SUMMARY OF STRUCTURES 261 



arboreal adaptations create the most marked variations in this neural 

 apparatus. The responsibility of the balancing mechanism in animals whose 

 life is spent largely in flight, soaring to great heights, or dropping swiftly from 

 the air to alight upon the branches of trees is most exacting. It necessitates 

 a mechanism capable of adjusting the body in postures most advantageous 

 for the use of wings. Adaptation to arboreal life imposes similar requirements 

 upon the balancing apparatus. Such of the apes, for example, as are able to 

 live almost exclusively in the trees and move from place to place by leaping, 

 climbing and swinging, require a most delicately adjusted mechanism in the 

 interest of equilibration. 



FUNCTION OF THE VESTIBULAR NUCLEI 



The primary centers associated with balancing are the vestibular nuclei. 

 These nuclei serve as the chief receiving stations for the impulses flowing 

 inward from the semicircular canals, the utricle and saccule. It is probable that 

 even from the earliest stages of life these nuclear groups in the oblongata 

 exert a pronounced influence upon the behavior of the young animal. With 

 rare exceptions among the mammals, the power to assume and maintain the 

 optimum physiological posture develops shortly after birth. The structural 

 substratum for the early maturing of this function is found m the lact that 

 the nerve fibers permitting communication between the proprioceptive organs 

 have consummated their connections with the vestibular nuclei. These 

 nuclei are in turn provided with efferent fibers necessary to the conduction of 

 impulses destined to the somatic muscles. In this manner the reflex arcs 

 essential to maintain the physiological optimum posture are completed. The 

 vestibular nuclei in the oblongata are, of themselves, sufficient to mediate 

 the reflex impulses necessary to the proper balance of the body. On the other 

 hand, there is evidence that many myelinized fibers, even at an early period 

 of development, pass to the region of the inferior colliculus. These axons 



