26o THE LOWER PRIMATES 



IV. The Vestibular Nuclei in Their Relation to the 

 Balancing Mechanism 



extreme sensitiveness of the balancing mechanism 



This group of nuclei is fundamentally associated with the function of 

 balancing, that is, the maintaining of the body in the optimum physiological 

 posture, or of righting the body in the event that it may for any reason be 

 forced out of this posture. This posture itself, although subject to numerous 

 modifications, presents a fairly well-generalized pattern in all vertebrates. 

 In accordance with this pattern the animal's best posture appears that in 

 which the ventral surface of the body is nearest to the surface of the earth. 

 This is true of fish, amphibia, reptiles and birds. From such a posture these 

 animals may most readily initiate locomotion or remain in a resting phase 

 preparatory to locomotion. This posture in mammals, with a few notable 

 exceptions, such as the sloth and the bat, is similar to that in the lower 

 vertelsrates. Any sHght deflection in this posture tends to cause extensive dis- 

 organization in the animal's behavior. Such disturbances may be artificially 

 induced by pathological lesions affecting the nervous system, more par- 

 ticularly that part of the nervous system connected with the proprioceptive 

 organization upon whicli balancing function depends. Tlius the destruction 

 of one semicircular canal will so thoroughly disorganize the animal's capacity 

 for assuming and maintaining the optimum physiological posture as to make 

 locomotion impossible. It also makes tlie animal incapable of maintaining 

 itself in any position. 



The mechanism upon which this important activity depends must needs 

 be highly organized. This applies to its receptor organs as well as its central 

 representation for receiving, converting and transmitting postural impulses 

 to the musculature. Furthermore, this mechanism is extremely sensitive to 

 many conditions of habitat and behavioral adjustments. Aquatic, aerial and 



