284 INTRODUCTION TO NEUROLOGY 



connection has been proved in several ways in addition to the 

 direct physiological experiments by electric stimulation already 

 referred to. First, if this area of the cortex (and a portion of 

 area 6 in front of it) is destroyed, the entire pyramidal tract will 

 degenerate, a result which follows from the destruction of no 

 other part of the cortex. Conversely, if the pyramidal tract is 

 interrupted, the giant pyramidal cells of this area are the only 

 neurons of the cortex to give clear pictures of chromatolysis of 

 their chromophilic substance. In the third place, these giant 

 cells of the human cortex have been counted, and a count of the 

 number of fibers in the pyramidal tract shows that the numbers 

 are in tolerably close agreement (nearly 80,000 on each side of 

 the body). Finally, a case of sclerotic degeneration involving 

 almost the entire cortex has been described by Spielmeyer, in 

 which these giant cells and the fibers of the pyramidal tract alone 

 escaped injury. 



The sensory projection centers of the cortex have also been 

 determined physiologically, though their limits are less precisely 

 known than are those of the motor cortex. The olfactory 

 receptive area has already been mentioned as comprised within 

 the archipallium (hippocampus and hippocampal gyms, see 

 p. 217), only a part of which is exposed on the surface of the 

 brain (the regio hippocampica of Fig. 129; areas 27, 28, 34, 35 of 

 Fig. 131). The visual projection center, which receives fibers 

 from the thalamic optic centers in the pulvinar and lateral genic- 

 ulate body (pp. 165, 212), is in the occipital region (Fig. 129). 

 Area 17 (Fig. 131) appears to be the chief center for the recep- 

 tion of these visual projection fibers, though the adjacent area 18 

 participates in this function, these areas together comprising 

 the area striata of the cortex (p. 268). The auditory projection 

 center is in the upper part of the temporal lobe (area 41, and 

 probably to some extent area 42 also, of Fig. 130). The tactual 

 projection center lies in the postcentral region (Fig. 128; areas 1, 

 2, and 3 of Fig. 130). The parts of the cerebral cortex which lie 

 between the sensory and motor projection centers which have 

 just been enumerated are the association centers (see pp. 287, 

 290). 



Within each general sensory sphere there is a focal area which 

 is exclusively receptive in function, such as area 17 (Fig. 131) in 



