MAMMALS 487 



already noted ^ to be present in whales, the hippopotamus and the 

 elephant ; some of the fibres are non-myelinated (Bruesch and Arey, 

 1942). A minute subdivision of the fibres into fasciculi is common 

 only among Mammals, and there is evidence that the complexity of 

 the glial framework increases in proportion to the visual development 

 of the animal in the evolutionary scale (Deyl. 1895). 



The inner architecture and septal system of the optic nerve throughout the 

 Vertebrates is interesting in this respect. As occurs ontogenetically in man, 

 Cyclostomes show merely a central column of ependymal cells which have 

 become invaginated within the developing nerve, and from them processes 

 radiate outwards towards the periphery. The same arrangement is seen in the 

 Dipnoan, Protopterus. In some Selachians and other Dipnoans and in snakes 

 this simple arrangement is reduplicated and the nerve is broken up into a number 

 of bundles each of which has a similar core of cells. In the remainder of the 

 Vertebrates the pattern is altered : oligodendi'oglial cells (derived from the 

 original ependymal cells) are scattered throughout the nerve. As the visual 

 functions become more highly developed in the higher Vertebrates and man, 

 the fascicvilation becomes progressively less obvious, the number of fibre -bundles 

 increasing and the original ependymal system becoming more uniformly dispersed 

 throughout the whole structure. 



It is interesting that the lamina cribrosa at the ojDtic nerve-head shows wide 

 variations. In general it may be said that in those Mammals which have good 

 day-vision this structure is well developed with many collagenous fibres (squirrel, 

 cat, monkey), while in species with a poor visual capacity (Rodents such as the 

 rat, mouse and rabbit) the lamina is absent and the retina may even herniate 

 in folds into the optic nerve sheath (Tansley, 1956) (Figs. 643-6). 



In all Vertebrates below Mammalia the decussation of the optic 

 nerve fibres at the cliiasma is complete (or jjractically so in some 

 Reptiles 2) so that each eye is connected solely with the opposite side 

 of the brain (Harris. 1904 : Kappers, 1921) ; in all Placentals it is 

 incomplete, but the crossed fibres always remain the more numerous. 

 In Vertebrates below Mammals the fibres remain in distinct and 

 separate fasciculi as they cross ; in Placentals they become intimately 

 intertwined and interlaced (Cajal. 1898 ; Bossalino. 1909). In general 

 the number of imcrossed fibres varies with the degree of frontality of 

 the eyes (Newton. 1704 ; J. Midler, 1826 ; Gudden, 1879) ^ ; in animals 

 with laterally directed eyes they are relatively few * ; they number 

 about 1/6 of the total in the horse. ^ 1/4 to 1/3 in the dog ^ and cat," 

 about 1/3 in the higher Primates, and about 1/2 in Man.^ TJiis arrange- 

 ment whereby corresponding half-fields of each retina are connected to 



1 p. 4.")1. 2 Snakes, p. 392. 



^ A relationship sometimes referred to as the Law of Xevvton-Miiller-Gudden. 



* Rodents such as the rat and rabbit, Bellonei (1884), Singer and Miinzer (1 

 Pick and Herrenheiser (1895), Brauwer and Zeeman (1925), Overbosch (1926). 



5 Dexler (1897). 



« Vitzou (1888). 



' Nieati (1878). Brauwer and Zeeman (1925), Overbosch (1926). 



* Brauwer and Zeeman (1925). 



