28o THE LOWER PRIMATES 



movements, affords the final common pathway for all ocular innervation 

 with the exception of the superior obHque and the external rectus muscles. 

 The important feature concerning the internuclear connection of this cell 

 group arises from the fact that in the higher grades of the primate series, the 

 nuclei of the two sides are most extensively connected by means of commis- 

 sural and decussating fibers. From the functional standpoint this interrela- 

 tionship between the nuclei denotes an increasing capacity for coordinating 

 the nerve impulses arising in the two nuclear groups. Thus, the oculomotor 

 nucleus which is provided with the least abundant intercommunication 

 between the groups on the two sides would, in comparison with a similar 

 nucleus having an extensive intercommunication, be capable of far less 

 conjugate control of the muscles moving the two eyes. 



RELATION OF CLOSE INTERNUCLEAR CONNECTIONS TO 

 BINOCULAR VISION 



The serial increase in this internuclear connection appears to be attended 

 by an increasing degree of binocular vision possessed by the animal. The 

 establishment of such vision primarily requires that the visual axes of the 

 two eyes shall be capable of maintaining the primary position of Listing, that 

 is to say, in parallel. Also for the purposes of near vision with close focusing 

 they should have the capacity of convergence to a certain degree at least. 

 These two positions of the visual axes, in parallel and in convergence, depend 

 upon the cooperative action of the oculomotor muscles of the two eyes. 

 There must be a simultaneous adjustment in the contractural tension, not 

 only in holding the gaze fixed upon distant and nearby objects, but in moving 

 the eyes through horizontal as well as vertical arcs. 



The more closely, therefore, the oculomotor nuclei of the two sides are 

 interrelated by means of connections between them, the more likely is such 

 harmony of action to be obtained in the oculomotor movements. Conse- 



