CHAP, ix.] NERVOUS SYSTEM AND ORGANS OF SENSE. 



291 



except at the ends of the ciliary processes, and at a certain consider- 

 able part which is called the tapetum, and is of a golden yellow 

 colour.* This tapetum is a roundish patch, occupying most of the 

 back of the inside of the choroid, and including within it the en- 

 trance of the optic nerve, which enters towards the lower margin of 

 the tapetum. It is this tapetum which gives the eyes of cats that 

 luminous appearance in obscurity, by reflecting the light a pro- 

 perty which is supposed to assist their nocturnal vision. The choroid 

 is an excessively vascular fibrous membrane, the vessels becoming 

 more minute as they advance forwards, and forming a fine capillary 

 network ending very near the margin of the cornea, in what is called 

 the ciliary ligament. This ligament is a circular band of connective 

 tissue and organic muscular fibre, which unites the choroid with the 

 sclerotic external to it. 



The arteries of the choroid come from the ophthalmic artery. 

 Within the outer vascular layer of the choroid a layer of capillary 

 vessels is distinguished by the name of tunica Ruyschiana, or chorio 

 capillaris. Within this, again, a structureless (or slightly fibrous) 

 membrane named the vitreous layer is described, while inmost of all 

 is the pigmentary layer, of closely placed polygonal pigment cells. 



The IRIS is that coloured part of the eye which is apparent around 

 the pupil, and which may be differently coloured on its front surface, 

 in different cats,f but is covered with dark pigment on its hinder or 

 deep surface. It consists of fibrous tissue and organic muscular 

 fibres. 



At its outer border the iris is continuous with the choroid and 

 ciliary ligament, and is connected with the cornea, while its free 

 inner border forms the outer margin of the pupil of the eye it being 

 a sort of curtain with a hole in it, about half an inch across, sus* 

 pended in the more anterior part of the interior of the eyeball, and 

 resting on the anterior surface of the lens. 



The iris is very, and very suddenly, contractile (in spite of the 

 organic nature of its muscular fibres), and has two muscles. The 

 first, or sphincter, is a flat band close to its free inner margin, and 

 on its posterior surface. The dilating muscle, on the other hand, 

 is formed of fibres which pass inwards on all sides towards the 

 margin of the pupil from the circumference of the iris. In ob- 

 scurity the pupil is widely open, with a circular opening through 

 contraction of the radiating fibres of the iris. In bright light the 



* At my request Mr. Henry Power has 

 been so kind as to examine the cat's eye 

 with the ophthalmoscope. As to the result 

 of his examination, after noting the non- 

 appearance of that spot known in man 

 as the fovca centralis or yellow spot, he 

 expresses himself as follows: "I owe 

 you thanks for directing my attention to 

 one of the most beautiful things I have 

 ever seen. Imagine a dense, yet lumi- 

 nous velvety-blackness below, bounded 

 by a nearly horizontal line, just above 



which is a pearly spot ; the entrance of 

 the optic nerve. This presents the usual 

 vessels emerging from it. The disc is 

 surrounded by a sapphire blue zone of 

 intense brilliancy, passing into metallic 

 green; and beyond this the tapetum 

 shines out with glorious colours of pink 

 and gold, with a shimmer of blue and of 

 green. It is really lovely. " 



f Or even in the two eyes of the same 

 cat. 



U 2 



