THE SENSORY CORTICAL AREAS 651 



we are often forced to be content merely with finding that the animal's move- 

 ments are influenced by some sensory stimulus, without being able definitely 

 to say how far the action may be regarded as the expression of a conscious 

 sensation, or whether it is not rather purely reflex. Our safest and most 

 important conclusions, therefore, we get from observations on man. 



We can obtain valuable information also by excitation experiments (of which 

 more further along), by the action current, and especially by anatomical study 

 of the afferent conducting pathways. In this section we shall limit ourselves 

 to the study of regions where the sensory paths end. These regions are known as 

 the sensory areas tactile area, olfactory area, auditory area, visual area, etc. 



A. AREA OF GENERAL SENSATION AND TOUCH 



Since, as we have seen, even the complete removal of a whole hemisphere 

 from a dog does not greatly inconvenience the animal, the locomotor movements 

 being surprisingly little affected, it follows that the regulation of the coarser 

 movements which goes on under the influence of the afferent nerves can be 

 accomplished independently of the cortex. On the other hand, observations 

 by Goltz, H. Munk, and others show that in the dog extirpation of the motor 

 region and of the cortical areas lying immediately adjacent thereto causes all 

 sorts of derangements of the tactile and the motor senses. It follows that 

 the afferent pathways from all parts of the body serving the tactile and motor 

 senses enter these regions. Similar sensory disturbances have been observed 

 in the monkey also after extirpation of the motor region. 



When the entire cortical area for the hinder extremity is removed, and as a 

 consequence the muscles of the opposite leg can no longer execute finely graded 

 movements, for some days after the operation there is complete insensibility in 

 this extremity, and a certain bluntness of sensibility becomes permanent. 



With still more extensive destruction, the finer movements of the hand and 

 foot are permanently arrested, and for some time after the operation the sensi- 

 tiveness of the paws is very much reduced, so that the animal reacts only to 

 very painful stimuli. In fact the sensitiveness of the hand and foot becomes 

 permanently so slight that a severe pinch produces no reaction at all (Mott). On 

 the other hand, Schafer has found that a monkey which does not react at all to 

 a painful pinch immediately notices a very light tactile stimulus applied to the 

 paralyzed extremity. 



The monkey (page 645) from which Goltz had removed the entire motor 

 region of the left hemisphere, took no notice of the gentle tactile stimuli applied 

 to the right extremity. Stronger pressure stimuli, however, were always felt. 

 The motor sensations were also somewhat diminished. 



Although the observations made on man differ from one another in many 

 points, on one point there is positive agreement, namely, that the motor region 

 and its immediate neighborhood is the cortical area for the sense of touch. 

 It is noteworthy that the motor and sensory disorders are not as a rule 

 coterminous. In some cases the paralysis involves most of the muscles of 

 the opposite side, whereas the disturbance in sensation going with it is of 

 but slight extent ; in other cases with a sharply circumscribed motor paralysis, 

 there goes a reduction of sensibility covering a very considerable area. 



