PHYLUM CHORDATA 



357 



iris. The rays of ligWt are brought to a focus on the retina 

 mainly by means of the crystalline lens, a firm, glassy body 

 situated within the iris. The cornea also assists in this, as 

 does a gelatinous substance, the vitreous humour, which fills 

 the part of the cavity of the eyeball internal to the lens. 



The ear in the dogfish is imbedded in the cartilage of 

 the posterior part of the skull (auditory region). It con- 

 sists of a somewhat complicated structure termed the 



Fig. 220. — Diagrammatic horizontal section of the eyeof man. c, cornea; ch, 

 choroid (dotted); C. P, ciliary processes; e. c, epithelium of cornea; e. cj, 

 conjunctiva; f. c, yellow spot; /, iris; L, lens; ON, optic nerve; OS, ora 

 serrata; o-x, optic axis; / c. R, anterior non-visual portion of retina; P. E, 

 pigmented epithelium (black); R, retina; sp. I, suspensory ligament; Scl, 

 sclerotic; V. H, vitreous body. (From Foster and Shore's Physiology.) 



membranous labyrinth, with soft walls and an internal 

 epithelium in which the fibres of the auditory nerve termi- 

 nate. Contained in the interior of the labyrinth is a fluid, 

 the endolymph, in which there are suspended particles of 



