274 THE GROWTH OF THE BRAIN. 



arrangement renders it possible for every sense organ 

 to control the centre for each group of muscles, in so 

 far as it is connected with it by paths of associating 

 fibres, and renders possible even the cross associations 

 between the different sensory areas, which are indicated 

 by coloured hearing and the like. 



Although anatomically the evidence is good that the 

 nerve tracts which put one hemisphere in connection 

 with one half of the spinal cord, cross the middle line at 

 the lower end of the bulb, yet physiological experiments, 

 in which one hemisphere of the cerebrum has been re- 

 moved from a dog, show that both halves of the body 

 may come under the voluntary control of the remaining 

 half of the cerebrum. This relation is not readily 

 explained. One difficulty resides in the fact that we 

 can by no means be sure that the manner in which the 

 impulses pass in the system after the operation is the 

 only one by which they pass under normal conditions, 

 for the central system gives evidence of possessing 

 supplementary pathways. Keeping this in mind, it 

 appears that the pathway by which one hemisphere can 

 control both halves of the body is dependent on a cross 

 connection between the appropriate lower centres in the 

 spinal cord. This cross connection is such in those cases 

 in which the muscles act in unison, as in the muscles of 

 mastication or phonation, that injury to one hemisphere 

 causes but little disturbance in the performance of these 

 muscles, the voluntary control of the centres on both 

 sides being easily maintained from either cerebral hemi- 

 sphere. On the other hand, where symmetrical groups 

 of muscles do not customarily react together, the control 

 of them by one hemisphere is but slowly attained, even 

 in the dog. 



It would appear to be most easily accomplished in 

 young and half-grown animals a fact which can be 



