272 IMIYsloUHJY FOR DKNTAL >TU>F.XTS. 



ing part of the cranium and through this hole the tumor or 

 blood clot is removed. 



III. These so-called motor areas are of course also *< nsm-ii 

 areas in the sense that the afferent stimuli which come up from 

 the spinal cord run to them. They are really sensori-motor 

 centers. For some of the more highly specialized proficient 

 sensations such as vision and hearing (see p. 279), there are. 

 however, special centers. These along with an extensive liehl 

 of associational or junctional grey matter constitute the third 

 main division of the cerebral cortex and occupy the greater 

 part of the parietal, the temporosphenoidal .and the occipital 

 lobes. The visual is the most definite of these centers. Thus 

 if the occipital lobe be removed or destroyed by disease on one 

 side, the corresponding half of each retina becomes blind. It 

 is by studying the exact nature of the involvement of vision in 

 such cases that the physician is able to locate the position of a 

 tumor, etc. 



The center for hearing is in the temporosphenoidal lobe, but 

 its location is not very definite. 



It will be seen, however, that the visual and auditory centers 

 take up but a small part of this third division of the cerebrum, 

 the most of it being occupied by associational areas. The nerve 

 cells of these areas do not, like those of the motor and sensory 

 centers, send fibers which run as pyramidal or optic fibers to 

 some lower nerve center, but only to other cerebral centers, 

 which they serve to link together. They are specialized to serve 

 as junction points for all the receiving and discharging centers 

 of the cerebrum, so that all actions may be properly correlated or 

 integrated. These junctional centers thus perform the great 

 function of adapting every action of the entire animal to some 

 definite purpose. Along with the nerve cells in the prefrontal 

 areas, the associational cells represent the highest development 

 of cerebral integration, so that we find the areas in which they 

 lie to become more and more pronounced, the higher we ascend 

 the animal scale. 



The Mental Process. The impression received by the visual 

 center when a young animal looks for the first time at, say a 



