672 THE BKAIN. 



ordinary bodily movements, 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 organization and habit. As we have said, stimulation 

 of the motor area of one hemisphere produces movements, 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 the unilateral crossed movement resulting from stimulation of 

 the cortex an accordance with natural 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 stimulation of the motor area for the 

 eyes on either hemisphere produces movements of both eyes, and stimula- 

 tion of the trunk area of one hemisphere is also very apt to produce bilat- 

 eral action of the trunk muscles; in such instances the movements on both 

 sides are quite normal movements. We may incidentally remark that re- 

 moval 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 shown by the relative magnitude of 

 the several cortical areas when they are mapped out according to parts 

 of the body. The area for the arm, for instance (c/. Figs. 149, 150), 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 be- 

 come 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 organization ; and the 

 results thus obtained lead us to the conclusion that the motor region is cor- 

 related 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- 

 ing 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 pyram- 

 idal 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 com- 

 plexity 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. 151). 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. 



