 
 his head, due to his erect posture, has increased his range 
 of circumferential vision. Having acquired during arboreal 
 life a more acute central form-sense than any other terrestrial 
 mammal, together with the most highly-developed stereo- 
 
INTRODUCTION 21 
 
 scopic vision, he has, by extended and more rapid movements 
 of both eyes and head, increased the range of his field of 
 fixation. 
 
 The different visual requirements thus outlined for dif- 
 ferent forms of environments have involved considerable 
 alterations in the structural architecture of the visual organs. 
 It is with these structural alterations that I now proceed 
 to deal, dividing them up for descriptive purposes under 
 the following headings: I. The Field of Vision; II. Light- 
 sense; III. Form-sense; IV. Accommodation and Con- 
 vergence; V. Color-sense; VI. The Protective Mechanisms 
 of the Eyeball. 
 
CHAPTER I. 
 THE FIELD OF VISION. 
 
 In studying the modifications met with in the visual 
 organs in the various species of Mammalia, to allow for 
 different requirements in connection with the visual field, 
 we have to take into consideration the monocular field, the 
 combined monocular fields, the binocular field and the 
 field of fixation. 
 
 The extent of the monocular field may be modified by 
 the degree of prominence of the eye in the head, and by 
 structural alterations in the eye itself. 
 
 In animals in whom a large monocular field is of the greatest 
 importance, such as those who depend for safety on their 
 rapidity of flight, the eyes are found set prominently out 
 from the surface of the head. In the Ungulata the outer 
 margin of the orbit is composed of a complete projecting 
 bony ring which serves to hold the eyeball prominently 
 forward (Fig. 1). In the Rodentia, the Insectivora and 
 the Carnivora the orbital ring is incomplete externally, in 
 the latter the malar and frontal apophysis being only united 
 by fibrous bands (Fig. 2). 
 
 The evolution from an incomplete to a complete bony 
 ring at the margin of the orbit can be well traced in the 
 ancestry of the horse. In the Meso-hippus baridi, its three- 
 toed ancestor, from the lower Miocene tertiary of North 
 America, the bony wall of the orbit is open behind. 
 
 In monkeys, anthropoid apes and in man the bony 
 
THE FIELD OF VISION 23 
 
 apophyses of the outer margin of the orbit are in contact, 
 and it is shut off from the temporal fossa by the orbital 
 plates of the malar and sphenoid bones, so that the orbit 
 
 Fig. 1. — Skull of a hornless sheep. (After Owen.) 
 
 becomes a more complete bone-lined recess than in any 
 other class of mammals. In some lemurs a similar separation 
 of the orbit from the temporal fossa is met with, but in the 
 majority of them no such complete separation has taken 
 
 Fig. 2.— Skull of a dog. 
 
 place. The closing in of the outer wall of the orbit in Pri- 
 mates does not give prominence to the eye and enlarge the 
 monocular field as in the Ungulata, but ensures steadiness 
 
24 EVOLUTION OF THE HUMAN EYE 
 
 of the movements of the eye in the interests of binocular 
 vision. 
 
 Some mammals have the power of markedly increasing 
 the prominence of the eyes in the orbit, and so temporarily 
 of increasing the extent of their monocular fields. In 
 burrowing animals, such as moles and hedgehogs, the eyes 
 are retracted, for the sake of protection, when they are 
 below ground, and are protruded, beyond their epidermal 
 coverings, when they come to the surface. In other mammals 
 increased prominence of the eyes becomes most noticeable 
 when they are alarmed, and when increased circumferential 
 vision is most likely to be of importance to them. The 
 increase in the prominence of the eyes is effected by relax- 
 ation of the retractor bulbi muscle, and contraction of the 
 muscle of Gegenbauer, which lines the fibro-elastic tissue 
 filling in the outer wall of the orbit in animals where it is 
 not bounded by bone. A vestige of this muscle is some- 
 times to be found in man opposite the sphenomaxillary 
 fissure, and is termed Muller's muscle. I shall have more 
 to say respecting it later, in speaking of the protective 
 mechanisms of the eyeball. I refer to it here because slight 
 protrusion of the eyes in human beings, as the result of 
 intense emotional excitement, has been observed; though 
 "starting of the eyes from the head" as the result of fright 
 is more often read of in fiction than actually seen in real 
 life. 
 
 An increase in the size of the cornea relatively to the size 
 of the eyeball must allow rays of light of greater obliquity 
 to be refracted into the eye, and an increase of the corneal 
 surface tends to increase not only the size of the field of 
 vision, but also the clearness of peripheral images. 
 
 In some rodents the cornea composes nearly half the 
 surface of the eyeball. The relations between the diameters 
 
jK-OxtllcrA* 
 
 Fig. 3. — Showing size of cornea in relation to size of eyeball. Drawn to scale. 
 
26 EVOLUTION OF THE HUMAN EYE 
 
 of the eyeball and the cornea of mammals are shown in the 
 following table (Fig. 3.) : 
 
 Diameters of the eyeball. Diameters of the cornea. 
 
 
 Antero- 
 
 
 1 
 
 
 
 
 
 posterior. 
 
 Lateral. 
 
 Lateral. 
 
 Vertical. 
 
 Man 
 
 24.8 mm. 
 
 24.4 mm. 
 
 11.6 mm. 
 
 11.0 mm. 
 
 Chimpanzee 
 
 19.0 
 
 it 
 
 18.0 
 
 " 
 
 1,0.5 
 
 it 
 
 
 Rhesus monkey 
 
 19.5 
 
 " 
 
 19.0 
 
 " 
 
 12.0 
 
 tt 
 
 11.0 " 
 
 Capuchin monkey 
 
 18.5 
 
 u 
 
 19.0 
 
 u 
 
 11.0 
 
 (( 
 
 10.5 " 
 
 Cat 
 
 22.0 
 
 tt 
 
 21.0 
 
 " 
 
 18.0 
 
 " 
 
 
 Horse 
 
 44.0 
 
 it 
 
 54.0 
 
 " 
 
 340 
 
 it 
 
 27.0 " 
 
 Ox . 
 
 36.0 
 
 it 
 
 38.0 
 
 tt 
 
 27.0 
 
 tt 
 
 22.0 " 
 
 Sheep . 
 
 27.0 
 
 it 
 
 28.0 
 
 tt 
 
 19.0 
 
 it 
 
 15.5 " 
 
 Antelope 
 
 27.0 
 
 tt 
 
 29.0 
 
 a 
 
 22.0 
 
 " 
 
 18.0 " 
 
 Pig 
 
 23.5 
 
 it 
 
 24.0 
 
 it 
 
 17.0 
 
 " 
 
 14.0 " 
 
 Wallaby 
 
 18.0 
 
 it 
 
 19.0 
 
 tt 
 
 14.0 
 
 tt 
 
 
 Rabbit 
 
 16.0 
 
 i. (( 
 
 20.0 
 
 " 
 
 15.0 
 
 it 
 
 
 Rat. 
 
 5.5 
 
 (< 
 
 5.5 
 
 u 
 
 5.0 
 
 tt 
 
 5.0 " 
 
 Hedgehog . 
 
 6.V 
 
 it 
 
 6.0 
 
 a 
 
 - '6.0 
 
 it 
 
 6.0 " 
 
 In man and monkeys, where a central spot of distinct 
 vision is of the greatest importance, the size of the cornea 
 relatively to the size of the eyeball is comparatively small. 
 Thus in all mammals below man the diameter of the cornea 
 measures more than half the antero-posterior diameter of 
 •the globe. In the chimpanzee it is about half and in man 
 considerably less than half. 
 
 Mammals that have a proportionally large cornea have 
 also a proportionally large lens which is nearly spherical in 
 shape, there being only a very slight increase in the size of 
 the lateral diameter over that of the antero-posterior (vide 
 table, p. 70). In man and monkeys, where the cornea is 
 proportionally small, the lens is also proportionally small 
 and considerably flattened from before backward. In 
 monkeys the antero-posterior diameter of the lens is a little 
 more than half the lateral diameter and in man about half. 
 
 Emmert 2 has estimated the relation of the size of the lens 
 to the size of the eyeball as follows: 
 
THE FIELD OF VISION 27 
 
 Horse . 6 to 100 
 
 Ox 7 to 100 
 
 Calf . . 5.5 to 100 
 
 Sheep 7.6 to 100 
 
 Pig 8 to 100 
 
 Dog 7.7 to 100 
 
 Cat 10 to 100 
 
 Rabbit 8 to 100 
 
 Man 4.2 to 100 
 
 As pointed out by Lindsay Johnson, 3 with a large spherical 
 lens, rays falling obliquely on the cornea will be brought to 
 a focus with comparatively little distortion, the image being 
 received upon a surface which is nearly equidistant from the 
 nodal points. This is a matter of considerable importance 
 in securing a large monocular field for moving objects, in 
 animals with a retina having a widely-extended sensitive 
 area. In the Primates, who require acute vision for small 
 fixed objects, and who have developed a highly sensitive, 
 small, central spot for this purpose, it is essential that the 
 image of the object looked at should be accurately focussed 
 on that spot only. It is advantageous, in the interests of 
 binocular vision, that the images of objects formed on parts 
 of the retina peripheral to the central spot should be but 
 dimly seen. Hence we find in the Primates the cornea 
 and lens so constructed as to sacrifice the interests of the 
 monocular field in favor of those of acute central fixation. 
 
 The alteration in the shape of the lens in Primates, so 
 that instead of being nearly spherical it is considerably 
 flattened antero-posteriorly, has considerably diminished 
 its refractive power. To compensate for this to some extent 
 we find that the refractive power of the cornea has been 
 considerably increased. The following interesting table 
 has been constructed by Kalt and Dufour; 4 it shows that 
 in the eyes of man and birds, which have flattened lenses, 
 the refraction of the cornea relative to that of the lens is 
 
28 
 
 EVOLUTION OF THE HUMAN EYE 
 
 considerably greater than it is in other mammals and in 
 fishes, which have rounder lenses. 
 
 Man 
 
 Crow 
 
 Horse 
 
 Dog . . . . . 
 
 Whale (out of water) 
 Carp 
 
 Focal distance 
 of the cornea. 
 
 Focal distance 
 of the crystal- 
 line lens. 
 
 Relation : 
 1 to— 
 
 31.2 mm. 
 
 49.2 mm. 
 
 1.60 mm 
 
 25.8 " 
 
 36.7 " 
 
 1.42 " 
 
 78.8 " 
 
 64.4 " 
 
 0.82 " 
 
 33.8 " 
 
 22.9 " 
 
 0,60 " 
 
 314.3 " 
 
 40.5 " 
 
 0.13 " 
 
 38.0 " 
 
 6.6 " 
 
 0.17 " 
 
 Most of the Uhgulata have markedly oval cornese, their 
 lateral diameters measuring more than the vertical. In 
 sheep there is a difference of 3.5 mm. between the two, and 
 in the ox 5 mm. Associated with oval cornese they have 
 oval pupils elongated laterally. According to Eversbusch, 5 
 this is due to the presence of a check ligament, extending 
 from the pupillary margin to the periphery, which can be 
 seen as an elevation on the posterior surface of the iris. I 
 have examined the iris of several different species of Ungu- 
 lates, but have been unable to find any such check ligament. 
 The elongation of the pupil can quite adequately be explained: 
 First, by the elongation of the cornea, to which the root of 
 the iris is attached laterally; and secondly, by the length 
 of the iris in the vertical plane being greater than in the 
 horizontal. In a horse's iris I find there is 2 mm. difference 
 in the length of the iris horizontally and vertically (Fig. 4). 
 There is also, in these animals with a horizontally oval pupil, 
 a difference in the length of the ciliary body horizontally 
 and vertically. In the horse the distance from the root of 
 the iris to the ora serrata measures 5 mm. horizontally and 
 10 mm. vertically. To put it another way, the retina extends 
 farther forward laterally than vertically in the Uhgulata, 
 which have pupils and cornese elongated laterally. The 
 combined effect of these three anatomical features is to pro- 
 
THE FIELD OF VISION 
 
 29 
 
 duce an extension of the monocular field of vision in the 
 horizontal plane. 
 
 In the center of these elongated oval pupils a deeply- 
 pigmented elevation is met with at both the upper and 
 lower borders, but considerably larger and more raised at 
 the upper than the lower (Fig. 5). This elevation has 
 received the name of the "corpus nigrum." In a horse, 
 
 A 
 
 ww 
 
 
 Fig. 4. — Sections through the iris of a horse. A, in the horizontal meridian. 
 B, in the vertical meridian. 
 
 the lateral diameter of whose pupil measured 17 mm., the 
 corpus nigrum at its upper border measured 8 mm. in length, 
 3 mm. in width and 3.5 mm. in height. In sections of the 
 iris it can be seen to be an extension forward from -the 
 pigment epithelium on its posterior surface, i. e., a growth 
 forward from the anterior extremity of the secondary optic 
 vesicle (Fig. 4 B). It is a vascularized structure and so 
 has some elements of mesoblastic tissue in it. Bleached 
 
30 
 
 EVOLUTION OF THE HUMAN EYE 
 
 sections show the mass to be made up of flattened epithelial 
 cells, which line and form columns between numerous, 
 variously-sized circular spaces. 
 
 The function of the corpus nigrum has so far not been 
 definitely determined. Lindsay Johnson 3 suggested that it 
 served as a shade for the large oval pupil. .Such a suggestion 
 does not explain the presence of a projection from the lower 
 margin of the pupil as well as the upper. In some animals, 
 
 
 Fig. 
 
 -Front of horse's eye from which cornea has been removed to expose 
 iris with the "corpus nigrum." 
 
 e. (/., gazelles, camels and llamas, the black bodies from the 
 upper and lower margins of the pupil have indentations 
 capable of fitting into one another. On examination of 
 horses ' eyes, when exposed to bright sunlight, I have observed 
 that contraction of the pupil causes the pigmented bodies 
 from the two borders to meet and fill up its center, leaving 
 the two extremities still open. So that the horse then 
 has, instead of one oval pupil, two circular ones, looking 
 
THE FIELD OF VISION 31 
 
 respectively forward and backward. A large oval pupil, while 
 affording a wide range of lateral vision, must detract very 
 considerably from the clearness of vision by not having the 
 stenopaic effect of a round pupil. I would suggest that one 
 function of the corpus nigrum is at times to compensate for 
 this defect by the conversion of the oval pupil into two 
 round ones. 
 
 Animals, such as the terrestrial herbivorous mammals, 
 who require panoramic vision, have their eyes set laterally 
 in the head so as to obtain the largest circumferential effect 
 of the combined monocular fields. Lindsay Johnson 3 
 measured the divergence of the optical axes in a large number 
 of representative animals, and constructed a diagram 
 graphically showing how it varies in the different natural 
 orders, families, genera and species. From this it is seen 
 that the most laterally placed eyes are met with among the 
 Rodentia, the Marsupialia and the Ungulata. The greatest 
 divergence is in hares, whose optical axis in each eye measures 
 85° of divergence from the middle line. It seems probable 
 that they, and some of the other rodents, have complete 
 panoramic vision, i. e., are able to see in their circumference 
 at one and the same time. 
 
 Among the Carnivora the smallest amount of divergence 
 is in lions and cats, in which it is less than 10° in each eye. 
 The Simise and man alone among mammals have parallel 
 optic axes. 
 
 This movement forward of the optical axes toward 
 parallelism is clearly in the interests of binocular vision, and 
 at the sacrifice of the range of simultaneous circumferential 
 vision. As ontogeny is a condensed recapitulation" of 
 phytogeny, it is interesting to note that in the human embryo 
 the optic vesicles when first formed are directly opposite 
 to one another, from which position they gradually turn 
 
32 EVOLUTION OF THE HUMAN EYE 
 
 forward, so that at the third month of fetal life the optic 
 axes of each eye diverge 45° from the middle line; before 
 birth they become parallel. 
 
 Wood Jones 1 has described how the alteration in the 
 position of the eyes in mammals is due to the recession of 
 the snout. He says: "When an animal has a fully elongated 
 snout region, it may be said to possess a long face with an 
 eye situated upon each side of it; but when the snout region 
 has undergone complete recession, it may be said to have a 
 flat face with two eyes situated upon the front of it. The 
 mere fact of the recession of the snout produces this change, 
 for the two eyes are turned to the front as the elongated 
 muzzle shrinks between them." 
 
 He further points out this recession of the snout is the 
 result of (a) the substitution of the hands for it as the chief 
 means of tactile exploration; (6) the decrease in the use of 
 the jaws for seizing and tearing food; (c) the diminished 
 importance of the sense of smell. 
 
 The three changes are all the outcome of the adoption 
 of arboreal life. It has resulted in the use of the forelimbs 
 for grasping purposes. From this they have gradually 
 acquired acute tactile sensibility, and have superseded the 
 snout as a means for exploration by the sense of touch; the 
 latter has, therefore, tended to atrophy from disuse. From 
 the grasping of boughs in arboreal life has arisen the power 
 of grasping food with the hands to convey it to the mouth 
 instead of seizing it primarily with the jaws. This diminished 
 use of the jaws has led to their loss of prominence and power. 
 "No trail of scent is laid among the branches of a tree" 
 (Wood Jones). "Life amid the branches limits the use- 
 fulness of the olfactory organs" (Elliot Smith). 
 
 Binocular vision is produced by the overlapping of the 
 fields of vision in the two eyes. In Cetaceans so massive 
 
THE FIELD OF VISION 33 
 
 is the head, and so far back are the eyes set, that it is 
 impossible for any such overlapping to occur (Fig. 6). In 
 rodents, such as rabbits and hares, which also have their 
 eyes set far back, the overlapping of the fields anteriorly 
 cannot exceed more than 15°. 
 
 It has been suggested by Grossman and Meyerhausen 6 
 that in these animals some overlapping of the fields may 
 occur posteriorly, and that in them a certain amount of pos- 
 terior binocular vision may be present. As all sportsmen 
 know, rabbits and hares see very imperfectly when danger is 
 immediately in front of them, and will run straight forward 
 
 Fig. 6. — Sperm whale. 
 
 toward a gun. I am told that it is a general belief in the 
 country, that if a hare lies in a furrow with the nose pointing 
 down it, .you can walk up the furrow to the hare's head. 
 
 In connection with the binocular vision of mammals it 
 is important to note the observations of Kalt 7 as to" the size 
 of the angle y in the different species. He states that "in 
 the human species the visual axis pierces the cornea 5° 
 inside its center. In the higher monkeys it is almost the 
 same, but one sees this angle increase as one descends the 
 animal scale. It is 15° in the Lemuridfe, 20° to 26° in the 
 Carnivora, and it varies between 55 ° to 63 ° in the Herbivora." 
 The increase of the angle tends to balance to some extent 
 3 
 
34 EVOLUTION OF THE HUMAN EYE 
 
 an increase in divergence of the optic axes, so that animals 
 with a wide divergence are still able to see binocularly. 
 
 A horse with eyes set prominently forward, large cornese 
 and laterally oval pupils, can see behind and observe the out- 
 kick of its hind leg without turning its head. With a visual 
 axis piercing the cornea 50° to 63° inside its center; with 
 the corpus nigrum capable of closing in the middle part of 
 the oval pupil and leaving a circular one looking forward, 
 and with about one-sixth of the fibers of the optic nerve 
 turning into the optic tract of the same side while the rest 
 decussate, a horse evidently possesses binocular stereoscopic 
 vision, which must be of the greatest assistance to it in the 
 estimation of size and distance in jumping. Every horse- 
 man knows that an object at which a horse is most likely 
 to shy is one situated at the side and of a light color. To 
 prevent shying we put blinkers on harness horses, so that 
 they may not see these laterally placed objects, but only 
 those which they view binocularly straight in front of them. 
 In training a shying horse it is customary to walk the animal 
 up to any object it has shied at with its head toward the 
 object, which it can then view binocularly. 
 
 Closely associated with the varying degrees of binocular 
 vision we meet with varying degrees of decussation of the 
 fibers of the optic nerve at the chiasma. This matter has 
 been dealt with very fully by Dr. Wilfred Harris. 8 He con- 
 siders that binocular vision is associated with carnivorous 
 habits; it is met with in carnivorous fishes, amphibia and 
 birds. In all these there is total decussation of the optic 
 nerve fibers at the chiasma, so that they do not have stereo- 
 scopic vision as in mammals, in many of whom, owing to 
 semi-decussation of the fibers at the chiasma, visual im- 
 pressions from the two eyes are received on the same side 
 of the brain. 
 
THE FIELD OF VISION 35 
 
 Rodents have no conjugate movements of the eyes, and 
 no consensual response of the pupils to light, only a direct 
 reaction. Singer and Miinzer 8 found that in mice and rats 
 the decussation of optic nerve fibers at the chiasma was 
 complete. Wilfred Harris examined the optic nerves, 
 chiasma and tracts of rabbits in sections stained with hema- 
 toxylin by Pal 's method, and also a fortnight after the experi- 
 mental enucleation of one eye, by staining with Marchi's 
 method, to trace the degenerated fibers. He found that 
 decussation was almost complete, only very few fibers, here 
 and there, turning round from the optic nerve into the 
 optic tract of the same side. Ramon y Cajal 10 obtained 
 similar results. 
 
 In the Ungulata it is difficult to observe the movements 
 of the two eyes at the same time, but it appears that they 
 are to some extent conjugate. In them the decussation of 
 the optic nerve fibers at the commissure is not complete. 
 In the horse and calf, two observers in Vienna, using Marchi 's 
 method, found that about one-sixth of the fibers turn into 
 the optic tract of the same side. 
 
 In the carnivorous mammals there is consensual reaction 
 of the pupils to light, and the movements of the two eyes 
 are more obviously conjugate than in the Ungulata. Wilfred 
 Harris found that experimental division of the optic tract 
 on one side in cats produced distinct homonymous hemi- 
 anopsia, with the hemianopic pupil reaction to direct light in 
 the opposite eye, though its consensual reaction was brisk 
 on exposure of the other eye to light. Microscopically he 
 found, in sections of the chiasma of a cat stained with 
 Marchi's method, after enucleation of one eye a fortnight 
 previously, that a large proportion of degenerate fibers were 
 present in the optic tract of the same side. These degenerate 
 fibers were spread equally through all parts of the tract and 
 not collected into any distinct bundle. 
 
36 EVOLUTION OF THE HUMAN EYE 
 
 In monkeys and in man there are conjugate movements 
 of the eyes, and consensual reaction of the pupils to light, 
 as in the Carnivora, but in addition they have a very highly 
 developed spot of central vision with a special bundle of 
 nerve fibers leading from it, and highly developed powers 
 of convergence and accommodation. 
 
 There is no very definite anatomical evidence as to the 
 course taken by the macular nerve fibers at the chiasma. 
 Cajal 10 discovered bifurcating fibers in the chiasma of the 
 rabbit, the divisions passing into the two optic tracts. 
 Kolliker 11 found similar bifurcations in the chiasma of a 
 cat a few days old. It was upon these observations that 
 Wilbrand based his hypothesis as to the escape of the macula 
 in cases of bilateral hemianopsia. 
 
 He attributed it to the bifurcation of fibers coming from 
 each macula at the chiasma, so that both maculae were 
 connected with the occipital cortex on each side. There 
 is, however, no evidence as to where the dichotomous fibers 
 found in embryo cats come from, and in rabbits' retinae 
 no specialized central area, such as a macula, can be dif- 
 ferentiated. 
 
 Usher and Dean 12 traced the course of degenerated nerve 
 fibers in the optic nerve and chiasma in monkeys after 
 experimental lesions in the retina at the macular region. 
 In summing up their results they say: "As regards the 
 question whether the macular fibers cross wholly or partially 
 in the chiasma, it is impossible from our experimental cases 
 to make a definite statement, owing to the difficulty that 
 exists in determining whether the wound was limited to 
 the macular fibers or, on the other hand, whether it had 
 damaged only a portion of these; while in Case 11 the decus- 
 sation was complete, in Case 10 there were uncrossed fibers, 
 even at the most posterior part of the chiasma." 
 
THE FIELD OF VISION 37 
 
 Though the anatomical evidence as to the course taken 
 by the macular fibers in the chiasma is very uncertain, it 
 seems highly probable, on physiological and clinical grounds, 
 that semi-decussation of them takes place. It would be 
 
 Fig. 7. — Diagram to show hypothetical semi-decussation of macular nerve 
 fibers at the chiasma. 
 
 difficult to understand how stereoscopic vision for detail 
 could be brought about, unless the images coming from 
 objects focussed on each macula were superimposed in the 
 visual cortex. It seems most probable that in man and 
 monkeys, not only are the visual impressions received from 
 
38 EVOLUTION OF THE HUMAN EYE 
 
 the left halves of the two retinae superimposed on the left 
 half on the occipital cortex, and those from the right halves 
 of the two retinas on the right half of the occipital cortex, 
 but that the impressions received from each macula are 
 similarly superimposed on each half of the brain (Fig. 7). 
 Clinically this view helps us not only in explaining the 
 "macula escape" in cases of bilateral hemianopsia, but 
 also affords an explanation of cases of central hemianopic 
 defects without any peripheral contraction of the fields, 
 an affection which, if right-sided, has the characteristic 
 symptom of the patient finding a difficulty in reading while 
 still retaining good central acuity of vision. 
 
 This semi-decussation of the nerve fibers from each macula 
 is also, I suggest, of interest in connection with the causation 
 of certain cases of congenital concomitant strabismus. As 
 we all know, there are cases in which children have per- 
 fect acuity of vision in each eye, with little or no refractive 
 error, who never at any time make an attempt to use the 
 two eyes together, nor can they by any means be induced 
 to do so. A very simple explanation of such cases might 
 be found in a congenital defect in the course of the macular 
 nerve fibers, whereby complete decussation, instead of semi- 
 decussation,, had taken place. With complete decussation 
 the impression received by the visual cortex from each 
 macula would be unilateral instead of bilateral, as with 
 semi-decussation. The acuteness of perception would not 
 be affected by the complete decussation, only the stereoscopic 
 effect. The anatomical evidence in proof of this hypothesis 
 would be exceedingly difficult to obtain. Clinical evidence 
 might, however, be forthcoming should a patient having 
 such a congenital concomitant strabismus subsequently 
 develop a homonymous bilateral hemianopsia, in which 
 case, if this hypothesis be true, we should expect to find the 
 
THE FIELD OF VISION 39 
 
 whole macular region on one side included in the blind 
 area. 
 
 Conjugate movements of the eyes in the vertical plane 
 are innate in the human infant, but in the horizontal plane 
 are only gradually acquired during the first six months of 
 life. 
 
 From the early development of conjugate movements in 
 the vertical plane we may infer that man's mammalian 
 forbears were exposed to dangers which attacked them from 
 overhead, in the way in which small rodents are now attacked 
 by carnivorous birds. The development of conjugate 
 movements in the lateral plane followed later, with the 
 shrinkage of the snout and the translation of the eyes 
 forward in the head. 
 
 The development of acute, central, stereoscopic vision 
 for form, together with corresponding points in the two 
 retinae, introduces a fresh factor in connection with the 
 field of vision— that is, the field of fixation. Though no 
 mammals below monkeys have a capacity for distinguishing 
 detail, such as is associated with the presence of a fovea 
 centralis, there is evidence to show that in some, especially 
 the Carnivora, there is an area centralis in the retina more 
 acutely sensitive to form than the periphery. Such animals 
 can fix an object and make movements of the eyes to main- 
 tain fixation, e. g., when a cat follows the movements of a 
 mouse. The field of fixation becomes, however, of greater 
 importance with the increased acuity of central stereoscopic 
 vision, such as has been developed in association with arboreal 
 life. Still more is it important to man, who has assumed a 
 fully erect posture and become exposed to the dangers of 
 terrestrial life by his descent from the trees. The rapid 
 movement of the highly-developed spots with acute form- 
 sense, from one object to another in his circumference, 
 
40 EVOLUTION OF THE HUMAN EYE 
 
 more than compensates man for the loss of the wide degree 
 of panoramic vision which the alteration of position of the 
 eyes from the side to the front of the head entails. For 
 these rapid changes of position of the spots with acute form- 
 sense, increased mobility, first, of the eyeballs, and secondly, 
 of the head, has been evolved. 
 
 Though nearly all mammals have, as in man, four recti 
 and two oblique muscles connected with the eyeballs, the 
 movements of the globes in all those below monkeys are 
 comparatively slight. It is in association with the develop- 
 ment of acute central stereoscopic vision, as in monkeys, that 
 the movements of the eyes become a conspicuous feature. 
 The range of such movements becomes considerably extended 
 in man, and may, I suggest, be correlated with the full 
 assumption of the erect posture and the greater need for 
 a wide, quick circumferential vision on the ground than 
 in the trees. 
 
 It is probably for the purpose of increasing the range of 
 movement of the eyes that man has developed a wider pal- 
 pebral fissure, in proportion to the width of the cornea, than 
 is met with in any other mammal. The effect of this pro- 
 portional difference is that a large part of the ocular con- 
 junctiva, with the sclerotic beneath, becomes exposed to 
 view. In no other mammal are "the whites of the eyes" 
 such a conspicuous feature as in man. An exposure of the 
 ocular conjunctiva in this way has rendered it liable to irri- 
 tation from atmospheric influences, more especially on the 
 inner side of the globe, where, due to the presence of the 
 lacrimal bay, the largest area is uncovered, and where, in 
 mammals other than Primates, a special protection is 
 afforded by the membrana nictitans. Irritation of this 
 sort from atmospheric influences, when excessive, gives 
 rise to the diseased conditions known as " Pingueculae " and 
 
THE FIELD OF VISION 41 
 
 "pterygium"— affections which are not met with in other 
 mammals than man. 
 
 Wood Jones 1 has described how the changed relations 
 between the long axis of the skull and face and the long 
 axis of the vertebral column, which is produced in monkeys 
 as the result of arboreal life from the partial assumption of 
 the upright posture, has resulted in a changed position of 
 the occipital condyles on the base of the skull, which hinge 
 it to the vertebral column. In quadrupeds these condyles 
 are situated at the posterior extremity of the skull. In 
 most monkeys they are situated well forward on the base; 
 in anthropoids still farther forward; in man, who has assumed 
 the permanently erect posture, the head is practically 
 balanced upon the first cervical vertebra. 
 
 As the head becomes more nearly balanced on the verte- 
 bral column, so the powerful ligamentous and muscular, 
 attachments which pass between its posterior surface and 
 the cervical spinous processes become reduced in strength, 
 and the spinous processes themselves become reduced in 
 size. The even balancing of the head on the spine, and the 
 reduction in size of the cervical spinous processes, both 
 tend to facilitate the range and rapidity of the lateral and 
 rotatory movements of the head. Such increased move- 
 ments of the head, like the increased movements of the 
 eyes, serve to increase the area of the fields of fixation. 
 
CHAPTER II. 
 LIGHT-SENSE. 
 
 The most primitive visual function is the perception of 
 light, and the eyes of some animals are so constructed that 
 this would seem to be the only purpose for which they 
 exist. 
 
 According to Prof. Elliot Smith, 1 in the forerunners of 
 the Mammalia, and in the primitive mammals themselves, 
 the senses of sight and hearing were but poorly developed 
 compared with that of smell. "This was due," he says, 
 "not only to the fact that the sense of smell had already 
 installed its instruments in, and taken possession of, the 
 cerebral hemisphere, long before the advent in this dominant 
 part of the brain of any adequate representation of the other 
 senses, but also, and chiefly, because to a small land-grubbing 
 animal the guidance of smell impressions, whether in search 
 for food or as a means of recognition of friends or enemies, 
 was much more serviceable than all other senses. Thus 
 the small creature's mental life was lived essentially in the 
 atmosphere of odors, and every object in the outside world 
 was judged primarily and predominantly by its smell, the 
 sense of touch, vision and hearing being merely auxiliary 
 to the compelling influence of smell." 
 
 The smallest and least developed eyes in existing mammals 
 are found in moles, hedgehogs, insectivorous bats and 
 cave-rats. All of them are nocturnal in their habits and 
 have only rods in their retinae— no cones. The visual 
 
LIGHT-SENSE 43 
 
 function in these animals may be attributed entirely to the 
 chemical reaction produced by light on the visual purple, 
 which, through stimulation of the rods, starts a nervous 
 impulse. These animals have a dread of light, or photo- 
 phobia; when it appears the moles and hedgehogs at once 
 return to their burrows, the rats retreat to the inner recesses 
 of their caves, and the bats into the darkness of the foliage. 
 
 The photo-chemical reaction of the visual purple, besides 
 providing for perception of light, could easily be amplified 
 to provide for adaptation for different degrees of light, by 
 the extent of the chemical reaction induced varying in 
 accordance with the intensity of the illumination. 
 
 Trendelenburg 13 showed that there is a very close corre- 
 spondence between the curve of scotopic luminosity in the 
 human eye and that obtained from the bleaching of the 
 visual purple of a frog 's eye. 
 
 The time which it takes for the visual purple to reform 
 in the rabbit's retina, after it has been bleached, coincides 
 very closely with that which it takes the human eye to regain 
 the approximately full power of dark adaptation after good 
 light adaptation. 
 
 A bleached, undamaged, detached retina will remain 
 bleached, unless brought back into contact with the pig- 
 ment epithelium, when it will regain its violet color. From 
 this, and from their anatomical arrangement, we may 
 assume that the pigment epithelial cells are concerned in 
 the manufacture of the visual purple. They are arranged 
 like the cells in other secreting structures on a basement 
 membrane, immediately beneath which is a rich vascular 
 plexus, the capillary layer of the choroid. The fanciful 
 suggestion made by Edridge Green, 14 that the visual purple 
 is secreted by the rods, has nothing to support it, either 
 anatomically or morphologically. The rods, from their 
 
44 EVOLUTION OF THE HUMAN EYE 
 
 structure and connections, are obviously not secreting cells, 
 but nerve end-organs. 
 
 Numerous careful and accurate observers have recorded 
 how the visual purple is only to be found in the outer limbs 
 of the rods and not in the cones. I have myself been able 
 to confirm its absence from the cones in the retinae of frogs 
 and birds. 
 
 If, then, the rods react to the stimulus of light through 
 the photo-chemical reaction of the visual purple, some other 
 explanation has to be found for the reaction of the cones. 
 A perception of light dependent entirely on changes in the 
 visual purple would become abolished when it was bleached 
 in bright light, and an animal with only rods in its retina 
 would be blind under such circumstances. 
 
 The theory suggested independently by Max Schultze, 15 
 Parinaud and v. Kries, 16 which assigns different functions 
 to the rods and cones, the so-called duplicity theory of 
 vision, helps to explain so many of the visual phenomena 
 met with in the human eye, both in health and disease, that 
 it may well be accepted as a working hypothesis for the 
 explanation of vision in other Vertebrata. 
 
 According to this theory the rods come into action in low 
 degrees of illumination. They can only distinguish different 
 degrees of light, not form or colors. The cones, on the other 
 hand, come into action in high degrees of illumination, are 
 capable of distinguishing the spectral colors, and of per- 
 ceiving clearly the details of form. As Parsons 17 very aptly 
 says: "Broadly speaking, vision with the dark-adapted eye, 
 i. e., scotopic vision, is monochromatic or tone-free. Vision 
 with the light-adapted eye, i. e., photopic vision, is poly- 
 chromatic or toned. In the former the threshold stimulus 
 intensity is low; in the latter relatively high." 
 
 Prisoners who have been confined in dark dungeons for a 
 
LIGHT-SENSE 45 
 
 long time, the congenitally total color blind, the mammals, 
 already referred to, who have only rods in their retinae, and 
 nocturnal birds, all have a dread of light— that is, photo- 
 phobia. They are all temporarily or permanently dependent 
 for vision on perceptions received through their rods and 
 the visual purple. 
 
 Those who have been exposed for a long time to glare, 
 like sailors becalmed on a tropical sea, those who have the 
 congenital affection which we call "night-blindness," or the 
 acquired affection which is spoken of as "retinitis pig- 
 mentosa," and diurnal birds, all have a dread of darkness; 
 that is a condition which I suggest may be conveniently 
 termed "scotophobia." They are all temporarily or per- 
 manently dependent for vision on perceptions received 
 through their cones. 
 
 No other photo-chemical substance has been discovered 
 in the retina besides the visual purple. It becomes, there- 
 fore, natural to inquire whether some reaction other than 
 chemical is not produced in the cones capable of originating 
 a nerve impulse on stimulation by light. 
 
 Stort and Englemann 18 have described how a shortening 
 of the inner segment of the cones takes place by the action 
 of light and an elongation in darkness. On exposure to 
 light, therefore, the outer segments of the cones are drawn 
 toward the outer limiting membrane and away from the 
 pigment-cells, while in darkness the opposite movement 
 takes place. This effect has been observed in the retina 
 of all animals hitherto examined, viz., fish, amphibians, 
 reptiles, birds and mammals, including man. The part 
 which moves actively is that which in its optical and chemical 
 properties is like protoplasm, viz., the part of the cone between 
 the ellipsoid and the cone nucleus, and to this they give the 
 name of "cone-myoid." This movement in the cones may 
 
46 EVOLUTION OF THE HUMAN EYE 
 
 be compared with the phototropic reactions met with in 
 plant life, in hydra and in some small worms. 
 
 It would be a simple way of explaining the difference in 
 the mode of stimulation of the rods and cones by light to 
 regard that of the former as a photo-chemical reaction, and 
 that of the latter as a phototropic reaction. Such an expla- 
 nation would serve not only to account for the differences 
 observed between rod and cone vision as regards light-sense, 
 but also as regards form-sense, as I shall attempt to show in 
 the consideration of that part of my subject. It" is true that 
 the contraction which has been observed in the cones resulted 
 from a strong stimulus of some duration, but so far it is the 
 only change of which we have any evidence as occurring 
 in the cones apart from the rods; it is well, therefore, to 
 see how far such a change will help in explaining visual 
 phenomena. 
 
 The degree of illumination required to excite a photo- 
 tropic reaction would be greater than that required to 
 produce a photo-chemical reaction, so that we should expect 
 rod vision, if it acted by the latter, to come into action with 
 lower degrees of illumination than cone vision, if it acted 
 by the former. Rod vision would fail in high degrees of 
 illumination, due to complete bleaching of the visual purple; 
 while cone vision, being capable of some degree of adaptation 
 due to a varying degree of contraction of the cone-myoid 
 in response to the amount of stimulating light, might con- 
 tinue active after the retina has been completely bleached. 
 
 As already mentioned, the mammals with the smallest 
 eyes, who only expose themselves to very low degrees of 
 illumination, have only rods in their retina?. Those mammals 
 who, though mainly nocturnal in their habits, enter rather 
 more boldly into the light of day, such as rats, mice and 
 rabbits, are found to have a few cones. Indeed, the amount 
 
LIGHT-SENSE 
 
 47 
 
 of daylight an animal exposes itself to and the number of 
 cones met with in its retina correspond fairly accurately. 
 The Simia?, who, to obtain their food in their arboreal resorts 
 (consisting, as it often does, of minute objects), require a 
 high degree of illumination, have an area of the retina in 
 which only cones are present, and in which they are closely 
 congregated together. It would seem probable that it is 
 due to this congregation of the cones together at the macula 
 that man and monkeys can stand exposure to brighter day- 
 light than any other mammal. Man 's eyes cannot, however, 
 stand exposure without damage to the same degree of 
 
 Fig. S. — Headlight fish. iSthoproru lucida. 
 
 direct sunlight as some birds; eagles, for example, as they 
 rise to fly will gaze directly at the sun without flinching. 
 In the diurnal rapacious birds the retina has been found to be 
 richer in cones than any other birds, or than any mammals. 
 Various means, supplementary to those in the retina itself, 
 have been acquired by mammals to fit them for an environ- 
 ment having low degrees of luminosity. Some deep-sea 
 fishes and cave-animals, who live in absolute darkness, have 
 no visual sense and no visual organs, their survival depend- 
 ing on the acuity of their other senses. Other deep-sea 
 fishes have evolved their own illuminating appliances, and 
 
48 EVOLUTION OF THE HUMAN EYE 
 
 carry in front of them an apparatus capable of emitting a 
 phosphorescent light (Fig. 8). In the eyes of pelagic fishes 
 reflecting membranes are met with; on the surface of the 
 iris is the membrana argentina, which gives it great brilliancy, 
 and acts by reflecting light on objects looked at in dark 
 places, much in the same way as that in which we employ 
 our ophthalmoscopic mirrors. On the inner surface of the 
 choroid they have another reflecting membrane, the tapetum 
 lucidum, which intensifies the power of seeing in dim lights 
 by reflecting rays back on the retina. The same device is 
 met with in several classes of mammals; in the carnivorous 
 Cetacese the reflecting membrane, as in fishes, covers the 
 whole fundus; in the terrestrial mammals, where a tapetum 
 . is found, it covers only a sector of the fundus external to 
 the optic disk. In the terrestrial jEarnivora the tapetum is 
 most brilliant and is composed of cells arranged in elongated 
 plates together with hexagonal crystals. In the Ungulata 
 the tapetum has an entirely different structure, being 
 composed of layers of fibrous tissue and not cells. Among 
 Primates a tapetum is met with in some lemurs with nocturnal 
 habits, and is similar in construction to that of the Carnivora. 
 Man and monkeys have no supplementary arrangement 
 for seeing in dim lights such as that afforded by a tapetum 
 lucidum. Monkeys, having resorted to arboreal life, where 
 they found their food by day, and where they were safe from 
 the attacks of carnivorous foes by night, have not had to 
 acquire nocturnal habits. Man, having inherited his visual 
 capacities from his arboreal ancestors, found himself, on his 
 descent from the trees, more than equal to his mammalian 
 foes by day, but inferior, and sadly handicapped, by night. 
 Not only had he not acquired the advantages of a tapetum, 
 butfrom prolonged life in the branches and reduction of 
 thesnout he had lost much of the protection afforded by 
 acuteness of smell. 
 
LIGHT-SENSE 49 
 
 All children instinctively dread darkness and many per- 
 sons can never free themselves from such dread throughout 
 their lives. O. G. S. Crawford, writing in a recent number 
 of the Cornhill Magazine on "Prehistoric Instincts," says: 
 
 "Primitive man is an animal that lives and works by day. 
 His habits are not nocturnal, and he seldom by choice goes 
 forth from his lair during the hours of darkness. This 
 love of daylight, and the corresponding aversion to dark- 
 ness, is probably due to the great reliance he places upon 
 sight; but, of course, the order of causation may have been 
 reversed. We do not, however, find it in dogs, which rely 
 on ear and nose. However this may be, there can be no 
 doubt about the existence of an instinctive dislike of dark- 
 ness in all of us today. This dislike is the pale survivor of 
 a very real and acute instinct of fear innate in us, which 
 dominates our childhood with all the vigor of its original 
 force." 
 
 Primitive man, a mighty hunter by day, through the 
 absence of a tapetum became a coward by night. A some- 
 what parallel instance may be referred to occurring among 
 birds. They have no tapetum lucidum in their choroids, 
 but in the retinse of nocturnal birds rods are found to pre- 
 dominate very largely over the cones, while in the retinae of 
 diurnal birds the reverse condition is met with, the cones 
 in them being far more numerous than the rods. The small 
 diurnal birds, sparrows, chaffinches, redbreasts and such 
 like, as soon as dusk begins to approach, seek concealment 
 in bushes and hedges, where they will be safe from the dreaded 
 attacks of the night owl. Should the owl prolong his hunt- 
 ing expeditions to such a distance that daylight overtakes 
 him before he can return to his usual retreat, he becomes 
 dazzled, is unable to proceed, and has to remain where he 
 is until darkness again ensues. If during the daylight he 
 4 
 
50 EVOLUTION OF THE HUMAN EYE 
 
 becomes discovered by the small diurnal birds upon which 
 he usually preys, they at once recognize his plight and give 
 him no mercy, mobbing him and pecking him with impunity. 
 Thus through the specialized character of the end-organs 
 of the retina the mighty hunter of the night becomes the 
 abject victim of the day. 
 
 Primitive man, having abandoned the safety of the trees, 
 sought protection from the perils that prowl by night in 
 the natural recesses of the cliffs, and became a cave-living 
 animal. Gradually he progressed to the hollowing out of 
 caves for himself, and thus acquired the first step which 
 led to the building habit. From this may we not trace the 
 construction of lake dwellings, which by their situation 
 afforded admirable protection from carnivorous foes, and 
 later the more substantial houses of brick and stone, ulti- 
 mately leading to all the glories of architecture? 
 
 It is probable also that fire, when first discovered by 
 primitive man through hacking iron pyrites with a flint 
 among dead leaves (Lubbock), was more valued for its 
 illuminating than its heating properties. By the illumina- 
 tion which it afforded him at night his range of vision would 
 become extended, and his struggle for existence against 
 carnivorous mammals considerably assisted; From this 
 first use of fire as an illuminant we can trace man's gradually 
 increasing means of overcoming the dangers of darkness, 
 until we arrive at the powerful searchlight of today which 
 he employs both for purposes of attack and defence. 
 
CHAPTER III. 
 FORM-SENSE. 
 
 Acuity of form-sense reaches its highest development 
 among mammals in the Primates. Mammals as a class are 
 characterized by the absence of a fovea, the Primates being 
 the only ones in which it is to be found. In many mammals 
 which do not possess a fovea there is an area centralis which 
 approaches in structure the yellow spot; this can be recog- 
 nized ophthalmoscopically, when the retinal bloodvessels 
 are present, by the absence of them in that region and by 
 their distribution around it. In sections this area centralis 
 is distinguishable by an increased thickness of the retina, 
 and microscopically by the arrangement of the cells. Tne 
 increased thickness is due to the greater length of the rods 
 and cones, their close congregation together, and the larger 
 number of cells in the nuclear and ganglion cell layers associ- 
 ated with them. 
 
 Chievitz 19 and Slonaker 20 have examined the eyes of a 
 number of different species of mammals anatomically, and 
 the latter has published their combined findings in tabular 
 form. In some animals, such as rats and bats, no central 
 area is discoverable. In rabbits, most of the Ungulates 
 and the fox, there is a horizontal bandlike area, which in 
 the horse measures 5 to 7 mm. in breadth. Cats and some 
 other Carnivora have a rounded central area. Krause 21 
 stated that cats possessed a fovea, but this was not found 
 to be the case by Ganser 22 and Chievitz. 111 Lindsay Johnson 3 
 
52 EVOLUTION OF THE HUMAN EYE 
 
 sums up the correlation between the area centralis, the 
 position of the eyes in the head and the shape of the cornea 
 and lens as follows : " Sensitive areas of restricted dimensions, 
 omitting those cases in which the area is limited to a macula, 
 exist in the Carnivora, in which order the divergence is not 
 great. In the Ungulates, Rodents, Edentates and Mar- 
 supials, where we find great divergence of the axes, large 
 corneas and nearly spherical lenses, the sensitive areas are 
 larger, and probably the degree of difference in perception 
 over such areas compared with the more peripheral parts 
 is but little." 
 
 Among Primates a fovea is met with in man, anthropoid 
 apes and monkeys, but not in lemurs. In Tarsius, which 
 it is generally agreed is more nearly allied to Primates than 
 it is to lemurs, Elliot Smith says, "There is no macula 
 lutea." It is found in some reptiles, and is present in 
 practically all birds. In writing of these latter, Casey 
 Wood 23 says: "The depth of the fovea may be regarded as 
 a •measure of the sharpness of vision. Slonaker classifies 
 fovea? as deep, medium and shallow. The round fovea is 
 especially 'deep' in swift fliers and birds of prey; 'medium' 
 to 'weak' in most birds, except that it is 'shallow' in the 
 domestic pigeon and probably lacking in the hen." 
 
 Anatomically the fovea consists in man of an area measur- 
 ing about 0.3 mm. in diameter, subtending an angle of about 
 70', and lying almost exactly in the optic axis of the re- 
 fractive media of the eye, at a point where the image pro- 
 jected by them is brought to the sharpest focus. It is 
 characterized by an unfolding of the inner layers of the retina, 
 which are spread apart so as to permit the direct unimpeded 
 access of rays of light to the percipient end-organs of the 
 retina. These end-organs consist exclusively of cones, and 
 in the very center of the fovea (Dimmers' foveola) of cones 
 
FORM-SENSE 53 
 
 of a specially slender variety. Ramon y Cajal showed 
 that each of the central cones is connected with but one 
 bipolar cell and but one ganglion cell, thus differing from 
 the end-organs in the more peripheral parts of the retina, 
 each of which is connected with several bipolar and several 
 ganglion cells. 
 
 The acuity of form-sense in man is most intense at the 
 fovea and rapidly decreases on proceeding from it toward 
 the periphery; thus it has been approximately estimated 
 that 5° eccentric from the center of the fovea the acuity is 
 reduced to \ and 20° eccentric to ^. 
 
 In the development of the human retina an area centralis 
 is recognizable before the fovea centralis makes its appear- 
 ance. According to von Hippel, 24 in the central area of the 
 retina at birth all the layers are recognizable, though in the 
 situation where the fovea ultimately forms, the cells of the 
 ganglion cell-layer and. of the inner nuclear layer are some- 
 what thin and spaced out. By the end of the fourth week 
 a depression is formed, but the full development of the fovea 
 is not completed until several months have elapsed. This 
 gradual unfolding of the fovea corresponds with what has 
 been observed regarding the development of an infant's 
 power of fixation. Worth 25 has observed that the pupil 
 contracts in a newborn infant if a light is suddenly flashed 
 on to its eye in the dark, and that the child will try to fix 
 it for an instant, as by a reflex act, showing that some pre- 
 ponderance in power at the macula is innate. He says: 
 "At the end of two or four weeks most infants will fix a 
 light steadily for several seconds at a time with one or other 
 eye, but will not converge both visual axes accurately in 
 looking at a near object. It is not until five or six weeks 
 that binocular fixation manifests itself." 
 
54 EVOLUTION OF THE HUMAN EYE 
 
 The high acuity of form-sense at the fovea may be due 
 to: 
 
 (a) The close congregation of cones together in that 
 region. 
 
 (b) The isolation of each cone in that region to one nerve 
 path. 
 
 (c) The opening out of the inner layers of the retina, 
 allowing the unimpeded access of rays of light to the cones. 
 
 Probably all three factors are essential for clearness of 
 definition in vision. 
 
 If, as suggested in the consideration of. the light-sense, 
 the response of the cones to light is a mechanical one, a 
 contraction produced in one of them being capable of start- 
 ing a stimulus in the isolated nerve path with which it is 
 connected, we have a very simple means of explaining the 
 first stage in the production of the perception of form. 
 
 At the center of the fovea in the human retina we have 
 an area in which 7000 cones are supposed to be crowded 
 together; 50 cones may be counted along a line of 200 n 
 in length; the average diameter of each cone at its widest 
 part is about 3 n, and the distance between two adjoining 
 cones 4 /*. Supposing now the image of a black letter on 
 a white ground, such as we have in our test-types, to be 
 focussed on this delicate mosaic of cones, those upon which 
 the white background is projected will be stimulated and 
 contract, those upon which the black letter fell, not being 
 stimulated, will remain uncontracted. Indeed, it seems 
 likely that if we could examine the outer surface of a retina 
 so stimulated, under sufficient magnification, a reproduction 
 of the letter looked at would be seen raised up and composed 
 of the unretracted outer limbs of the cones. 
 
 An abnormally low acuity of central form-sense dating 
 from birth, and unaccompanied by any obvious ophthal- 
 
FORM-SENSE 55 
 
 moscopic changes, might be due to an absence of any one 
 of the three above-mentioned characteristic features at the 
 macula, so that the condition we describe as "congenital 
 amblyopia" might be brought about in different ways; we 
 have, however, definite anatomical evidence that in con- 
 nection with it the fovea is sometimes found to be absent. 
 
 Seefelder 26 recorded the absence of the fovea in a case 
 of congenital amblyopia and nystagmus associated with 
 aniridia. Its absence has also been noted in congenital 
 amblyopia accompanying albinism. Further investigations 
 as to the structure of the retina at the macula in connection 
 with congenital amblyopia is, I suggest, much to be desired. 
 
 Monkeys, the class of mammals which first developed a 
 fovea, like birds, in whom a fovea is almost always present, 
 live an arboreal life. They, like birds, subsist on a diet of 
 fruit and insects, which has to be picked up, picked off or 
 picked out. This picking process requires considerable 
 concentration and sharpness of vision. In the case of birds 
 it is done with the bill, and in the case of monkeys with the 
 fingers. 
 
 Other mammals who have adopted the arboreal habit, 
 such as lemurs, some of the Insectivora and rodents, have 
 not developed a fovea, the reason probably being that they 
 make more use of their teeth and comparatively elongated 
 snouts for the picking up and preparation of their food 
 than the higher Primates, and therefore require less acute- 
 ness of vision for detail. 
 
 The adoption by monkeys of feeding habits, intimately 
 associated with the use of the hands and the formation of 
 a central spot of acute form-sense, endowed them with the 
 capacity of picking up objects and examining them closely. 
 Such a capacity would tend to excite feelings of curiosity, 
 the marked development of which is one of the characteristics 
 
56 EVOLUTION OF THE HUMAN EYE 
 
 of all apes, and from this would arise a desire for investi- 
 gation. It must have been the overpowering influence of 
 such a desire which induced man's ancestors to descend 
 from the safety of the branches to explore the earth beneath, 
 and thereby encounter new and unknown dangers. The 
 appetite for exploration and research, thus started, grew 
 by what it fed on, and continues to grow even up to the 
 present time. 
 
CHARTER IV. 
 ACCOMMODATION AND CONVERGENCE. 
 
 Intimately associated with the evolution of the fovea 
 centralis, and therefore with the adoption of arboreal life, is 
 an increased capacity for accommodation and convergence. 
 
 The accommodative power of the eyes of mammals has 
 been investigated by several different observers. All are 
 agreed that its range is greatest in man, slightly less in apes, 
 and very considerably less in all the lower orders. 
 
 Hess and Heine 27 found its range equal to 10 to 12 D. in 
 apes; 2.5 to 3.5 D. in dogs; 1.25 D. in cats; 2.5 to 3 D. in a 
 young wolf; and in rabbits. 
 
 Barrett 28 was unable to detect any alteration of refraction, 
 during examination by retinoscopy or With the ophthal- 
 moscope, in any mammal except man and monkeys. Those 
 which he examined were rabbits, guinea-pigs, rats, mice, 
 cows, horses and dogs among domestic animals; and deer, 
 jackal, pecari, various cats, hyena, opossum, porcupine, 
 mongoose and thirteen monkeys among wild animals. He 
 found that electrical stimulation of the eyes of cats and dogs 
 produced no result, and that the voluntary accommodation 
 of monkeys was equal to 4 to 5 D. 
 
 Beer 29 estimated the range of accommodation in Macacus 
 rhesus as equal to 10 D., and that in the anthropoid and 
 some other apes it was still greater, approaching even in 
 the untailed apes to the degree present in man. By electrical 
 stimulation of the freshly enucleated eye of the common cat 
 
58 EVOLUTION OF THE HUMAN EYE 
 
 he observed an increased projection of the anterior surface 
 of the lens, but the increased refraction amounted only to 
 2 or 3 P. In the solipedes and ruminants he estimated the 
 range as from § to 2 D. In the hare and rabbit he totally 
 failed to discover any change, either after administration 
 of miotics or after electric stimulation. 
 
 .-'■' J. * r /.- 
 
 Fig. 9. — Ciliary body of the echidna. 
 
 Priestley Smith, 30 by examining rabbits' eyes with the 
 shadow test and the faradic current, convinced himself 
 that there was some accommodative change, though in 
 several it was slight and very transient. 
 
 In the horse and dog and probably in most wild mammals 
 the refraction is hypermetropic, so that, their accommo- 
 dation being feeble, their near point cannot be nearer than 
 one or two meters away. Barrett thoroughly atropinized 
 
ACCOMMODATION AND CONVERGENCE 
 
 59 
 
 the eyes of a cat and then allowed it to pursue a mouse, 
 which it caught without any difficulty. 
 
 I have examined microscopically sections of the ciliary 
 body from the eyes of mammals belonging to various dif- 
 ferent natural orders, and find that the degree of develop- 
 ment of the ciliary muscle corresponds fairly accurately 
 with the degree of capacity for accommodation which'' the 
 animal has been found to possess. 
 
 Fig. 10. — Ciliary body of the groat ant-cater. 
 
 In some of the lowest mammals, such as the echidna 
 (Fig. 9) and the great ant-eater (Fig. 10), the ciliary muscle 
 is practically non-existent. In rabbits (Fig. 11) only a 
 few scanty fibers are recognizable. In the marsupials 
 (Fig. 12) there is a small definite bundle of longitudinally 
 placed fibers. In the L'ngulata (Fig. 13) and Carnivora 
 (Fig. 14) the longitudinally directed fibers are more numer- 
 
60 
 
 EVOLUTION OF THE HUMAN EYE 
 
 ous and form a considerably broader band. The ciliary 
 muscle in apes (bigs. 15 and 16) and man shows an extensive 
 
 ^jglSSSSSS 
 
 Fig. 11. — Ciliary body of the rabbit. 
 
 • ; i -- 
 
 Fig. 12. — Ciliary body of the wallaby. 
 
 increase in development on that met with in any of the 
 other mammals. Not only is it wider and longer, but its 
 
ACCOMMODATION AND CONVERGENCE 
 
 Gl 
 
 fibers show a more complex arrangement. In all the other 
 mammals which I have examined it is composed entirely 
 of longitudinal fibers; in apes and man it possesses also what 
 have been described as radial and circular fibers (Fig. 17). 
 
 Fig. 13. — Ciliary body of the ox. 
 
 Fig. 14. — Ciliary body of the cat. 
 
 At the International Ophthalmological Congress held 
 at Utrecht in 1S99, I demonstrated how the arrangement 
 of the ligamentum pectinatum also differs very considerably 
 
62 
 
 EVOLUTION OF THE HUMAN EYE 
 
 in man and apes from that met with in other mammals. 
 In the human eye the fibers of which it is composed radiate 
 out in a fan-like fashion from the position where Descemet 's 
 membrane ends, and the angle of the anterior chamber is 
 prolonged outward 0.8 mm. beyond that point. So that 
 if a line vertical to the surface of the eye be drawn back- 
 
 FiG. 15. — Ciliary body of the Rhesus monkey. 
 
 ward through the canal of Schlemm it passes into the angle 
 of the anterior chamber (Fig. 17). If the same line be 
 continued backward it almost passes through the large 
 circular artery at the base of the iris, and only a small portion 
 of the ciliary processes lie to its inner side. In the lower 
 mammals the ligamentum pectinatum is a much more 
 
ACCOMMODATION AND CONVERGENCE 
 
 63 
 
 extensive structure, filling up a large part of what forms 
 the angle of the anterior chamber in the human eye It 
 is composed of an external laminated zone, attached on its 
 
 Fig. 16. — Ciliary body of the chimpanzee. 
 
 outer surface to the sclerotic, which is wider and more spaced 
 out than in the human eye, and an inner cavernous zone 
 
 Fig. 17. — Ciliary body of the human eye. 
 
 with irregularly shaped spaces between its fibers (Fig. 13). 
 The inner limit of the ligament, that facing the anterior 
 chamber, in these animals, is bounded in places by what 
 
64 EVOLUTION OF THE HUMAN EYE 
 
 are. called the pillars of the iris. These consist of prolon- 
 gations forward of the stroma of the iris to the back of the 
 cornea, to which they are adherent just where Descemet's 
 membrane terminates (Figs. 11 and 12). It is these bands 
 of iris tissue, which are not present in man's eye, that give 
 the toothed appearance to the ligament, and which led to 
 its being compared to a comb. It is the disappearance 
 of these pillars of the iris in man's eye which has allowed 
 the anterior part of the ciliary body to become displaced 
 backward. As the angle of the anterior chamber in the 
 lower mammals does not extend beyond the point where 
 Descemet's membrane ends, a line drawn backward ver- 
 tically to the surface of the eyeball through the canal of 
 Schlemm does not pass through the angle of the anterior 
 chamber but some distance external to it, and the large 
 circular artery of the iris, together with a large portion of 
 the ciliary processes, lies to its inner side (Figs. 13 and 14). 
 
 The concentration of the fibers of the ligamentum pectin- 
 atum together in the human eye forms a fixed point, or ten- 
 don of origin, for the ciliary muscle. In the lower mammals, 
 where the area occupied by the ligamentum pectinatum is 
 broader and more spaced out, the fibers of the ciliary muscle 
 do not have any fixed point of origin. Running longi- 
 tudinally they terminate anteriorly in the spaces between 
 the fibers of the ligamentum pectinatum, in the same way 
 as they terminate posteriorly between the fibers of the 
 lamina suprachoroidea (Fig. 14). 
 
 A comparison of the ciliary muscle in its different degrees 
 of development in different mammals shows that the spindle- 
 shaped cells of which it is composed lie between the fibers 
 of the lamina suprachoroidea. 
 
 In the human eye the fibers of the lamina, if traced for- 
 ward, can be seen to enter the posterior border of the ciliary ' 
 
ACCOMMODATION AND CONVERGENCE 65 
 
 muscle, so that the most anterior portion of the perichoroidal 
 space is entirely free from any such fibers. If the uvea be 
 separated from the sclerotic some of the anterior and external 
 of the cells of the ciliary muscle may be found attached 
 to the latter coat, being situated between the layers of 
 what elsewhere forms the lamina fusca. 
 
 In some animals, as pointed out by Kalt, at the termi- 
 nation of the ciliary body posteriorly, in the spaces of the 
 lamina suprachoroidea left empty by undeveloped muscle 
 fibers, a layer of large cells is met with continuous behind 
 with the endothelial cells of the choroid. 
 
 In pathological specimens, where, as the result of edema 
 of the ciliary body, or as the result of traction from fibrous 
 bands on its inner surface, its tissue has been much spaced 
 out, I have met with large cells between the muscle fibers, 
 suggesting in their appearance transitional stages between 
 endothelial cells and muscle cells. 
 
 Muscle fibers, or at any rate cells which cannot be dis- 
 tinguished from them, are met with in isolated patches of 
 the external surface of the choroid; thus Salzmann 31 writes: 
 "If one follows the suprachoroidea from behind forward 
 new structural elements, smooth muscle fibers, appear even 
 in the equatorial region, or, at times, still farther posterior, 
 therefore in the territory of the choroidea. They are grouped 
 in bundles singly, or for the most part branched, and then 
 forming three or more rayed stellate little figures— 'muscle 
 stars. ' " 
 
 Fuchs, 32 in a recent article, has described how he found 
 muscle fibers in the lamina suprachoroidea in the neighbor- 
 hood of the papilla in frontal sections through the optic 
 disk. 
 
 The above facts, taken together, all tend to show that 
 the spindle-shaped cells composing the ciliary muscle are 
 5 
 
66 EVOLUTION OF THE HUMAN EYE 
 
 morphologically the same as the endothelial cells found in 
 the spaces of the lamina suprachoroidea and in the pectinate 
 ligament, and that the development of the ciliary muscle 
 is due to the proliferation and lengthening out of such 
 endothelial cells. 
 
 Anatomists have frequently described Descemet's mem- 
 brane as the uveal layer of the cornea, and I think such 
 a description is both accurate and useful. Descemet's 
 membrane is composed of the same elastic tissue as that 
 of the pectinate ligament, both staining in a similar way 
 with acid orcein. Elastic fibers are also found in the lamina 
 suprachoroidea. The elastic tissue of Descemet's mem- 
 brane has been proved to be produced as a kind of secretion 
 of the endothelial cells lining it. In the human fetus, before 
 the iris is formed, and before there is any anterior chamber, 
 two parallel rows of cells are seen lying interposed between 
 the cornea and the anterior capsule of the lens. The anterior 
 chamber is developed as a space between these two rows of 
 cells, the anterior of which becomes the lining endothelium 
 of Descemet's membrane, and the posterior the endothelium 
 on the surface of the iris and of the pupillary membrane. 
 It may therefore be claimed that not only does Descemet's 
 membrane represent the uveal layer in the cornea, but 
 that the anterior chamber is analogous to an opened-out 
 and enlarged potential space of the lamina suprachoroidea. 
 
 These morphological analogies of the fibers of the ciliary 
 muscle and its surroundings are of considerable assistance 
 in understanding the distribution of its fibers in the human 
 eye. Having recognized that the fibers of which the muscle 
 is composed are developed from cells contained in spaces on 
 the outer surface of the ciliary body, it becomes evident 
 that the shape and direction of these muscle fibers will 
 depend on that of the spaces in which they are formed. 
 
ACCOMMODATION AND CONVERGENCE 67 
 
 In all mammals in which the ciliary muscle is rudimentary 
 its fibers run longitudinally (Figs. 11, 12, 13 and 14). It 
 is only when we come to the more highly developed ciliary 
 muscle of Primates that we meet with the addition of radial 
 and circular fibers internal to the longitudinal ones (Figs. 
 15, 16 and 17). It is in these animals also that we meet 
 with a considerable displacement backward of the anterior 
 part of the ciliary body, due to disappearance of the pillars 
 of the iris, and alteration in the arrangement of the fibers 
 of the pectinatum already referred to. The ciliary processes 
 in Primates, instead of projecting out from the back of the 
 iris; have become displaced outward and backward beyond 
 its root, thus altering the shape and direction of the lymph 
 spaces immediately external to them. The innermost of 
 these spaces assume a circular direction, and those more 
 external a radial course; the muscle fibers developing in 
 these spaces from the cells lining them take the same 
 direction. 
 
 Donders 33 pointed out that the construction of the ciliary 
 muscle varies with the refractive condition of the eye. He 
 found that in actual myopia it was flatter and longer than 
 in emmetropia or hypermetropia, and that its point of 
 origin from the fibers of the ligamentum pectinatum . was 
 situated farther back. 
 
 Iwanoff 34 described how, in myopia, the ciliary muscle 
 consists almost entirely of longitudinal fibers, "Briicke's 
 muscle," the circular fibers, "Miiller's muscle," being 
 absent, while in hypermetropia he found that the circular 
 fibers were more numerous than in emmetropia, and formed 
 a considerable inward projection from the anterior part of 
 the muscle. This inward projection of the muscle is accom- 
 panied by an inward projection of the ciliary processes 
 behind the root of the iris. In myopia the ciliary processes 
 
68 
 
 EVOLUTION OF THE HUMAN EYE 
 
 are much less prominent, and are separated by a consider- 
 able distance from the root of the iris (Fig. 18). 
 
 No doubt, as has frequently been pointed out, the ciliary 
 muscle in hypermetropia, having more work to do, is hyper- 
 
 Fig. 18. — Varieties of the ciliary muscle. A, emmetropic; B, myopic; 
 C, hypermetropic. (From Iwanoff.) 
 
 trophied, while in myopia, having but little work to do, 
 it is atrophic. I think, however, the direction of its fibers 
 is largely influenced by the length of the antero-posterior* 
 axis of the eyeball. Where this has been abnormally in- 
 creased, as in axial myopia, the uveal tissue is considerably 
 
ACCOMMODATION AND CONVERGENCE 69 
 
 stretched, the ciliary processes are drawn backward, and 
 the spaces in the tissue immediately external to them are 
 also drawn backward and elongated out. The fibers of the 
 ciliary muscle contained in these spaces become likewise 
 altered in their direction, and assume a longitudinal position 
 antero-posteriorly instead of running circularly. 
 
 In hypermetropia, where the eyeball is abnormally short 
 from before backward and not fully developed, the ciliary 
 processes, as in the embryonic condition and as in the 
 lower animals, project farther forward and inward, so that 
 the spaces external to them, and the muscle fibers contained 
 therein, have a circular direction. 
 
 In animals below Primates in which the ciliary muscle 
 has scarcely any more fixed point of attachment anteriorly 
 than it has posteriorly, and where its fibers have all a longi- 
 tudinal direction, the effect of their contraction would appear 
 to produce a drawing together, or rucking up, of the inner 
 surface of the ciliary body, much in the same way as con- 
 traction of the dilator muscle fibers of the iris draws together 
 or rucks up the iris in dilation of the pupil (Fig. 14). In 
 apes and in man the concentration together of the fibers of 
 the pectinate ligament at the sclero-corneal margin pro- 
 duces a sort of tendon, or fixed point of origin, to the ciliary 
 muscle. So that on its contraction the tissue into which 
 it is inserted is drawn forward toward this fixed point. 
 
 A study of the morphology and embryology of the lens 
 and its suspensory ligament seems to leave no doubt that 
 the effect of contraction of the ciliary muscle is, as Helm- 
 holtz pointed out, a slacking .of the suspensory ligament 
 and a relaxation of tension in the lens fibers, 
 v The following table gives the measurements of the antero- 
 posterior and lateral diameters of the lens in the eyes of 
 several different mammals: 
 
70 EVOLUTION OF THE HUMAN EYE 
 
 DIAMETERS OF THE LENS IN MAMMALS. 
 
 
 Anteroposterior. Lat 
 
 Man 
 
 4.5 mm. 9 
 
 Chimpanzee . 
 
 . . 4 
 
 7 
 
 Monkey . 
 
 3.5 
 
 6 
 
 Civet cat 
 
 5 
 
 6.5 
 
 Virginian fox 
 
 6 
 
 8 
 
 Horse 
 
 . 11 
 
 20 
 
 Antelope . 
 
 . . 11 
 
 14 
 
 Sheep .... 
 
 10 
 
 12.5 
 
 Pig . 
 
 6 
 
 9 
 
 Rabbit 
 
 . . 7 
 
 9 
 
 Rat . . . . 
 
 4 
 
 ' » 4.5 
 
 Hedgehog 
 
 3.5 
 
 3.5 
 
 Sooty phalanger 
 
 . 7 
 
 9.5 
 
 Great ant-eater . 
 
 . 6 
 
 8 
 
 Rat kangaroo 
 
 . . 8 
 
 10 
 
 From this it will be seen that in man the anteroposterior 
 diameter of the lens measures nearly half that of the lateral 
 diameter. In monkeys it is a little more than half, while 
 in all the other mammals the proportional difference between 
 the two diameters is considerably less, and in some of them 
 the lens is almost spherical (Fig. 19). 
 
 In man and monkeys the ciliary processes are separated 
 from the sides of the lens by a definite space— the so-called 
 circumlental space. In all other mammals the ciliary pro- 
 cesses lie in contact with the sides of the lens. 
 
 It seems natural to infer that the flattening of the lens 
 from before backward in man's eye has resulted from the 
 growth of the ciliary body away from the sides of the lens, 
 and the traction which is thereby produced on its anterior 
 and posterior capsule through the suspensory ligament. 
 An examination of embryonic human eyes in different 
 stages of development confirms this view. The ciliary 
 processes, when first formed, are in contact with the sides 
 of the lens, and with the growth of the eyeball gradually 
 become separated from it. In association with the gradual 
 
ACCOMMODATION AND CONVERGENCE 71 
 
 JiavM. 
 
 Fig. 19. — Lenses in mammals drawn three times natural size. 
 
72 
 
 EVOLUTION OF THE HUMAN EYE 
 
 separation of the two structures there is alteration in the 
 shape of the lens from that of a sphere to that which it 
 assumes in the fully formed eye. 
 
 The following table gives the measurements of the antero- 
 posterior and lateral diameters of the lens in a series of 
 human fetal eyes of different ages, and those recorded by 
 Dub 35 from the ninth month to the twelfth year: 
 
 Age. 
 
 Anteroposterior. 
 
 Lateral. 
 
 Fourth month 
 
 2.8 mm. 
 
 3.3 mm 
 
 Fifth 
 
 3.5 " 
 
 4 
 
 Sixth 
 
 3.8 " 
 
 4.5 " 
 
 Seventh " 
 
 4 
 
 5 
 
 Ninth " 
 
 4.3 " 
 
 5.75 " 
 
 To these may be added those given by Dub from the 
 ninth month to the twelfth year: 
 
 Age. 
 
 Antero-posterior. 
 
 Lateral. 
 
 9 to 12 months 
 
 . 2.46 mm. 
 
 7.46 mm 
 
 1 to 2 years 
 
 . 2.57 " 
 
 7.87 
 
 I 
 
 2 to 3 " . . 
 
 . . 2.72 " 
 
 8.2 
 
 ' 
 
 3 to 4 " . . 
 
 . . 2.83 " 
 
 8.46 
 
 1 
 
 4 to 5 " 
 
 . . 3.0 " 
 
 7.8 
 
 I 
 
 5 to 6 " 
 
 3.2 " 
 
 8.4 
 
 i 
 
 7 " 
 
 2.9 " 
 
 8.2 
 
 I 
 
 12 " 
 
 3.6 " 
 
 8.8 
 
 t 
 
 Adult 
 
 4.5 " 
 
 9 
 
 I 
 
 From this it will be seen that the lens, when first formed, 
 is spherical in shape, and gradually assumes its flattened 
 condition with the growth of the eyeball. After birth the 
 traction on the sides of the lens causes a diminution in its 
 antero-posterior diameter (Fig. 20). 
 
 If further confirmation is required that flattening of the 
 lens and consequent bowing of its fibers occurs, as the result 
 of traction on its sides, it is to be found in the effects of 
 pathological enlargement of the eyeball, such as occurs in 
 cases of congenital glaucoma— so-called buphthalmos. In 
 
ACCOMMODATION AND CONVERGENCE 73 
 
 ii-.li *<•!£■ 
 
 Collin -f- 3 
 
 4* fc.ft- 
 
 j-'f-Vft 
 
 ^H.tf. 
 
 ?£»*./* 
 
 <)*W*. 
 
 i-l 
 
 U\1. 
 
 1-J 1 ^-. 
 
 3-^yv.. 
 
 li.-->y< 
 
 S- b yU. 
 
 7->**>- 
 
 /LV^ - 
 
 AcL.lt 
 
 Fig. 20. — Lenses in human eyes at different ages, drawn three times natural 
 
 size. 
 
74 EVOLUTION OF THE HUMAN EYE 
 
 this disease there is considerable expansion of all the diameters 
 of the eyeball, and considerable widening of the ciliary 
 region, so that the ciliary processess become separated 
 farther than normally from the sides of the lens, and the 
 circumlental space is increased in width. At the same time 
 the antero-posterior diameter of the lens is diminished and 
 the lateral diameter increased. In one specimen which I 
 have the antero-posterior diameter of the lens measures 
 3.5 mm., the lateral 9 mm., and in another the antero- 
 posterior diameter is 4 mm. while the lateral is 10 mm. 
 
 Investigations into the anatomy of the suspensory 
 ligament of the lens have shown that it is composed of 
 separate bundles of fibers, which run in different directions, 
 and to which specific names have been attached. The two 
 chief bundles are: The orbiculo-antero-capsular fibers, lying 
 in the valleys between the ciliary processes, which attach 
 the posterior non-plicated part of the ciliary body to the 
 anterior capsule of the lens; and the cilio-postero-capsular 
 fibers, passing between the bundles of orbiculo-antero- 
 capsular fibers, which attach the anterior part of the ciliary 
 processes to the posterior capsule of the lens. A third set 
 of fibers— the equatorial— attaches the center of the ciliary 
 body to the side of the lens capsule. 
 
 To understand how these different bundles of fibers of 
 the suspensory ligament are developed, it is necessary to 
 examine the relations which the ciliary body, or the part 
 of the eye which ultimately becomes the ciliary body, bears 
 to the side of the lens at different periods of fetal life. For 
 some time the two structures lie in contact, and it is while 
 they are in contact that adhesions form between them, 
 which ultimately develop into the fibers of the suspensory 
 ligament. At an early stage of fetal life, before the iris and 
 ciliary processes have commenced to form, the part of the 
 
ACCOMMODATION AND CONVERGENCE 
 
 75 
 
 globe where the non-plicated part of the ciliary body ulti- 
 mately develops lies in contact with the anterior capsule 
 of the lens, and forms adhesions to it (Fig. 21, A). On 
 
 Fig. 21. 
 
 -Diagram to show relation of ciliary body to the side of the lens at 
 different times in fetal life. 
 
 the growth of the globe this part of the ciliary body gradually 
 recedes, the ciliary processes budding out in front of it. 
 The direction in which the ciliary processes grow is at first 
 inward and backward, so that they come to lie in contact 
 
76 EVOLUTION OF THE HUMAN EYE 
 
 with the posterior capsule of the lens, and while there form 
 adhesions to it (Fig. 21, B). On the expansion of the 
 globe the ciliary processes become separated from the sides 
 of the lens— a separation which is still further increased 
 on the formation of the anterior chamber, which displaces 
 the lens backward (Fig. 21, C). The growth of the ciliary- 
 body away from the side of the lens causes the adhesions 
 which have formed between them to be lengthened out into 
 the fibers of the suspensory ligament. It also, by exerting 
 traction on the anterior and posterior capsule and sides 
 of the lens, causes that structure to alter its shape, so that 
 it loses its spherical form and becomes flattened antero- 
 posteriorly; the flattening is more marked on the anterior 
 surface than the posterior because the traction is greater 
 on the anterior capsule than the posterior, due to there 
 being a wider separation of the non-plicated parts of the 
 ciliary body from the anterior capsule of the lens than of 
 the ciliary processes from the posterior capsule. Flattening 
 of the lens antero-posteriorly, due to traction on its anterior 
 and posterior surfaces, increases the curve of its fibers, and 
 keeps them so curved until the traction on -the two surfaces 
 is relaxed, i. e., until the suspensory is rendered slack by the 
 contraction of the ciliary muscle. The effect of such slacken- 
 ing must necessarily be most on the anterior surface upon 
 which the traction is greatest, and most of all in the central 
 circular area on the anterior surface around which the 
 most anterior of the fibers of the suspensory ligament are 
 attached. 
 
 The farther the backward growth of the non-plicated 
 part of the ciliary body, with the orbiculo-antero-capsular 
 fibers attached to it, the greater will be the traction on the 
 anterior capsule of the lens and the increase of latent accom- 
 modative power. One of the features in the architecture 
 
ACCOMMODATION AND CONVERGENCE 77 
 
 of the eye in man and monkeys, which is strikingly distinctive 
 from that in lower mammals, is the elongation of the pars 
 plana of the ciliary body, i. e., the region situated between 
 the hindermost of the ciliary processes and the ora serrata. 
 This is brought out to some extent in the following table, 
 which shows in a series of mammals' eyes the length of a 
 straight line drawn between the root of the iris and the ora 
 serrata in one column and the antero-posterior diameter 
 of the eyeball in the other: 
 
 
 Antero- 
 
 Distance 
 
 
 posterior 
 
 between root 
 
 
 diameter of 
 
 of iris and ora 
 
 
 eyeball. 
 
 serrata. 
 
 
 mm. 
 
 mm. 
 
 Man 
 
 24.8 
 
 6 
 
 Chimpanzee . 
 
 19 
 
 4.25 
 
 Rhesus monkey 
 
 19.5 
 
 5 
 
 Capuchin monkey 
 
 18.5 
 
 4.25 
 
 Cat 
 
 22 
 
 3.5 
 
 Horse 
 
 44 
 
 5 to 10 
 
 Ox 
 
 35 
 
 6.5 
 
 Antelope . 
 
 27 
 
 3 to 5 
 
 Sheep 
 
 28 
 
 3 to 6 
 
 Pig . 
 
 22 
 
 4 
 
 Rabbit 
 
 19 
 
 2 
 
 In the Ungulata two measurements are given, because 
 in animals with horizontal oval pupils the distance of the 
 root of the iris from the ora serrata is considerably less 
 where the iris is narrow than where it is broad. 
 
 From this table it will be seen that in man and monkeys 
 the distance between the root of the iris and the ora serrata 
 is about one-quarter of the antero-posterior .diameter of 
 the globe, while in all other mammals, except the Ungulata, 
 it is considerably less. In the Ungulata, while it measures 
 nearly one-fourth in the vertical diameter, it is considerably 
 less in the lateral. 
 
 A lens flattened antero-posteriorly has considerably less 
 refractive power than a spherical or nearly spherical one. 
 The lesser refractive power of the lens in man and monkeys, 
 due to flattening of it antero-posteriorly, is to some extent 
 
78 EVOLUTION OF THE HUMAN EYE 
 
 compensated for, as already mentioned, by • increased 
 refractive power in the cornea. Further compensation is 
 effected by increase in the depth of the vitreous chamber 
 and displacement backward of the retina, on which the 
 image of the object looked at has to be focussed. The 
 following table gives the depth of the vitreous chamber 
 from the posterior pole of the lens directly backward to the 
 retina, and the diameters of the lens, in a series of mamma- 
 lian eyes. From this it will be seen that a decrease in the 
 sphericity of the latter is accompanied by an increase in 
 the depth of the former. 
 
 Lens. 
 
 
 Vitreous chamber. 
 
 Antero-posterior. Lateral. 
 
 Man . 
 
 15.4 mm. 
 
 4.5 
 
 mm. 9 mm. 
 
 Chimpanzee . 
 
 11.5 " 
 
 4 
 
 7 
 
 Horse 
 
 20 
 
 11 
 
 20 
 
 Antelope 
 
 12 
 
 14 
 
 17.5 " 
 
 Pig 
 
 9 
 
 6 
 
 9 
 
 Rabbit 
 
 6 
 
 7 
 
 9 
 
 Thus, while in the rabbit and antelope the depth of the 
 vitreous chamber is less than the width of the antero- 
 posterior diameter of the lens, in the horse and pig it is 
 considerably greater. In man and the chimpanzee the 
 depth of the vitreous chamber is nearly three times as great 
 as the width of the antero-posterior diameter of the lens. 
 
 A similar comparison may be made in the changes which 
 take place in the shape of the lens and in the depth of the 
 vitreous chamber during the process of development of the 
 human eye. The following table shows the depth of the 
 vitreous chamber and the diameters of the lens of the devel- 
 oping human eye up to the time of birth. 
 
 Lens. 
 
 Age. 
 
 Vitreous chamber. 
 
 Antero-posterior. Lateral. 
 
 Fourth month 
 
 4.5 mm. 
 
 2.8 mm. 3.3 mm. 
 
 Sixth month 
 
 5 
 
 3.8 " 4 
 
 Seventh month 
 
 7.5 " 
 
 4 " 4.5 " 
 
 Ninth month 
 
 9.5 " 
 
 4.3 " 5.75 " 
 
 At birth . . 
 
 U 
 
 3.75 " 7 
 
ACCOMMODATION AND CONVERGENCE 79 
 
 Weiss 36 has shown that the anterior segment of the eye- 
 ball grows rapidly during the first year of life and attains 
 its full dimensions by the end of the second year. The 
 subsequent growth of the eye, which continues into the 
 twenties, is entirely due to an expansion of the posterior 
 segment. At birth the depth of the vitreous is 11 mm. and 
 in the adult eye 15.4 mm. 
 
 This difference in the depth of the vitreous chamber in 
 different classes of mammals, and at different periods in 
 the course of development in man's eye, may be correlated 
 with the varying states of refraction met with. 
 
 The refraction of the eyes of a large number of mammals 
 has been investigated by Lang and Barrett 37 and by Lindsay 
 Johnson. 3 Their investigations agree in showing that wild 
 species of mammals are always hypermetropic. Lindsay 
 Johnson says: "A slight degree of hypermetropia, i. e., 
 under 1 D. may be said to be the rule throughout the 
 higher mammals, while higher hypermetropia, i. e., 2 D. 
 to 5 D., is found in the wild species of rodents, the Edentata 
 and the Marsupials." 
 
 Among domesticated mammals myopia is not uncommonly 
 met with. In a series of mammals Lang and Barrett found 
 it in the following proportions of the eyes examined : Rabbits, 
 5 out of 52; guinea-pigs, 5 out of 28; rats, no myopia, but 
 
 1 mixed astigmatism out of 10; cows, no myopia in 10; 
 horses, no myopia, but 1 mixed astigmatism out of 6; cats, 
 
 2 out of 12; dogs, 2 slight myopia out of 6. Lindsay Johnson 
 says : " Myopia is frequently met with both with and without 
 astigmatism among domesticated animals, especially guinea- 
 pigs, rabbits and other rodents kept in confinement in small 
 hutches." He also states that he found myopia to be a 
 persistent characteristic in mandrils and baboons. Lang 
 and Barrett found that 2 monkeys out of 11 which they 
 examined were myopic. There is no note as to how long 
 
80 EVOLUTION OF THE HUMAN EYE 
 
 these myopic monkeys had been kept in confinement pre- 
 vious to having their refraction tested. 
 
 The tendency to the development of myopia as an out- 
 come of confinement in restricted spaces seems to be an 
 example of adaptation of structure to function, i. e., an 
 elongation of the posteriorpart of the eyeball to meet require- 
 ments of vision for near objects. It is similar to the con- 
 dition of things which is met with in human beings. The 
 incompletely developed eye of the newborn is nearly always 
 hypermetropic, the mean amount met with having been 
 estimated by different observers as 2.3 D. and 3 D. Dur- 
 ing the first decade of life the hypermetropia curve rapidly 
 falls, while the percentage of myopes increases during the 
 first two decades. 
 
 Man's eyes at birth are comparable to those of terrestrial 
 mammals in the natural state, who have not adopted arboreal 
 life. His lenses are then nearly spherical and his vitreous 
 chambers of comparatively little depth, so that his eyes 
 are adapted for vision at long distances, with but little 
 capacity for the focussing of near objects. By expansion 
 of the globes, flattening of the lenses antero-posteriorly and 
 increase in the depth of the vitreous chambers, man's eyes 
 gradually reach that condition which is most suitable for 
 his environment under normal conditions, i. e., where parallel 
 rays are brought accurately to a focus on the macula when 
 the ciliary muscle is at rest, and where there is an ample 
 capacity of accommodation for near objects. If, while this 
 developmental process, which lasts up to the twentieth year, 
 is still proceeding, man's vision becomes unduly restricted 
 to use at short ranges, then, as in the lower mammals which 
 are kept confined in small compartments, adaptation of 
 structure to function tends to take place, the vitreous 
 chambers become deeper than normal and myopia is 
 developed. 
 
ACCOMMODATION AND CONVERGENCE 81 
 
 As already pointed out, the development of a highly acute 
 spot of central vision for form-sense is correlated in mammals 
 with a change in feeding habits, viz., the use of the hands 
 for picking up food and conveying it to the mouth, instead 
 of in the first place seizing it with the mouth. When the 
 food consists of small things, such as fruits, grains, nuts 
 and insects, acute form-sense, unless accompanied by active 
 accommodative power, would be of very limited use. Hence, 
 like the development of the fovea centralis, the increased 
 range of accommodation, which has necessitated the exten- 
 sive changes in the shape and structure of the eye just men- 
 tioned, has been the outcome of the adoption of arboreal 
 life. 
 
 Just as we find the development of active accommodation 
 linked up with the development of acute central form-sense, 
 so we find the power of convergence linked up in its for- 
 mation with the development of active accommodation. 
 Indeed, these three visual factors appear to have evolved, 
 pari passu, with life in the trees. 
 
 As has already been pointed out, the evolution of con- 
 jugate movements of the eyes is associated with the presence 
 of semi-decussation of the fibers of the optic nerves at the 
 chiasma, the receipt of visual impressions from the two 
 eyes on the same side of the brain, and the development of 
 corresponding points in the retina. These conjugate move- 
 ments reach to a high degree of perfection in the Carnivora, 
 but it is only when we come to the Primates that we meet 
 with a highly developed power of convergence in association 
 with stereoscopic vision. 
 
 Some reptiles and birds are able to converge the axes of 
 the two eyes on objects when feeding, but they have com- 
 plete decussation of the fibers of the two optic nerves. Visual 
 impressions from the two eyes are not received in them on 
 the same side of the brain, so that they do not have true 
 6 
 
82 EVOLUTION OF THE HUMAN EYE 
 
 stereoscopic vision. A movement of the eyes may be 
 observed in some of the Ungulata, when their attention is 
 suddenly attracted, which is an approach toward conver- 
 gence, but which is really more a movement from diver- 
 gence toward parallelism. It is exceedingly well depicted 
 in a plate in Chalmers Mitchell's book on The Childhood of 
 Animals, which shows a group of spring-bok facing the 
 artist, in the characteristic attitude of attention, at the 
 London Zoological Gardens a few hours after a kid was born. 
 In the Carnivora more obvious attempts at convergence 
 may be noted; when holding food with their front paws 
 they tear it with their teeth. Dogs can be trained to con- 
 verge by having a lump of sugar placed on their nose "on 
 trust." 
 
 The associated evolution of acute central form-sense, 
 accommodation and convergence, is of considerable interest 
 when we consider the intimate dependence of each one upon 
 the others for purposes of stereoscopic vision. If any one 
 of these three faculties becomes seriously interfered with, 
 so that the balance between them is disturbed, then the 
 recently acquired capacity of fusing the images seen with the 
 two eyes is given up, and one eye only is employed for fix- 
 ation, the other being allowed to wander in the direction of 
 least resistance, the condition we speak of as concomitant 
 strabismus being established. Concomitant strabismus may 
 be the outcome of several different causes, but three well- 
 established predisposing factors are: (1) A defect in the 
 development of the central form-sense in one eye, i. e., con- 
 genital amblyopia; (2) the presence of hypermetropia, 
 which leads to an excessive amount of accommodative effort 
 being made in the focussing of the images of near objects; 
 (3) the presence of heterophoria, which leads to undue 
 amount of effort on the part of the recti muscles to maintain 
 the requisite amount of convergence. 
 
CHAPTER V. 
 COLOR-SENSE. 
 
 The color-sense of mammals has been investigated by 
 experiments and by a' study of their natural history. I 
 propose briefly to summarize the conclusions these investi- 
 gations have led to, discuss the way in which the color- 
 sense has been influenced by the adoption of arboreal life, 
 and endeavor to correlate its varying degrees of acuteness 
 with the different anatomical appearances met with in the 
 percipient end-organs of the retina. 
 
 Many carefully thought-out experiments have been per- 
 formed to test the chromatic sense in mammals, and in some 
 elaborate precautions have been taken to exclude any fallacy 
 which might be occasioned by unequal degrees of luminosity. 
 These experiments have mostly been applied to dogs and 
 monkeys. The details of them will be found well epitomized 
 in Parsons' 17 book on Color Vision. It will suffice for my 
 purpose here to quote only the results obtained. 
 
 Graber 38 found that the pig and the dog could distinguish 
 between blue and red, showing a distinct preference for the 
 former; it is, however, doubtful if this preference was not 
 due to brightness rather than color. 
 
 Kalischer 39 came to the conclusion, from feeding experi- 
 ments he carried out on dogs, that there is no doubt of their 
 ability to perceive differences of hue as well as differences 
 of brightness; but that there exist considerable differences 
 in their sensibility to color. 
 
84 EVOLUTION OF THE HUMAN EYE 
 
 Miss E. M. Smith, 40 from an elaborate series of experi- 
 ments on dogs, summed up her conclusions as follows: 
 
 "1. That while evidence has been obtained to show that 
 some dogs possess a rudimentary power of color discrimi- 
 nation, such discrimination is highly unstable and cannot 
 be supposed to play any part in the animals' normal exist- 
 ence. 
 
 "2. That the color-sense is, as shown by the high color 
 threshold, very weak. 
 
 "3. That color discrimination, even where clearly 
 established, may be readily inhibited by differences of 
 luminosity or position." 
 
 Kinnaman 41 carried out experiments on monkeys with 
 food in glass tumblers covered with different colored cards, 
 and came to the conclusion that their capacity to distinguish 
 color as such was undoubted. 
 
 Dahl 42 colored some sweets and some bitter substance 
 with different colored dyes; he found that after a few 
 attempts a monkey learned to leave without even tasting 
 those which were colored with the dye indicating a bitter 
 substance, and to seize at once upon those colored to indicate 
 sweets. Varying the experiments sufficiently he found that 
 the monkey distinguished all the different colors readily 
 except dark blue. 
 
 Hess 43 tested a monkey in the following manner: Grain 
 was scattered over a black surface upon part of which a 
 spectrum was thrown. The monkey gathered all the grains 
 from the extreme red to the extreme violet, leaving those 
 unilluminated. When the animal was dark-adapted and 
 the intensity of the light diminished until the grains were 
 visible to the dark-adapted experimenter only in the yellow 
 and green, only those grains were gathered. Hess con- 
 cludes that the extent of the spectrum is the same for the 
 
COLOR-SENSE 85 
 
 monkey as for man, and that it is brightest for the dark- 
 adapted monkey in the region where it is also brightest for 
 the dark-adapted man. 
 
 From the results obtained by these various experimenters 
 it seems evident that the color-sense of monkeys is much 
 the same as in man, but that that of dogs and of other 
 mammals experimented on is much less highly developed. 
 
 The origin of the color-sense and of the coloring of animals 
 received much attention from the great pioneers of evolution 
 of the last century— Darwin, Alfred Russell Wallace, 
 Lubbock and Grant Allen. Much of what they wrote, 
 brings out very clearly what I am now endeavoring to show, 
 viz., that the color vision of Primates is far more acute than 
 that of other mammals, and that this is due to their arboreal 
 life with its natural accompaniment of a frugiferous diet. 
 I cannot do better than quote from their writings, especially 
 from Grant Allen's 44 fascinating book on The Color Sense: 
 its Origin and Development, which has not, I think, attracted 
 the amount of attention which it deserves. What is to be 
 learned regarding the color-vision of mammals from a study 
 of their natural history he sums up as follows : 
 
 "When we come to the highest class of vertebrates— 
 the Mammalia— strangely enough the evidence of a color- 
 sense almost fails us. The antipathy of male ruminants 
 for scarlet and the curiosity which certain monkeys display 
 with regard to bright-colored objects are the only facts 
 in -point which come under ordinary observation. This 
 result, so contrary to what we might have expected, appears 
 quite natural when we examine more closely the cir- 
 cumstances of the case. By far the larger part of the mam- 
 mals are herbivorous like the ruminants and pachyderms, 
 or carnivorous like the technical carnivores, insectivores 
 and whales. Only a small portion of the class subsists 
 
86 EVOLUTION OF THE HUMAN EYE 
 
 upon fruits, while none of them are very specially connected 
 with flowers. Hence a large set of possible tests which 
 we can employ in the case of insects and birds are wholly 
 inapplicable to mammals. Moreover, the want of close 
 relations with the colored parts of plants has probably 
 resulted in a want of any peculiar love for bright color, 
 such as we see reason to suspect in the butterflies, humming- 
 birds and parrots. This absence of taste for brilliancy is 
 probably marked by the absence of brilliant hues in the 
 animals themselves; the result of sexual selection for these 
 hues, as we shall see hereafter, only appears among the 
 Mammalia in a few higher arboreal and frugivorous species 
 such as the mandril and certain squirrels." 
 
 Alfred Russell Wallace, 45 in discussing the origin of the 
 color-sense, said: "The primary necessity which led to the 
 development of 'the sense of color was probably the need of 
 distinguishing objects much alike in form and size but 
 differing in important properties, such as ripe and unripe 
 or eatable and poisonous fruits, flowers with honey or with- 
 out, the sexes of the same or of closely allied species. In 
 most cases the strongest contrast would be the most useful, 
 especially as the colors of the objects to be distinguished 
 would form but minute spots or points when compared 
 with the broad masses of tint of sky, earth or foliage against 
 which they would be set." 
 
 The association of brilliant hues in the external coverings 
 of animals with those of the objects from which they obtain 
 their food is very striking. Humming-birds, which are 
 the most highly colored of all birds, obtain their food from 
 flowers; they have a bifid tubular tongue which they insert 
 into the corolla of flowers to suck up the honey. Parrots, 
 whose plumage is generally extremely bright and gaudy, 
 live for the most part on brightly colored fruits. The 
 
COLOR-SENSE 87 
 
 coloring of rapacious and carrion birds, on the other hand, 
 is always dull and destitute of any decorative hues. 
 
 With regard to colors in the external coverings of mammals, 
 Grant Allen 44 says: "The whole series, whether among' 
 marsupials, pachyderms, Cetacea, ruminants, Carnivora 
 or Insectivora, show almost uniform tints of black, brown, 
 gray or dingy yellow. It is true that many animals, like the 
 zebras, tigers, spotted deer and giraffes, have very notice- 
 able alterations of light and dark shades, but they do not 
 yield us pure spots of green, blue, red or yellow. When 
 we come to the essentially arboreal mammals, however, the 
 tree rodents and the Quadrumana, we get many com- 
 paratively brilliant species. The squirrels are often remark- 
 able for their beautiful colors, and the so-called flying squirrels 
 call for special notice in this respect." 
 
 Darwin, 46 in his Descent of Man, described the various 
 decorative colorings met with in monkeys. He said: "In 
 very many species the beard, whiskers and crests of hair 
 round the face are of a different color from the rest of the 
 head, and when different are always of a lighter tint, being 
 often pure white, sometimes bright yellow or reddish. The 
 whole face of the South American Brachyurus cahms is of a 
 'glowing scarlet hue'; but this color does not appear until 
 the animal is nearly mature. The naked skin of the face 
 differs wonderfully in color in the various species. It is 
 often brown or flesh-color, with parts perfectly white, and 
 often as black as that of the most sooty negro. In the 
 Brachyurus the scarlet tint is brighter than that of the 
 most blushing Caucasian damsel. It is sometimes more 
 distinctly orange than in any Mongolian, and in several 
 species it is blue, passing into violet or gray. In all the 
 species known to Mr. Bartlett, in which the adults of both 
 sexes have strongly-colored faces, the colors are dull or 
 
88 EVOLUTION OF THE HUMAN EYE 
 
 absent during early youth. This likewise holds good with 
 the mandril and rhesus, in which the face and the posterior 
 parts of the body are brilliantly colored in one sex only." 
 
 There is no member of the whole class of mammals which 
 shows such resplendent colors as the adult male mandril; 
 they have been compared with those of the most brilliant 
 birds. The following is the description of one of these 
 animals now in the Zoological Society's Gardens: The 
 rather long, harsh hair is dark over the body, but each 
 separate hair is ringed with black and yellow toward the 
 tip, the pattern in certain lights giving a greenish tinge to 
 the fur similar to the shade of other baboons and many other 
 African monkeys. On the forehead there is a crest of hair 
 and on the chin a yellow beard. The naked patches on the 
 buttocks are bright red in the adults. On each side of 
 the face, reaching from the eyes to the nostrils, is a cor- 
 rugated swelling, divided into a number of longitudinal 
 ridges. These ridges are bright blue and furrows between 
 them purple. The muzzle, of a bright scarlet, is surrounded 
 by a raised border like that of a pig (see Frontispiece). 
 
 At first it seems strange that the buttocks should be the 
 part of the body which in some monkeys has acquired the 
 most highly decorative color. In a supplemental note 
 "on sexual selection in relation to monkeys" at the con- 
 clusion of his book on The Descent of Man, Darwin brings 
 together evidence to show that monkeys so ornamented 
 are proud of their bright red rumps; they turn and display 
 them to old friends and new acquaintances as a form of 
 greeting. These naked parts become more turgid and of a 
 brighter color during the season of love; their display before 
 the female probably acts as a sexual excitement and their 
 proximity to the sexual organs may in this way be explained. 
 
 From the foregoing it will be seen that in mammals as 
 
COLOR-SENSE 89 
 
 in birds there is a very close correlation between the character 
 of their food and their external coloring. It is only those 
 who have adopted a frugivorous diet that have acquired 
 brilliant hues. How important a good sense of color must 
 be to a fruit-eating animal became clearly demonstrated to 
 me while I was on a visit to Norway; the wild strawberries 
 were then ripe, and several of my party began to gather 
 them; one man, however, could not find any, and it was 
 afterward discovered that he was red-green blind. Had 
 he had to depend for his sustenance on such small fruits, 
 which he gathered for himself, he must soon have perished. 
 In both birds and mammals bright coloring is often sex- 
 limited, and most frequently it is the male which is the more 
 resplendent. Physiologists are generally agreed that such 
 secondary sexual characters are dependent on internal 
 secretions of the genital organs— testes or ovaries. This 
 point lends considerable support to the Darwinian hypothesis 
 of sexual selection. It is an hypothesis which, if accepted, 
 recognizes the presence of the color-sense in those animals 
 which have acquired sex-limited decorative hues. The theory 
 of sexual selection raised so much controversial discussion 
 that it will be well here to quote what such an authority as 
 Prof. Weissman 47 wrote concerning it fifty years after the 
 publication of The Origin of Species: " I really see no reason 
 why we should doubt the power of sexual selection, and I 
 myself stand wholly on Darwin's side. Even though we 
 certainly cannot assume that the females exercise a con- 
 scious choice of the handsomest mate and deliberate like 
 the judges in a court of justice over the perfections of their 
 wooers, we have no reason to doubt that distinctive forms 
 (decorative feathers) and colors have a particularly exciting 
 effect upon the female, just as certain odors have among 
 animals of so many different groups, including the butter- 
 
90 EVOLUTION OF THE HUMAN EYE 
 
 flies. The doubts which existed for a considerable time, 
 as a result of fallacious experiments, as to whether the colors 
 of flowers really had any influence in attracting butterflies, 
 have now been set at rest through a series of more careful 
 investigations. We now know that the colors of flowers 
 are there on account of butterflies, as Sprengel first showed, 
 and that the blossoms of phanerogams are selected in relation 
 to them, as Darwin pointed out. Certainly it is not possible 
 to bring forward any convincing proof of the origin of 
 decorative colors through sexual selection, but there are 
 many weighty arguments in favor of it, and therefore a 
 body of presumptive evidence so strong that it almost 
 amounts to certainty." 
 
 At the conclusion of his book, Grant Allen 44 summarizes 
 the whole evolution of color-sense in the following glowing 
 passage: "Insects produce flowers. Flowers produce the 
 color-sense in insects. The color-sense produces a taste 
 for color. The taste for color produces butterflies and 
 brilliant beetles. Birds and mammals produce fruits. 
 Fruits produce a taste for color in birds and mammals. 
 The taste for color produces the external hues of humming- 
 birds, parrots and monkeys. Man's frugivorous ancestry 
 produces in him a similar taste, and that taste produces 
 the various final results of human chromatic art. 
 
 "What a splendid and a noble prospect for humanity in 
 its future evolution may we not find in this thought, that 
 from the coarse animal pleasure of beholding food, man- 
 kind has already developed through delicate gradations 
 our modern disinterested love for the glories of sunset and 
 the melting shades of ocean, for the gorgeous pageantry of 
 summer flowers, and the dying beauty of autumn leaves, 
 for the exquisite harmony which reposes on the canvas of 
 Titian, and the golden haze which glimmers over the dreamy 
 
COLOR-SENSE 91 
 
 visions of Turner! If man, base as he yet is, can neverthe- ' 
 less rise today in his highest moments so far above his 
 sensuous self, what may he not hope to achieve hereafter, 
 under the hallowing influence of those chaster and purer 
 aspirations which are welling up within him even now toward 
 the perfect day!" 
 
 It would be interesting to be able to correlate with these 
 views on the evolution of color-sense definite changes in 
 the structure of the retina. Such knowledge as we at 
 present possess permits of this being done in only a tentative 
 manner. The color-sense in man, like the form-sense, is 
 most acute at the macula, where cones only are present. 
 Though the final appreciation of both form and color is 
 evidently a psychological process, it seems to be an essential 
 preliminary step in such appreciation for some change to 
 take place in the end-organs of the nerve fibers in the retina. 
 That the change which gives rise to the sensation of form 
 is essentially different from that which gives rise to the 
 sensation of color is shown by the fact that the one may be 
 deficient without the other being impaired. In cases of 
 congenital amblyopia the form-sense is defective while the 
 color-sense is normal, and in the case of congenital dichro- 
 mats the color-vision is defective while the form-sense is 
 normal. 
 
 The essential factor in the perception of color must be 
 due to some at present unknown reaction, which waves of 
 light of varying wave-lengths produce in the individual 
 cones. As the perception of hues is more acute at the 
 macula than elsewhere, we may infer that the cones in that 
 situation are more capable of undergoing this unknown 
 reaction than those situated in the more peripheral parts 
 of the retina. A comparison of the microscopical appear- 
 ances of the cones at the macula with those at the periphery 
 
92 EVOLUTION OF THE HUMAN EYE 
 
 of the retina shows very striking differences in them, both 
 as regards shape and size. The foveal cones, especially 
 those in the very center, are long and slender. The extra- 
 foveal cones decrease in length gradually as they extend 
 toward the periphery, the change being most marked in 
 the outer segment, which also gradually becomes wider. 
 
 Careful histological examination of the cones in the retina 
 of congenitally color-blind persons might, I suggest, add 
 considerably to our knowledge of color perception. I have 
 been for some years on the lookout for a specimen in which 
 to make such an examination, but so far have not been 
 successful in obtaining it. 
 
 For the perception of colors in the peripheral parts of the 
 field a larger object, intenser light and greater saturation 
 of color is required than in the center. Given cones with 
 a hue-perceiving capacity, these requirements might be 
 largely accounted for by varying degrees of concentration 
 of the cones in different parts of the retina. The retinae 
 of Primates differ from those of lower mammals in having 
 an area, the macula, in which only cones, closely packed 
 together, are present; we should, therefore, expect that the 
 hue-perceiving faculty of Primates would be endowed with a 
 capacity of seeing small patches of color, color less brilliantly 
 illuminated, and less saturated, than in those mammals in 
 which the cones are more widely scattered. To put it in 
 another way, we should expect that the color-sense of mam- 
 mals, with widely diffused cones, would be comparable to 
 that of the peripheral portions of the retina of Primates. 
 
 The perception of small colored objects is essential for 
 the existence of arboreal animals which live mainly on 
 fruits that they pick up with their fingers. It is by no 
 means essential for the existence of carnivorous or herbivo- 
 rous mammals. For the former even the capacity of 
 
COLOR-SENSE 93 
 
 perceiving color in large masses can have but little use, 
 though in the latter the power of seeing green in large areas 
 might be of some assistance in their search for food. There 
 can be no doubt that bulls in bull-fights are excited into a 
 state of rage by the banderilleros flaunting red cloaks in 
 front of them. I would suggest that the perception of red, 
 presented in this way, excites in them the same instinctive 
 feelings and reactions as that produced by the sight of 
 flowing blood— the inevitable accompaniment of combats 
 in which the flesh is gored with horns. We ourselves use 
 the expression "seeing red" to describe the feelings which 
 excite us to uncontrollable vengeance. 
 
CHAPTER VI. 
 
 THE PROTECTIVE MECHANISMS OF THE 
 EYEBALL. 
 
 For the development of the conjunctiva it seems essential 
 that the membrane should for a time be converted into a 
 closed sac by the union of the margins of the eyelids 
 together in front of the eyeball. In mammals, like the 
 ruminants, who have to fend for themselves and run beside 
 their mothers directly they are born, the lid margins become 
 separated before birth, so that they are born into the world 
 with their eyes open. The young of nearly all the Carnivora, 
 of most rodents and Insectivora, who spend the first weeks 
 of life in some prepared lair or nest, are born blind, due to 
 the eyelids being still united in front of the eyes. The 
 same applies to the marsupials, whose young for some time 
 after birth are carried about in the maternal pouch. The 
 young of arboreal mammals, such as the lemurs, monkeys 
 and the higher apes, travel about with their mothers from 
 bough to bough directly after birth. The baby monkeys 
 are often carried by their mothers, while the baby lemurs 
 cling tightly to their ventral fur. When born into such an 
 environment it is obviously desirable to be endowed with 
 sight, and we find that these arboreal mammals, like the 
 ruminants, enter the world with their eyes open. Newborn 
 children are in many ways quite as helpless as the newborn 
 Carnivora, but, unlike them, the epithelial union of the 
 margins of their eyelids breaks through before birth; this 
 is doubtless an inheritance from their arboreal ancestors. 
 
PROTECTIVE MECHANISMS OF THE, EYEBALL 95 
 
 All terrestrial mammals, with the exception of man and 
 monkeys, possess three eyelids for the protection of the 
 surface of the eye. Man and monkeys have two, the tbird 
 eyelid being reduced to a vestigial structure, the semilunar 
 fold. 
 
 In birds the third eyelid, or nictitating membrane, is met 
 with in the most highly developed condition; it has in them 
 a muscle directly attached to it which draws it forward in 
 front of the eyeball. In the Mammifera the movements 
 of the third eyelid are most extensive in the Ungulata, and 
 are produced indirectly by the contraction of a muscle 
 which draws backward the eyeball and pushes forward the 
 orbital fat. The third eyelid in mammals is composed of 
 a plate of cartilage covered on both surfaces by mucous 
 membrane. Anteriorly it has a free even margin fitting 
 accurately the surface of the cornea, over which it glides. 
 Posteriorly it terminates in a considerably thickened edge 
 which penetrates into the depths of the orbit on the inner 
 surface of the globe, where it is continuous with a mass of 
 fat. The muscle which draws back the eyeball— the retractor 
 bulbi— arises from the bones at the apex of the orbit, and 
 is inserted into the sclerotic inside the attachments of the 
 recti muscles, between them and the optic nerve. It encircles 
 the latter in a cave-like fashion, hence it is often spoken of 
 as "the choanoid muscle" (Fig. 22). When the eyeball is 
 drawn back in the orbit by this muscle the fat of the orbit, 
 with the base of the third eyelid attached to it, is pressed 
 forward, so that the anterior margin of the eyelid becomes 
 swept across the front of the eye. 
 
 In mammals which possess this choanoid muscle the bony 
 outer wall of the orbit is incomplete, the space between the 
 bones being filled up by a fibrous membrane termed the 
 "cornet," which contains muscular tissue, "the orbital 
 
9G 
 
 EVOLUTION OF THE HUMAN EYE 
 
 Fig. 22. — Eye of an antelope showing the retractor bulbi or choanoid 
 muscle (C). S, sheath of Tenon's capsule external to choanoid. R, external 
 rectus. 0, superior oblique. T, Tenon's capsule. 
 
 Fig. 23. — Section through Gogcnbauer's muscle from the inner surface q( 
 the cornea of a sheep. 
 
PROTECTIVE MECHANISMS OF THE EYEBALL 97 
 
 muscle of Gegenbauer" (Fig. 23). In the Primates, who 
 have no third eyelid or choanoid muscle, there is an outer 
 bony wall to the orbit, the only gap in its continuity being 
 the sphenomaxillary fissure. 
 
 The retraction of the eyeball by the choanoid muscle 
 causes the contents of the orbit to become pressed against 
 the fibro-muscular membrane, filling in its outer wall. It 
 is partly the elastic recoil of this membrane, and partly the 
 contraction of Gegenbauer 's muscle, which restores the 
 eyeball to its usual position, and brings about the retraction 
 of the third eyelid. 
 
 In association with the third eyelid there is a special 
 gland, "the Harderian gland," whose duct opens on its 
 inner surface, and whose secretion is emptied when the 
 eyelid is pressed forward, forming a lubricating fluid for 
 the surface of the cornea. Among mammals this gland is 
 largest in the Ungulata, whose third eyelid is most mobile, 
 smaller in the Carnivora, and absent altogether in monkeys 
 and man who have no third eyelid. 
 
 Some mammals possess both a lacrimal gland and a 
 Harderian gland; where, however, the latter is large and 
 the third eyelid is efficient the former is usually small. In 
 monkeys and man, who have no third eyelid or Harderian 
 gland, the lacrimal gland is larger than in other mammals, 
 and largest of all in man. 
 
 The functions of the third eyelid are obvious from its 
 anatomical arrangements, it protects the front of the eye 
 from injurious influences, lubricates its surface, and removes 
 any foreign substance which may become lodged on it. In 
 animals, such as the Herbivora, who when feeding hold 
 their heads close to the ground, and in the Carnivora, who 
 do the same in tracking their prey by scent, the presence 
 of such a protective mechanism is of the greatest service 
 7 
 
98 EVOLUTION OF THE HUMAN EYE 
 
 in guarding the cornea from coming in contact with foreign 
 substances, or in their removal should they gain access to 
 the eye. 
 
 The grasping of the food in the first place with the hands 
 and the conveyance of it by them to the mouth, as is done 
 by animals who have adopted arboreal life, saves the eyes 
 from much of the risk to which they are exposed when, as 
 in grazing, the food is seized directly with the mouth. The 
 development of the forelimbs into organs of delicate tactile 
 sensibility, capable of picking up minute particles, has 
 made it possible for them to be used for the removal of 
 foreign bodies from the eye, and thus, as suggested by 
 Le Coq, to play the part of the third eyelid in a way which 
 is not possible for hoofs or claws. 
 
 The greater flow of lacrimal secretion in response to the 
 irritation of foreign bodies supplies also an hydraulic 
 apparatus for removal of foreign bodies, to replace the 
 mechanical arrangement of the third eyelid. 
 
 Not only as the third eyelid becomes a superfluous structure 
 in Primates, but the retraction of the eye which its use entails 
 would be a source of trouble and inconvenience to them. 
 The movement of the eyes backward and forward in the 
 orbit in animals who have assumed the erect or semierect 
 posture would considerably interfere with the accuracy of 
 stereoscopic vision and of accommodation and convergence. 
 
 The vestigial remains of the third eyelid persist in man 
 and monkeys as the semilunar fold. Owen 48 says that a 
 remnant of the choanoid muscle is to be found in the lower 
 Quadrumana, in the form of a few fibers detached from the 
 inner part of the recti to be inserted into the sclerotic nearer 
 the entry of the optic nerve. Sir John Bland-Sutton 49 has 
 recently suggested that Tenon's capsule is really the repre- 
 sentative of this muscle in man. He says: "A critical 
 
PROTECTIVE MECHANISMS OF THE EYEBALL 99 
 
 examination of Tenon's capsule in the orbit of man and 
 monkeys convinces me that it is the degenerated represent- 
 ative of the retractor bulbi as surely as the little pleat of 
 mucous membrane, known as the semilunar fold, is the 
 vestige or remnant of the third eyelid." 
 
 Motais 50 has found the capsule of Tenon constructed in 
 a uniform manner in all classes of vertebrates; it presents, 
 he says, only slight variations due to differences in the 
 arrangements of the ocular muscles. He has published 
 pictures showing its arrangements in the eyes of a horse 
 and of a cow, in which also the choanoid muscle is present. 
 I have found it present myself in dissections I have made 
 of an antelope (Fig. 22) and of a sheep. It ensheaths, and 
 stretches between, the recti muscles, and forms, moreover, 
 a sheath to the choanoid muscle itself. Inasmuch, then, as 
 Tenon's capsule is found in animals in whom the choanoid 
 muscle is present, it would seem impossible to regard it as 
 a vestigial remnant of the muscle when absent. 
 
 Lindsay Johnson 3 states that he met with a case of an 
 obvious membrana nictitans in a youth which was capable 
 of slight movement and extended in a crescent form nearly 
 as far as the cornea. He does not say, however, whether 
 this movement of the membrane was effected by any accom- 
 panying displacement of the eyeball backward. In 1912, 
 S. E. Whitwall 61 recorded an instance of a retractor bulbi 
 muscle present in both orbits of a man, aged fifty years. 
 He states that vestiges of it are also met with in some monkeys 
 (e. g., Macacus rhesus). 
 
 Though the evidence of any vestigial remains of the 
 choanoid, or retractor bulbi muscle, is very doubtful, the 
 presence of vestigial remains of Gegenbauer's orbital muscle, 
 or, as I suggest it might be well termed, "the protrusio 
 bulbi muscle," is clear and conclusive. 
 
100 EVOLUTION OF THE HUMAN EYE 
 
 H. Miiller 62 in 1858 first described how he had found flat 
 muscle fibers in the inferior orbital fissure in man, corre- 
 sponding to that met with in the orbital membrane of Mam- 
 malia. He also described how it was the antagonistic con- 
 traction of this membrane, by muscular action, which restored 
 the eyeball to its position in the socket after it had been 
 drawn back by the retractor. The muscle he found was 
 composed of unstriated fibers, and was stimulated into 
 action by irritation of the cervical sympathetic nerve. 
 
 Sir William Turner 63 in 1862 wrote an article entitled 
 "Upon a Nonstriped Muscle Connected with the Orbital 
 Periosteum of Man and Mammals." In it he says: "While 
 engaged in making a dissection in the human subject dur- 
 ing the winter session of last year, of the inferior maxillary, 
 or second division of the fifth cranial nerve, my attention 
 was attracted to a pale reddish, soft mass, filling up the 
 narrow chink of the sphenomaxillary fissure, and extending 
 from the sphenoid fissure in the sphenoid bone to the infra- 
 orbital canal in the superior maxillary bone. It was evi- 
 dently connected to the superior (ocular) aspect of the 
 periosteum of the orbit, and it was pierced by the orbital 
 branch of the superior maxillary nerve. Since the period 
 of making the above observation I have availed myself of 
 several opportunities of examining the same region in other 
 subjects, and have constantly observed appearances of a 
 nature similar to that just described. The amount of the 
 reddish mass and the depth of its tint varied slightly in 
 different instances. Frequently it was so pale as scarcely 
 to attract attention, which may perhaps be the reason why 
 it has so long been neglected by anatomists. When care- 
 fully examined with the naked eye, or still better with a 
 single lens, it was seen to exhib.it a fibrous appearance." 
 
 The existence of these unstriated-muscle fibers in man, 
 
PROTECTIVE MECHANISMS OF THE EYEBALL 101 
 
 which may be regarded as the analogue of the "protrusio 
 bulbi muscle" in lower animals, and which is supplied by 
 the cervical sympathetic nerve, is of considerable clinical 
 interest. 
 
 First, because it has been noted that slight protrusion of 
 the eyeball, or exophthalmos, may occur from stimulation 
 of the cervical sympathetic, and slight recession or enoph- 
 thalmos from its paralysis. Secondly, because overaction 
 of this muscle offers the most satisfactory explanation of 
 the exophthalmos which occurs in connection with Graves' 
 disease. As stated by Andre Crotti 54 the exophthalmos 
 in this affection generally develops slowly, but may come 
 on rapidly. It is liable to fluctuations varying with the 
 physical and mental condition of the patient. It may 
 subside entirely after operation, on the same day as the 
 operation, to reappear again a few days later. It also 
 disappears after death. All these circumstances are con- 
 sistent with the contraction or relaxation of a muscle which 
 has the power of pushing forward the globe. The amount 
 of protrusion which its contraction produces would naturally 
 vary with the state of its development; like other muscles 
 it would tend to hypertrophy and increase in power by pro- 
 longed stimulation. By such increase in its power the 
 increase in the amount of the exophthalmos might be 
 accounted for in the cases of long standing. 
 
 Another symptom of the disease, viz., retraction of the 
 upper lid, is due to contraction of muscle fibers supplied 
 by the cervical sympathetic nerve. This muscle, which 
 was also first described by Midler, is also named after him, 
 and, like the muscle about the sphenomaxillary fissure, is 
 composed of unstriated fibers. 
 
 A number of other theories have been suggested to account 
 for the exophthalmos in this disease, which may be sum- 
 marized as follows: 
 
102 EVOLUTION OF THE HUMAN EYE 
 
 (a) Abnormal deposit of retrobulbar fat. 
 
 (b) Relaxation of recti muscles from fatty degeneration. 
 
 (c) Serous infiltration of the retrobulbar connective and 
 fatty tissue of the orbit from vasomotor disturbance caused 
 by toxemia from thyroid disturbance. 
 
 (d) Venous congestion. 
 
 (e) Localized arterial dilation. 
 
 Postmortem examinations have failed to substantiate 
 the presence of any of these conditions. Localized arterial 
 dilation would produce pulsating exophthalmos. Venous 
 orbital congestion would, if it produced proptosis, also pro- 
 duce retinal venous dilation— a condition which is never 
 seen. None of these theories will account for the fluctu- 
 ations in the degree of the proptosis in the way in which 
 contraction and relaxation of a muscle tissue will do. 
 
 The effect of the contraction of Muller's muscle in the 
 human eye, situated as it is in the loose periosteum of the 
 orbit, would be, as in other animals, to press forward the 
 fat of the orbit and consequently the eyeball which lies in 
 front of the fat. Under such circumstances, if, in a case 
 of exophthalmos, an incision was made through the orbito- 
 tarsal ligament, we should expect the fat, being pushed 
 forward and kept in a state of tension, to well up through 
 the incision. We should also expect that in extreme cases 
 of exophthalmos, in which ulceration of the cornea was 
 threatened, recession of the eye and beneficial results might 
 attend removal of some of the fat. Foster Moore 55 has 
 recently recorded how he performed an operation of this 
 description, removing a "heaped-up tablespoon of fat" 
 through an incision which extended the whole length of 
 the lower conjunctival fornix. Afterward the lids were 
 easily approximated and sewed together over the front of 
 the eye, so that the eye was preserved. From the result 
 
PROTECTIVE MECHAXISMS OF THE EYEBALL 103 
 
 produced in this case he infers, as I think wrongly, that 
 the proptosis must have been due to an excess of fat. The 
 circumstances of the case may equally well be accounted 
 for by a protrusion forward of orbital fat due to muscular 
 contraction. 
 
 Though the vestigial character of the plica semilunaris 
 is well recognized, the vestigial character of the caruncle 
 seems to have been overlooked. A study of the morphol- 
 ogy of the parts about the inner canthus of the eye in mam- 
 mals shows variations of a marked character, not only in the 
 different natural orders, but also in the different species. 
 In kangaroos the inner canthus is prolonged into a triangular 
 flat area, kept moist by the overflow of tears; it is con- 
 tinuous with the skin of the nose and is fringed with fur. 
 A somewhat similar bald area, fringed with fur, is met with 
 in the Carnivora, varying, however, in size and shape in 
 different species. It is among the Ungulata that the most 
 conspicuous developments are met with in this region, 
 more especially among the ruminants (Fig. 24). In them 
 the inner canthus is prolonged for a considerable distance 
 inward and downward, sometimes as a thin streak and 
 sometimes as a broad band. The band or streak ends 
 usually in an inverted fold, for the lodgment of which there 
 is, in some animals, a depression in the lacrimal bone. 
 This fold or recess is termed the "suborbital pit," and by 
 French naturalists, as it is kept moist by lacrimal secretion, 
 "larmier." The whole of the integumentary area leading 
 frOm the eye to the recess and the recess itself is composed 
 of numerous sebaceous glands opening into dwarfed hair- 
 follicles (Fig. 25); at the bottom of the recess a few short 
 hairs are frequently seen projecting from among the glands. 
 Around this glandular structure are bands of muscle fibers, 
 which by their contraction bring about a protrusion of the 
 
104 
 
 EVOLUTION OF THE HUMAN EYE 
 
 secreting surface. The secretion from the glands has a 
 strong musky odor, and these suborbital pits reach their 
 greatest size in animals which possess an acute sense of 
 smell; especially is this so in animals such as deer and 
 
 >Ln t-e tope 
 
 JFecd Xleer. 
 
 Fig. 24. — Showing the arrangement of parts at the inner canthus in different 
 
 mammals. 
 
 antelopes, in whom smell is what may be termed a " distance- 
 sense," in contradistinction to a contact-sense, such as it is 
 in most of the Carnivora. These odoriferous glandular 
 organs probably form a means by which a stray member 
 
PROTECTIVE MECHANISMS OF THE EYEBALL 105 
 
 of a herd may be guided to rejoin its fellows. They seem 
 also to have some connection with the sexual function, as 
 they are usually larger in the male than in the female, and 
 their growth is said to be arrested by castration. In arboreal 
 
 
 
 Fig. 25. — Lining membrane of the suborbital pit of a sheep. 
 
 mammals, like monkeys and anthropoid apes, the sense of 
 smell is of but little use; the olfactory organs are com- 
 paratively poorly developed, and the odoriferous organs 
 at the inner angles of eves have dwindled into vestigial 
 
106 EVOLUTION OF THE HUMAN EYE 
 
 structures. , The caruncle, both in man and monkeys, has 
 become contracted up into a small elevation cut off from 
 any continuity with the skin of the face by the margins of 
 the eyelids, which have completely closed round it, so that 
 it is entirely located in the lacrimal bay. That it really is 
 a vestige of the secreting odoriferous areas met with in 
 other mammals seems evident from its containing rem- 
 nants of all the constituents of such areas. The following 
 is the description of the structure of the caruncle given in 
 Quain's Anatomy: "Occupying the recess of the angle at 
 the border of the plica semilunaris is a spongy-looking 
 reddish elevation (caruncula lacrimalis), formed by a small 
 insulated portion of skin containing a few large, modified 
 sweat-glands, also a group of sebaceous glands which open 
 into the follicles of very fine hairs. There is further found 
 in it a small amount of plain muscular tissue (H. Miiller) 
 as well as a few cross-striated muscular fibers." It is also 
 stated that in man, as in some animals, the caruncula lacri- 
 malis retains its connection with the skin at the inner canthus. 
 It is interesting to note in connection with this matter, that 
 the reduction of what must have been a large suborbital 
 pit to a comparatively small bald area at the inner canthus, 
 can be traced in the evolution of the horse. In the skull of 
 the modern horse no depression is found in the lacrimal 
 bone in front of the orbit for such a glandular structure. 
 In the fossil remains of the skulls, from the Pliocene epoch, 
 of the E. sivalensis of India and of the E. stenois of Europe, 
 traces of the depression or gland-cavity are met with, while 
 in the fossil skulls of the Hipparion, a three-toed horse from 
 Miocene beds, there is a very large depression in the bone 
 in front of the eye. 
 
 In conclusion, I would claim that the changes in the 
 visual organs of mammals, produced by the adoption of 
 
PROTECTIVE MECHANISMS OF THE EYEBALL 107 
 
 arboreal life, increased both the range and accuracy of 
 their powers of observation for natural phenomena. It 
 was in this way that the portal became opened for the 
 evolution of the mental faculties of memory and inference— 
 faculties which enabled man's anthropoid ancestors, on 
 descent from the trees, to subdue their foes, to become 
 successful hunters and to adopt an omnivorous diet. It 
 would seem, therefore, that some such study of the visual 
 organs as I have attempted in this lecture is necessary for 
 fully understanding the way in which man gained his 
 predominant position in the animal kingdom. 
 
 REFERENCES. 
 
 1. Jones, F. Wood: Arboreal Man, London, 1918. 
 
 2. Emmert: Ztschr. f. Vergl. Augenheilk., 1886. 
 
 3. Johnson* G. Lindsay: The Comparative Anatomy of the Mamma- 
 lian Eye, 1901. 
 
 4. Kalt and Dufour: Enoyclopedie francaise d'Ophtalmologie, vol. ii, 
 p. 917. 
 
 5. Eversbusoh: Ztsehr. f. Augenheilk., 1882. 
 
 6. Grossman and Meyerhausen: Arch. f. Ophth., 1877, xxiii, 3. 
 
 7. Kalt: Enoyclopedie francaise d'Ophtalmologie, vol. ii, p. 903. 
 
 8. Harris, Wilfred: Brain, 1904, xxvii, 105. 
 
 9. Singer and Miinzer: Beitrage zur Kenntniss der Sehnervenkreuzung, 
 Wien, 1888. 
 
 10. Cajal, Ramon y: Der Structure des Chiasma Opticum. German 
 translated from the Spanish, Leipzig, 1899. 
 
 11. Kolliker: Festschrift; Ueber Sehnerven-Degeneration und Sehnerven 
 Kreiizung, Wiesbaden, 1887. 
 
 12. Usher and Dean: Brain, 1903, part civ, 539. 
 
 13. Trendelenburg: Centralbl f. Physiol., 1904, vol. xvii. 
 
 14. Edridge Green: Color-blindness and Color-perception, 1909, p. 318. 
 
 15. Schultze, Max: Arch. f. Mikro. Anat., 1866, vol. ii. 
 
 16. Kries, v.: Nagel's Handbuch der Physiologie d. Menschen. 
 
 17. Parsons, J. H.: An Introduction to the Study of Color Vision, Cam- 
 bridge, 1915. 
 
 18. Stort and Englemann: Archiv. f. Ophthal., 1887, xxxiii, 3, p. 229; also 
 Ophth. Review, vol. vii, p. 142. 
 
 19. Chievitz: Arch. f. Anat., 1891, Hefte iv, v and vi. 
 
 20. Slonaker, J. R.: Journal of Morphology, 1897, xiii, 445. 
 
 21. Krause: Internat. Monatsschr. f. Anat. u. Physiol., 1891, viii, 414. 
 
 22. Ganser: Ztschr. f. Vergleich. Augenheilk., Heft ii, p. 139. 
 
 23. Wood, Casey: The Fundus Oculi of Birds, Chicago, 1917. 
 
 24. Hippel, v.: Arch. f. Ophth., 1898, xlv 286. 
 
108 EVOLUTION OF THE HUMAN EYE 
 
 25. Worth: Squint: Its Causes, Pathology and Treatment, London, 1903. 
 
 26. Seefelder: -Arch. f. Ophth., 1909, lxx, i, 39. 
 
 27. Hess, C, and Heine, L.: Ibid., vol. xlvi, 2, p. 243; also Ophth. 
 Review, 1898, xvii, 253. 
 
 28. Barrett, J. W.: Ophth. Review, 1898, xvii, 255. 
 
 29. Beer, Th.: Wien. klin. Wohnschr., 1898; also Ophth. Review, 1898, 
 xvii, 272. 
 
 30. Smith, Priestley: Ophth. Review, 1898, xvii, 287. 
 
 31. Salzmann, M.: The Anatomy and Histology of the Human Eyeball. 
 Translated by Dr. E. V. L. Brown, Chicago, 1912. 
 
 32. Fuchs, E.: Arch. f. Ophth., 1918, xcv, 4. 
 
 33. Donders: Accommodation and Refraction of the Eye, London, 1866. 
 
 34. Iwanoff : Arch. f. Ophth., 1869, vol. xiv, 3, 284. 
 
 35. Dub, B.: Ibid., vol. xxxvii, 4, p. 26. 
 
 36. Weiss: Anatom. Hefte, 1897, vol. viii. 
 
 37. Lang, W., and Barrett: Roy. Lond. Ophth. Hosp. Rep., 1886, xi, 103. 
 
 38. Graber : Grundinien zur Erf orschung des Helligkeits u. Farbensinnes 
 der Thiere Prag., 1884. 
 
 39. Kalischer: Arch. f. Anat., 1909, p. 316. 
 
 40. Smith, Miss E. M.: British Jour, of Psychol., 1912, v, 119. 
 
 41. Kinnaman: Am. Jour, of Psychol., 1902, xviii, 173. 
 
 42. Dahl: Scrapbook, February, 1907. 
 
 43. Hess: Vergleichende Physiologie des Gesichtsinnes, Jena, 1912. 
 
 44. Allen, Grant: The Color Sense, its Origin and Development, London, 
 1879. 
 
 45. Wallace, Alfred Russell: Darwinism, 1889. 
 
 46. Darwin, Ch.: Descent of Man, London, 1882, 2d edit. 
 
 47. Weissman, A. : Darwin and Modern Science : The Selection Theory, 
 1909. 
 
 48. Owen, Sir R. : On the Anatomy of Vertebrates, 1868, vol. iii. 
 
 49. Bland-Sutton, Sir J.: Lancet, October, 1919, p. 673. 
 
 50. Motais: Encyclopedie francaise d'Ophtalmologie, ii, 665. 
 
 51. Whitwall, S. E.: Jour, of Ahat. and Physiol., vol. xlvi. 
 
 52. Miiller, H.: Siebold u. Kblliker's Zeitschrift, 1858, p. 541. 
 
 53. Turner, Sir W.: Natural History Review, 1862. 
 
 54. Crotti, Andre: Thyroid and Thymus, 1918. 
 
 55. Moore, Foster: Lancet, October 20, 1920.