252 THE LOWER PRIMATES 



operation of this function, the cerebellum must be in touch from instant to 

 instant with the varying degrees of muscle tension existing in each muscle 

 group of the body. Cerebellopetal fd^ers furnish a physical means by which 

 such communication is established. A continuous stream of atl'erent impulses 

 is thus passing to the cerebeUum during all phases of muscular activity. It is 

 interesting in this connection to note that most of the afferent pathways to 

 the cerebellum from the musculature of the body pass upward in the spino- 

 cerebellar tracts whose destination is the vermis cerebelli. This portion of 

 cerebellar organization is chiefly concerned with the axial and paraxial muscu- 

 lature, while the lateral lobes are much more engaged with the coordinative 

 control of the appendicular muscles in the limbs. All of the ascending cere- 

 bellar fibers are believed to receive intermediate relay in precerebellar nuclei. 

 The column of Clark in the spinal cord is an example of such a precere- 

 bellar nucleus. It receives peripheral afterent fibers and in turn gives rise to 

 fibers which constitute the dorsal spinocerebellar tract. A similar longi- 

 tudinal nucleus, perhaps not so discretely limited as the column of Clark, 

 provides a relay for the libers constituting the ventral spinocerebellar tract. 

 Both of these precerebellar nuclei have the form of long cell columns extend- 

 ing through many segments of the spinal cord. In this respect they differ 

 from the inferior olive which is also a conspicuous precerebellar nucleus. This 

 nucleus is saccular in form and limited to the segments of the oblongata. 



EVOLUTIONAL EXPANSION IN THE INFERIOR OLIVE 



Such expansion as occurs in the spinal precerebellar nuclei must in the 

 main be longitudinal. It is for this reason less conspicuous than the expan- 

 sion in the inferior olive which is largely in the transverse diameters. The 

 progressive enlargement of the inferior olivary nucleus in passing from 

 the lowest of the primate order to man gives the impression of a much more 

 striking evolutional process than is the case with other precerebellar nuclei. 

 The olive is also more impressive from the fact that in its expansion there is 



