ORGAN OF SIGHT IN MAMMALIA. 251 



describes as ' processes having a muscular appearance, with the 

 fibres running forwards in a radiated direction.' 1 On removing 

 the anterior part of the sclerotica, whilst the eye was suspended 

 in spirit, both the vitreous humour and the lens rolled out ; and 

 the capsule of the lens showed no particular mark of the inser- 

 tion or fixation of the ciliary processes ; their impressions, in 

 remains of pigmental matter, were perceptible on the anterior 

 part of the ' canal of Petit.' The transverse diameter of the lens 

 was six lines, the antero-posterior diameter four lines. The pig- 

 ment was not confined to the inside of the choroid ; but in both 

 Rhinoceroses dissected by me, I found on the outside of the 

 chorion much loose cellular tissue, with dark pigment : this 

 coloured flocculent tissue concealed at first the venas vorticosae, 

 even when injected. The sclerotica is one line thick at the back 

 part of the eyeball ; and is thinnest near the middle of the ball, 

 becoming thicker towards the cornea, which is two lines thick. 

 The choroid adheres pretty strongly to the back part of the 

 sclerotic, around the entry of the optic nerve, both by the enter- 

 ing vessels and by the tenacity of its outer flocculent coat, 

 especially where the vessels penetrate the sclerotica. There is 

 no tapetum lucidum. The lower eyelid has a special depressor 

 muscle. 2 



The Tapir has a proportionally small eyeball. Of the Perisso- 

 dactyle group the Horse has the largest eyes, in relation to its 

 greater powers of locomotion. They are lateral, prominent, 

 capable of directing against any object in the rear, without turn 

 of the head, the outkick of the hind-leg. The cornea inclines to 

 an oval figure, the larger end being toward the nose. The tape- 

 turn is of a light blue colour, and fibrous structure : the ciliary 

 processes are long ; more numerous than in the ox : the pupil is 

 transversely oblong, rather wider on the nasal side, with a few 

 processes from the upper margin. 



In the Hog-tribe the cornea is oval, with the large end in- 

 ternal, or toward the nose ; the sclerotic is thin ; the pupil is 

 round ; the eyeball rather larger than the palpebral opening 

 would indicate ; the inner figure of the choroid is of a shining 

 chocolate colour in the common Hog, but much darker in the 

 Babyroussa. The eyes in Ruminants are large, lateral; the 

 transverse exceeds the fore-and-aft diameter of the eyeball. In 

 the Ox the latter is to the transverse diameter as 43 to 49 ; in 

 the sheep as 32 to 35. The ciliary processes are short in most, 

 especially in some Antelopes: the retina extends far forward. 



1 cvi", p. 157, pi. x., figs. 1-3. 2 v", p. 56. 



