INTERNAL STRUCTURE OF THE BRAIN STEM 1029 



specialization in the course of its own evolutionary progress; its advances 

 in this respect gain c\en more emphasis because of their association with the 

 simultaneous increase in size of the lateral cerebellar lobes. These lobes, as 

 well as the dentate nucleus, expand in direct proportion as manual differ- 

 entiation becomes more definitive. 



In a snnilar tnaniier, the red nucleus, serving as the intermediate relay 

 for impulses from the cerebellum, allords an index of the coordinati\e capac- 

 ity of the lateral cerebellar lobes. Attention has already been called to the 

 probable dual nature of the red nucleus as a relaying station. That it serves 

 other offices than the conduction of coordinati\e control seems likely. 

 On the other hand, the concomitant expansion of the dentate and red nuclei 

 in passing from the lowest to the highest of the primates speaks strongly in 

 favor of its relation to the impulses essential to coordinativc control of the 

 upper and lower extremities. Ilie red mieleus does not itself disclose an}' 

 specific participation in the neokinetic ad\ances of the upper extremity. 

 Its close incorporation in the cerebellar mechanism, however, makes it 

 probable that it receives its chief incentive toward expansion from the acces- 

 sion of coordinativc impulses essential to control the muscles in the upper 

 extremity. The difference in the planimetric coeflicients of the red nucleus 

 between the lowest of the primates and man is a little over 10 hundredths. In 

 this respect it coincides closely with the similar difference existing between the 

 dentate nucleus at the two extremities of the order. 



The volumetric comparison of the superior cerebellar peduncle which 

 connects the dentate with the red nucleus corroborates the conclusions con- 

 cerning the progressive increment in coordinativc control. This structure also 

 shows a progressive augmentation in passing from the lower end of the series 

 to its upper extremity. The significance of all of these elements related to the 

 cerebellum bears defmitely upon tlie field of neokinesis inasmuch as they are 

 all alike in\ol\ed in the coordinativc control of the appendicular musculature. 



