AMPHIBIA. 



221 



elongated auditory-cells, closely con- 

 nected with fibres of the auditory 

 nerve. From each a tapering auditory 

 "hair" projects into the labyrinth- 

 cavity. The sensory patches are 

 disposed as follows : One in each 

 ampulla on a projecting fold, crista 

 acustica,znd the remaining five, macula:, 

 in utriculus, sacculus (2), and three 

 cochlear projections. 



(f>) Visual Organs (Fig. 63). The 

 eye is relatively large, and projects 

 not only externally but also internally 

 into the mouth-cavity. It is lodged 

 in the orbit, on the side of the skull. 

 The eyeball is rounded internally but 

 flattened externally, and its wall is 

 made up of three concentric coatings. 

 The outermost of these, the firm, white 

 sclerotic, strengthened by cartilage, 

 exhibits externally a circular trans- 

 parent area, the cornea. The second 

 much thinner coating, the cJwroid, is 

 pigmented externally and very vas- 

 cular internally. This layer does not 

 line the cornea, but, at its margin, 

 becomes continuous with a vertical 

 partition, the iris, which presents a 

 central perforation, the pupil. By 

 the iris the eye is divided into a 

 small external and a large internal 

 chamber. The latter is lined by the 

 retina, which is the third and most 

 delicate coating. In it eight distinct 

 layers (Fig. 64) can be distinguished. 

 The optic nerve pierces the scler- 

 otic and choroid coats on the inner 

 side of the eyeball to reach the 

 retina, in which it breaks up into 

 a feltwork of fibres. These form a 



JtA 



Fig. 64. DIAGRAM OF 

 LAYERS OF RETINA (from 

 Landois and Stirling). 

 Pi, Pigment -cells ; St, rods 

 and cones continuous by 

 various intermediate struc- 

 tures with Ggl, ganglion- 

 cells connected with o, 

 fibres of optic nerve; Li, 

 internal limiting mem- 

 brane which conies just 

 outside vitreous humour 

 (not shown). 



