1048 MOVEMENTS OF CORTICAL ORIGIN. [BOOK in. 



stimulation 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 move- 

 ments. 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, 

 nimbleness 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 

 organisation ; and the results thus obtained lead us to the con- 

 clusion 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 between 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 

 quantity 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 cor- 

 respondence 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 



