272 ORGANS OF THE SENSES. 



of the eye their thick and tough choroid, destitute of the 

 pigment which lines the other parts, shines with a blue 

 or green coloured metallic lustre, a tapetum lucidum which 

 is not seen in the lowest nor in the highest terrestrial 

 animals of this class, and which often presents a red colour, 

 as in albinos, from the exposed vascular layer of the choroid. 

 The ciliary ligament is less and the processes are more de- 

 veloped than in the inferior classes. The iris presents less 

 varied and deep hues than in birds, its free edge, in the em- 

 bryo, supports the membrana pupillaris which closes the 

 pupil as the inter-pulpibral membrane closes the eye-lids 

 at the same period of life, and where the pupil is elongated 

 transversely, as in the horse, the upper free margin of the 

 iris presents pendent processes, as in the cartilagenous fishes, 

 which hang down more or less over the pupil, and sometimes 

 they rise up also from the lower edge. Between the sclerotic 

 and the choroid is a thin brown coloured layer connecting 

 these coats and considered as continuous with the delicate 

 arachnoid covering of the brain and optic nerve, at the ciliary 

 ligament the minute canal of Fontana is still often per- 

 ceptible though less than in birds, and the canal of Petit 

 surrounds the lens between the vitreous and the aqueous 

 humour. The optic nerve penetrates the choroid by a round 

 aperture on the nasal side of the axis of vision ; but in some 

 of the rodentia, as the marmot, it enters by a lineal slit in the 

 choroid, as in birds, and sometimes a rudimentary trans- 

 parent pecten is observed in the embryos of mammalia 

 extending forwards through the vitreous humour, as in the 

 lower oviparous vertebrata. As we ascend through the quadru- 

 manous animals to man, the eye becomes smaller and more 

 spherical, its membranes thinner, the vitreous humour more 

 abundant, the crystalline lens smaller and more compressed, 

 the pupil more circular, the ophthalmic ganglion larger, the 

 retinal surface more extensive, the eyes more approximated 

 and parallel, their orbits more completely surrounded with 

 bone, and the circular suspensory muscle extending forwards 

 from around the optic foramen to the sclerotic in the inclined 

 heads of inferior mammalia, is no longer developed or required 

 for the small eyes and elevated orbits of the quadrumana and 

 man. It is however in the visual apparatus of man (Fig. 105.) 



