THE SPECIAL NERVOUS MECHANISMS 127 



Fig. 28. Other fibres, commissural or projection ones, enter 

 the part of the cortex in question and break up into terminal 

 dendrites, which then come into relation with the apical and 

 basal dendrites of the cell by means of synapses. 



Thus each cell in the cortex can make a very great number of 

 connections with other cortical cells, or with nerve cells in other 

 parts of the central nervous system. Each of the collateral 

 axons, for instance, can become a commissural or projection 

 fibre, and a number of fibres coming from anywhere else can form 

 synapses with the dendrites of any cortical cell. This is why 

 the latter are so much more crowded in the canine, and still 

 more so in the amphibian, than in the human cortex : there is the 

 increasing tendency, as we raise in the scale of evolution, for the 

 connections between the cells to become more and more numerous 

 and complex, so that the number of paths which an impulse 

 leaving a pyramidal cell may take tends always to increase. 



The higher is tJie type of brain, the more manifold are the ways 

 in which the various centres may communicate with each other. 



Localisation o£ Function in the Cortex. — We must think of 

 the activity of the cortex cerebri in a twofold way: it is one 

 organ in so far as every part of it is connected with every other 

 part, and it is multiple inasmuch as functions are specialised 

 or localised in it. This specialisation has become known to us, 

 partly by experiment upon the brains of the higher mammals 

 (other than man), partly by the extension of these results to the 

 human brain (which is, of course, very similar in structure to 

 that of the anthropoid apes), and partly by observations made 

 on human subjects suffering from disease and accidents. 



Thus it is possible to distinguish in the human cortex a " motor 

 region," which is concerned with the initiation and elaboration of 

 willed movements; a " sensory region," upon which is dependent 

 the full psychical development of the sensations arising from 

 stimulation of the receptor organs; and a "prefrontal region," 

 about the functions of which we have very httle positive knowledge. 



Fig. 38 represents the approximate positions and boundaries 

 of the regions, and the particular areas into which the latter are 

 divided. The two great fissures — those of Rolando and Sylvius — 

 serve as lines which enable us to divide up the cortex into these 

 regions and areas. One hemisphere (the left one) is seen from 

 the side, but the reader must understand that the cortex dips 

 down into the great median fissure that separates right and left 



