CUAP. XVI.] SOME OTHER MAMMALS. 273 



transverse fibres (known as ' psalterial fibres ') exist, which 

 form a reflected part of the great transverse commissure or 

 Corpus Callosum. This body is, in fact, bent upon itself 

 behind, and it is the portion (thence prolonged forwards to 

 the posterior pillars of the Fornix, somewhat triangular 

 in shape) which, in higher Mammals, is commonly called 

 the ' psalterium ' or ' lyra.' Beneath it is a membrane 

 (' velum interpositum ') lying on the surface of the 

 Thalami (a great part of which it hides), and forming 

 a kind of roof over the Third Ventricle. 



Opposite the anterior extremities of the Thalami, the 

 two halves of the Fornix again separate so as to constitute 

 its ' anterior pillars,' which dip downwards just behind 

 the Anterior Commissure, along the side of the third 

 ventricle to its floor, where each, after twisting upon itself, 

 so as, with its fellow, to cause a single white projection 

 ' Corpus albicans ') near the centre of the base of the 

 brain (fig. 74, p), again passes upwards and penetrates the 

 inner side of the corresponding Thalamus. 



The Fornix exists in all Quadrupeds, and has a much 

 larger relative size in some of the lower forms than in the 

 Quadrumana or Man. It is, for instance, extremely well 

 developed in the Beaver, the Rabbit, and other Rodents. 



The Corpus Callosum was formerly believed not to 

 exist in the Monotremes and Marsupials ; and, in fact, it 

 is present in them only as a very rudimentary structure. 

 In Insectivora it is larger ; while in some Rodents it has 

 already attained a considerable development, as may be 

 seen from the brain of the Beaver (fig. 71), where it is 

 thick and comparatively long from before backwards. In 

 this animal it has also attained the more horizontal direc- 

 tion commonly met with in higher forms, though in some 

 other Rodents it is a notably less developed structure 

 being short, thin, and nearly vertical in direction. The 



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