H^'LOBATES HOOLOCK, THE GIBBON 421 



cerebral peduncles bounding the optico-peduncular space. From this space 

 emerge the two oculc^motor nerves and in it are contained the mammillary 

 bodies, the attachment of the infunchbular stalk, and the tuber cinereum. 



Internal Structure of the Brain Stem in Hylobates Hoolock 



In some particulars the cross sections through the brain stem of gibbon 

 show a slight advance, particularly in the definition ot the structures when 

 compared with the baboon and macacus. This applies more especially to 

 the inferior olive and pontile nuclei. In other structural details the definition 

 in gibbon does not show any marked degree of progress over the other inter- 

 mediate primates. 



LE\EL OF THE PYRAMIDAL DECUSSATION (FIG. I Q9 ) 



Here the crossing libers of the pyramid { Pyx) determine a conspicu- 

 ous rearrangement in the axis. The ellects of this decussation are seen in 

 several notable features. In the lirst place, the usual separation ot the ventral 

 gray column ( Ven) from the central gray matter (Cen) is clearly shown. 

 This behavior on the part of the pyramidal fibers is ot particular interest. 

 It is primarily a mammalian characteristic. Furthermore, the pallio-spinal 

 connection represented by the pyramid is peculiar to mammals. Decussation 

 in conduction pathways of all ^'arIetIes, whether intersegmental or projec- 

 tional, is the almost invariable rule in the vertebrate neuraxis. Whether, as 

 Cajal believes, such crossing was determined by the universal decussation 

 ot the optic libers throughout this phylum, or whether some other factors have 

 been operative, the irresistible tendency for long fiber tracts to pass from 

 one side of the brain to the other has exerted its influence on the pyramidal 

 system. The nerve fibers in this system are all newcomers among the conduct- 

 ing axons of the neuraxis. They were introduced by the mammals and repre- 

 sent the last accessional elements in the control of motor reactions. Thev are 



