116 THE \'ERTEBR.\TE ANI^LU.: IRRITABIUTY 



going impulses. It is in the central s^'stem that afferent neurones 

 have s\-napse^ with adjuster neurones and these in turn with effer- 

 ent neurones. The multiphcity of connections thus made possible 

 furnishes the most important part of the mechanism of integration. 

 It is desirable to consider the nature of the adjustment in the dif- 

 ferent regions of the central ner\-ous system (Fig. 63). 



Adjustor neurones in the spinal cord are concerned with the 

 simpler and less comphcated of the reflex arcs. In the " scratch "' 

 reflex, for instance, adjustor neurones carr*- the impulse down the 

 spinal cord or may transmit it from side to side. Impulses enter- 

 ing the cord over spinal nerv^es can also pass upward to the medulla, 

 cerebellmn, and diencephalon. The cell bodies of these adjustor 

 neurones are located lq the gray matter of the spinal cord, while 

 their processes, which transmit impulses up and down the cord, are 

 to be found in the fiber tracts of the white matter. The white mat- 

 ter also contains groups of nene fibers which arise from adjustor 

 neurones located in the cerebral cortex, mesencephalon, and 

 medulla. The gray matter of the cord is, therefore, the seat of 

 adjustor neurones which connect dift'erent levels of the cord with 

 one another and with parts of the brain, and which carr^- mipulses 

 from one side of the cord to the other. In addition, the efferent 

 neurones, which send processes out through the ventral roots of 

 spinal nerves, are found in the gray matter of the cord. The 

 spinal cord adjusts simple reflexes and transmits impulses to and 

 from the brain. 



The primitive brain in vertebrates is comp>osed of the telen- 

 cephalon, diencephalon, mesencephalon, metencephalon, and myel- 

 encephalon. These parts constitute the so-called " brain stem." 

 The changes in the direction of greater brain complexity occur 

 chiefly in the region of the telencephalon and metencephalon. The 

 cerebral hemispheres arise from the dorsal wall of the telencephalon 

 and, by outgrowth and folding, become the most conspicuous part 

 of the brain in mammals {cf. Figs. 39, p. 67, and 288, p. 532). The 

 cerebellum is the dorsal development from the metencephalon. The 

 cerebral hemispheres and cerebellum are the parts in which new 

 functions are added; while the functions of the brain stem remain, 

 for the purposes of this discussion, constant throughout the verte- 

 brate group. In contrast to the arrangement of the white and 

 gray matter of the spinal cord, ■^'ithin the brain the gray matter, 

 which consists of groups of adjustor neurones, occurs in masses, 



