SUMMARY OF STRUCTURES 253 



a tendency for it to become more definitely convoluted. This feature goes 

 hand in hand with the progressive development of the lateral cerebellar 

 lobes. Those forms having the largest cerebellar hemispheres have also the 

 most highly convoluted inferior olivary nuclei. 



The parallelism in development of olive and lateral lobes of the cere- 

 bellum depends upon the fact that the inferior olivary nucleus, in its capacity 

 of a precerebellar relay station, sends its fibers not only to the vermis, but in 

 very large measure to the lateral lobes as well. Such is not the case with the 

 precerebellar nuclei situated in the spinal cord. Most of the fibers arising in 

 Clark's column, as well as those which give origin to the ventral spinocere- 

 bellar tract, have their destination in the vermis of the cerebellum. The 

 contrasts drawn by these morphological facts are not without evolutional 

 significance. The spinocerebellar tracts represent a portion of the musculature 

 with striking phyletic constancy, namely, the axial muscles of the body. The 

 representation of these muscles when projected upon the cerebellar cortex 

 requires for its elaboration the limited areas of the vermis only. This muscu- 

 lature is restricted to the trunk, the neck and such axial structures as those 

 innervated by certain of the cranial nerves. No such functional limitation 

 prescribes the representation of the inferior olivary body as a precerebellar 

 nucleus. It expands as the cerebellar lobes expand, and having a widespread 

 connection with them, exists in response to portions of the muscular system 

 whose representation when projected upon the cerebellar cortex reaches all 

 areas both in the vermis and in the lateral cerebellar lobes. 



Factors Underlying Progressive Expansion of Inferior Olivary 

 Nucleus. Apparently the same dynamic influences which have produced 

 expansion in the cerebellar hemispheres are operative in the progressive 

 expansion of the inferior olivary nucleus. Were it possible to select the one 

 most compelling factor underlying such influence, it would doubtless be the 

 progressive differentiation of the forelimb. This differentiation does not alone 



