196 



PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



shown that lesions in this part of the cortex are accompanied 

 by a paralysis of the muscles on the other side, particularly 



StdA Central, '^''^f * V<^rua, 

 SvlacaHoso \ — ^ SuU.preca*Crmarg. 



SulcpaneCo 



occip. 



Stdccalcarin. 



Fig. 88. — ^To show extension of motor areas on to the mesial surface, brain of chim- 

 panzee. — (Sherrington and Oreenbaum). Mesial surface of left hemisphere: Stippled region 

 marked L EG gives themotor area for lower limb; /, s, and /i indicate regions from which 

 movements were obtained occasionally with strong stimuli; f, foot and leg; s, shoulder and 

 chest; h, thumb and fingers. 'Ihe shaded area marked EYES indicates a region stimulation 

 of which gives conjugate movements of the eyes. 



in the limbs. Pathological or experimental lesions here, more- 

 over, are followed by a degeneration of the pyramidal neurons, 

 — a degeneration which extends to the termination of the neurons 

 in the cord. With these data we can construct a fairly complete 

 account of the mechanism of voluntary or non-automatic move- 

 ments. The initial outgoing or efferent impulses arise in the large 

 pyramidal cells of the motor areas, and proceed along the axons of 

 their neurons to the motor nuclei of the cranial or spinal nerves. 

 The neurons of the pyramidal tract constitute the motor tract for 

 voluntary movements; a lesion anywhere along this tract causes 

 paralysis, more or less complete, and on the other side of the body in 

 general, if the lesion is anterior to the decussation. The path of the 

 motor fibers is represented in the schema given in Fig. 89. Arising 

 in the cortex, they take the following route (see also Fig. 82, B) : 



1. Corona radiata. 



2 Internal capsule. 



3. Peduncle of cerebrum. 



4. Pons Varolii, in which they are broken into a number of 



smaller bundles by the fibers of the middle peduncle of 

 the cerebellum (brachium pontis). In this region, also, 

 some of the fibers cross the mid-line, to end in the 

 motor nuclei of the cranial nerves: Third, fourth, fifth, 

 sixth, and seventh. 



