Chapter IX 



COMPARATIVE SUMMARY OF STRUCTURES HAVING 



EVOLUTIONAL SIGNIFICANCE IN THE BRAIN STEMS 



OF THE LOWER PRIMATES 



A Critical Comparisoii oj the Pyramidal Tract, Olivary Body, Dorsal Sensory 

 Nuclei, Vestibular, Cerebellar and Pontile Nuclei, the Midbrain Colliculi and 

 Oculoinotor Decussation. Their EvolutKmal Significance in Relation to Behavior 



CoMPARAXn E Re\TE\V OF StKUCTURAL AND BeHA\ lORAL ADAPTATIONS 



CERTAIN homologous constituents in the brain stem of the lemur, 

 tarsier, marmoset and howling monkey manifest a measurable vari- 

 ability in structural clelinition and relative size. Such variations 

 seem to harmonize with equally well-defined modifications in the sensory and 

 motor equipment utilized by these animals in their highly differentiated 

 behavioral adjustments. That these variables should particularly involve the 

 neokinetic elements of behavior might be presupposed from the fact that the 

 progressive addition of new motor complexes paved the way to the highest 

 evolutional development. 



I. The Pyramidal Tract in Relation to the Vollintary Control of 

 THE Extremities with Especial Reference to the Hand 



Especially striking are the variations indicative of modification in 

 volitional inlluence over the somatic muscles. The pyramidal system has 

 quite as much significance with regard to the degree of neokinetic activity 

 as the cortex of the cerebral hemispheres. In direct proportion as the phyletic 

 expansion of the neopallium has made possible new accessions of highly 



