210 THE LOWER PRIMATES 



speak of as a fifth hand to the animal's motor and sensory equipment. 

 Although the prehensile adaptabihty of the tail in mycctes is hardly to be 

 compared with that of the woolly monkey (Lagothrix) or the spider monkey 

 (Ateles), the caudal appendage of the howhng monkey is an important organ 

 which the animal uses with great deftness and skill. The increment in the 

 column of Burdach is also significant. It rellects a further difl'erentiation of 

 the hand with the consequent development of new motor capacities as well 

 as acquisitions in manual dexterity and precision. In proportion as the fore- 

 limb has emancipated itself from its limitations as a locomotor organ, it has 

 expanded its potentialities in mastery of the environment and has added 

 immeasurably to the upbuilding of new behavioral reactions. 



One feature of the brain stem which dilferentiates mycctes from lemur 

 and marmoset is the size of the cerebellum. 



The pons Varolii is also considerably larger than in the lower forms. 

 The inference from the size of the cerebellum in conjunction with that of the 

 pons seems to be that mycctes possesses a fairly wide range of acquired skilled 

 movements and employs the forelimbs for other purposes than those of 

 locomotion. 



DORSAL ASPECT OF THE MIDBRAIN 



The dorsal aspect of the midbrain presents two well-defined sets of 

 collicular eminences whose prominence suggests the persistence of certain 

 visual and auditory functions primordially vested in the mesencephalon. 



Internal Structure of the Brain Stem in Mycetes Seniculus 



The internal structure of the brain stem in mycetes gives the impression 

 that all of the structures thus far recognized as criteria in estimating behav- 

 ioral reactions stand out with a clearness of definition not observed in either 

 the marmoset or the lemur. 



