284 COMPARATIVE ANATOMY 



is not always round, but may be transversely oval (e.g. 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. 203), and gradually 

 decreases 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 on its outer periphery by a structureless 

 hyaline membrane (limitans externa)} 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 

 part and a nervous part. The former is stretched as on a frame 

 between the limitans externa and hyaloid membrane. 



The nervous elements are arranged in the following concentric 

 layers : 



I. Developed from the internal layer of the secondary optic vesicle, 



A. Cerebral layer. 



1. Layer of nerve-fibres (of optic nerve). 



2. Layer of ganglion-cells. 



3. Inner reticular layer. 



4. Granular layer (inner). 



5. Outer reticular or subepithelial layer. 



B. Epithelial layer. 



6. Layer of visual cells (outer granular layer with the rods 



and cones). 



II. Developed from the external layer of the secondary optic vesicle. 



7. Pigment epithelium (retinal epithelium). 



These layers are so arranged that the nerve-fibres lie next to 

 the vitreous humour, that is, internally, while the rods and cones 

 are situated towards the choroid, i.e. are external. Thus the terminal 

 membeis of the neuro-epithelium are turned away from the rays 

 of light falling upon the retina, and the rays must therefore pass 

 through all the other layers before they reach the rods and cones.' 2 



1 The limitans externa encloses the entire retina externally in the embryo, but 

 later the rods and cones come to project through it (cf. Fig. 209). 



2 Fishes possess the longest, Amphibians the thickest rods, so that in the 

 latter there are only about 30, 000 to a square millimetre, -while in Man there are 

 from 250,000 to 1,000,000. In Fishes, Birds, and Mammals the rods (which are 

 the phyletically older structures) far exceed the cones in number, while in 

 Reptiles the reverse is the case : in Petromyzon and Elasmobranchs, for example, 

 there are no cones at all. In some Reptiles there are no rods, and in nocturnal 



