ESSENTIALS OF PHYSIOLOGY. 



of the thigh, of another in flexion of the leg, and so on. The repre- 

 sentation of the movements of the leg, trunk, arm, and face is in that 

 order on the convolution from above downwards, and each of these 

 sub-areas may be further subdivided into points for the movements of 

 particular groups of muscles. 



Correlated with the development of the cerebral cortex is the 

 increased size of the pyramidal tract, which in monkeys, and still more 

 in man, is the principal path for the rapid conduction of voluntary 

 impulses to the motor neurons of the spinal cord. Other motor paths 

 from the cerebral cortex probably exist in the human nervous system, 

 but have fallen into disuse, and it is for this reason that the effects of 

 lesions of the cortex or of the pyramidal tracts in any part of their 

 course are so much more severe and permanent in man than in the 

 lower animals. 



It must not be supposed that the motor area of the cerebral cortex 

 is the actual seat of voluntary impulses. On the contrary, its activity 

 is aroused by impulses from other parts of the brain, and represents 

 only a small fraction of the total process of which it forms a part. 

 Generally speaking, the movements produced are on the opposite side 

 of the body to that stimulated, but in some cases muscles on both sides 

 of the body may be simultaneously affected ; for example, stimulation 

 of a point concerned with the movements of the eyes will result in both 

 eyes being turned towards the opposite side. If the right side be 

 stimulated, the eyes are turned to the left by the contraction of the 

 right internal rectus and the left external rectus, with simultaneous 

 relaxation of the right external and left internal recti. Similarly the 

 areas for the trunk and neck govern movements of both sides of the 

 body. In all cases in which a movement is carried out by the combined 

 action of muscles on the two sides of the body, the muscles of both sides 

 are bilaterally represented in the cortex. 



If .a stronger stimulus is applied to a motor point on the cortex than 

 is necessary to elicit the movement peculiar to that point, the excitation 

 will spread to adjacent areas, just as irradiation occurs in the spinal 

 cord. By increasing the strength of the stimulus it is possible to throw 

 all the muscles of the body into convulsive contractions. A similar 

 phenomenon is exhibited in Jacksonian epilepsy, in which a localised 

 irritation of a motor area, such as that due to the pressure of a spicule 

 of bone, causes a general convulsion beginning in the part of the body 

 represented at the site of the lesion. 



The motor area has been stimulated in conscious human beings ; the 

 stimuli elicited movements without any sensation other than a conscious- 

 ness of the movements which took place. There is therefore no reason 



