888 CENTRAL NERVOUS SYSTEM 



skull over the affected center and removing the meningeal tumor or 

 spicule of bone which is responsible for the stimulation. To enable 

 the surgeon to locate exactly the position of the irritating body, it is 

 necessary to examine the patient very closely as to the muscular group 

 which is initially affected during the convulsions, and then to examine 

 an outline map of the cerebral hemisphere indicating the position of 

 the various motor and sensory areas as deduced mainly from experi- 

 ments on the higher monkeys and verified by the experience gain'ed 

 by previous operations. Topographic maps indicating the surface mark- 

 ings corresponding to the various convolutions of the cerebrum must 

 also be used. In such operations the surgeon often has the opportunity 

 of experimentally verifying the position of various centers. 



The entire cortical representation of motor acts does not follow 

 this strictly crossed unilateral arrangement. Voluntary movements which 

 involve muscle groups limited totally to one half of the body for their 

 completion are generally represented in the cortex of the opposite side. 

 Certain symmetrical muscle groups which commonly operate in syn- 

 chrony such as those involved in mastication also are represented in 

 and controlled by the contralateral cortex. On the other hand stimula- 

 tion of the frontal visuo-motor area of one side causes deviation of 

 both eyes towards the opposite side so that the movement of each eye 

 must be represented on both halves of the brain. When the motor 

 area of one side of the brain is destroyed, the movements of certain 

 bilaterally acting muscles, such as those of inspiration, of movements 

 in the diaphragm, intercostal and abdominal muscles and those of the 

 larynx are not affected on either side. This may be due to a bilateral 

 representation of these muscles in the cortex, as in the case of the ocular 

 movements, to the fact that certain of the efferent fibers connecting the 

 cortex, with the lower motor neurons do not cross to the opposite side 

 of the cord, or to the commissural connections which link one half of 

 the cortex with the other. 



The Efferent Pathway in the Brain and Cord 



The crossed connection betAveen the cerebrum and the functional 

 groups of muscles which it controls is due to the arrangement of the 

 efferent neurons which link it with the motor neurons of the cord and 

 brain stem. The most important tract involved in conducting efferent 

 impulses from the motor areas arises from the large pyramidal cells 

 (Betz cells) which are characteristic of the motor areas of the cortex 

 and do not occur in its other parts. This is the pyramidal tract (tractus 

 cortico-spinalis). Its fibers converge from the various parts of the motor 

 area as they approach the internal capsule of the cerebrum, and pass 

 through the midbrain without crossing. At the level of the pons fibers 



