HEAD AND NECK OF THE HORSE 



119 



This is the tapetum,^ and the characteristic appearance of the choroid 

 in this region is due to the presence of a layer of fibrous tissue.'^ The 

 colour of the tapetum varies somewhat in different specimens, but is 

 mostly composed of shades of green and blue merging into yellow. 



The choroid is composed of four layers: — (1) The outermost, the 

 lamina suprachorioidea, having a close resemblance to the lamina 

 fusca of the sclera; (2) the lamina vasculosa, consisting of a super- 

 ficial stratum containing whorls of veins that drain into the venai 

 vorticoste, and a deeper layer in which are branches of the short 

 posterior ciliary arteries ; (3) the laTnina choriocapillaris, consisting 

 mainly of a very rich mesh work of capillary vessels ; and (4) a trans- 

 parent lamina hasalis. The tapetum occurs between the lamina 

 vasculosa and the lamina choriocapillaris. 



Vorticose veins. 



Tapetum 



Papilla of optic nerve. 



Fig. 49.— The fundus of the eyeball. 



Dissection. — To display the ciliary body, cut an eyeball across a little 

 distance in front of the equator, and remove the vitreous body and the 

 crystalline lens from the anterior segment. 



The ciliary body (corpus ciliare) connects the choroid to the 

 periphery of the iris and consists of the ciliary processes internally 

 and the ciliary muscle externally. In a longitudinal section of the 

 eyeball it has the form of an elongated, curved triangle, the base of 

 which is directed towards the iris. The ciliary processes (processus 

 ciliares), over a hundred in number, are radially^>rranged ridge-like 

 thickenings of the ciliary body; each ridge becoming taller from the 

 periphery centralwards and ending abruptly as a slightly expanded 

 prominence. The length of the ridges, that is, the breadth of the 

 zone formed by the ciliary processes, is less on the medial side of the 



1 Tafetum [L.], a carpet. 



2 The tapetum varies structurally in different mammals. For example, in the 

 horse, as above stated, it is composed of fibrous tissue (tapetum fibrosum) ; while in 

 the dog it consists of polyhedral cells (tapetum cellulosum). 



