CENTRAL NERVOUS SYSTEM. 



H 



FIG. 197. Lateral view of a human hemisphere ; cor- 

 tical area V, damage to which produces "mind-blind- 

 ness;" cortical area H, damage to which produces 

 "mind-deafness;" cortical area S, damage to which 

 causes the loss of audible speech ; cortical area W, dam- 

 age to which abolishes the power of writing. 



the sense that he gets a visual impression, but because of the interruption of 

 the association fibres the object is not recognized, and the impulses reaching this 

 sensory area elicit no response 

 from the muscles, the motor 

 areas for which are located else- 

 where. 



Of these connections between 

 sensory and motor areas a suffi- 

 cient number have been studied 

 to suggest that the typical ar- 

 rangement of the cells in the 

 cerebral cortex is the following : 

 The afferent impulses are dis- 

 tributed in the sensory cortical 

 areas among several classes of 

 cells. Some of these, through 

 their neurons, form association 

 tracts by which the impulses are 

 transferred from the sensory to 

 the motor regions. Concerning 



the exact manner in which the impulses arrive at these associating cells, or 

 concerning the layer in the cortex which represents them, information is 

 meagre, but the observations on the distribution of the fibres in the cortex 

 suggest that the short association tracts must be at the level of the superficial 

 fibre-layers, while the longer tracts extend far below the cortex, and would 

 most naturally be associated with the deepest layers of cells. 1 Upon attempt- 

 ing to carry out this arrangement to anything like the completeness demanded 

 by the physiological reactions, it is necessary to postulate the existence of such 

 pathways between each sensory and each motor area, and thus there must be a 

 pathway extending from every sensory to every motor area. This arrange- 

 ment is of course to be pictured as modified in several ways. 



In the first place, the connection between a given motor and a given sen- 

 sory area is by no means proportionate in the several instances. The connec- 

 tion, for example, between the visual area and the motor area for the arm is 

 probably represented by more nerve-elements, and these better organized, than 

 the connection between the gustatory area and that for the movements of the 

 leg. 



When, therefore, it is said that such connections exist, it must be added 

 always that the nexus is different for the several regions concerned, and, what 

 is more, that in man, at least, it is different for the two- hemispheres. 



The Relative Functions of the Two Hemispheres. When the subject 

 is right-handed, it appears that in man injury to the left cerebral hemisphere 

 is more productive of disturbance than injury to the right hemisphere. At 

 the same time, lesion of the left hemisphere is far more frequent than that of 



1 Andriezen : Brain, 1894. 



