514 THE SOLIDS. 



the cells of tapetum lucidum, but also in those of the hexa- 

 gonal and stellate choroidal epithelium. 



The tapetum lucidum is a layer of fibrous tissue implanted 

 upon the choroid, possessing the remarkable property of re- 

 fracting unequally the rays of light which fall upon it, and 

 hence its brilliancy and metallic lustre : acetic acid destroys 

 to some extent this peculiarity ; the stellate pigment is con- 

 tinued behind the tapetum lucidum, which singular and 

 beautiful structure acts as a concave reflector, its use being to 

 economise light and to cause the rays to traverse the retina a 

 second time, by which means animals possessing it are enabled 

 to discern objects in a light which would be insufficient for 

 the purpose in the absence of such a provision. 



The description of the choroid now given includes that 

 portion of it only which corresponds to the retina, and 

 which ceases at a line known as the ora serrata ; about the 

 eighth of an inch behind the margin of the cornea, in front 

 of this line as far as the iris, the choroid is known as the 

 ciliary body ; this is covered behind by a layer of non-striated 

 muscular fibre, the ciliary muscle (see Plate L XVIII. Jig. 4.), 

 and from it the ciliary processes descend. 



Ciliary processes. These processes, usually reckoned at 

 about sixty in number, are received into corresponding folds 

 or plaitings of the hyaloid membrane, called the secondary 

 ciliary processes, and which taken altogether form a circle 

 around the crystalline lens named after their discoverer the 

 Zone of Zinn : they are each composed of numerous blood- 

 vessels (see Plate LXYII. fig. 4.), fibrous tissue, irregular 

 pigment cells ; and " on their inner surface is a tough colour- 

 less lamina, composed of ill-defined nucleated cells continuous 

 with the border of the retina, but clearly not composed of 

 nervous matter, by means of which they are immediately con- 

 nected with the hyaloid membrane." 



The iris may be regarded as an extension of the choroid, 

 although it does not exhibit all the anatomical characters of 

 that membrane ; it is made up of a considerable quantity of 

 pigment cells, of blood-vessels, and of fibres of unstriped 

 muscle. (See Plate LXVIII. fig. 9.) 



