760 MOVEMENTS OF CORTICAL ORIGIN. [BOOK m. 



are, so to speak, the elementary factors of ordinary bodily move- 

 ments, the detached and imperfect chords of a musical piece ; and 

 in the following facts relating to their production we can recog- 

 nize the influences of organisation and habit. As we have said, 

 stimulation of the motor area of one hemisphere produces move- 

 ments, as a rule, which are limited to one side of the body, and 

 that the opposite side. Now both in ourselves and in the higher 

 animals a large number of bodily movements, especially of the 

 limbs, are habitually unilateral ; and, putting aside the question 

 why there should be two halves of the brain, and why the one 

 half of the brain should be associated with the cross half of the 

 body, we may recognize in them unilateral crossed movements 

 resulting from stimulation of the cortex in accordance with natu- 

 ral habits. But some movements of the body are ordinarily 

 bilateral; the two eyes, for instance, are ordinarily moved 

 together, and the two sides of the trunk move together very 

 much more frequently than do the two fore limbs or the two 

 hind limbs. And in accordance with this we find that stimula- 

 tion of the motor area for the eyes on either hemisphere produces 

 movements of both eyes, and 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 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 com- 

 paratively 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 rela- 

 tive 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. 123, 124, 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 forefinger 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 

 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 pass- 



