673 



EYE. 



retina ; and it terminates in the posterior margin of the ciliary liga- 

 ment, circle, or more properly muscle (fly. 5, i) a flattened band of 

 gray matter, about the seventh part of an inch in breadth, attached 

 to the sclerotic internally near its junction with the cornea. In these 

 situations the two membranes adhere with some firmness ; they are 

 elsewhere connected by vessels which pierce the outer and ramify upon 

 the inner membrane, and by the filaments of a fine intermediate 

 cellular tissue. But the connection is so slight that it may be readily 

 broken by gentle inflation with a blow-pipe insinuated through a 

 puncture in the sclerotic, without injury to the fragile texture of the 

 choroid. The choroid consists almost entirely of a multitude of 

 minute vessels, curiously interlaced, and communicating freely with 

 each other. It is supplied with blood by 15 or 20 branches of the 

 ophthalmic artery, which pierce the sclerotic round the entrance of 

 the nerve, and are at first distributed externally on the posterior part 

 of the sphere ; but they finally pass inwards, and terminate in a close 

 and uniform vascular expansion over the whole concave surface. This 

 is called the tunic of Ruysch, who erroneously considered it as a 

 distinct membrane. The innumerable veins of the choroid, or vensc 

 vorticosse, are arranged with great elegance and regularity in arched 

 and drooping branches, like the boughs of the weeping willow ; they 

 are very conspicuous upon the outer surface, above the first exterior 

 ramifications of the arteries. (Fig. 6.) They unite in four or five 

 common trunks, which emerge through the sclerotic at equal intervals 

 behind the middle of the eye-ball. The outer surface of the choroid 

 is somewhat rough and flocculent ; the inner surface, upon which the 

 retina is expanded, is delicately smooth and even. Both are abund- 

 antly covered with the pigment, which is secreted by every part of the 

 choroid, and pervades its loose and porous texture. 



Kig. o. 



The sclerotic partly removed, and the rest turned back, showing the Choroid 

 Coat and Iris, a, optic nerve. 



The choroid may thug be divided into three layers : 1, an external 

 brown soft lamella supporting the ciliary nerves and long ciliary 

 vessel, and in front containing the ciliary muscle the outer pigment 

 layer ; 2, the less deeply coloured proper vascular layer, with the 

 larger arteries and veins ; and 3, a colourless, delicate, internal layer, 

 containing an extremely abundant capillary plexus, the membrana 

 choris-capillaris, which however does not extend farther in front than 

 the ora serrate. The tissue of which the choroid is composed, inde- 

 pendent of its vessels and nerves and the ciliary muscle, is intermediate 

 between elastic and connective tissue. Bowman was the first to demon- 

 strate the true muscular fctructure of the ciliary ligament or muscle 

 aa it is now more properly called. 



Pigment In man this matter is of a deep-brown colour, in most 

 other animals it is black, and hence is very commonly called the 

 Pigmentum Nigrum. It appears under the microscope to consist of 

 almost regularly hexahedral contiguous cells, 0'006"' O'OOS"' in 

 diameter, 0'004'" thick, disposed in an elegant mosaic manner, in 

 which the large quantity of brownish-black pigment usually prevents 

 the nucleus being apparent as more than a clear spot in the interior. 

 It adheres very loosely, so that when the surfaces covered with it are 

 drawn to and fro in water, it becomes diffused, and may be washed 

 off. The choroid thus treated is found to be of the same whitish or 

 gray colour which characterises the ciliary ligament. In the natural 

 state of the parts, not only the choroid, but the cellular tissue on its 

 external surface, and the inside of the sclerotic, are deeply stained by 

 the pigment, which shows through, and occasions the bluish tint of 

 the white of the eye in persons of delicate complexion. But on the 

 inner surface of the choroid the pigment is retained by an expansion 

 finer than a spider's web, yet of close texture, which may be called 

 after its discoverer the membrane of Dalrymple. By this means the 

 transparency of the retina is preserved. It is probable that this 

 membrane of the pigment is of a serous kind, and that it is reflected 

 at the optic and ciliary margins of the choroid, and passes over the 

 whole posterior surface of the retina thus doubly defended from 

 absolute contact with the pigment. The choroid is not the only part 

 which secretes this colouring matter. It is found in equal or greater 

 abundance upon the back of the iris, on the surfaces of the ciliary 

 processes, and in fact wherever it is wanted to facilitate vision. The 

 pigment cells are absent in the eyes of albinoes, as well as, at any 

 rate partially, in the region of the tapetum in animals. 



In many animals, especially the nocturnal and carnivorous tribes, 

 the pigment is deficient at the bottom of the eye, and the surface of 

 the choroid in that situation presents a brilliant colour and almost 

 metallic lustre. This is called the Tapetum (tapestry of divers colours). 



KAT. HIST. DIV. VOl. II. 



It is of various shades of blue, green, and yellow ; sometimes change- 

 able like shot silk, and sometimes of a silvery whiteness. The tint 

 occupies various proportions of the surface ; it is most brilliant 

 immediately opposite the pupil, and passes gradually into the dark 

 hue of the pigment. There is no vestige of a tapetum iu the human 

 eye. The use of it is not well known. It probably causes the animal 

 to see better in the dusk and less clearly in the day, by reflecting the 

 rays a second time through the retina. This reflection from a very 

 effective concave mirror produces a strong convergence of the rays 

 which come back through the pupil, and is the cause of the well- 

 known glare of the eyes of cats and other animals seen in an obscure 

 light from that particular distance at which the emerging rays are 

 most completely brought to a focus upon the eye of the observer. 

 The breadth of this luminous appearance arises from the great 

 dilatation of the pupil under the circumstances in which it is seen. 



Retina. -The optic nerve, having entered the interior of the globe 

 through the sclerotic and choroid membranes, forms a slight promi- 

 nence at the point of union of its several portions, and thence spreads 

 out in the form of a fine transparent membrane over the whole 

 concave surface of the choroid, embracing the translucent body 

 called the Vitreous Humour. Towards the choroid it appears to 

 consist of a mere homogeneous pulp, not very different from the 

 medullary matter of the brain ; but it is undoubtedly most elabo- 

 rately and minutely organised. Towards the vitreous humour it has 

 the structure of a most delicate vascular web, consisting of innumer- 

 able ramifications of the central artery (which, as we have already 

 mentioned, accompanies it into the interior of the globe) and of its 

 associated veins. Its name may have been derived from the net-work 

 formed by the visible branches of these vessels ; at least it is not 

 otherwise applicable to the structure of the membrane. The distri- 

 bution of the central artery may be made visible to an observer in 

 his own eye by a curious experiment first suggested by Purkiuje. 

 One eye being steadily directed to a surface of some uniform dark or 

 neutral tint, such as a wall painted of a lead colour, and the other 

 eye closed by the hand, the flame of a small wax-taper is to be slowly 

 waved round and round, so as to be brought at every turn at a. little 

 distance over the front of the eye. The central artery will gradually 

 come into view, at first obscurely, and afterwards more clearly. The 

 experiment succeeds best after the experience of several trials on 

 successive nights. The form is such as might be expected from a 

 branching net-work of vessels : the lines are dark, with bright edges 

 on a faintly illuminated ground. There are other modes of making 

 the experiment, which show the appearance more distinctly, but they 

 are less simple. The retina terminates anteriorly in a thin scalloped 

 edge, fitting into corresponding irregularities called the ora serrata in 

 the posterior margin of the ciliary body. (Fiys. 5, I ; 9, d.) Exactly 

 opposite the pupil there is a bright yellow spot, fading gradually off 

 at the edges, and having a black point in the centre precisely where 

 the axis of direct vision intersects the back of the eye. ( Fig. 5, n.) 

 This central point was believed by its discoverer, Soemmering, to be 

 an actual deficiency of the substance of the retina ; and it is generally 

 called in consequence the foramen of Soemmering. But it is now 

 known to be merely a central absence of the yellow colour of that 

 part of the retina rendered conspicuous by the pigment seen through 

 the ordinary transparent texture. These appearances are lost very 

 shortly after death, and are replaced by a minute fold, into which the 

 retina gathers itself, reaching from the place of the central point to 

 the prominence which marks the union of the divided portions of the 

 nerve. The use of this yellow spot and central point, and of the 

 tendency of the retina to assume a folded shape in this situation is 

 not understood. 



The microscopic construction of the retina has been recently 

 studied by Pacini, Kolliker, and Miiller. Although of variable thick- 

 ness in all parts it may be divided into the five following layers : 



1. The layer of rods and cones called the bacillar layer. 



2. The granular layer. 



3. The layer of gray nerve-substance. 



4. The expansion of the optic nerve. 



5. The limitary membrane. 



" The bacillar layer, stratum bacillorum, seu membrana Jacobi, 

 presents a very remarkable structure, being composed of innumerable 

 rod-like and conical corpuscles, disposed with the utmost regularity, 

 and reflecting the light very strongly. With the exception of H. 

 Miiller, this structure in animals has been understood quite erroneously, 

 and even in man it has been but very superficially known. It consists 

 of two elements, the rods (bacilli) and the cones (coni), which 

 together constitute a single layer 0'036'" thick at the bottom of the 

 eye, more anteriorly 0'024"', and quite in front not more than O'OIS'" 

 in thickness. In general these bodies are so arranged that the more 

 numerous rods have their largest ends directed outwards, whilst the 

 cones are disposed in the reverse direction, whence the latter when 

 imperfectly examined appear to constitute an inner, distinct, 

 thinner layer, lying between the inner extremities of the ' rods.' " 

 (Kolliker.) 



The granular layer is composed of opaque granular corpuscles, 

 reflecting the light tolerably strongly, of a round or oval figure, and 

 0'002"' 0'004'" in size, sometimes looking like free nuclei, sometimes 

 like minute cells almost entirely filled by large nuclei. 



2 X 



