220 ESSENTIALS OF ZOOLOGY 



deeply pigmented choroid coat lines the sclerotic, and opposite 

 the rim of the cornea it is produced as the iris, the margin 

 of which defines the pupil, and into ciliary processes. The 

 choroid, like the sclerotic, is pierced by the optic nerve. The 

 transparent lens is large and almost globular. It is enclosed 

 in a thin, transparent capsule. The retina is a delicate cup- 

 shaped membrane internal to the choroid, and ends in a rim 

 about the level of the ciliary processes. It is made up internally 

 of the nervous layer, and externally, next the pigment layer, of 

 rods and cones. Between the lens and cornea the cavity is 

 occupied by aqueous humour, and between the lens and the 

 retina the cavity is filled with the transparent, jelly-like 

 vitreous humour. The eyeball is moved within the orbit 

 by the usual four recti and two oblique muscles. The optic 

 nerve is surrounded by the fibres of the retractor bulbi which 

 arise from the parasphenoid. Another set of muscle fibres, 

 the levator bulbi, runs from the top of the skull under the eye 

 to the maxilla, and the contraction of these fibres raises the 

 eye into the projecting, prominent position. The conjunctival 

 space gives an opening to a gland, the Harderian gland, the 

 products of which are conveyed into the nose by the lachrymal 

 duct. The eye originates in the manner already described in 

 the previous chapter. The lens is almost fish-like in character. 

 The eye is well developed and is designed to focus the rays of 

 light coming from the field of vision on that occupied by the 

 rods and cones of the retina. The rays pass through the trans- 

 parent elements mentioned, and the focus is adapted to distance 

 by changing the shape of the lens. This is accomplished by 

 relaxing or compressing the capsule of the lens by means of 

 the muscles of the ciliary processes' which lie all round the lens 

 and to which the capsule is attached. The quantity of light 

 entering the eye is regulated by the dilatation or constriction 

 of the iris. Moreover, the eye is brought into position by the 

 contraction of the levator bulbi muscle and rotated as required 

 by the other muscles. 



The internal auditory labyrinth is like that of the skate, 

 and it has a similar development from the ectoderm. It 

 consists of three semicircular canals, each expanded into an 

 ampulla. The canals open into a common sac, the utriculus, 



