128 



THE MECHANISM OF LIFE 



hemispheres, as well as into the Kolandic and Sylvian fissures, so 

 that such a view as that of Fig. 38 does not show its entire area. 

 Round about, but mainly in front and in the depths of the 

 fissure of Rolando, is the motor region, and this has been divided 

 up into a number of particular areas, each of which controls 

 the movements of some small part of the body. Thus, directly 

 in front of the fissure are the " centres " or " areas " for the 

 movements of the trunk, arm, hand, head and eyes, face, etc. 

 These areas are not very well defined (for instance, they do not 

 coincide exactly with the areas of the various " gyri " or con- 

 volutions), and they overlap to some extent. When any one of 

 them is stimulated in a suitable manner, usually by application of 



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ry imK 



Fig. 38. — Diagbam of the Principal Coktical Centres in the Left 

 Human Cebebeal Hemisphere. 



a feeble electric current, certain groups of muscles twitch or con- 

 tract, and this is the evidence that the area in question controls 

 these parts. The movements that are so elicited are not the finished 

 ones that occur in the normal behaviour of the animal, but they 

 indicate none the less the nature of the nervous controls and paths. 

 The cortex is, as we have seen, a thin sheet of grey matter 

 overlying the core of nerve fibres that forms the greater part of 

 the mass of the hemispheres. Directly beneath it we come 

 upon the white matter, and much of this consists of the axons 

 proceeding from the overlying pyramidal cells of the cortex. 

 Tracing these fibres by various means, we find that they run 

 down into the crura or cerebral peduncles towards the medulla. 

 There, as Fig. 28 shows, these " pjrramidal tracts " mainly cross 

 each other, that one coming from the left-hand side of the brain 



