MOTOR AREAS OF THE CEREBRUM 887 



on the frontal lobe. Its excitation causes conjugate deviation of both 

 eyes to the opposite side. The second area lies in the occipital lobe 

 coinciding approximately with the center for visual sensation. Stimula- 

 tion of it also causes eye movements, and these are not simply the result 

 of referred sensations which result in movements initiated from the 

 frontal visuo-motor center, since they persist after the latter has been 

 destroyed. Consequently the occipital visual center is both sensory and 

 motor in function. 



Physiological Bases of the Clinical Effects of Lesions in the Motor 

 Area of the Cortex. Stimulation of the motor area of the cortex causes 

 usually a response on the part of the muscles on the opposite side of the 

 body. From this it appears that the connections between the two halves of 

 the cortex and the muscles they control is a crossed one. In agreement with 

 this is the fact which has been known since very ancient times that in- 

 jury to one side of the brain frequently results iri paralysis of certain 

 voluntary movements on the opposite side of the body. 



But since cortical lesions are usually small in comparison to the area 

 of the motor regions, the derangement more commonly affects muscular 

 acts represented in a limited area of the cortex and as a result the move- 

 ments of a single arm, of one side of the face, or of one leg are removed 

 from voluntary control. Such a limited, unilateral paralysis is called 

 monoplegia. 



Jacksonian Epilepsy. When the strength of the stimulus applied lo- 

 cally to the cortex is increased, its influence spreads so that adjoining 

 motor cells are brought into activity and larger and larger groups of 

 muscles take part in the response. This progressive " march" of the 

 response to cortical stimulation is usually limited in its spread to the 

 opposite half of the body, in this way differing markedly from the 

 spread of spinal reflexes, which we have seen occurs when the stimulus is 

 increased (page 842). 



The foregoing results obtained by experimental stimulation in animals, 

 are very similar to the symptoms observed in man when the cerebral 

 cortex is stimulated by the pressure on it of a meningeal tumor or a 

 spicule of bone. Such stimulation causes contraction in the correspond- 

 ing muscular area; the contraction then spreads to neighboring groups 

 of muscles, and may ultimately involve the whole musculature of the 

 body in a convulsive fit. This is known as Jacksonian epilepsy, and 

 it is to be distinguished from ordinary epilepsy by the fact that the 

 patient does not become unconscious during the fit. Like ordinary 

 epilepsy, however, the Jacksonian type is usually preceded by a pe- 

 culiar sensation of numbness or tingling in the area that is to show 

 the first contraction. One of the greatest achievements of modern 

 brain surgery is the cure of a Jacksonian epilepsy, by trephining the 



