THE BEAIN IN THE EDENTATA. 305 



into three azygos lobes by means of the two fissures prima and secunda. These three 

 lobes will be called lohus antlcus, loltiis centralis, and lobus posticus respectively. The 

 coHA' enience and clearness of the term " centralis " induce me to adojit it, for there is 

 little danj^er of this term ever being confused with that denoting the small " lobiUus 

 centralis," a term api^lied in human anatomy to the cephalic part of the lohus anticus. 



The Jissnra secimcla cuts deei^ly in a horizontal direction into the cerebellum in the 

 mesial plane, from a point upon the caudal surface a short distance above the sharp 

 ventro-caudal angle. It follows a transverse course of 10 mm., i. e. about 5 ram. on each 

 side of the mesial plane, and then bends suddenly at a right angle into the vertical 

 direction, and reaching the ventral surftice it extends in the cephalic direction as far as 

 the posterior medullary velum, /. e. into close proximity to the cerebellar peduncles. 



By means of this 2:)eculiar fissure, a little mesial tongue-like process — the lohus posticus 

 — is completely cut off from the rest of tlie cerebellum. 



ThQ posterior lohe is divided by a deep fissure upon the ventral surface into two main 

 groujis of folia. The anterior of these is a small insignificant group of three folia 

 intimately associated with the posterior medullary velum and corresponding to the 

 nodulus of human anatomy. The rest of the lobe is a large, complex, triangular mass, 

 which includes the postero-ventral angle of the cerebellum and corresponds to the uvula 

 of human anatomy. 



The lohns centralis constitutes the chief part of the cerebellum. It is limited in front 

 by the fissura prima, behind by the Jissura secunda, and laterally by the Jlssurce Jlocculi. 

 It is a large, irregular, and complicated region, which, in the exuberance of its growth, 

 has bulged obliquely forward and laterally on each side, and, wedging its way between 

 the lateral part of the lohus anticus and the front of the lohus Jlocculi, it has extended on 

 the anterior surface as far in the ventral direction as the columna cerehelli. 



In the caudal direction it shows a similarly exuberant growth, for the lateral parts of 

 the lohus centralis bulge downward upon the caudal surface of the organ between the 

 lohus posticus and the lobus Jlocculi. 



Thus the central lohe, which is comparatively narrow in the mesial j)lane, expands 

 considerably in its lateral parts and embraces the lobus Jlocculi. 



This expanded lateral part of the lobus centralis on each side may be distinguished 

 as the area crescens, for it will be found that in the higher mammals it is the exuberant 

 increase of this part of the cerebellum which is the main factor in the evolutionary 

 process. 



The most caudal part of the lohus centralis in the mesial plane is naturally separable 

 from the rest as a distinct wedge-shaped sector, which is probably the homologue of the 

 pyramid of hrmian anatomy. In Orycteropus Wixs, pyramid does not appear to belong to 

 the central lobe, but to be quite distinct from it ; and we shall see subsequently that in 

 many mammals the pyramid is quite a subsidiary twig of the large tree-like branch 

 which forms the central lobe. This view is, moreover, supported by developmental 

 evidence, in the case of animals the pyramid of aa hich is independent in the adult. 



"We shall see later that in the simpler cerebellum of small mammals the pyramid is 

 directly continued in the lateral direction, on each side, into the caudal extremity of the 



SECOND SERIES. — ZOOLOGY, VOL. VII. 50 



