266 THE LOWER PRIMATES 



passing from the lowest apes to man. Concomitant with this increase of the 

 lateral cerebellar lobes, there is an expansion in the dentate nucleus. The 

 reason for this growth in the cerebellar hemispheres and the corresponding 

 development of the dentate nucleus is found in the connections of the coor- 

 dinating organ. The vermis of the cerebelhim has a much more restricted 

 connection than the lateral lobes. It is in communication by means of atTerent 

 spinocerebellar tracts \\ith the spinal cord and by olivo-cerebellar libers as 

 well as vestibulo-cerebellar libers, with the oblongata. The lateral lobes, on 

 the other hand, although connected with the axial portions ol the brain by 

 olivo-cerebellar fibers, receive their main tributaries from the pallio-ponto- 

 cerebellar fibers. These axons connect the lateral lobes of the cerebellum with 

 the hemispheres of the cerebrum. In other words, the vermis ol the cerebel- 

 lum responds to such inllux ol impulses as may arise Irom cerebellar 

 representation in the spinal cord and oblongata; whereas the cerebellar 

 hemispheres are responsive to more complex neural syntheses created within 

 the cerebral cortex as well as in the oblongata. Impulses trom these latter 

 sources appear to require more expansive cerebellar receiving areas tor their 

 elaboration than is afforded by the vermal cortex. It is apparent that the 

 phyletic growth of the cerebral hemispheres has determined cerebellar 

 expansion. In proportion as the cerebral cortex becomes more highly con- 

 voluted, the connections between the cerebral and cerebellar hemispheres 

 enlarge and the lateral lobes of the cerebellum expand correspondingly. 



The case of the vermis is dillerent. Its connections are exclusively seg- 

 mental, that is to say, with the definitely segmented portions of the central 

 axis. The lateral cerebellar lobes, although possessing certain segmental 

 connections, establish their preeminent communications by means of supra- 

 segmental fibers. These latter fibers arise in portions of the central nervous 

 system which have developed as superstructures over and above the primor- 

 dial segmented neuraxis. 



