376 THE BRAIN OF MAMMALS, BY TH. MEYNERT. 



The double nature of the tracts by which the projection sys- 

 tem begins to be reduced in the cms cerebri, as crusta and 

 tegmentum, indicates a natural division of their ganglia of 

 origin into the ganglia of the crusta and the ganglia of the 

 tegmentum of the crus cerebri. Of the former, the most im- 

 portant are the caudate nucleus (intra-ventricular corpus 

 striatum) and the lenticular nucleus (extra-ventricular corpus 

 striatum) ; of the latter, the optic thalami } the corpora quadri- 

 gemina, and the corpora geniculata interna. In point of 

 size there is no necessary relation between the development 

 of the tract of the crusta with its ganglia, and the develop- 

 ment of the tract of the tegmentum with its ganglia ; a fact 

 which is very significant, and is the fundamental explanation 

 of the variations in form of mammalian brains. The tract 

 of the crusta only of the cerebral peduncle rises and falls 

 with the development of the hemispheres, and hence Man, 

 for example, possesses the largest lenticular nucleus and crusta 

 of the crus cerebri. The tegmentum of the crus, with its 

 ganglia, especially the corpora quadrigemina and corpora 

 geniculata interna, on the other hand, are the more largely 

 developed in proportion to the smallness of the cerebral lobes. 

 Hence the defective proportion which, in the small cerebral- 

 lobed brain of the Bat (figs. 230, 231, and 232), the pedun- 

 cular tract (P 2 , P 2 a) bears to the tegmentum (Tg and P 2 r). The 

 important pathological fact that lesion of the ganglia of the 

 crusta of the cerebral peduncle occasions complete hemiplegia 

 and the equally noteworthy physiological experiment that on 

 the application of an external stimulus all forms of movements 

 occur in technically complete course, when only the ganglia of 

 the tegmentum in the brain of the Mammal are left, speak 

 strongly in favour of the view that the mass of the animal body 

 is represented in the brain by a twofold projection, on the 

 one hand by the crusta, and on the other through the teg- 

 mentum of the crus cerebri, but are only brought into relation 

 through the pedal tract of the crus with the functional activity 

 of the cerebral hemispheres. 



The third principal mass of grey matter, the grey sub- 

 stance of the central cavities (as stated above, p. 374), so 

 far from causing any reduction in the mass of fibres belonging 



