190 DISSECTION OF THE DOG 



(spatium perichorioideale), crossed by the fine ciliary vessels and nerves and a 

 loose network of pigmented connective tissue known as the lamina fusca. 



Cornea. — The cornea is transparent, of great density, and nearly circular in 

 outline. Its thickness is greatest in the central part and becomes less towards 

 the periphery. The edge of the cornea is overlapped by the sclera in much 

 the same manner as a watch-glass is overlapped by the metal rim into which 

 it fits. 



Chorioid. — The choroid forms the greater part of the middle tunic of the 

 eyeball, and is thickest about the entrance of the optic nerve. If the choroid 

 be viewed from the front, a triangular or crescentic area, beginning either 

 above or medial to the entrance of the optic nerve, is readily distinguished 

 from the rest. This is the tapetum, and the peculiar appearance of the choroid in 

 this position is caused by the presence of a number of superposed layers of 

 cells (tapetum cellulosum). In the dog the tapetum has a metallic lustre and 

 is often of a golden colour with a whitish or bluish periphery. 



The main thickness of the choroid consists of two layers of blood vessels. 

 The more superficial layer, the lamina vasculosa, contains the larger vessels ; 

 while the inner layer, the lamina choriocapillaris, is composed of a network 

 of capillaries. The tapetum occurs between the two laminae. 



Dissection. — In order to display the ciliary body an eyeball should be cut 

 across a little distance in front of the equator, and the vitreous body and 

 crystalline lens carefully removed from the anterior segment. 



Corpus ciliare. — The ciliary body consists of the ciliary processes and the 

 ciliary muscle. The ciliary processes (processus ciliares) are some seventy to 

 eighty radially arranged ridge-like thickenings of the body ; each ridge 

 becoming taller from the periphery centralwards and ending abruptly as a 

 slightly expanded prominence. A portion of the hyaloid membrane (membrana 

 hyaloidea), which surrounds the vitreous body, is applied to the ciliary pro- 

 cesses, and is radially thickened and folded, so as to present grooves and 

 elevations intimately connected with the processes and the depressions between 

 them. Immediately behind the ciliary processes there is a circular area, 

 almost entirely smooth, knoAvn as the orbiculus ciliaris, which, however, is 

 very narrow or even absent in the medial half of the eye. 



M. ciliaris. — The ciliary muscle consists of involuntary fibres arranged, 

 for the most part, in a meridional or radiating manner. Some fibres, however, 

 are circular in direction. In a sagittal section of the eyeball the ciliary muscle 

 forms a triangular mass in the neighbourhood of the junction of the sclera and 

 cornea. The radiating fibres arise from the sclera close to the rima cornealis 

 and are attached to the ciliary processes and the orbiculus ciliaris. 



Dissection. — The iris must now be examined. Its posterior surface can be 

 studied in the preparation used to show the ciliary body. To reveal its 

 anterior surface the cornea must be removed from another eveball. 



