EYE 



283 



Ch 



Jit 



quickly to visual impressions: it is often brightly coloured, 



owing to the presence, not only of pigment, but also of 



coloured fat globules. The pupil is as a rule round, but in many 



Reptiles and in Owls has the form of a vertical slit. The form, 



structure, and relative size of the lens varies considerably : 



amongst Reptiles, its refractive index is lowest in Snakes, and 



highest in Lizards (except 



Geckos). In many Birds it has 



a very peculiar form. The 



mechanism of accommodation 



in some Snakes resembles that 



seen in Amphibians (p. 281), 



while in the other Sauropsida 



the ciliary muscle, acting on 



the capsule of the lens, is able 



to alter the curvature of the 



latter, as is also the case in 



Mammals. 



Mammals. In Mammals 

 the eyeball, especially in Prim- 

 ates, is more completely enclosed 

 within the bony orbit than is 

 the case in most other Verte- 

 brates, and this may partially 

 account for the fact that, except 

 in Monotremes, the sclerotic no 

 longer shows traces of cartilage 

 or bone, but is entirely of a 

 fibrous character : in Whales it 

 is extremely tough. Excepting 

 in aquatic Mammals, in which 

 it is somewhat flattened, the 

 cornea is moderately convex, and 

 the whole eyeball is of a more 

 or less rounded form. 



A tapctum lucidum, consisting either of cells or fibres, exists in 

 the choroid of numerous Mammals, and gives rise by interference 

 to a glistening appearance when seen in the dark (Carnivora, 

 Ruminants, Perissodactyla, &c.). In some Mammals there is a 

 differentiation of the capillary vessels (membrana choriocapillaris) 

 from the larger arterial trunks of the choroid : the veins are 

 situated externally to the arteries. Certain structures homologous 

 with the processus falciformis or pecten are present in Mammals in 

 the embryo only. 



The ciliary muscle consists of smooth elements. The external 

 surface of the lens is less convex than the internal, which latter lies 

 in the so-called fossa patellaris of the vitreous humour. The pupil 



FIG. 208. EYE OF AN OWL. 



Ch, choroid ; CM, ciliary muscle ; Co, 

 cornea ; Co, vitreous chamber ; Ir, 

 iris ; L, lens ; Op, OS, optic nerve 

 and sheath; P, "fan" (pecten); Rt, 

 retina ; Sc, sclerotic, with its bony 

 ring at t ; VK, aqueous chamber ; 

 VN, point of junction between 

 sclerotic and cornea. The dotted 

 line around the broadest portion of 

 the circumference, divides the eye 

 into an inner and an outer segment. 



