148 Mammalia. Organ of Sight. 



Crepuscular and Nocturnal Mammals {e.g. Pteromys, Aye- 

 Aye, Lemur) have a large convex Cornea, very wide Pupil, 

 and convex Lens (esp. Bats and Nocturnal Eodents). 

 Aquatic Mammals, on the contrary, have an excessively 

 convex Lens, but very slightly projecting Cornea. 



In many Mammals the Choroid^ instead of being uniformly 

 lined with dark pigment, as in Man, presents, on a greater or 

 less extent of its back part, a brightly coloured layer, named 

 Tapetum, on the inner surface of which is the tunic of Ruysch 

 {i.e. the inner part of the Choroid Coat formed by the capil- 

 laries of the Choroidal vessels), as well as the layer of hexago- 

 nal cells, which, however, is here, as in the eye of the Albino, 

 destitute of pigment. The Tapetum consists sometimes of 

 tendinous fibres, sometimes of cells, filled with granular 

 matter *: it is of various colours, e.g. silvery, white, white 

 edged with blue, light blue, green, amber, chocolate : it occu- 

 pies a broad transverse tract of the Choroid ; and may present 

 a Crescentic form, the greater extent being below the entrance 

 of the Optic nerve, and only a small portion above. No Ta- 

 petum is found in Bimana, nor in Quadrumana, above the 

 Lemurs. 



The Macula lutea of the Petina, or * yellow spot ' of Soem- 

 merins:, appears to be confined to the eye of Bimana, and of 

 the higher Quadrumana. 



The eye is occasionally closed by a single circular Eyelid 

 with a small round opening in it, and protected by a car- 

 tilaginous plate continued from the upper part of the Orbit, 

 comparable to the palpebral plates in the Crocodile : such 

 circular eyelid may however have the division of the hori- 

 zontal eyelids indicated by an external groove at the inner 

 Canthus*. As a rule there are three eyelids present, two being 

 horizontal : of these the lower one is generally the largest 



* Leidig. * jtnvhs, the coi-ner of the eye. 



