878 CENTRAL NERVOUS SYSTEM 



dividual whose cortical area corresponding to the hand is damaged, he 

 will recognize the contact of the object, and will know that it is unpleas- 

 antly hot. He will be unable, however, to perceive the roundness, size, 

 or weight of the vessel, to know how hot it is, or to recognize the posi- 

 tion of the hand which grasps it. 



. The Sensory Centers of the Cerebral Cortex 



The Area of Cutaneous and Deep Sensibility. Impulses giving rise to 

 cutaneous and deep sensibility which pass from the termination of the 

 secondary afferent neurons in the optic thalamus to the cerebral cortex 

 divide themselves into seven afferent streams, which may be affected by 

 cortical lesions more or less independently. These comprise impulses 

 concerned with the appreciation of (1) touch, (2) one dimensional local- 

 ization, (3) two dimensional localization, (4) three dimensional local- 

 ization, (5) pain, (6) heat, (7) cold. They pass to areas in the cortex 

 which are more or less distinct for sensations arising from different parts 

 of the body, and for sensations of different psychical quality. The sen- 

 sory area for these sensations consists of the pre and post central convo- 

 lutions, the anterior part of the superior parietal lobule and the angular 

 gyri. Afferent impulses arising from one half of the body cross the 

 cord or medulla in their ascent and affect this part of the cortex of the 

 opposite side of the brain, so that the sensory disturbance which arises 

 from a unilateral injury of the cortex expresses itself by a loss in sensory 

 discrimination in contralateral parts of the body. Each of these parts 

 is represented in a definite part of this sensory cortex. The lower ex- 

 tremity is represented in the upper part of this area, the upper extrem- 

 ity in the middle portion and the head in the lower portion. The extent 

 of the cortical area corresponding to the different parts of the body is 

 proportional to the functional complexity of their acts and sensations. 

 Consequently the hands are represented by a very large region; the feet 

 and face by extensive areas compared with which the representation of 

 the proximal parts of the limbs and trunk is insignificant. The chances 

 are consequently greatly in favor of an injury to the sensory cortex, af- 

 fecting one of these parts of the body, and it is very rare that a prox- 

 imal part of a limb is affected without the distal part sharing in the dis- 

 turbance. In the sensory area for the hand, distinct regions exist for 

 each finger and the, corresponding proximal part of the hand. The area 

 for the little finger adjoins the area for the lower extremity, with the 

 result that loss of sensation involving the little finger also usually is 

 accompanied by a disturbance of sensation in the foot. The ring, mid- 

 dle, index fingers and thumb are represented one below the other in the 

 sensory cortex, the thumb area adjoining that for the face, so that sen- 

 sory disturbances in the thumb and face are apt to occur together. 



