456 THE BRAIN OF MAMMALS, BY TH. MEYNERT. 



lobes in Man and Mammals, it is impossible to overlook the 

 existence of a widely extended morphological harmony between 

 the form of the highest and of the lowest divisions of the cere- 

 brum. 



We obtain the key to this insight into the morphology of 

 these parts from the fact above illustrated, that the crusta of 

 the crus cerebri is directly proportionate in size to the lobes of 

 the cerebrum. 



If the size of the crusta be dependent on the size of the 

 hemispheres, and if fasciculi of the crusta pass through the 

 anterior division of the pons into the cerebellum, as appears 

 to occur from the diminution in thickness of this tract in the 

 pyramids, the pons must become correspondingly prominent, 

 the larger the crusta, and vice versa. The pons is therefore 

 prominent in Man, and inconspicuous in Mammals. Since, 

 however, the deeper layers of the transverse fasciculi in the 

 pons of Man are independent of the size of the crusta, these 

 become,if by reason of the small development of the crusta the 

 pons is short, uncovered at a certain height behind and near 

 the pyramids. This is actually the case to a small extent in 

 the brains of Monkeys with feebly developed cerebral lobes, 

 and very completely in the Lemurs. They thus become visible 

 as the corpus trapezoides in the brains of Mammals. On 

 the other hand, in Man, the larger prolongation of the crusta, 

 the pyramid, demands space for the increase of its trans- 

 verse section in the median region of the oblongata, and the 

 olivary bodies situated behind the pyramids in the mam- 

 malian type consequently become displaced outwards from 

 the median line, and therefore appear free at the side of the 

 pyramids. 



In addition it may also be remarked that the hemispheres 

 of the cerebellum augment in direct proportion to the cerebral 

 hemispheres, and that a proportionately enlarging ganglion of 

 the cerebellar hemispheres, the nucleus dentatus, attains a 

 development of size that keeps pace with that of the inferior 

 olivary bodies. Thus the absolute size of these increases with 

 the development of the cerebral lobes, and their projection from 

 the medulla oblongata augments in the same ratio. 



