644 THE NERVOUS SYSTEM 



fact was conclusively proven by Mott and Sherrington who cut the sensory 

 roots of the brachial plexus and found that the monkey so operated lost the 

 power of voluntary use of the muscles on the side operated. The cortical 

 motor apparatus here remains intact but lacking the necessary sensory stim- 

 ulation the cortical cells are unable to initiate the motor impulses required for 

 voluntary action. 



It has already been shown that the motor fibers of the internal capsule of 

 one side cross over to the opposite side in the decussation of the pyramids in 

 the medulla. This decussation is not quite complete, as some fibers pass 

 down on the same side in the direct pyramidal tract. A small portion of 

 these direct fibers end around the motor neurones of the same side, but the 

 great majority cross to the opposite side in the anterior commissure at some 

 lower level of the cord. It follows that the motor areas of the cortex on one 

 side control the muscular movements of the opposite side of the body, but 

 also to a slight extent those of the same side. As a matter of fact disease in 

 the region of the fissure of Rolando is usually accompanied by a disturbance 

 of the motor function on the opposite side of the body, although there is 

 some slight motor disturbance on the same side. 



LOCALIZATION OF SENSORY FUNCTION IN THE CEREBRAL 



CORTEX. 



There is evidence that fibers from the nerves of special sense are specially 

 connected with definite and distinct parts of the cerebral cortex, the sensory 

 areas. 



The fibers from the sensory nerves, we have found, are connected with 

 the cerebral cortex by chains of neurones. These sensory paths, although 

 complex, are definite and distinct. Their cortical connections have been 

 mapped out with considerable definiteness. 



The Body Sensory or Somesthetic Area. The demonstrated motor 

 function around the pre-Rolandic region for a long time obscured the fact 

 that this region, especially the pre- and post-central convolutions, is inti- 

 mately connected with the perception of general body sensations. Physi- 

 ological and pathological observations supported this view, and recently 

 Flechsig has much strengthened the view by his method of studying the 

 progressive development of the brain. In figure 405 we produce Flechsig's 

 diagram showing the body sensory area, aptly designated by Barker the 

 somesthetic area. The borders of the area are more or less indefinite and less 

 distinct than the main portion. This is indicated in the figure by the lighter 

 shading. Lesions of this area in the cortex lead to loss of sensibility in defi- 

 nite regions of the opposite side of the body. Reaction in this cortical area 

 evidently forms a necessary link in the chain of nerve impulses initiated by 

 cutaneous sensory stimulation and leading to conscious sensations. 



