128 HUMAN PHYSIOLOGY 



gray matter of the motor regions contains many large 

 nerve cells, of a type called pyramidal, from which the 

 axons of nerve fibers pass inward. The fibers which 

 thus have their origin in the motor areas become con- 

 densed into well-defined bundles which can be recognized 

 at each successive level in the brain-stem. In the medulla 

 most of these fibers sweep across into the opposite half 

 of the nervous system and descend the spinal cord to 

 connect with the motor cells in its gray core. It will be 

 noted that a fiber from the cortex never reaches a muscle; 

 it plays upon a cluster of cells of a lower order and the 

 contractile elements are governed by these. Something 

 like this has been seen to be true of the respiratory 

 center in the medulla. 



When the motor areas in man are extensively damaged, 

 or when the fibers carrying impulses down the axis 

 from these areas are interrupted, a disabling paralysis 

 results. This is to be contrasted with the coordinating 

 powers of the dog which can lose not merely the motor 

 regions of the cerebrum but the whole of that division 

 and still balance and walk. We have here another il- 

 lustration of the concentration of functions in the cere- 

 brum of the higher forms and the reduction of capacity 

 in the cord and brain-stem. 



Movements which we call voluntary are assumed to 

 be preceded by processes in the cerebral motor areas. 

 In all probability this is equally true of many move- 

 ments which we class as involuntary. The line of 

 demarcation is arbitrary and of little value. Even 

 when an act is as clearly as possible deliberate we can- 

 not say that the process originated here or there in the 

 cerebrum. The excitation of a motor spot is presum- 

 ably to be referred to some other part of the brain and 

 as we attempt to trace the action to its source we are 

 baffled and left unsatisfied. We are forced toward the 

 conclusion that the ultimate cause of every movement 

 is to be sought on the afferent side of the nervous sys- 



