EYE. 



179 





Tins membrane is of a deep brown or black 

 colour, being stained with the colouring matter 

 called I lir black moment ; but when this is 

 removed, it exhibits a high degree of arterial 

 and venous vascularity. Its external surface 

 is comparatively rough, coarse, and flocculent, 

 and obscured by the cellular membrane which 

 connects it to the sclerotic. The inner surface, 

 which is in contact with the retina, presents a 

 very different appearance. It is soft and smooth, 

 and when minutely injected, resembles the 

 more delicate mucous membranes, and exhibits 

 a remarkable degree of minute villous vascu- 

 larity. The external surface being composed 

 of the larger branches of arteries, veins, and 

 nerves, may be torn away from the soft, smooth, 

 and more closely interwoven inner layer, or 

 the inner layer may be partially dissected up 

 from it, with some care, especially in the eyes 

 of the larger quadrupeds. This manoeuvre 

 having been executed by lluysch, and prepara- 

 tions so formed displayed by him, the inner 

 layer has been denominated the tunica Ruys- 

 chiana. but this is a mere anatomical artifice. 

 There is no natural division into two layers, 

 the soft, smooth, and highly vascular inner 

 surface being formed by the ultimate subdivi- 

 sion and distribution of the larger branches of 

 vessels, which exhibit themselves separately on 

 the outside. It is a condition somewhat analo- 

 gous to that of the skin, where the soft, smooth, 

 villous external surface presents so remarkable 

 a contrast to the rough internal surface with its 

 layer of cellular membrane uniting it to the 

 subjacent parts. 



The choroid is supplied with blood from the 

 ophthalmic artery by the short ciliary arteries, 

 which penetrate the sclerotic at a short distance 

 from the entrance of the optic nerve, and are 

 distributed to it in nearly twenty small branches. 

 These branches ramify and inosculate freely on 

 the outside of the membrane, and are visible as 

 distinct vessels, especially on the posterior part 

 of the sphere. They finally terminate on the 

 inner surface, forming a beautiful vascular 

 expansion. The long ciliary arteries give 

 scarcely any twig to the choroid, being distri- 

 buted to the iris, and the anterior branches 

 furnished to the sclerotic, as described in 

 speaking of that membrane, do not penetrate 

 to the choroid. The veins of the choroid pre- 

 sent a peculiar appearance. The ramifications 

 are arranged in the form of arches or portions 

 of a circle, bending round to a common trunk 

 like those of certain trees with pendulous 

 branches. They discharge their blood into 

 four or rive larger branches which penetrate 

 the sclerotic at nearly equal distances from each 

 other behind the middle of the eyeball. On 

 account of this peculiar arrangement they have 

 received the name of vasa vorticosa. They lie 

 external to the ciliary arteries, but the ultimate 

 ramifications pervade the inner surface in the 

 same manner as the arteries ; and if the venous 

 system of the eye be minutely injected, the 

 same beautiful uniform villous vascularity is 

 displayed as in the arterial injections. 



The annexed figure is a copy of Zinn's re- 

 presentation of the vasa vorticosa. 



Fig. 104. 



The numerous nerves which pierce the scle- 

 rotic and run forward between that membrane 

 and the choroid, called ciliary nerves, being 

 distributed almost exclusively to the iris, are 

 to be noticed when that organ is described ; 

 small branches of them are, however, probably 

 distributed to the choroid and its appendages, 

 and possibly even to the retina and hyaloid 

 membrane. 



The inner villous surface of the choroid, 

 which in man is stained with the black pig- 

 ment, in several other animals presents a bril- 

 liant colour and metallic lustre. This is called 

 the tii/iit inn. It is not a superadded material nor 

 dependent on any imposed or separable colour- 

 ing matter, but is merely a different condition 

 of the surface of the choroid or tunica Ruys- 

 chiana, by means of which rays of light of a 

 certain colour only are reflected. It exists in 

 the form of a large irregular patch, occupying 

 the bottom of the eye toward the outside of 

 the entrance of the optic nerve. It is of a 

 beautiful blue, green, or yellow colour, with 

 splendid metallic lustre, and sometimes white 

 as silver. It is not obscured by the black 

 pigment which covers the rest of the surface 

 and even encroaches a little on its margin, and 

 consequently it acts most perfectly as a concave 

 reflector, causing the rays of light previously 

 concentrated on the bottom of the eye by the 

 lens to be returned, and to produce that re- 

 markable luminous appearance observed in the 

 eyes of cats and other animals when seen in 

 obscure situations. This provision is absent in 

 man, the quadrumanous animals, bats, the 

 insectivorous order, perhaps all the rodentia,lhe 

 sloths and many other of the class mammalia ; 

 while it is present in the majority if not all of 

 the ruminants, as well as in the horse, the 

 cetacea, and most of the carnivorous tribe. It 

 does not appear to exist in birds or reptiles, 

 and is absent in the osseous, although present 

 in the cartilaginous fishes. I must here, how- 

 ever, state that I am obliged to speak loosely 

 respecting this matter, as the subject has not 

 yet been thoroughly investigated. The use of 

 this tapetum has not been ascertained, or the 

 reason why it exists in some and is absent in 

 other animals explained. It is obvious that 

 where it is present the rays of light are trans- 

 mitted through the retina, and again when 

 reflected by the tapetum are returned through 

 the same retina, thus twice pervading that 

 structure. 



H 2 



