214 COMPARATIVE ANATOMY 



as a rule does not reach so far. In Birds it is always more or 

 less folded, and consists rnainly of a closely-felted network of 

 capillaries. In both Reptiles and Birds, the pecten appears to 

 be important in the nutrition of the contents of the eyeball 

 and of the retina : it has nothing to do with accommodation. 



The iris, which is regulated by striated muscle, by means of which it is 

 able to respond very quickly to visual impressions, is often brightly coloured, 

 and this colour is due 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. 



Mammals. In Mammals the eyeball is always more com- 

 pletely enclosed within the bony orbit than is the case in most 

 other Vertebrates, 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 (Fig. 168), 



With the exception of aquatic Mammals, in which it is some- 

 what flattened, the cornea is moderately convex, and the whole 

 eyeball is of a more or less rounded form. 



A tapetum lucidum, consisting either of cells or fibres, exists in the choroid 

 of numerous Mammals, and gives rise by interference to a glistening appear- 

 ance when seen in the dark (Carnivores, Ruminants, Perissodactyles, &c. ). 



Certain structures homologous with the processus falciformis and 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 is not always round, but may be transversely oval (Ungulates, 

 Kangaroos, Cetaceans), or slit-like and vertical (e.g., Cat). 



Retina. 



The fibres of the optic nerve, which pass into the eyeball at a 

 right or acute angle, cross one another at the point of entrance, 

 and are then distributed to the sensitive elements of the retina. 

 The latter is thus thickest at the point of entrance of the nerve, 

 which is known as the "blind spot" (Fig. 168), and gradually de- 

 creases in thickness towards the ciliary processes, until, at the point 

 of origin of the iris, it consists of a single layer of cells. 



The retina is bounded externally by a structureless hyaline 

 membrane (limitans externa), 1 while on its inner side it is covered 

 by the hyaloid membrane, which, strictly speaking, belongs 

 to the vitreous humour. The retina is quite transparent in the 

 fresh condition, and consists of two portions which are histo- 

 logically and physiologically quite distinct: they are, a supporting 



1 The membrana limitans encloses the entire retina externally in the embryo, 

 but later the rods and cones come to project through it (see Fig. 172). 



