

ON VOLUNTARY MOVEMENTS. 669 



If, however, not a Maeacus or other ordinary monkey, but the more 

 highly developed orang-outang be taken as the subject of experiments, the 

 differentiation is found to be distinctly advanced ; the several areas are 

 more sharply defined, and what is important to note, the respective areas 

 tend to be separated from each other by portions of cortex stimulation of 

 which gives rise to no movement at all. 



The opportunities of stimulating the cortex of man himself have been 

 few and far between, and have, for the most part, been conducted under 

 unfavorable circumstances ; but as far as the results so obtained go, they 

 show that the topographical distribution of areas for the several movements 

 is carried out on the same plan as in the monkey (we are purposely confin- 

 ing ourselves now to the results of artificial stimulation) ; and, moreover, 

 justify the conclusion, which a priori reasons would lead us to adopt, that 

 in man the differentiation is advanced still further than in the monkey. 



Thus, when we survey a series of brains in succession, from the 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 gray 

 matter gives rise to a particular movement, and to that alone. 



570. In treating of the structure of the brain we spoke ( 545) 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 develop- 

 ment of the movements which we have just described. When the move- 

 ments 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 origin 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 gray matter, eventuate in similarly 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 elec- 

 trodes 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 elec- 

 trometer shows a similar sustained current of action ; this is followed by 

 rhythmic movements of the muscles, accompanied by corresponding rhyth- 

 mic movements of the mercury of the electrometer. Without insisting too 

 much on the exact interpretation of these results, we may take them as at 

 least showing that, when the motor region of the cortex is excited, nervous 



