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 A Comparative Study 
 
 Area of Acute Vision in Vertebrates 
 
 JAMES ROLLIN SLONAKER 
 
 Approz'cd as a Thesis for the degree of Doctor of Philosophy 
 
 in the department of Biology at 
 
 Clark University 
 June 20, 1896 C. F. HODGE 
 
 Reprinted from Journal of Morphology, Vol. XIII, No. 3 
 
 BOSTON 
 QINN & CONIPANY 
 
 W^z Sltfjetiteum press 
 1897 
 
A Comparative Study 
 
 Area of Acute Vision in Vertebrates 
 
 JAMES ROLLIN SLONAKER 
 
 Approved as a Thesis for the degree of Doctor of Philosophy 
 
 in the departtnetit of Biology at 
 
 Clark University 
 June 20, 1S96 C. F. HODGE 
 
 Reprinted from Journal of Morphology, Vol. XIII, No. 3 
 
 BOSTON 
 
 QITMN & CO^vIPANY 
 
 2ri)e aitfienafum pregg 
 
 1897 
 

A COMPARATIVE STUDY OF THE AREA OF 
 ACUTE VISION IN VERTEBRATES. 
 
 JAMES ROLLIN SLONAKER, 
 Fellow in Biology, Clark University. 
 
 Introduction. 
 
 This investigation has been pursued during the past three 
 years in the Neurological Laboratory of Clark University under 
 the direction of Dr. C. F. Hodge, to whom I am under great 
 obligations for his assistance and encouragement. I am also 
 greatly indebted to Clark University for the apparatus and 
 material which has made this work possible. 
 
 Up to the present I have been engaged chiefly in a gross 
 comparison of the retina rather than in its minute histology, 
 therefore my aim will be, first, to sum up the results of others 
 and also to add my own; second, to correlate as far as possible 
 the habits of the animal with its visual apparatus. 
 
 Since there are so many investigators who have written on 
 various phases of the eye, it will be impossible to mention all. 
 Reference, therefore, will be made to only a few of the most 
 important in the historical resume and literature on the subject. 
 
 I have adopted the nomenclature of the German investigators 
 and called the structure corresponding to the macula lutea of 
 man the area. According to the position of the area or fovea 
 on the nasal or temporal side of the optic nerve entrance, it is 
 called area or fovea nasalis or temporalis. 
 
 Historical.^ 
 
 On the basis of the methods of investigation employed, the 
 literature may be divided into three periods: (i) from the ear- 
 
 1 The literature on this subject has been fully presented by J. H. Chievitz 
 (Ueber das Vorkommen der Area centralis retinae in den vier hoheren Wirbel- 
 thierklassen, Arch. f. Anat. u. Entwickelungsgeschichte, 1891, Heft 4, 5, u. 6, pp. 
 311-321), but as I have not found it anywhere in English, I will devote some space 
 to it. 
 
446 SLONAKER. [Vol. XIII. 
 
 liest investigations till about 1830, or previous to the common 
 use of the microscope; (2) from the use of the microscope till 
 1887, or a period when the old methods of hardening and 
 staining were employed, which made only the nuclei and larger 
 processes visible; and (3) from 1887 to the present time, or 
 since the use of the silver chromate and the methyl-blue methods 
 of staining, which make clear not only the cells, but the finest 
 processes of both neurites and dendrites. 
 
 Although Francesco Buzzi (3) is given the credit of having 
 discovered the yellow spot in the human eye in 1782, it was 
 not until 1791 that the fovea centralis was noticed. This dis- 
 covery was made by the celebrated German anatomist, Sm. Th. 
 V. Soemmerring (2), and was called for a number of years the 
 " Foramen of Soemmerring," he having considered it a per- 
 foration. Buzzi, on the contrary, thought it merely a thin and 
 transparent part of the retina. Michaelis (4) favored Buzzi's 
 theory, while Reil (5), Meckel (6), and Home (7) considered it 
 a foramen. 
 
 The discovery of the foramen of Soemmerring in man natu- 
 rally led to many investigations in other classes of vertebrates. 
 Michaelis examined the eyes of the dog, swine, and calf, but 
 found no trace of a fovea. Home (7), however, was more for- 
 tunate. Knowing the great similarity which existed in the 
 anatomy of man and the monkey family, he wisely chose one of 
 the latter, and consequently was the first to discover the fovea 
 in the ape in 1798. He considered it a real foramen for the 
 passage of a lymphatic vessel, and tried to correlate it with 
 such a vessel in the optic nerve of the sheep and calf. Cuvier 
 (8) confirmed the presence of the fovea in the ape family, but 
 he considered it a thinning of the retina. This view gained 
 ground, but it was not firmly established till 1830, when v. 
 Ammon (9) demonstrated by the aid of the microscope that the 
 retina was continuous through the fovea. 
 
 Albers (10) found in 1808 "a central hole surrounded by a 
 yellow border " in the giant tortoise (Testudo mydas), but was 
 not able to confirm such an appearance in the second eye. 
 
 Knox (11) in 1823 was the first to demonstrate the presence 
 of a fovea in animals other than the primates. He examined 
 
No. 3-] ACUTE VISION IN VERTEBRATES. 447 
 
 the eyes of some reptiles and demonstrated the presence of a 
 well-defined fovea in the lizard (Lacerta scutata) and the cha- 
 meleon, Joh. Muller(i2) says: " Aforamen centrale is present 
 in the middle point of the retina of other reptiles, which is not 
 visible as in man, where the limiting membrane is unharmed, 
 but where the choroid shows through." H. Miiller {13), who 
 so admirably describes the eye of the chameleon and the retina 
 of the different classes of vertebrates, describes a well-defined 
 fovea found in many birds, while in some birds two foveae are 
 present. He also states that in mammals an area centralis is 
 present, which approaches in structure the yellow spot of the 
 human retina. 
 
 Three things were always sought for by the early investiga- 
 tors: the yellow spot, the foramen, and the folds of the retina, 
 which more or less concealed the foramen. Though Home had 
 described these folds as due to post mortem changes and not 
 present in the fresh eye, they were considered as normal by 
 many writers even as late as the middle of the present cen- 
 tury (i). 
 
 The old theory that the fovea was a foramen which enlarged 
 and contracted with the intensity of the light, thus protecting 
 the retina from injury, rapidly gave place with the use of the 
 microscope to the opposite view, that it was the place of acute 
 vision. The microscope further brought out the fact that the 
 cells of the yellow spot had a definite arrangement, and that 
 this arrangement might be present without a fovea. With this 
 thought in view investigations were made in all classes of ver- 
 tebrates with the result that a fovea has been found in each 
 class, and that an area centralis is quite common. 
 
 Hulke (14) has described the presence of a point in the retina 
 of several amphibians and reptiles which, owing to a similar 
 arrangement of cells, he thinks corresponds to the human fovea. 
 Gulliver (15) has described the presence of a fovea in the fish, 
 and Carriere (16) in Hippocampus. Hoffmann (17) describes 
 an arrangement in the crocodile which corresponds to that in 
 the fovea, Krause (18) treats of the eyes of different verte- 
 brates, and states that the dove and cat possess a fovea, while 
 the chicken and dog do not. He seems to be the only person 
 
448 SLO MAKER. [Vol. XIII. 
 
 who has found a fovea in the cat. Ganser (19) and Chievitz 
 (3 1 ) have found only an area. 
 
 Chievitz has described and pictured the area and fovea cen- 
 tralis in many animals, and put his results in a concise tabu- 
 lated form. Other investigators of this period will be mentioned 
 later in a similar tabulation. 
 
 Many obscure points were made clear by these numerous 
 investigators. However, two points still remained unsolved: 
 the structure of the molecular layers and the endings of the 
 fine branches of the retinal cells. The solution of these points 
 depended on a new method of research. This new method 
 was inaugurated by Tartuferi (21) in 1887, who used the quick 
 method of Golgi and succeeded in showing that the cell pro- 
 cesses end in more or less fine tufts which did not anastomose 
 with other bunches. Later on he discovered and described the 
 structure of the molecular layers, 
 
 Dogiel (22) in 1888 so modified the Ehrlich method that it 
 would stain the fresh retina. He was thus able to confirm 
 almost all the results of Tartuferi. He found that the branches 
 from different cells anastomose, but other investigators have 
 not confirmed this. The works of Baquis (23) and Ramon y 
 Cajal (24), who used the Golgi method, in general confirm the 
 results previously obtained. Ramon y Cajal has made clear 
 the endings of the rod and cone fibres in the outer molecular 
 layer. He finds that there are certain cells of the inner nuclear 
 layer which are related to the cones. That is, their terminal 
 branches come in contact only with the processes from the 
 cones, while other cells of this layer send their dendrites to the 
 rods. In general, each cell communicates with many rods or 
 cones, excepting in the fovea, where the process from each cell 
 branches very little and comes in contact with but one cone. 
 
 Methods. 
 
 As I have only attempted a gross comparison of the areas of 
 acute vision in this study, I have used only those hardening 
 fluids and methods of research which will preserve the eye with 
 the least possible distortion. For fine histological study of the 
 
No. 3-] 
 
 ACUTE VISION IN VERTEBRATES. 
 
 449 
 
 retinal cells, other methods such as that of Golgi or Ehrlich are 
 preferable. 
 
 For my purposes it is necessary to obtain the eye fresh, at 
 least not later than an hour after death, and subject it to the 
 action of certain hardening fluids which will permeate and pre- 
 serve without distorting the eye. Post vioj'tetn changes occur 
 in the retina very soon, such as wrinkling in the neighborhood 
 of the fovea, which obscure its .shape and size and make sec- 
 tions through it of little value. The eye is carefully oriented 
 in every case before it is removed from the head by sewing a 
 small tag to the outer layers of the sclerotic (Fig. i). In no 
 
 Fig. I. 
 
 case should the eye be punctured in removal, for this invariably 
 causes wrinkling of the retina and distortion of the ball. 
 
 I have tried many hardening fluids, but find that Perenyi's 
 fluid works the best. It not only preserves the eye with little 
 distortion, but also decalcifies all bone, thus making sections 
 even through the whole head with eyes in situ possible. The 
 different per cents of formaline which I have used have not 
 proved satisfactory, as they caused wrinkHng of the retina. 
 
 The former injection method ^ is now wholly replaced by that 
 of simple immersion, which is as follows : after the eye is prop- 
 erly tagged, it is carefully removed and immersed for from 24 
 to 36 hours in Perenyi's fluid. The time depends upon the 
 size of the eye and the amount of bone to be decalcified. It is 
 then changed to 70^ alcohol, and allowed to remain 24 hours. 
 Quite frequently when this change is made the ball caves in 
 and becomes somewhat distorted. This may be prevented or 
 
 1 American Naturalist, January, 1S96, p. 24. 
 
450 SLONAKER. [Vol. XIII. 
 
 the eye again made perfect by injecting into the vitreous 
 chamber with a hypodermic syringe enough 70^ alcohol to fill 
 out the eye. It is kept 24 hours in each of the following 
 liquids: 80, 90, 95, loo/o alcohol, and a mixture of absolute 
 alcohol and ether (one part each). 
 
 The eye is now well hardened and the front half may be cut 
 off, leaving the posterior half uninjured. After the hardened 
 vitreous humor is removed the retina is exposed to view. The 
 entrance of the optic nerve, area and fovea centralis, if present, 
 and the larger blood-vessels will be easily seen. In many cases 
 the area is very indistinct and the blood-vessels wanting or so 
 meagre as to be invisible to the naked eye. 
 
 When one wishes to section the eye, a window is cut in the 
 same plane of the desired sections and the hardened vitreous 
 humor carefully removed without injury to the retina or other 
 structures. It is then changed to celloidin. Best results are 
 obtained when three grades of celloidin are used: (i) very 
 dilute ; (2) less dilute ; (3) as thick as will run. It is allowed 
 to remain from four to six days in the first, six to eight days in 
 the second, and ten to fifteen days or longer in the third. It 
 is then mounted on a block and cut in 80^0 alcohol. In every 
 case when sufficient material was at hand sections were made 
 in vertical and horizontal planes. Serial sections were always 
 saved through the fovea, so that the central section could be 
 readily distinguished. Sections were stained in haematoxylin 
 and eosin and mounted in balsam. 
 
 In order to demonstrate more quickly the presence or 
 absence of an area or fovea centralis, the whole head of small 
 
 Fig. 2.: — Snow-bird (Junco hyemalis) 3/1. 
 /. Fovea centralis. .A''. Nerve entrance. P. Pecten. 
 
No. 3-] ACUTE VISION IN VERTEBRATES. 451 
 
 animals was immersed in the Perenyi for from three to six 
 hours, when the anterior part of the eye was hardened so that 
 the cornea, lens, and vitreous humor were easily removed, 
 leaving the posterior half in situ. Better results are obtained 
 when the skin is removed from the head before immersion. 
 
 a f<S .M 
 
 Fig. 3. — Tern (Sterna hirundo) i/i. 
 
 •V. Nerve entrance. a. Band-like area. ft. Area and fovea temporalis. 
 
 P. Pecten. fn. Area and fovea nasalis. 
 
 With birds I have had good results, the retina lying back 
 smoothly so that the fovea and entrance of the optic nerve, 
 marked by the pecten, may be easily seen. Figs. 2 and 3 
 represent the appearance of the retina after the front of the 
 eye has been removed. With other animals, especially 
 mammals, there is a greater tendency for the retina to 
 wrinkle. 
 
 Permanent demonstration material may be prepared by 
 subjecting the whole head to the different fluids as described 
 for the hardening of the eye. It is not necessary, however, to 
 carry it farther than 8ofo alcohol when the front half of the 
 eye and vitreous humor may be removed. Such material is 
 preserved in 8ofo alcohol. 
 
 Sections were made through the whole head of several 
 animals (fishes, amphibians, reptiles, birds, and some small 
 mammals) in order to determine approximately the angles 
 which the lines of vision make with the median plane. The 
 plane of section passed through each fovea or center of area 
 centralis and the center of the pupil. Fig. 4 represents such 
 a section through the foveae, /,_/", of a chickadee's head (Parus 
 atricapillus), while the lines GH and GI show the axes of 
 vision. The axes of vision, owing to the mobility of the eye, 
 
452 
 
 SLONAKER. 
 
 [Vol. XIII. 
 
 Fig. 4. — Chickadee (Panis atricapillus) 3/1. 
 f Fovea. / Pecten. GH and GI Axes of vision. 
 
 may be greater or less during life. The pecten, p, marks the 
 entrance of the optic nerve. When a second fovea is present 
 it is situated on the temporal side of the nerve entrance, as 
 shown in Fig. 5. 
 
 In order to show the relation of the retinal arteries to the 
 area and fovea centralis, they were injected with the gelatine- 
 carmine mass of Ranvier. In small animals this injection was 
 made in the carotid arteries, while with large animals the eyes 
 were removed and the injection made into that branch of the 
 ophthalmic artery which supplies the retina. After injection 
 the eye was at once cooled and hardened in alcohol. When 
 hardened, the front half of the globe and the vitreous humor 
 were carefully removed, exposing to view the retina, arteries, 
 entrance of nerve, and area and fovea centralis, when present. 
 Usually the fovea is readily seen if it is present, but the area 
 is sometimes very difficult to discern, and were it not for the 
 blood-vessels acting as landmarks, it might be overlooked 
 altogether. Drawings were made of the posterior half, great 
 care being taken to orient it so that one would look into it 
 
No. 3-] ACUTE VISION IN VERTEBRATES. 
 
 453 
 
 Fig. 5. — White-bellied Swallow (Tachycineta bicolor) 3/1. 
 
 NH and NI. Axes of vision of fovea nasalis. 
 TR and TL. " " " temporalis. 
 
 along the axis of vision. I have not attempted an exact repre- 
 sentation of the area, but have only indicated by dotted lines 
 its position and extent as I have found it (PI, XXVII, Figs. 3, 
 4, 8, 9). 
 
 The results of these injections only serve to substantiate 
 Miiller's observations (25). He states that mammals are the 
 only class of vertebrates which possess, in the true sense, a 
 retinal circulation, while in many mammals only a meagre cir- 
 culation is present (horse and rabbit). Denissenko (32) has 
 found an exception to this statement. He describes and 
 pictures retinal blood-vessels in the eel, which penetrate to the 
 outer nuclear layer. This is the only exception which I have 
 so far been able to find. In the sections which I have of 
 the eel's eye, owing to their thickness I have not been able 
 to demonstrate the presence of capillaries beyond the inner 
 boundary of the outer nuclear layer. Fishes and amphibians 
 possess a good circulation in the hyaloid membrane, while birds 
 
454 SLONAKER. [Vol. XIII. 
 
 and many reptiles have the circulation of the pecten. Huschke 
 126) states that these vessels of the hyaloid membrane and the 
 pecten correspond to the retinal vessels in mammals. They do 
 not, however, penetrate the retina. 
 
 The photographic representations of the fovea and area of 
 the different animals were all taken with the same magnification 
 so that they are directly comparable. In every case the section 
 through the center of the fovea was selected. In some cases 
 the section includes not only the bottom of the fovea, but also 
 some cells of the inner edge of the area beyond. In this case 
 the bottom of the fovea is more or less obscured by these cells. 
 In case the retina does not lie smoothly on the choroid, its 
 position is to be considered abnormal and due either to post 
 mortem changes or to the reaction of the reagents. 
 
 Description. 
 
 Before giving a description of the areas of acute vision in 
 the animals examined, a few words may be necessary regarding 
 the development, form, position, and prevalence of the area and 
 fovea centralis in different vertebrates. 
 
 In the development of a fovea an area centralis is first differ- 
 entiated (27). This stage, according to Chievitz, is present in 
 the human foetus about the sixth month, after which time the 
 fovea begins to appear (29). This takes place by a pitting in 
 of the vitreal surface, or a crowding to the sides, as it were, of 
 the cells in the center of the area. The area differs from the 
 rest of the retina either in thickness or in compactness of cells. 
 In some cases the difference in thickness is easily seen and 
 sharply marked off (PI. XXVII, Figs, i, 3, 6, 7, 8), while 
 in others the increase is very gradual and slight (PL XXVII, 
 ^'g- 5)- The increase in thickness is due usually to a greater 
 number of nerve cells, cells of the inner and outer nuclear 
 layers, and greater length of the rods and cones. But an area 
 is not necessarily designated by an increase in thickness of 
 any of these layers because the cells may be more numerous 
 and packed more closely together. This is well illustrated in 
 the nerve-cell layer of the frog's area, which is but a single cell 
 
No. 3-] ACUTE VISION JN VERTEBRATES. 455 
 
 deep over the entire retina. In the center of the area, however, 
 the cells lie closely together, while in other parts of the retina 
 they are some distance apart. 
 
 The rods and cones in the area have a less diameter and are 
 more numerous per given area than elsewhere. Hence the 
 cells which form their connection with the nerve fibres (nerve 
 cells and cells of the inner nuclear layer, or bipolar cells) must 
 also be more numerous. But this is not the only reason for an 
 increase in the number of cells in the area. Ramon y Cajal 
 (28) has described and pictured the manner in which these cells 
 form the connection between the nerve fibres and the rods and 
 cones. Numerous processes (dendrites) from the nerve cells 
 pass outward and branch profusely in the inner molecular layer 
 among the ingoing branches (neurites) of the cells of the inner 
 nuclear layer, A similar relation exists in the outer molecular 
 layer between the outgoing branches (dendrites) of these cells, 
 which are bipolar, and the ingoing branches (neurites) of the 
 cells of the outer nuclear layer. These fine branches from the 
 cells of different layers only come in close relation, and in no 
 case were they found to anastomose with each other. He 
 divides these bipolar cells into two classes: (i) those whose 
 dendrites come in contact with the neurites from the cone cells; 
 and (2) those whose dendrites come in contact with neurites 
 from the rod cells. In the periphery of the retina each bipolar 
 rod cell may come in contact with from 10 to 30 rods. Likewise 
 each bipolar cone cell is related to several cones. But toward 
 the center of vision the number of rods and cones which con- 
 nect with a single bipolar cell becomes rapidly less, and in the 
 center of the fovea each bipolar cell comes in contact with but 
 a single cone. Ramon y Cajal also finds a similar relation 
 existing between the cells of the nerve-cell layer and the 
 bipolar cells of the inner nuclear layer. 
 
 The number of cells in the outer nuclear layer is directly 
 dependent on the number of rods and cones, each rod and cone 
 having but a single nucleus. In fact a rod, or cone, with its 
 nucleus, has long been considered as a much drawn-out cell 
 whose dendritic end (the rod or cone) is more or less distant 
 from the nucleus, and is in some cases connected only by a 
 
456 SLONAKER. [Vol. XIII. 
 
 fibre. In the periphery of the retina each rod and cone nucleus 
 lies vertically over the rod or cone to which it belongs, and the 
 cone nucleus is in the base of the cone. But in the region of 
 the fovea these nuclei are crowded toward the periphery and lie 
 some distance from the rods and cones to which they belong. 
 This causes a diagonal appearance of these fibres in a cross 
 section of the retina. The cone nuclei here lie three or four 
 deep. The rods gradually decrease in relative number toward 
 the area or fovea, and in the center of the fovea they are wanting, 
 cones only being present. Borysiekiewicz (30) states, however: 
 " Within the fovea centralis all distinguishing characteristics be- 
 tween the rods and cones are lacking ; the so-called cones of the 
 fovea entirely resemble the rods in the periphery of the retina, 
 it is therefore correct to speak, not of the cones, but of the rods 
 of the fovea." Most investigators do not uphold his view. 
 
 The length of the rods and cones in the fovea and area 
 varies considerably in different animals. They may be longer 
 than in other parts of the retina, as in the crow (PI. XXIX, 
 Fig. 52), or shorter, as in the ring-neck plover (PI. XXIX, Fig. 
 47). This difference in length of rods and cones is shown by 
 a greater or less thickness of the pigment layer. Dimmer (29) 
 thinks in the human macula the rod and cone layer is of about 
 the same thickness as elsewhere. 
 
 If the front half and vitreous humor is removed from the 
 hardened eye, in many animals the area is readily seen as a 
 whiter region which has various shapes and positions. It is 
 never sharply marked off from the surrounding retina, but 
 gradually blends with it. Its form may be circular, oval, or 
 band-like. In the latter case it may also contain a circular 
 area, as in the ring-neck plover, goose, and tern (PI. XXVII, 
 Figs. 8, 12). The round area may be situated on the nasal side 
 of the nerve entrance, in which case it is designated area 
 nasalis ; or it may be on the temporal side, and designated 
 area temporalis. Usually at least one of these different kinds 
 of areas is present, and all three may be present as in the tern. 
 In many eyes, however, when examined in this macroscopical 
 way, no area is visible. It is not until sections are made and 
 subjected to careful microscopical examination and measure- 
 
No. 3-] ACUTE VISION IN VERTEBRATES. 457 
 
 merit that an area is discerned. Even with the aid of the 
 microscope the presence of an area is often very doubtful. 
 
 The fovea is also found to have a variety of forms and posi- 
 tions. In depth it may vary from the very questionable de- 
 pression found in the guinea hen (PI. XXVIII, Fig. 36) to the 
 very deep pit of the crow or hawk (PI. XXIX, Figs. 48, 50, 52). 
 It has also been found to vary in the same species, but this 
 may be due to slight swelling produced in hardening (pigeon, 
 PI. XXVIII, Figs. 37, 38). In size it varies from the very broad 
 fovea in the human (PI. XXVIII, Figs. 24, 25) to the exceedingly 
 sharp depression in the sparrow hawk (fovea nasalis) and the 
 horned toad (PI. XXX, Fig. 56), Chievitz (31, a) claims that 
 certain animals have also a trough-like fovea, such as the tern, 
 etc. I have not been able to procure any of the species which 
 he describes as having a trough-like fovea, but have succeeded 
 in obtaining a species from the same genus. Sterna. In a 
 macroscopical examination one would at once conclude that 
 there was a trough-like fovea present, but when sections were 
 made across this trough-like appearance, no depression was 
 found (PI. XXIX, Figs. 40, 42). In my researches I have seen 
 nothing excepting the macroscopical appearance which might 
 be taken to indicate the presence of a trough-like fovea in any 
 of the animals examined. Chievitz has pictured only the 
 macroscopical view of his trough-like fovea, and in no place 
 have I found that he has made cross sections and microscopical 
 examination. In the tabulation which follows I have used his 
 descriptions, as I have not been able to examine the species 
 which he has described. 
 
 The relations of the fovea correspond to the positions above 
 described for the area. That is, we find a fovea nasalis, as in 
 the crow, blue jay, robin, snow-bird, etc., and z. fovea temporalis, 
 as in man, gorilla, owl, tern, hawks, etc. One or two foveas 
 may be present, but in each case where two are present the 
 nasal fovea is always deeper than the temporal. 
 
 A very noticeable and important difference in the position of 
 the fovea in various birds has been observed. Very little vari- 
 ation is found in the position of the fovea nasalis, but the loca- 
 tion of the fovea temporalis depends wholly upon the position 
 
458 SLONAKER. [Vol. XIII. 
 
 of the eye in the head. As the eyes are turned more and more 
 forward, the fovea temporalis approaches the fovea nasalis, and 
 as binocular vision becomes more frequent, the nasal fovea 
 becomes less distinct or merges into the temporal. This change 
 is shown in Plate XXXI. The change to an asymmetrical 
 form, and the position of the lens in the eyes of the birds which 
 possess the power of binocular vision, is also quite marked. In 
 many of these birds, as the tern and white-bellied swallow 
 (PI. XXIX, Figs. 40, 41, 46, and PI. XXXI, Figs. 64, 65), 
 where the fovea nasalis is sharp and deep and the fovea tem- 
 poralis quite shallow, the eyes are almost symmetrical. But 
 in those birds which use binocular vision more as shown by a 
 greater depth of the temporal fovea, as in the hawks (PI. 
 XXIX, Figs. 48-51, and PI. XXXI, Figs, m, 67), the eye 
 becomes more asymmetrical, and finally reaches the most 
 irregular form in the owl (PI. XXXI, Fig. 68), where binocular 
 vision only occurs. 
 
 Various combinations of area and fovea are found in different 
 animals. The most simple is a single fovea surrounded by a 
 circular area, as in the primates and most birds. Again we 
 find a simple fovea surrounded by a round area which is con- 
 tinuous with a band-like area extending horizontally across the 
 retina, as in the goose and ring-neck plover. One or two foveae 
 may be present, each surrounded by a round area and connected 
 by a short, slight, band-like area, as in the sparrow hawk, red- 
 tailed buzzard, and kingfisher. The most complex combination 
 which I have found is in the tern. Here the fovea temporalis 
 surrounded by a small round area is not connected with the 
 band-like area which extends horizontally across the retina, and 
 near its middle widens out into a round area surrounding the 
 fovea nasalis (PI. XXVII, Fig. 12). 
 
 In my researches I have been able to examine 93 different 
 species, of which 18 were mammals, 41 birds, 6 reptiles, 3 am- 
 phibians, and 25 fishes. In some cases the results were doubt- 
 ful, and sufficient material was not available to ascertain all 
 points with certainty. Such cases I have indicated. Many of 
 the species examined have been observed by others, in which 
 case I have always aimed to give credit to the first observer. 
 
No. 3-] ACUTE VISION IN VERTEBRATES. 459 
 
 In the animals which I have examined the space for the 
 observer is left blank, excepting when the species has been 
 previously examined, in which case I have inserted (S) after 
 the name of the first investigator. 
 
 In the following tabulation I have adopted the form of 
 Chievitz, and have endeavored to present the results of all 
 investigators up to the present time. Some results have 
 necessarily been omitted, as the investigators in their descrip- 
 tions have given only the common name and not the species of 
 the animal. 
 
460 
 
 SLONAKER. 
 
 [Vol. XIII. 
 
 
 
 
 
 
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No. 3-] ACUTE VISION IN VERTEBRATES. 463 
 
 
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 ACUTE VISION IN VERTEBRATES. 
 
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468 
 
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No. 3-] ACUTE VISION IN VERTEBRATES. 469 
 
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470 
 
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No. 3-] ACUTE VISION IN VERTEBRATES. 47 1 
 
 
 
 
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472 
 
 SLONAKER. 
 
 [Vol. XIII. 
 
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No. 3.] 
 
 ACUTE VISION IN VERTEBRATES. 
 
 473 
 
 The following tabulation, condensed from the foregoing, will 
 show at a glance the prevalence of an area and fovea centralis 
 in the different classes of vertebrates. 
 
 It 
 
 II 
 
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 One 
 
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 51 
 
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 10 
 
 38 
 
 31 
 
 
 8 
 
 18 
 
 
 
 102 
 
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 I 
 
 59 
 
 II 
 
 36 
 
 7^ 
 
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 22 
 
 25 
 
 Reptiles 
 
 3(?) 
 
 17 
 
 20 
 
 
 3 
 
 6 
 
 
 2 
 
 14 
 
 Amphibians 
 
 3 
 
 " 
 
 3 
 
 
 8 
 
 2 
 
 
 
 30 
 
 Fishes 
 
 10 
 
 25 
 
 20 
 
 
 
 5 
 
 
 
 Mammals. 
 
 Mammals as a class are characterized by the absence of a 
 fovea, the primates being the only ones in which it has been 
 found. As a rule an area is present, though in some cases 
 even an area has not been demonstrated. H. Miiller (13) says: 
 " In mammals there is at least an area centralis present which 
 approaches in structure the yellow spot, and is made known by 
 a similar course of the blood-vessels as in man." If such is 
 the case, I have failed in some instances to demonstrate the 
 presence of such an area. 
 
 In some mammals the area is readily seen with the naked 
 eye, but in the majority of those I have been able to examine 
 such is not the case. In many instances vertical and horizontal 
 sections have to be made and subjected to microscopical exam- 
 ination and measurement before a thickening or an arrangement 
 of cells indicating an area is found. In some the very slight 
 thickening is marked also by an increase in thickness of the 
 tapetum. 
 
 I will now proceed to a more detailed description of the 
 area and fovea in the mammals which I have studied. I shall 
 not, however, enter into the histological arrangement of the 
 cells. 
 
474 SLONAKER. [Vol. XIII. 
 
 _4< ^•hcrr^a^j 
 
 Human and Gorilla. 
 
 The fovea and macula lutea are readily seen, located about 
 4 mm. toward the naoft^ side of the entrance of the optic nerve. 
 The macula is rather sharply marked off from the surrounding 
 retina, and is of small extent compared with other mammals. 
 The blood-vessels in either of these cases were not injected, 
 but they could be traced as far as represented in PI. XXVII, 
 Figs. I, 2. Figs. 24, 25, PI. XXVIII, represent horizontal sec- 
 tions through a child's eye and an adult's respectively. The 
 foveola described by Dimmer (29) is much more noticeable 
 in the adult (Fig. 25) than in the child's fovea. In the case of 
 the gorilla, which was about nine hours post mortem, folds had 
 formed about the fovea so that its appearance is not well rep- 
 resented in sections. PI. XXVIII, Fig. 26, represents the 
 horizontal section and Fig. 27 the vertical section through 
 the center of the fovea. 
 
 Rabbit (Lepus sylvaticus). 
 
 The nerve entrance is readily seen above the center and a 
 little toward the temporal side. From it two large bundles of 
 nerve fibres branch out horizontally. In the injected specimen 
 the blood-vessels are seen to lie in these bundles, and do not 
 branch over the rest of the retina. The band-like area is seen 
 to extend horizontally across the retina, immediately below the 
 nerve entrance, and to gradually fade out just before reaching 
 the ora serrata. It is from ^ to i mm. broad (PI. XXVII, 
 Fig. 13). 
 
 Rat (Mus rattus). 
 
 I have not succeeded in demonstrating the presence of a 
 definite area in this animal. 
 
 WoODCHUCK (Arctomys monax). 
 Red Squirrel (Sciurus hudsonicus). 
 Fox Squirrel (Sciurus niger). 
 Chipmunk (Tamias striatus). 
 
 No area is visible to the naked eye, but in horizontal and 
 vertical sections a slightly thicker oblong or oval area is dis- 
 
No. 3-] ACUTE VISION IN VERTEBRATES. 475 
 
 cernible. This I have called the area centralis. It is situated 
 near the center of the retina, but slightly above and toward 
 the temporal side. The nerve entrance is very noticeable and 
 of unusual shape. The nerve is flattened out fan-like just 
 before piercing the sclerotic, so that the papilla is narrow and 
 elongated. PI. XXVII, Fig. 14, represents the entrance of 
 nerve and the position of the area as nearly as I can ascertain 
 it in Sciurus niger. 
 
 Bat (Vespertilio subulatus). 
 
 I have not been able with the material at hand to demonstrate 
 an area. 
 
 Sheep (Ovis avies). 
 
 Chievitz (31, b) has described this area as not visible to the 
 naked eye, round, about 4 mm. in diameter, and located about 
 8 mm. toward the temporal side of the nerve entrance. I have 
 examined more than 20 eyes, and in every case find a white, 
 band-like region, about 1-2 mm. broad, extending horizontally 
 across the retina, gradually becoming invisible to the naked 
 eye just before reaching the ora serrata. It compares favorably 
 in every respect with the area centralis of the cow. It lies 
 above the nerve entrance, which is below the center and toward 
 the temporal side. PI. XXVII, Fig. 8, represents the position 
 and extent of the area and its relation to the blood-vessels and 
 nerve entrance in the left eye. 
 
 Cow (Bos taurus domesticus). 
 
 A horizontal band-like area 1-2 mm. broad is present, hav- 
 ing the same general relation to the nerve entrance and blood- 
 vessels as found in the sheep (PI. XXVII, Fig. 6). 
 
 Pig (Sus domesticus). 
 
 A band-like area about i mm. broad passes horizontally across 
 the retina, and has the same relation to the blood-vessels and 
 nerve as that described for the sheep and cow. The nerve 
 entrance is nearer the center of the retina (PI. XXVII, 
 Fig- 9)- 
 
476 SLONAKER. [Vol. XIII. 
 
 Horse (Equus caballus). 
 
 The band-like area is here very broad, 5-7 mm., and extends 
 horizontally across the retina. The nerve entrance is below 
 the center and slightly toward the temporal side. The blood- 
 vessels are very meagre, and, according to Miiller, extend over 
 but a small portion of the retina, leaving the area centralis and 
 the entire upper part of the retina free from blood-vessels. The 
 blood-vessels are usually not visible unless they are injected 
 (PI. XXVII, Fig. 7). 
 
 Cat (Felis catus domesticus). 
 
 Chievitz (31, c) has described the area as round and not visi- 
 ble to the naked eye and located toward the temporal side of 
 the nerve entrance. Ganser (19) has described it as round, 
 and Krause (18) has stated that the cat possesses a fovea as 
 well as an area. The retinal blood-vessels (PI. XXVII, Fig. 4) 
 would indicate as much as those of the sheep or cow the pres- 
 ence of a band-like area. Or the finer branches radiating 
 toward a common point on the temporal side of the nerve 
 entrance would suggest a round area similarly located as that 
 described by Chievitz. In most cases a region similar to that 
 indicated by the dotted lines and having the same macroscopical 
 appearance as an area is observed. This appearance may be 
 due to the tapetum lying behind. The lower margin of this area- 
 like region corresponds very closely with that of the tapetum. 
 In sections I have not found a well-defined round area, but a 
 general thickening over the greater part of the region indicated. 
 
 Skunk (Mephitis mephitica). 
 Mink (Putorius vison). 
 
 No area is visible to the naked eye, but in horizontal and 
 vertical sections an oblong or oval thickening is found located 
 a little above and on the temporal side of the nerve entrance, 
 which in these animals is central. 
 
 Fox (Vulpes vulpes). 
 
 A horizontal, band-like region extending across the retina 
 just above the nerve entrance may be seen with the naked eye 
 
No. 3-] ACUTE VISION IN VERTEBRATES. 477 
 
 (PL XXVII, Fig. 5). In the representation the retinal vessels 
 were not injected, and the smaller branches could not be accu- 
 rately made out. In cross sections only a slight thickening of 
 the retina is noticed, and the lower edge of the indicated region 
 corresponds with the lower margin of the tapetum. 
 
 Dog (Canis familiaris). 
 
 The retinal blood-vessels (PL XXVII, Fig. 3) indicate the 
 presence of a band-like area. Again the finer and more numer- 
 ous branches radiating toward a common spot on the temporal 
 side of the nerve entrance points to the presence of a round 
 area. Neither are visible to the naked eye, but Chievitz (31, b) 
 has described the presence of a round area in this latter position. 
 I have not succeeded in demonstrating it. 
 
 Birds. 
 
 Birds are characterized by the presence of a fovea, although 
 a few cases are very doubtful (hen and guinea hen). Chievitz 
 says (31, c) that at least a round area is always present which 
 regularly possesses a fovea, sometimes very clearly seen, and 
 in other cases so shallow as to be very doubtful (duck and 
 hen). 
 
 Where but a single fovea is present the position and form 
 are so similar, as shown in the tabulation, that a large number 
 may be described together. As a rule it is situated about the 
 center of the retina, a short distance above and toward the nasal 
 side of the optic nerve entrance. The nerve entrance is always 
 more or less obscured from view by the pecten, which extends 
 obliquely from the point of entrance downward and forward, so 
 that a line joining the fovea and nerve entrance forms about a 
 right angle with the pecten (PL XXVII, Figs. 17, 23). The 
 fovea, with but few exceptions which will be described separately, 
 is surrounded by a simple round area more or less sharply 
 marked off from the surrounding retina. The fovea varies 
 considerably in depth. In the tabulation I have classified them 
 as deep, medium, and shallow. 
 
 Most birds possess a deep and well-defined fovea, as seen in 
 the following: 
 
478 SLO MAKER. [Vol. XIII. 
 
 Robin (Merula migratoria, PI. XXVII, Fig. 17, and PI. XXVIII, Fig. 28). 
 
 Blue-Bird (Sialia sialis, PI. XXVIII, Fig. 29). 
 
 Kinglet (Regulus satrapa, PI. XXVIII, Fig. 30). 
 
 Sx\ow-BiRD (Junco hyemalis, PL XXVIII, Fig. 31). 
 
 Crow (Corvus americanus, PI. XXIX, Fig. 52). 
 
 Blue Jay (Cyanocitta cristata, PL XXIX, Fig. 53). 
 
 Night Heron (Nycticorax nycticorax, PL XXVII, Fig. 23). 
 
 A number of birds possess a medium fovea, as seen in the 
 pigeon (Columba livia domestica, PI. XXVIII, Figs. 37, 38). 
 It is readily observed surrounded by a well-defined area. It 
 varies somewhat in depth in the same species, as is shown in 
 this case. Fig. 37 represents a medium fovea, while Fig. 38 
 would be classed as shallow. 
 
 Most of the Gallinas which I have examined possess medium 
 to very shallow fovea. The quail (Colinus virginianus) and the 
 partridge (Bonasa umbellus) each possess a medium fovea, 
 while in the turkey (Meleagris gallopavo, PI. XXVIII, Fig. 35) 
 and the guinea hen (Numida pucherani, PI. XXVIII, Fig. 36) 
 it is shallow. In the last case the depression is so slight as to 
 scarcely deserve the name of fovea. Chievitz mentions an area 
 nasalis and a questionable fovea in the hen (Gallus doraesticus). 
 I have succeeded in finding only a very slight thickening. 
 
 Screech Owl (Megascops asio). 
 Barred Owl (Syrnium nebulosum). 
 
 These owls possess a single deep fovea surrounded by a 
 sharply defined round area which differs from those just 
 described only in position. It is located on the temporal side 
 and above the nerve entrance in such a position as to function 
 in binocular vision. The nerve entrance is similar in position 
 to that of other birds, but the pecten is much smaller in pro- 
 portion to the size of the eye (PI. XXIX, Fig. 55, and PI. 
 XXVII, Fig. 10). 
 
 Goose (Anser cinereus domesticus). 
 
 The goose possesses a shallow fovea nasalis surrounded by a 
 round area situated on a band-like area extending horizontally 
 through the retina. The fovea and round area are easily 
 observed with the naked eye, but the band-like area is much 
 
No. 3-] ACUTE VISION IX VERTEBRATES. ^jg 
 
 less distinct. Vertical sections across this area show only a 
 slight increase in thickness, both on the nasal and temporal side 
 of the fovea (PL XXVIII, Figs. 32-34)- 
 
 Tame Duck (Anas boschas domesticus). 
 Surf Duck (Oidemia deglandi). 
 
 Similar relations exist here as in the goose. The fovea is 
 quite shallow, and is surrounded by a distinct round area which 
 is situated on a band-like horizontal area (PI. XXVIII, Fig. 39). 
 
 Rixg-Xeck Plover (iEgialitis semipalmata). 
 
 A very distinct band-like area is seen passing obliquely 
 through the retina. A dark line, resembling a trough-like 
 fovea, extends almost the full length through the center of this 
 area. Cross sections reveal, however, no trough-like fovea. 
 The single fovea nasalis, surrounded by a sharply bounded 
 round area, is observed located about the middle of the band- 
 like area. It is of medium depth and- readily seen by the naked 
 eye (PI. XXVII, Fig. 20, and PI. XXIX, Fig. 47). 
 
 Sparrow Hawk (Falco sparverius). 
 
 A fovea nasalis and a fovea temporalis, each surrounded by 
 a sharp round area connected by a short band-like area, are 
 easily observed. The fovea nasalis is very deep and sharp and 
 is situated about the center of the retina. The fovea tempo- 
 ralis, somewhat shallower, is situated near the ora serrata about 
 the same distance from the nerve entrance as the fovea nasalis, 
 but in a lower plane. The area temporalis is likewise smaller 
 than the area nasalis. The band-like area is not sharply 
 bounded, is of slight thickness, and extends only between the 
 two round areas. The fovea temporalis is similar in position 
 to that of the owl, and the fovea nasalis to that of the crow, 
 robin, etc. (PI. XXVII, Fig. 19, and PI. XXIX, Figs. 48, 49). 
 
 Red-Tailed Buzzard (Buteo borealis). 
 
 Almost the same conditions exist as found in the sparrow 
 hawk, excepting the two foveae are relatively nearer together 
 (PL XXVII, Fig. II, and PL XXIX, Figs. 50, 51). 
 
480 SLONAKER. [Vol. XIII. 
 
 Kingfisher (Ceryle alcyon). 
 The same conditions exist as are found in the hawk (PI. 
 XXIX, Figs. 44, 45). 
 
 White-Bellied Swallow (Tachycineta bicolor). 
 
 A fovea nasalis and a fovea temporalis are easily seen, each 
 surrounded by a round area situated on a band-like area extend- 
 ing obliquely across the retina. The positions of the foveae 
 are very similar to those described in the hawks. The fovea 
 and area temporalis are likewise smaller, and are situated nearer 
 the ora serrata than the fovea and area nasalis in the hawks. 
 The area and fovea nasalis are shown in PI. XXIX, Fig. 46. 
 
 Common Tern (Sterna hirundo). 
 
 In this case both nasal and temporal foveas surrounded by 
 round areas are present, and in addition a band-like area. The 
 area nasalis is located on the band-like area about the center of 
 the retina, but the area temporalis is above the band-like area, 
 and apparently in no way connected with it. A dark line, 
 resembling a trough-like depression, extends through the center 
 of the band-like area, through the fovea nasalis, and terminates 
 near the entrance of the optic nerve (PI. XXVTI, Fig. 12). A 
 cross section of this area, given in PI. XXIX, Figs. 40, 42, 
 fails to demonstrate such a depression. The temporal end of 
 the band-like area widens and soon becomes indistinct. The 
 fovea temporalis is very shallow and might be overlooked. It 
 is located near the ora serrata a little above the median hori- 
 zontal plane. The fovea nasalis is deep and easily observed 
 (PL XXIX, Figs. 41, 43). 
 
 Reptiles. 
 
 In the tabulation twenty-five species are mentioned. All but 
 three are described as having an area, and these three are ques- 
 tionable. Eight of the number possess a well-defined fovea, 
 while two are doubtful. 
 
 In snakes an area seems to be the rule. In the three 
 species I have examined, the retina was not sufficiently well 
 
No. 3] ACUTE VISION IN VERTEBRATES. 48 1 
 
 preserved to make certain the presence of an area. It is only 
 visible in sections. 
 
 In the lizard an area has been described in every case, and a 
 fovea in all but two, which are doubtful. The only lizard which 
 I have examined, the horned toad (Phrynosoma cornutum), pos- 
 sesses a deep and sharp fovea, situated on a broad band-like 
 area. The fovea is situated about the center of the retina, just 
 above the entrance of the optic nerve, which is marked by a 
 slender conical pecten. The band-like area is broadest in the 
 region of the fovea, and extends horizontally across the retina. 
 A dark line extends about i mm. to either side from the fovea 
 and gives the appearance of a trough-like fovea, as seen in the 
 tern, but cross sections reveal no depression. The band-like 
 area gradually becomes indistinct some distance from the ora 
 serrata (PI. XXVII, Fig. 15, and PI. XXX, Figs. 56, 57). 
 
 In the turtles only an area has been found which is oval or 
 round in shape, and lies about the center of the retina, just 
 above the nerve entrance. It is not visible to the naked eye, 
 and in sections is noticed rather as a closer arrangement of 
 the cells than as a thickening. PI. XXX, Fig. 61, represents 
 a section through the area of Chelydra serpentina. A repre- 
 sentation of the entire retinal section would be necessary to 
 show any difference in thickness. In an injected specimen of 
 Chelopus insculptus, a short and seemingly rudimentary blood- 
 vessel was noticed (PI. XXVII, Fig. 16) which seemed to be 
 an approach to a retinal circulation. In the other eye it was 
 not so long but similarly located. 
 
 In the crocodiles Chievitz has described and pictured a band- 
 like area and shallow trough-like fovea which extend horizontally 
 through the entire retina. I have not been able to examine any 
 species of this order. 
 
 Amphibians. 
 
 The presence of an area and absence of a fovea seems to be 
 the rule. Hulke and Chievitz, however, have described a shal- 
 low fovea in Bufo vulgaris and Bufo calamitia, though in some 
 cases it is wanting. I have found a band-like area in Bufo 
 lentiginosus, Rana virescens and catesbiana. It is not visible 
 
482 . SLONAKER. [Vol. XIII. 
 
 to the naked eye and is demonstrated only in vertical sections 
 by a slight and gradual increase in thickness, principally in the 
 inner nuclear layer and in the closer arrangement of the nerve 
 cells. The position of the area is outlined in PI. XXVII, 
 Fig. 18, as found in Rana catesbiana. PI. XXX, Fig. 62, 
 represents the vertical section through the area. 
 
 Fishes. 
 
 Fishes seem to be characterized, as a rule, by the absence of 
 both a fovea and a well-defined area. Nothing is visible to the 
 naked eye excepting in a few cases, which will receive special 
 mention. If sections of the eye, however, are subject to micro- 
 scopical measurement, an oblong or oval region, slightly thicker 
 than the rest of the retina, is found located on the temporal 
 side and a little above the center. In fact, the whole upper 
 half of the retina is somewhat thicker than the lower half in 
 all fishes which I have examined. That region indicated above, 
 however, is the thickest, and I have designated it the area 
 centralis. It also corresponds in position to that of the fovea 
 when a fovea is present. Some of the material at hand was not 
 sufficient to demonstrate clearly the presence of such an area. 
 Such cases I have indicated as doubtful. PI. XXX, Fig. 63, 
 represents a section through the area of the flounder (Para- 
 lichthys dentatus), but no increase in thickness is visible in so 
 small a portion of the retina. 
 
 Krause (37) has described the presence of a round area and 
 shallow fovea in Syngnathus typhle, and Carriere (16) has 
 described and pictured a similar area and fovea in Hippocampus. 
 Gulliver (15) has described a round area and shallow fovea in 
 Pagellus centrodonpus (.?). Schiefferdecker (38) has described 
 a similar area and fovea in Pleuromectes platessa. I have not 
 been able to procure any of these species, but have found an 
 area and fovea in another species. 
 
 Pipefish (Siphostoma fuscum). 
 
 The eye of this fish being so small, I have not attempted a 
 macroscopical examination. The area and fovea are, however, 
 
No. 3-] ACUTE VISION IN VERTEBRATES. 483 
 
 probably visible to the naked eye. In horizontal sections the 
 area and fovea may be readily seen. The fovea is broad and 
 shallow when compared with that of most birds and some 
 reptiles. It is located on the temporal side, about midway 
 between the nerve entrance and the ora serrata, and a little 
 above. A horizontal section through the nerve passes through 
 the area below the fovea, as shown in PI. XXX, Fig. 59 (i). 
 A section through the fovea is shown in Fig. 58 (i). 
 
 Physiological. 
 
 In order to make a physiological comparison of the areas of 
 acute vision in the different vertebrates, the exact function of 
 the different elements of the retina must be known. Most 
 physiologists agree on the functions of all the elements except- 
 ing the rods and cones. All are agreed, however, that the rods 
 and cones are the elements which give the sensation of sight, 
 but just the function of each is very obscure. The source of 
 information regarding the functions of the rods and cones has 
 necessarily been confined to man. When this has been finally 
 settled, a more accurate comparison of the powers of sight in 
 the different vertebrates can be made. 
 
 A great many theories have been advanced regarding the 
 functions of the rods and cones, and as these theories cannot 
 be fully verified or tested by physiological experiments, they 
 will have to be accepted as such. 
 
 What the changes are which take place in the retina during 
 an act of sight had long been a mystery till the visual purple 
 was discovered in the rod and cone layer by Boll. This, how- 
 ever, sufficed for only a short time, as it was soon found that 
 the cones possessed no visual purple, or at least none could be 
 demonstrated in them. Since the cones are the only sensitive 
 elements in the fovea, some other photo-chemical substance 
 must be present in them. The theories of Young-Helmholtz, 
 Herring, Mrs. Franklin, etc., agree generally in the functions 
 of the rods and cones, but differ in the photo-chemical sub- 
 stance and its change in an act of sight. Since the theories of 
 Young-Helmholtz and Herring do not ascribe different func- 
 
484 SLONAKER. [Vol. XIII. 
 
 tions to the rods and cones, I shall not refer to them further. 
 Mrs. Franklin (39) bases her theory on carefully conducted 
 experiments testing the sensitiveness of different regions of 
 the retina to various colors and intensities of light. She 
 assumes the presence of two kinds of molecules in the photo- 
 chemical substance of the retina: (i) gray molecules which 
 give rise to the sensation of gray; (2) color molecules which 
 have been differentiated from the gray, and whose atoms of 
 the external layer are arranged in three directions at right 
 angles to each other. These give the sensation of color. She 
 would thus attribute to the rods the perception of uncolored 
 light, for she says (40, a) : " In the very eccentric part of the 
 retina the differentiation of the color molecule out of the gray 
 molecule has not taken place; these parts of the retina are 
 chiefly useful to us in warning us of danger from moving 
 insects and other enemies, and for this the power to detect dif- 
 ferences of brightness is sufficient." Again (41): ^^ Only the 
 cones are sensitive to variations of color; they must be extremely 
 sensitive to variations of intensity in white light as well, — 
 otherwise the fovea would not be the place with which we make 
 out the minutest variations of line and shade in an intricate 
 drawing. If the cones only give color, they do not give color 
 only." Her experiments, as well as those of Konig (44), 
 further show that the fovea is blind to blue, and is not able to 
 perceive other colors when the illumination is faint, seeing them 
 only as "different intensities of gray" (40, b). In color-blind 
 people she finds that they are blind in the center of the fovea, 
 but have come to use a small spot on the edge of the fovea as 
 the point of acute vision (42). The maximum sensitiveness 
 of the retina to faint impression is found to be about 25° from 
 the fovea where it is four times as sensitive, and at 50° it is 
 still twice as sensitive as the fovea. These gray and color 
 molecules are, of course, only theoretical and cannot be demon- 
 strated. The gray molecules, without doubt, correspond to 
 the visual purple of other writers, which is found only in the 
 rods. The results of the various experiments on the sensitive- 
 ness of the retina to different colors correspond closely with 
 the arrangement of the rods and cones. In the center of the 
 
No. 3-] ACUTE VISION IN VERTEBRATES. 485 
 
 fovea, where only cones exist, colors are most easily perceived, 
 while in the periphery, where there are few cones, it is difficult 
 to distinguish them. 
 
 M. H. Parinaud (43) has found by experiment on the excised 
 retina that the visual purple (hence the rods) cannot be demon- 
 strated nearer the center of the fovea than two millimeters. 
 From this place the rods are found to increase in number 
 toward the periphery and the cones to decrease. 
 
 Again, the retina being four times as sensitive to faint 
 impressions 25° from the fovea as at the fovea, and since at 
 this distance the rods are far more numerous than the cones, 
 we can consider the functions of the rods fairly well determined 
 to be the perception of diffuse and gray lights. 
 
 To sum up: (i) the rods and cones are the sensitive elements 
 of sight; (2) the rods give us the sensation of gray, while the 
 cones give us the sensation of color and gray; (3) the rods are 
 more sensitive to faint impressions than the cones; (4) the 
 elements of the other layers form the connection with the optic 
 nerve. 
 
 With this in view concerning the functions of the retinal 
 elements in man, and supposing the functions to be the same 
 in the other vertebrates, a physiological comparison may be 
 attempted. 
 
 Quite a difference is noticed in the relative thickness of the 
 layers of the retina of the different vertebrates. This is shown 
 diagrammatically in PI. XXVII, Fig. 22. The layers which 
 exhibit the greatest difference are the inner and outer nuclear 
 layers and the rod and cone layer. In mammals the outer 
 nuclear layer is much thicker than the inner, while in birds, 
 reptiles, amphibians, and fishes the reverse is true. 
 
 The layer which shows the greatest diversity, however, is 
 that of the rods and cones. A great difference exists in their 
 size, length, shape, and relative number. Fishes possess the 
 longest rods, while amphibians have not only long rods, but 
 also the thickest found in the vertebrates. The rods of mam- 
 mals are long but very slender, while in birds they are compar- 
 atively short and thick. The cones are the longest and most 
 slender in some of the reptiles (chameleon), and of greatest 
 
486 
 
 SLONAKER. 
 
 [Vol. XIII. 
 
 diameter in the mammals. They are about the same length in 
 mammals, birds, and amphibians, while in fishes they are 
 shorter. In birds the diameter of the cones approaches very 
 closely to that found in the reptiles. The following tabulation 
 of measurements compiled from Miiller's descriptions (20) of 
 the rods and- cones of different animals will make clear these 
 relations. These measurements are in millimeters. 
 
 
 Rods 
 
 Cones 
 
 Diameter 
 
 Length 
 
 Diameter 
 
 Length 
 
 Human 
 
 .OOI5-.OO18 
 
 .04-.06 
 
 .OO4-.O06 
 
 .032-.036 
 
 Pigeon 
 
 .O026-.OO33 
 
 .02-.028 .001-005 
 
 .O25-.03 
 
 Chameleon 
 
 
 
 .00 1 -.00 1 3 
 
 .06-. 08 
 
 Frog 
 
 .006-007 
 
 .04-.06 
 
 .005 
 
 .02-028 
 
 Perch 
 
 .0026 
 
 .04-.05 
 
 
 .008-012 
 
 The diameter of the rods and cones is of great importance 
 when the sensitiveness of the retina of different animals is con- 
 sidered. Since these sensitive elements always lie as closely 
 together as possible, the animals in which their diameter is 
 small would have more per given area, hence a more sensitive 
 retina. 
 
 Another important difference is the relative number of rods 
 and cones. In mammals and amphibians the rods far surpass 
 the cones in number. In birds the reverse is true, while in 
 reptiles few or no rods are found. In fishes the rods and cones 
 are more equally divided. A few exceptions to this are of 
 great importance in substantiating the theories of the functions 
 of the rods and cones. It has been stated (45) that in the bat 
 and mole there are no cones in the yellow spot and in the 
 rabbit only a few. The same is true of other nocturnal mam- 
 mals which I have examined. I have not been able to demon- 
 strate the presence of cones in the mink, skunk, or rat, while 
 they are present in the squirrels. In the night birds and in 
 the eel very few or no cones have been demonstrated. This 
 accords completely with the theory that the rods function in 
 
\ 
 
 No. 3.] ACUTE VISION IN VERTEBRATES. , 487 
 
 the perception of uncolored and diffuse light. Since all colors 
 appear as gray by diffuse light, even though perceived by the 
 cones, and since the rods are more sensitive to faint impressions 
 than the cones, the presence of rods and almost complete 
 absence of cones in night animals is no more than can be 
 expected. Again, since the perception of color is one of the 
 important functions in day animals, and as this is done only by 
 the cones, the relatively greater number of cones in these 
 animals is readily accounted for. 
 
 Acute vision, however, seems to depend on the presence of 
 a fovea. In man the power to see distinctly grows rapidly less 
 from the fovea to the ora serrata. The macula, it is true, sees 
 objects more distinctly than the peripheral parts of the retina, 
 but even this functions with the peripheral part more as a sen- 
 tinel for moving objects than as a point of acute vision. It is 
 true that all animals are attracted more quickly by moving 
 objects than by stationary ones, and it is especially true in those 
 animals whose retinal development has not proceeded beyond 
 the differentiation of an area. The power of quiet and close 
 discrimination of objects at rest seems to be present only with 
 those animals which possess a fovea. 
 
 Fishes as a rule depend upon sight for their food, excepting 
 such as the shark, which depends almost wholly on its smell. 
 This class of vertebrates does not, however, usually possess a 
 fovea. How distinctly they see we cannot say, but we know 
 that the trout quickly takes the fly when thrown on the water, 
 or the pickerel the whirling spoon as it is drawn before it. 
 They see the objects while in motion, and are apparently unable 
 to distinguish them from the real article of food. An experi- 
 ence in fishing confirms the fact that a pickerel will not bite at 
 a motionless spoon-hook. The retina of these fish has simply 
 a thickening or area at the axis of vision. 
 
 A somewhat similar experiment can be tried with the frog or 
 toad. If one attaches a bit of red flannel, a green leaf, or any 
 other small object to a thread and dangles it before a hungry 
 frog, he will quickly jump for it. A toad may be fed on meat 
 in a similar way, but in no case will the meat be taken unless it 
 is in motion. Neither do these animals show any marked power 
 
488 SLONAKER. [Vol. XIII. 
 
 of discrimination by sight. They will jump at any small 
 moving object, and are apparently not able to distinguish till 
 they have it in their mouths whether it is an article of food or 
 a pebble. Investigations again show the presence of an area 
 and absence of a fovea. 
 
 In some of the reptiles, however, a marked difference in 
 power of discrimination by sight is noticed. Experiments were 
 made wholly on a small lizard (horned toad). If a dead fly 
 were put before him when he was hungry he would eye it 
 closely for a brief time, then quickly take it. His aim was 
 always certain, never missing his mark, while that of the ordi- 
 nary toad was more at random, throwing out her tongue indis- 
 criminately at moving objects. It is true the lizard was 
 attracted more by a live and moving fly than by a dead, motion- 
 less one, but he also had the power of perceiving things at 
 rest. This little creature possessed a sharp and well-defined 
 fovea. 
 
 In general, birds' eyes are almost as perfect as man's, and 
 likewise the optic lobes are even greater in proportion to the 
 size of the body than that of man. It is true that the bird 
 often catches flies as they buzz about, but it also inspects each 
 leaf carefully above and below for a worm or bug which may 
 be there in hiding, and which it seldom fails to recognize. The 
 hawk as it soars high in the heavens sees the snake, rat, or 
 mouse in the grass, and is frequently seen to dart and secure 
 its prey. Very acute sight is present in all birds and especially 
 in birds of prey. 
 
 A great difference exists in the power of sight in mammals. 
 The primates possess the power of most acute vision. Many 
 of the mammals depend on smell and hearing more than on 
 sight. The dog picks his master out of the crowd by smell; so 
 does the sheep her lamb. Sight in these cases being only par- 
 tial recognition, as they are not sure until they have confirmed 
 their sight by the sense of smell. The same is true of the 
 cow, for she must smell of the strange cow when introduced into 
 the herd. The horse is cured of his fright by smelling of the 
 object which caused it. In all these cases we have a motion of 
 the ears, showing that the animal is not only using sight and 
 
No. 3-] ACUTE VISION IN VERTEBRATES. 489 
 
 smell, but also hearing. Mammals in general do not see a man 
 if he remains quiet, but the crow easily recognizes him, and 
 can distinguish his stick from a gun. The dog looks into your 
 face, but you cannot tell whether he is looking into your eyes 
 or at your mouth. He has an indefinite gaze, and, like most 
 mammals, is not satisfied with the sense of sight alone, but 
 must confirm and improve by the sense of smell and hearing. 
 
 In the present study it is impossible to make a more definite 
 comparison of the powers of \'ision in the different vertebrates. 
 Many years of careful observation of the visual habits and 
 related histological structure of each animal will be necessary. 
 But so far as experiments have gone, the power of quiet and 
 close discrimination of an object at rest seems to be present 
 only in those animals whose development of the retina has pro- 
 ceeded a stage farther above that of the simple area — to the 
 fovea centralis. 
 
I. 
 
 1851. 
 
 2. 
 
 1798. 
 
 3- 
 
 1796. 
 
 4- 
 
 1796, 
 
 490 SLONAKER. [Vol. XIII. 
 
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 10. 1808. Albers, J. F. Bemerkungen iiber den Bau der Augen ver- 
 
 schiedener Thiere. Denkschr. der Kon. Acad, der Wiss. 
 Miinchen, pp. 81-90. 
 
 11. 1823. Knox, R. On the Discovery of the Foramen Centrale of the 
 
 Retina in the Eyes of Reptiles. The Edinburgh Philos. 
 Journ., vol. ix, pp. 358, 359. 
 
 12. 1826. MuLLER, JOH. Zur Vergl. Physiol, des Gesichtssinnes, Leipzig, 
 
 p. 102. 
 
 13. 1872. MuLLER, H. Ueber das ausgedehnte Vorkommen einer dem 
 
 gelben Fleck der Retina entsprechenden Stelle bei Thieren. 
 Miiller''s Anatomie und Physiologic des Auges. Zusammen- 
 gestellt von Otto Becker, Leipzig, p. 138. 
 
 14. 1867. HuLKE, J. W. On the Retina of Amphibia and Reptilia. 
 
 Jour7i. of Anat. and Physiol., vol. i, pp. 94-106. 
 
 15. 1868. Gulliver, G. Fovea Centrahs in the Eye of the Fish. Journ. 
 
 of Anat. and Physiol., vol. ii, p. 12. 
 
 16. 1885. Carriere, J. Die Sehorgane der Thiere, Miinchen und 
 
 Leipzig, p. 57. 
 
No. 3-] ACUTE VISION IN VERTEBRATES. 491 
 
 17. Hoffmann. On the Eye of the Crocodile. Bronn-Klassen und 
 
 Ordnungen des Thierreiches, Bd. vi, erste Abtheil., p. 819. 
 
 18. 1891. Krause, W. Die Retina. Internat. Monatsschrift fiir Anat. 
 
 u. Physiol., Bd. viii, pp. 414, 415. 
 
 19. 1882. Ganser. Zur Anatomie der Katzenretina. Zeitschrift f. ver- 
 
 gleichende Augenheilkujide, Heft 2, pp. 139, 140. 
 
 20. 1856. MtJLLER, H. Anatomisch-physiologische Untersuchungen iiber 
 
 die Retina des Menschen und der Wirbelthiere. Anat. u. 
 Physiol, des Aiiges, pp. 52-134. 
 
 21. 1887. Tartuferi, F. Suir anatomia della retina. Intertiat. Zeit- 
 
 schrift f. Anat. n. Physiol, v. Krause, Bd. iv, pp. 421-441. 
 
 22. 1 888. DoGiEL, A. Ueber das Verhalten der nervosen Elemente in 
 
 der Retina der Ganoiden, Reptilien, Vogel, u. Saugethiere. 
 Anat. Anzeiger, pp. 133-144. — Arch./. mik.Anat., Bd. xU, 
 pp. 62-87. 
 
 23. 1890. Baquis, E. La Retina della Faina. Anat. Anzeiger, Nos. 13 
 
 und 14, pp. 366-371. 
 
 24. 1893. Ramon Y Cajal. Neue Darstellung vom histologischen Bau 
 
 des Centralnervensystems. Arch. f. Anat. u. Physiol., 
 Anatomische Abtheilung, Heft v und vi, pp. 399-410. — 
 Sur la morphologic et les connexions des ^Idments de la 
 rdtine des oiseaux. Anat. Anzeiger, No. 4, pp. 111-121, 
 1889. 
 
 25. 1 861. MtJLLER, H. Notiz iiber die Netzhautgefasse bei einigen 
 
 Thieren. Anat. icnd Physiol, des Attges, pp. 137, 138, 141. 
 
 26. HuscHKE. Eingeweidelehre, S. 748 und 749. (H. Miiller.) 
 
 27. 1890. Chievitz, J. H. Ueber die Entwickelung der Area und Fovea 
 
 centralis retina. Arch. f. Anat. n. Entwick., Leipzig, Heft 
 V, vi, pp. 232-366. 
 
 28. 1894. Ramon y Cajal. Retina der Wirbelthieren. Uebersetzt und 
 
 herausgegeben von Richard Greeff, {a) pp. 149-154; {b') 
 
 pp. 6-14. 
 Dimmer, F. Beitrage zur Anat. und Physiol, der Macula lutea 
 
 des Menschen, Leipzig und Wien, p. 54. 
 Borysiekiewicz, M. Weitere Untersuchungen iiber den 
 
 feineren Bau der Netzhaut, Leipzig und Wien, p. 56. 
 Chievitz, J. H. Ueber das Vorkommen der Area centralis 
 
 retinae in den vier hoheren Wirbelthierklassen. Arch. f. 
 
 Attat. n. Entwickelungsgeschichte, Leipzig, Heft iv, v, und 
 
 vi, {a) pp. 322-324 ; {b) p. 327 ; {c) p. 326. 
 
 32. 1880. Denissenko, G. Mittheilung iiber die Gefasse der Netzhaut 
 
 der Fische. Arch. f. mikr. Anat., Bd. x\-iii, pp. 480- 
 485. 
 
 33. 1887. Borysiekiewicz, M. Untersuchungen iiber den feineren Bau 
 
 der Netzhaut, Leipzig und Wien, p. 70. 
 
 29. 
 
 1894, 
 
 30. 
 
 1894, 
 
 31- 
 
 1891. 
 
492 SLONAKER. [Vol. XIII. 
 
 34. 1 81 8. SOEMMERRING, D. W. De oculorum hominis animalium que 
 
 sectione horizontali. Commentatio. Gottingen. (Chievitz.) 
 
 35. 1805. Blumenbach, J. F. Handbuch der Vergl. Anat, Gottingen, 
 
 p. 547. 
 SCHWALBE. Lehrbuch Anatomie der Sinnesorgane, p. 90. 
 Krause, W. Die Retina. Internat. Monatsschrift f. Anat. 
 
 u. Physiol., Bd. vi. 
 SCHIEFFERDECKER. Anat. Anzeiger, No. 12. 
 Franklin, Mrs. C. Ladd. Eine neue Theorie der Licht- 
 
 empfindungen. Zeitschr. f. Psychologic u. Physiologic der 
 
 Sinnesorgane, iv. 
 Franklin, Mrs. C. Ladd. On Theories of Light-Sensation. 
 
 Mind, N. S., 2, pp. 473-489 ; («) P- 484 ; (^) P- 477- 
 Franklin, Mrs. C. Ladd. Psychological Review, vol. iii, 
 
 No. 2, p. 230. 
 Franklin, Mrs. C. Ladd. The Normal Defect of Vision 
 
 in the Fovea. Psychological Review, vol. ii, p. 143. 
 Parinaud, M. H. La sensibility de I'ceil aux couleurs spec- 
 
 trales. Revtie Scientifique, Ser. 4, tome iv, pp. 134-141, 
 
 Aug. 3. 
 KoNiG, Dr. Arthur. Sitzungsberichte der Koniglichen Preus- 
 
 sischen Akademie der Wissenschaften zu Berlin, xxx. 
 Stewart. Manual of Physiology, p. 726. 
 
 36. 
 
 1887. 
 
 37- 
 
 1889. 
 
 38. 
 
 1887. 
 
 39- 
 
 
 40. 
 
 1893. 
 
 41. 
 
 1896. 
 
 42. 
 
 1895. 
 
 43- 
 
 1895. 
 
 44. 
 
 1894. 
 
 45- 
 
 1895. 
 
No. 3-] ACUTE VISION IN VERTEBRATES. 493 
 
 EXPLANATION OF PLATE XXVII. 
 
 Fig. I. Human, left eye, ^, showing nerve entrance {N), blood-vessels, and 
 macula and fovea (^ F). 
 
 Fig. 2. Gorilla, left eye, \. N, nerve entrance; A F, area and fovea. 
 
 Fig. 3. Dog (Canis familiaris), left eye, \. Shows nerve entrance {N) and 
 blood-vessels which were injected. 
 
 Fig. 4. Cat (Felis catus domesticus), left eye, \. N, nerve entrance ; Ab, 
 white, band-like region, which appears as an area. The blood-vessels were in- 
 jected. 
 
 Fig. 5. Fox (Vulpes vulpes), left eye, \. N, nerve entrance ; Ab, band-like 
 area. Blood-vessels were not injected. 
 
 Fig. 6. Cow (Bos taurus domesticus), left eye, \. N, nerve entrance ; Ab, 
 band-like area. 
 
 Fig. 7. Horse (Equus caballus), left eye, \. 7V^, nerve entrance ; Ab, band- 
 like area. 
 
 Fig. 8. Sheep (Ovis avies), left eye, \. N, nerve entrance ; Ab, band-like 
 area. Blood-vessels were injected. 
 
 Fig. 9. Pig (Sus domesticus), left eye, \. N, nerve entrance ; Ab, band-like 
 area. 
 
 Fig. 10. Barred Owl (Syrnium nebulosum), left eye, \. N, nerve entrance ; 
 P, pecten ; A F, area and fovea. 
 
 Fig. II. Red-Tailed Buzzard (Buteo borealis), left eye, \. N, nerve entrance; 
 F, pecten; Ft, At, fovea and area temporalis; Fn, An, fovea and area nasalis; 
 Ab, band-like area. 
 
 Fig. 12. Tern (Sterna hirundo), left eye, \. TV, nerve entrance ; P, pecten ; 
 Ft, At, fovea and area temporalis ; Fn, Ati, fovea and area nasalis ; Ab, band-like 
 area. A dark line extending along the band-like area corresponds to Chievitz's 
 trough-like fovea. In cross sections no such fovea is found. 
 
 Fig. 13. Rabbit (Lepus sylvaticus), left eye, \. A'', nerve entrance; Ab, band- 
 like area. The blood-vessels were injected. 
 
 Fig. 14. Fox Squirrel (Sciurus niger), left eye, ^. N, nerve entrance; Ab, 
 area not visible to the naked eye. 
 
 Fig. 15. Homed Toad (Phrynosoma cornutum), left eye, \. N, nerve en- 
 trance ; P, conical pecten; F, fovea; Ab, band-like area. 
 
 Fig. 16. Turtle (Chelopus insculptus), left eye, \. N, nerve entrance ; A, 
 area, not \isible to the naked eye ; Bv, an apparent rudimentary blood-vessel 
 which was injected. 
 
 Fig. 17. Robin (Merula migratoria), left eye, \. N, nerve entrance; P, 
 pecten ; A F, area and fovea. 
 
 Fig. 18. Frog (Rana catesbiana), left eye, -J^. TV, nerve entrance; /4(5, band- 
 like area, not visible to the naked eye. 
 
 Fig. 19. Sparrow Hawk (Falco sparverius), left eye, |. A^, nerve entrance ; 
 P, pecten ; An, Fn, area and fovea nasalis ; At, Ft, area and fovea temporalis ; 
 Ab, band-like area. 
 
 Fig. 20. Ring-Neck Plover (^Egialitis semipalmata), left eye, \. N, nerve en- 
 trance ; P, pecten ; A F, area and fovea ; Ab, band-like area. 
 
494 
 
 SLONAKER. 
 
 Fig. 21. Chicken (Gallus domesticus), left eye, \. N, nerve entrance ; P, 
 pecten. 
 
 Fig. 22. Diagrammatic representation of the comparative thickness of the layers 
 of the retina in the different vertebrates. Measurements were taken of the retina 
 at corresponding positions and magnified 130 diameters, i. Nerve-fibre layer; 
 2. Nerve-cell layer ; 3. Inner molecular layer ; 4. Inner nuclear layer ; 5. Outer 
 molecular layer ; 6. Outer nuclear layer ; 7. Rod and cone layer ; 8. Pigment 
 layer. The last two layers generally overlap. I. Human. II. Cat (Felis catus 
 domesticus). III. Blue Jay (Cyanocitta cristata). IV. Snake (Eutainia sirtalis). 
 V. Frog (Rana catesbiana). VI. Pickerel (Esox reticulatus) . 
 
 Fig. 23. Black-Crowned Night Heron (Nycticorax nycticorax), left eye, \. 
 N, nerve entrance ; F, pecten ; AF, area and fovea. 
 
lournal of Morphology Plate XXVII 
 
 First /fa If 
 
 a' 
 
 -ft 
 
 -T 
 
 -t* 
 
 Atel- 
 
 Fig. 1. Fig. 2. I'ig- 3. ^^S. 4. 
 
 Fig. 11. 
 
 Fig. 12. 
 
 \ 
 
Journal vf Morphofogt/ P/ale XYl/JI 
 
 Second Half 
 
 .flb 
 
 Fig, 13. 
 
 Fig. 18. 
 
 Fig. 21. 
 
 J 
 
 IL 
 
 m. 
 Fig. 22. jjL 
 
 Fig. 19. 
 
 ^r 
 
 / 2 3 f S 6 7 y 
 
 
 
 
 
 
 
 
 / X3 -^ 6' 6 7 V 
 
 
 
 
 
 
 
 
 
 H 
 
 U-Vw, a w . 
 
 / ,ir, 2_ 
 
 6-6 7 Y 
 
 3 _ ■¥ f-f, 
 
 .'■'^.'^'■i 
 
 / 2 J -(^ ^ tf 7 V f 
 
 A vv\\3V\ vh I 0. 
 
 v-v 
 
 Fig. 23. 
 
ACUTE VISIOX IN VERTEBRATES. 495 
 
 EXPLANATION OF PLATE XXVIIL 
 
 Fig. 24. Human, age 4. Horizontal section through the center of fovea of 
 the right eye. The eye was enucleated during life and immersed at once in the 
 hardening fluid. Section 36^ thick. X 32.3. 
 
 Fig. 25. Human, adult. Horizontal section of right eye through center of 
 fovea. Eye was enucleated during life and subjected at once to the hardening 
 fluids. Section 36^11 thick. X 32.3. 
 
 Fig. 26. Gorilla. Horizontal section of right eye through center of fovea. 
 The eye was about nine hours /^j^ mortem and the apparent depth of fovea is due 
 to the folds of the retina in the macula. Section 36/x thick. X 32.3. 
 
 Fig. 27. Gorilla. Vertical section through the center of the fovea and the 
 folds of the macula, showing the folds as they appear due to post mortevi changes. 
 Section 36M thick. X 32.3. 
 
 Fig. 28. Robin (Merula migratoria). Horizontal section through center of the 
 fovea of right eye. Eye was subjected to hardening fluids immediately after death. 
 Section 24M thick. X 32.3. 
 
 Fig. 29. Blue-Bird (Sialia sialis). Horizontal section through center of the 
 fovea of the left eye. Subjected to hardening fluids immediately after death. 
 Section iSjot thick. X 32.3. 
 
 Fig. 30. Kinglet (Regulus satrapa). Horizontal section through center of 
 the fovea of the right eye. The head was subjected at once after death to hard- 
 ening fluids and sections afterwards made through whole head mth eyes in situ. 
 This section includes not only the bottom of the fovea, but also some of the cells 
 of the area beyond. Section 24^ thick. X 32.3. 
 
 Fig. 31. Snow-Bird (Junco hyemalis). Horizontal section through center of 
 the fovea of right eye. This eye was hardened by the injection method. The 
 retina in the region of the fovea floated off from the choroid. Section 36/x 
 thick. X 32.3. 
 
 Fig. 32. Goose (Anser cinereus domesticus). Vertical section of right eye. 
 Across band-like area on the nasal side of the fovea about midway to the ora 
 serrata. The arrow points to the center of the area. Eye was subjected to hard- 
 ening fluids immediately after death. Section 36,u thick. X 32.3. 
 
 Fig. 33. Goose. Vertical section through the center of the fovea of right 
 eye. Section 36^1 thick. X 32.3. 
 
 Fig. 34. Goose. Vertical section of right eye across band-like area on the 
 temporal side of fovea about midway to ora serrata. The arrow points to the 
 center of the area. A fold in the section partiy obscures the area. Section 36^ 
 thick. X 32.3. 
 
 Fig. 35. Turkey (Meleagris gallopavo). Horizontal section through center 
 of the fovea of right eye. Eye was immersed at once after death in hardening 
 fluid. Section 36^ thick. X 32.3. 
 
 Fig. 36. Guinea Hen (Numida pucherani). Horizontal section through center 
 of area of right eye. The eye was subjected to hardening fluids at once after 
 death. The arrow points to the center of the area where a very slight pitting may 
 be seen, which may possibly be called a fovea. Section 24^ thick. X 32.3. 
 
496 SLONAKER. 
 
 Fig. 37. Pigeon (Columba livia domestica). Adult. Horizontal section 
 through center of the fovea of right eye. Section i8/U thick. X 32.3. 
 
 Fig. 38. Pigeon. Horizontal section through center of fovea of right eye. 
 Section 24,14 thick. X 32.3. 
 
 Fig. 39. Surf Duck (Oidema deglandi). Horizontal section through center 
 of the fovea of left eye. Eye about three hours post mortem. Section 30/t thick. 
 X 32-3- 
 
JniiriniJ of Mt'r-pholciijj ]oJ. X//I 
 
 m 
 
 tuf. M. 
 
 Fiff. i'.V. 
 
■ lonruaJ ,>f McrphoUxji] ]ol. Slfl 
 
 PlnJ,^ XXVIII ' h'hiiir) 
 
 Fif/. 26. 
 
 WrVf TSFTimW] 
 
 ./ 
 
 Fiq. ?r. 
 
 F-lrj. -^1). 
 
 F,g.31. 
 
JoiiT-nal of Morphology Jo/, xm 
 
 Fu,. 32. 
 
 Ficf. 3ft 
 
Journal of Morphology IbJ.xm 
 
 Plate XXl'in (Sfhalf) 
 
 
 
 Fig. 33. 
 
 ■*ifp»\\t1 ^p^lm-J^l^ 
 
 Tig. 34. 
 
 Fiq. 36. 
 
 Fie,. .37. 
 
 H</. .3A 
 
 Fiy.Sa. 
 
498 SLONAKER. 
 
 EXPLANATION OF PLATE XXIX. 
 
 . Fig. 40. Tern (Sterna hirundo). Vertical section across band-like area (a) on 
 the nasalis side of the fovea nasalis. Section 36/* thick. X 32.3. 
 
 Fig. 41. Tern (Sterna hirundo). Horizontal section through center of fovea 
 nasalis. Section 30^ thick. X 32.3. 
 
 Fig. 42. Tern (Sterna hirundo). Vertical section across band-like area {a) 
 about midway between fovea nasalis and temporalis. Section 36/a thick. X 32.3. 
 
 Fig. 43. Tern (Sterna hirundo). Horizontal section through center of fovea 
 temporalis. Section 30/i thick. X 32.3. 
 
 Fig. 44. Kingfisher (Ceryle alcyon). Horizontal section through center of 
 fovea nasalis. Section 30/t thick. X 32.3. 
 
 Fig. 45. Kingfisher (Ceryle alcyon). Horizontal section through center of 
 fovea temporalis. Section 30/i thick. 32.3. 
 
 Fig. 46. White-Bellied Swallow (Tachycineta bicolor). Horizontal section 
 through center of fovea nasalis. Section 24/11 thick. X 32.3. 
 
 Fig. 47. Ring-Neck Plover (^gialitis semipalmata). Horizontal section 
 through fovea of left eye. Section i8,u thick. X 32.3. 
 
 Figs. 48, 49. Sparrow Hawk (Falco sparverius). Section passed through 
 each fovea and denter of pupil. Fig. 48, fovea nasalis and Fig. 49, fovea tempo- 
 ralis. Section 42/^ thick. X 32.3. 
 
 Figs. 50, 51. Red-Tailed Buzzard (Buteo borealis). Section passed in plane 
 of both foveas and center of pupil. Fig. 50, fovea nasalis and Fig. 51, fovea 
 temporalis. Section 42jU thick. X 32.3. 
 
 Fig. 52. Crow (Corvus americanus). Horizontal section through center of 
 fovea. Section 36/x thick. X 32.3. 
 
 Fig. 53. Blue Jay (Cyanocitta cristata). Horizontal section through center 
 of the fovea. Section 48/x thick. X 32.3. 
 
 Fig. 54. Shorepeep (Ereunetes pusillus). Horizontal section through center 
 of fovea of left eye. Shows some cells of the area lying beyond the center of the 
 fovea. Section 24/^1 thick. X 32.3. 
 
 Fig. 55. Barred Owl (Syrnium nebulosum). Horizontal section of right eye 
 through center of fovea (fovea temporalis). Section 48/u thick. X 32.3. 
 
Journal of Moi-phology Vol. XIII 
 
 Fig. 40. 
 
 
 i. ? 
 
 Fig. 44. 
 
Jounnt] of Moi'phology Vol.xlJl 
 
 Phile XXIX (I'^fhalf) 
 
 Fiff. -td. 
 
 V ^^ I 
 
 F 
 
 ^.^.jM 
 
 Fig. 42. 
 
 Fig. 43. 
 
 Fill. 44. 
 
 Fitf. 43. 
 
 Fi^. 46. 
 
 Fii,.4l 
 
'TournaJ of Mor'pholiKjjj loI.X/f/ 
 
 % 
 
 
 1 
 
 ' 
 
 
 1 
 
 *!■ 
 
 
 1 
 
 t 
 
 1 
 
 1 
 
 ' * 
 
 
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 <■■■ '.Sff 
 
 Fi(j. 4^. 
 
 Fii 
 
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 iiV/. j^. 
 
Journnl of Morphology Vol.xm 
 
 P/,rh' XXIX (k"^Iirilf) 
 
 M 
 
 Fig. 50. 
 
 Fig. 51. 
 
 Fill. 5^. 
 
 Fig. .W. 
 
 Fig. .U. 
 
 Fig. .5.i . 
 
500 SLONAKER. 
 
 EXPLANATION OF PLATE XXX. 
 
 Figs. 56, 57. Horned Toad (Phrynosoma cornutum). Fig. 56, vertical sec- 
 tion and Fig. 57, horizontal section through the center of each fovea. Fig. 56 
 shows also the width of the band-like area. Sections iZfj. thick. X 32.3. 
 
 Fig. 58. Pipefish (Siphostoma fuscum). Horizontal section through center 
 of fovea (i) of left eye. 2 indicates the position of nerve entrance. Section i8/x 
 thick. X 32.3. 
 
 Fig. 59. Pipefish (Siphostoma fuscum). Section in a lower plane, showing 
 entrance of nerve (2) and the area (i). Section iS/x thick. X 32.3. 
 
 Fig. 60. Chipmunk (Tamias striatus). Vertical section across area (a). 
 Section 24;U thick. X 32.3. 
 
 Fig. 61. Turtle (Chelydra serpentina). Horizontal section through band-like 
 area. Section i8;ti thick. X 32.3. 
 
 Fig. 62. Frog (Rana catesbiana). Vertical section across band-like area (a). 
 Section 24^ thick. X 32.3. 
 
 Fig. 63. Flounder (Paralichthys dentatus). Vertical section, showing com- 
 parative thickness of different layers of the retina. Section 30^14 thick. X 32.3. 
 
vToui'inil oi' Mor'phohujij f()/. XIII 
 
 fia. 60. 
 
Klounial oi' Morpholoijy Vol.xni 
 
 Plate XXX 
 
 Fuj. 57. 
 
 Fig. no. 
 
 Fifj. HI. 
 
 Fm-t''^ 
 
 Fiy.ea 
 
502 , SLONAKER. 
 
 EXPLANATION OF PLATE XXXL 
 
 The following figures were made from sections through the whole head with 
 eyes in situ. i^A^ marks the axis of vision for the fovea nasalip ; FT, fovea tem- 
 poralis ; and Op, the entrance of the optic nerve indicated by the pecten. 
 
 The slight divergence of the lines of binocular vision {FT) is probably due to 
 the relaxation of the internal recti muscles after death. 
 
 Fig. 64. White-Bellied Swallow (Tachycineta bicolor), \. 
 
 Fig. 65. Common Tern (Sterna hirundo), \. 
 
 Fig. 66. Sparrow Hawk (Falco sparverius), \. 
 
 Fig. 67. Broad- Winged Hawk (Buteo latissimus), \. 
 
 Fig. 68. Great Horned Owl (Bubo virginianus), \. 
 
Joarnal of Morphologfi/ 
 
 P/ate XXXI 
 
 Fig-. 64, 
 
 Fig. 6 5. 
 
 Fig. 67. 
 
 Fig. 66 
 
 Fig-. 68 
 
Date Due 
 
 i 
 
SL5 
 QP479 
 
 jof acute vision a^*