Mammalia: Nervous System 715 



seems evident, therefore, that incomplete decussation is related to 

 binocular vision. It provides that the visual centers on each side of the 

 brain shall receive impressions from both eyes. Precisely how it works 

 to give the animal a unified picture of the outside world is the subject 

 of many theories and is one of the many unsolved problems presented 

 by vertebrate eyes. 



The eighth cranial nerve (inaccurately named "auditory" 

 or "acoustic") clearly demonstrates, by its central connections, that 

 the ear is physiologically two organs. The nerve consists of two quite 

 distinct trunks. The vestibular trunk passes to the sensory cristae of 

 the semicircular canals and to the maculas of the utriculus and sac- 

 culus. The cochlear trunk goes to the organ of Corti. The two trunks 

 pass together into the medulla, where each becomes related to primary 

 reflex centers. The vestibular centers are connected with coordinating 

 centers in the cerebellum but apparently have, at most, only slight 

 connection forward into the cerebral cortex. The cochlear trunk is 

 connected not only with various reflex mechanisms in the medulla and 

 cerebellum and with others in the posterior pair of lobes of the mesen- 

 cephalic corpora quadrigemina, but also has important connections 

 forward with the higher coordinating centers of the thalami and, 

 finally, with centers in the temporal region of the cerebral cortex. 



MECHANISM OF BEHAVIOR 



The foregoing account of the mammalian brain sketches only in 

 barest outline the overwhelming intricacies of its structure. Even so, 

 the meager outline may suffice for a little appreciation of its general 

 mode of operation. The brain has been likened (p. 667) to the office of 

 a manufacturing establishment. The office must receive information 

 both from the outside and from the inside of the factory. From outside 

 must come information (exteroceptive) concerning needed raw ma- 

 terials and orders for products. From inside must come reports (intero- 

 ceptive) on the progress of the work and various other matters. Also, 

 the office must be kept informed about the condition and working 

 efficiency of its machinery (proprioceptive information). Information 

 of the various sorts comes at first to the hands of secretaries and sub- 

 ordinate employees, each of whom transmits it to one of higher author- 

 ity who, after some analysis and coordinating of information received, 

 forwards the results to still higher officials. In a secluded private office 

 may be found the person or persons who plan, make major decisions, 

 and give orders as to what is to be done and when and how it shall be 

 done. These orders are transmitted to a group of executive officers who 

 see that the specified work is properly carried out in the factory. But 

 the executives have no authority to initiate anything unusual or new 



