250 ANATOMY OF VERTEBRATES. 



anterior surface of the iris. The quantity of the aqueous humour 

 is small : the lens, d, is subspherical, flatter in front than behind. 

 The nucleus is seen in the posterior half and the surrounding la- 

 mina) are reflected inward and backward toward the middle of the 

 anterior surface of the nucleus, leaving a funnel-shaped cavity in 

 front of it which is filled by less dense substance. In Hyperoodon 

 the pupil is transversely oblong with a moderate projection of 

 the upper margin, reminding one of the skate's pupillary curtain 

 (vol. i. p. 334). In the Grampus the choroid presents a greenish 

 tinge : in the Porpoise it is a bluish white. In both, the pupil 

 resembles that of Hyperoodon. The retina is thick. 



In the Seals the sclerotic is chiefly remarkable for the sudden 

 thinning at the part corresponding Avith the ciliary zone ; it is 

 moderately thick both in front and behind : the cornea is thin and 

 flabby. The muscles of the eye-ball being inserted into the an- 

 terior part of the sclerotic may shorten the axis of the eye and 

 bring the lens nearer to the back of the globe, thus adapting it to 

 vision in air and water. In the Sirenia the eye is very small. 

 In a Rhytina of 25 feet in length the eye-ball was but 1 inch 

 in diameter : it is about 1 inch in diameter in the Dugong : the 

 pupil is circular. 



The eye of the Elephant is about 2 inches in diameter, re- 

 minding one of that in the Whale by its small relative size : 

 there is likewise an unusual thickness of fibrous or sclerotic sub- 

 stance at the entry of the optic nerve, and a similar extent of 

 light-coloured tapetum within the choroid, which tapetum presents 

 the fibrous type of structure: the pupil is round, the cornea is 

 larger and more convex than in Cetacea. 



o 



In the Rhinoceros the eyeballs are of small comparative size ; 

 in the Indian species which I dissected, 1 each measured in 

 aiitero-posterior diameter one inch five lines, and in transverse 

 diameter one inch three lines. Some dark-brown pigment lies 

 under the conjunctiva for the extent of about a line from the 

 circumference of the cornea : the same kind of pigment is also 

 deposited upon the outside of the nictitating eyelid, and over a 

 great part of the inner surface of the same part, covered of course 

 by a reflection of the conjunctiva. The trunks of the vena? 

 vorticosa? perforate the sclerotica half-way between the entry of 

 the optic nerve and the edge of the cornea : their disposition, with 

 the flocculent but somewhat firm connecting tissue of their 

 radiating branches, presented that structure which most nearly 

 resembled the figures given by Mr. Thomas of the parts he 



1 v", p. 56. 



