592 TEXT-BOOK OF PHYSIOLOGY 



clinical symptoms with pathologic processes of the central nerve system, 

 and from observation of the developmental stages of the embryonic brain , 

 it has been established in recent years : 



1. That the general and special sense-organs of the body are associated 



through afferent nerve-tracts with definite though perhaps not sharply 

 delimited areas of the cerebral cortex; and 



2. That certain areas of the cortex are associated through efferent nerve- 



tracts with special groups of skeletal or voluntary muscles. 



Experimental excitation of a cortical area associated with a sense-organ is 

 undoubtedly attended by the production of a sensation at least similar to 

 that produced by peripheral excitation of the sense-organ itself; destruction 

 of the area is followed by an abolition of all the sensations associated with the 

 sense-organ. For these reasons such areas are termed sensor. 



Experimental excitation of a cortical area associated with a group of skele- 

 tal muscles is attended by their contraction; destruction of the area is followed 

 by their relaxation or paralysis. For these reasons such areas are termed 

 motor. 



Since the sense-organs are remote from the brain and the impressions 

 made upon them by the objective world can be utilized in the mental life 

 only after they have been reproduced in the cortical areas, it may be said that 

 each sense-organ has its special area in the cortex by which it is represented, 

 or, in other words, each sense-organ has a cortical area of representation. 



Since the muscles are remote from the brain and since they contract in 

 response to the discharge of nerve impulses from the cells of the cortical motor 

 areas, it may be said that the mental activities associated with the motor 

 areas are represented by the contractions of the muscles; in other words, 

 that the cortical motor areas have areas of representation in the general 

 skeletal musculature. It is usually stated, however, in the reverse way: viz., 

 that the muscle movements have areas of representation in the cortex. 



The cortex of the cerebrum may therefore be compared to a mosaic 

 made up, partially at least, of both sensor and motor areas which respec- 

 tively represent sense-organs and motor organs, and which bear a definite 

 anatomic and physiologic relation one to the other. Their cooperation 

 is essential to the normal performance of many forms of cerebral activity. 



A knowledge of the situation of these areas, the order of their develop- 

 ment, the effects that arise from their stimulation or follow their destruction, 

 are matters of the highest importance in the study of cerebral activity and 

 indispensable to the physician in the localization of lesions which manifest 

 themselves in perversions or abolition of sensations and in convulsive seizures 

 or paralyses. 



The Sensor Areas. The sensor areas which should theoretically be 

 present in the cortex are primarily those which receive and translate into con- 

 scious sensations nerve impulses, developed by changes going on in the body 

 itself; and secondarily those which receive and translate into conscious sensa- 

 tions the nerve impulses developed in the special sense-organs by the impact of 

 the external or objective world. In the former areas, are received the nerve 

 impulses that come from the mucous membranes, muscles, joints, viscera, etc., 

 and give rise to muscle, and visceral sensations. In the latter areas are 

 received the nerve impulses that come from the sense-organs and give rise 

 to cutaneous, gustatory, olfactory, auditory, and visual sensations. A num- 



