1044 MOVEMENTS OF CORTICAL ORIGIN. [BOOK m. 



more lowly frog, through the bird, the rabbit, the dog, and other 

 lower mammals up to the monkey, the anthropoid ape, and so to 

 man himself, we find an increasing differentiation of the cerebral 

 cortex, by which certain areas of the cortex are brought into 

 special connection with certain skeletal or other muscles in such 

 a way that stimulation of a particular portion of the grey matter 

 gives rise to a particular movement and to that alone. 



657. In treating of the structure of the brain we spoke 

 (632) of the pyramidal tract as starting from the motor region 

 of the cortex ; and it is obvious that the fibres of this tract must 

 be concerned in the development of the movements which we 

 have just described. When the movements are brought about 

 by stimulation of the fibres in some part of their course, in the 

 internal capsule for instance, there can be no doubt that the 

 stimulation starts impulses which, travelling down the tract to 

 the origins of certain cranial or spinal nerves, in some way give 

 rise to coordinate motor impulses along the motor fibres of the 

 nerves ; and we may with reason speak of the impulses then 

 passing along the tract as motor or efferent in nature. When the 

 stimulus is applied direct to the cortex, we may assume that 

 processes, started in the grey matter, eventuate in similar efferent 

 impulses along the fibres of the tract. All the evidence leads us 

 to regard this tract as an efferent tract. 



When the spinal cord is divided in the lower dorsal region and 

 the electrodes of an electrometer are brought into connection with 

 the transverse cut surface and with some point of the longitudinal 

 surface above, the electrometer gives evidence of currents of 

 action (manifested as negative variations of a demarcation current 

 or current of rest, 67) whenever the motor area of the hind 

 limb is stimulated, but not when other parts of the cortex are 

 stimulated. We have already said that stimulation of any part of 

 the motor region may under abnormal conditions give rise to 

 general epileptiform convulsions; when these occur during such 

 an experiment as the above, currents of action manifest themselves 

 in the lower dorsal cord, whether the stimulation giving rise to 

 the convulsions be applied to the area for the hind limb or to any 

 part of the motor region. It has been further observed that the 

 currents of action developed within the spinal cord tally in a very 

 exact manner with the muscular movements. The convulsions 

 begin with a sustained ' tonic ' contraction of the muscles, and the 

 electrometer shews a similar sustained current of action ; this is 

 followed by rhythmic movements of the muscles, accompanied by 

 corresponding rhythmic movements of the mercury of the electro- 

 meter. Without insisting too much on the exact interpretation 

 of these results we may take them as at least shewing that, when 

 the motor region of the cortex is excited, nervous impulses accom- 

 panied by " currents of action " pass downward along the fibres of 

 the pyramidal tract. 



