1048 MOVEMENTS OF CORTICAL ORIGIN. [BOOK HI. 



lation of the trunk area of one hemisphere is also very apt to 

 produce bilateral action of the trunk muscles ; in such instances 

 the movements on both sides are quite normal movements. We 

 may incidentally remark that removal of the trunk area leads to a 

 good deal of bilateral degeneration, that is, to degeneration of 

 strands in the pyramidal tracts of both sides, whereas such a 

 bilateral degeneration is comparatively scanty after removal of the 

 leg or arm area. 



That it is the movement and not the part moved which is, so 

 to speak, represented on the cortex is further shewn by the relative 

 magnitudes of the several cortical areas when they are mapped 

 out according to parts of the body. The area for the arm, for 

 instance, cf. Figs. 126, 127, is, so to speak, enormous compared to 

 that of the trunk when the relative bulks of these two parts of 

 the body are considered ; and within the arm area itself the space 

 occupied by the thumb and fore-finger and digits is, bulk for bulk, 

 out of proportion to the space allotted to the shoulder ; so also the 

 area for the eyes or for the mouth is out of proportion to the size 

 of those organs. But these relative sizes of the respective areas 

 become intelligible when we bear in mind relative mobility, nim- 

 bleness and delicacy of execution ; in these respects the shoulder is 

 far behind the thumb, while the eyes and mouth surpass most 

 other parts of the body. 



We are brought yet a step further when we compare, in respect 

 of the cortical motor region, animals of different grades of organi- 

 sation ; and the results thus obtained lead us to the conclusion 

 that the motor region is correlated not to movements in general, 

 but to movements of a particular kind. Taking in series the 

 rabbit, the dog, the monkey and man, we find in passing from 

 one to the other, an increase in prominence and in differentiation 

 of the motor region accompanied by an increase in the bulk of 

 the pyramidal tract ; among the many striking differences be- 

 tween the brains of these several animals, these two features, 

 the increasing complexity of the motor region, and the increasing 

 size of the pyramidal tract, are among the most striking. The 

 size of the pyramidal tract is itself correlated to the complexity 

 of the motor region, and, being the more easily determined, 

 may be used as indicating both ; the difference in the size 

 of the pyramidal tract in these animals is seen all along the 

 whole length of the cord (Fig. 128). Now as regards mere quan- 

 tity of movement, if we may use such an expression, the differences 

 between these animals are of no great moment. If we were 

 to take the amount of energy expended as movement in twenty- 

 four hours per gramme of muscle present in the body in each 

 of the four cases, we should certainly not find any correspon- 

 dence between that and the size of the pyramidal tract. If 

 however we take a particular kind of movement, what we may 

 perhaps call skilled movement, that is movement carried out by 



