THE MOTOR AREAS 



647 



G. THE COURSE OF THE CONDUCTING PATHWAYS FROM THE MOTOR 

 CORTICAL FIELDS TO THE NUCLEI OF THE MOTOR NERVES 



The nerve paths which originate in the great pyramidal cells of the cerebral 

 cortex proceed through the corona radiata to the internal capsule, through this 

 to the crus cerehri and then continue distalward to the nuclei of origin of the 

 motor nerves, with which they are connected. The pyramidal pathivays are 

 connected with motor nerves of the opposite side. The fibers belonging to 

 the cranial motor nerves pass to the opposite side in different parts of the 

 brain-stem, while the pyramidal fibers which reach the spinal cord cross for 

 the most part in the medulla (crossed pyramidal tracts), but in part also in 

 the spinal cord itself (direct pyramidal tracts). All these paths degenerate 

 after destruction of the cerebral cortex (see Fig. 262, page 590). 



Clinical evidence has shown that these paths pass through the corona radiata, 

 forming, as we might expect, a pretty compact bundle. Lesions in the corona 



FIG. 292. The motor tract (dark) at various levels of the internal capsule, after Beevor and 

 Horsley. L, lenticular nucleus; T, optic thalamus; C, caudate nucleus; a, anterior commis- 

 sure; F, point of junction of the lenticular and caudate nuclei. 



produce isolated paralyses of the face, arm or leg musculature, showing that the 

 paths proceeding from the motor cortical areas are distinct from one another also 

 in their further course. 



In the internal capsule the pyramidal paths are drawn closer together, the 

 deeper they go. According to Beevor and Horsley, at a high level they fill the 

 entire cross section of the capsule with the exception of its most anterior and 

 most posterior sections. Further down they are restricted more and more to the 

 posterior limb of the capsule, as may be seen from Fig. 292. 



The separate tracts can be fairly well stimulated in the internal capsule. 

 Some responses are bilateral just as in the case of not overstrong stimulation of 

 the cortex ; but most are unilateral. The bilateral responses are : eversion of the 

 lips, movements of mastication, swallowing, adduction of the vocal cords all of 

 them equally strong on both sides ; opening and closing of the eyelids, protrusion 

 of the lips, retraction of the angle of the mouth all stronger on the opposite 

 side; the rest are strictly unilateral. 



Beginning at the most anterior part of the capsule which can be stimulated, 

 and moving the electrodes gradually backward, the following responses in the 

 order named are obtained in the monkeys (Beevor and Horsley) : opening of 

 the eyelids, turning the eyes to the opposite side, opening of the angle of the 

 mouth, rotation of the head and eyes to the opposite side, rotation of the head 

 alone to the opposite side, movements of the tongue, of the angle of the mouth, 



