650 THE NERVOUS SYSTEM 



The Parietal Association Center. Special mention is made of this 

 association area because there is increasing evidence that it is the parietal 

 region of the brain, rather than the frontal, as popularly believed, that is 

 most intimately concerned with the higher acts and powers of imagination, 

 idealization, and reasoning. It is the region through which the individual 

 maintains his interests and relations with the external world as against his 

 own body. The parietal association center is more closely related to the 

 visual, auditory, and speech centers of the cortex. The great musician 

 Bach had an exceptionally well developed parietal region. 



On the Cortical Centers in General. For purposes of clearness in 

 presentation, the cortical centers have been discussed one by one, but the 

 reader is guarded against the thought that their activities are in any sense 

 isolated. A motor area does not usually act in the absence of sensory or af- 

 ferent stimulation in the actual living body, whether it may do so on occa- 

 sion or not. Neither do sensory impressions arising in the peripheral sense 

 organ make their way over definite tracts to the brain and cortex and arouse 

 sensations alone. Sensations do not occur independent of motor activities 

 on the one hand, and of intellectual acts through the association centers on 

 the other. 



The association centers are the highest co-ordinating regions of the nervous 

 system. They are to the sensory and motor centers what these latter are to 

 the reflex centers of the cord. The difference is one of degree and not of 

 kind. Further, the association centers are probably set into activity by the 

 complex of inflowing or afferent impulses in just the same sense that the 

 spinal reflex centers are set in activity by sensory or afferent stimuli; the 

 condition is, of course, a thousand times more complex. 



THE CRANIAL NERVES. 



The cranial nerves consist of twelve pairs; they appear to arise (super- 

 ficial origin) from the base of the brain in a bilateral series, which extends 

 from the under surface of the anterior part of the cerebrum to the lower end 

 of the medulla oblongata. Traced into the substance of the brain and 

 medulla, the roots of the nerves are found to take origin from various masses 

 of gray matter. 



The roots of the first or olfactory and of the second or optic nerves are 

 discussed elsewhere. The third and fourth nerves arise from gray matter 

 beneath the corpora quadrigemina; and the roots of origin of the remainder 

 of the cranial nerves can be traced to gray matter in the floor of the fourth 

 ventricle, and in the more central part of the medulla, around its central 

 canal, as low down as the decussation of the pyramids. 



According to their several functions the cranial nerves may be thus 

 arranged : 



