THE MOTOR AREA 213 



injury to the spinal and pyramidal neurons. Lesions of the 

 anterior horn cells causes complete paralysis of the correspond- 

 ing muscles, so that in time atrophy results. When the pyram- 

 idal neurons are affected, as in hemiplegia from unilateral 

 lesions of the motor cortex, there is paralysis as regards volun- 

 tary control, but the spinal neurons being intact the muscles 

 are open to reflex stimulation, which, being uncontrolled, owing 



FIG. 13 



Plan of the human brain in profile (Dalton), showing its fissures and 

 convolutions: S, fissure of Sylvius; S' y anterior branch; S", posterior 

 branch; R, fissure of Rolando; P, parietal fissure. 



to lack of inhibition from the cortex, may lead to spastic rigidity. 

 This condition, however, may be due in other cases to impulses 

 coming byway of the rubrospinal tract. It is very suggestive, 

 in this connection, that stimulation of the cortex leads to inhi- 

 bition of tone, and to inhibition of active contraction, as well 

 as to active muscular contraction. When, for example, in a 

 monkey the part of the arm area which presides over extension 

 of the elbow is stimulated, the biceps relaxes while the triceps 



