THE SENSES. O'.MI 



the ciliary processes. It is a iibro-mnscular membrane perforated by a 

 central aperture, the pupil. It is made up chiefly of blood-vessels and 

 connective tissue with pigment and unstriated muscle. 



Posteriorly are two layers of pigment cells (uvea), in which are repre- 

 sented the two layers of cells of which the optic vesicle is originally 

 formed, and behind which are the retina proper and its pigment layer. 

 In the iris representatives of both layers are deeply pigmented. The 

 structure of the iris proper is made of connective tissue in front with 

 corpuscles which may or may not be pigmented, and behind of similar 

 tissue supporting blood-vessels inclosed in connective tissue. The pig- 

 ment cells are usually well developed here, as are also many nerve-fibres 

 radiating toward the pupil. Surrounding the pupil is a layer of circu- 

 lar unstriped muscle, the sphincter pup illce. In some animals there are 

 also muscle-fibres which radiate from the sphincter in the substance of 

 the iris forming the dilator pupillce. The iris is covered anteriorly by 

 a layer of endothelium continued upon it from the posterior surface of 

 the cornea; posteriorly there is a very fine layer which is a continuation 

 of the membrana limitans interna of the retina. 



The Lens. The lens is situated behind the iris, being inclosed in a 

 distinct capsule, the posterior surface of which is less thick than the 

 anterior. It is supported in place by the suspensory ligament, fused 

 to the anterior surface of the capsule. The suspensory ligament is 

 derived from the hyaloid membrane, which incloses the vitreous humor. 



Structure. The lens is made up of a series of concentric lamina? 

 (fig. 414), which when it has been hardened, can be peeled off like the 



Fig. 413. Fig. 414. 



Fit? 413 Ciliary processes, as seen from behind. 1, posterior surface of the iris, with the 

 sphincter muscle of th?pSpil; 2, anterior part of the choroid coat ; 3, one of the ciliary processes, 



f ^KSjffld^?Stte crystalline lens. The laminae are split ur> after hard- 

 ening fn alcoh^ 1 the denser central part or nucleus; 2, the successive external layers. X 4. 

 (Arnold.) 



leaves of an onion. The laminae consist of long ribbon-shaped fibres, 

 which in the course of development have originated from cells. 



