72 THE NERVOUS SYSTEM 



especially the hands, give a sense of shape in space without help from 

 vision. Thus the whole middle and upper part of the lateral aspect of 

 the hemispheres seems to be connected with the general bodily sensitivity. 

 The somesthetic areas, which may be defined as those representing the 

 visceral and dermal sensations as well as the sensations arising in the 

 muscles (touch plus kinesthesia), have not been definitely determined. 

 Indeed, frequent injuries to the motor areas, accompanied with no les- 

 sening or derangement of general sensitivity (somesthesia), suggest that 

 the whole' matter is still indefinite. When this knowledge is attained, 

 the workings of the nervous system will become clear. 



The cortical representations of the other four of the "five senses" 

 ("feeling" has just been discussed) are somewhat more definite. The 

 visual centers seem to be on both sides of the calcarine fissure in the 

 mesial surface of the occipital lobe, and affect an area in the cortex of 

 the lateral aspect of the occipital lobe in the first convolution. We should 

 suppose that one ill-defined region represented that part of the retina, 

 the macula lutea, concerned with the ever-changing focus of sight. 

 Schafer (also Henschen) thinks that this spot is at the anterior end of the 

 calcarine fissure on the mesial surface. We should look for another 

 center for color-vision, but so far none has been found. We might sup- 

 pose that other small regions were interested merely with perception of 

 light, and that still another, as Wilbrand suggests, was concerned with the 

 appreciation of perspective. At present, however, the visual cortical area 

 cannot be thus divided. 



Schafer concisely summarizes somewhat as follows the probable rela- 

 tions between the cortical areas of both hemispheres and the retina: The 

 visual area of one hemisphere is connected with the corresponding lateral 

 half of both retinse, while the upper, lower, and intermediate zones of the 

 area represent the zones of the corresponding lateral halves of both retinse; 

 the focal point of the area, located as above defined, is connected with 

 more than the corresponding half of the macula lutea of each retina. 

 Thus, the areas for the focus of seeing are each concerned with both 

 foveae centrales. This fact enables one to understand why the movements 

 of the two eyes in focusing on a new point are so perfectly controlled and 

 also the perfection of sight when this particular retinal region of the 

 fovea is concerned. 



The auditory centers in man are in the superior temporal gyrus and 

 also in the island of Reil within the Sylvian fissure. It is probable that 

 each center represents both cochleae, for when these areas of only one 

 hemisphere are destroyed both ears are made partially deaf. The supe- 

 rior temporal convolution of the left hemisphere is concerned with the 

 hearing of words, and the form of aphasia known as word -deaf ness 

 is associated with disturbance or removal of the posterior two-thirds 

 (Naunyn) of this gyrus. (See page 396 and Chapter XL) 



The sound of the speaking voice is heard, but the words are not recog- 

 nized, nor appreciated as having any meaning. Thus the speech-center 

 is in the left hemisphere in right-handed persons. It remains to be learned 



