VISION 335 



which this sense alone is lost, those of the skin, for example, remaining 

 unharmed. On the other hand the skin's sensations are sometimes 

 lost without disturbance of the kinesthetic sense. The location of these 

 centers is still in doubt, but evidence is accumulating that it is in the 

 posterior central convolutions, and possibly also in front of the Rolandic 

 fissure on the cortex cerebri. 



One would expect these centers, moreover, to be very closely allied 

 with the centers of voluntary movement. These impulses probably pass 

 up in the long paths of the dorsal columns and in the direct cerebellar 

 tracts. 



(For further facts as to muscular control see Chapter XII.) 

 Partial loss of the kinesthetic sense produces a greater or less degree of 

 ataxia or imperfect muscular adjustment and coordination such as is 

 seen, e.g., in ataxic aphasia, in locomotor ataxia, and in paralytic dementia 

 ("paresis"). The spinal afferent fibers are disturbed or destroyed in 

 these conditions. A heightening of this sense beyond its normal state is 

 produced by small doses of alcohol, and is felt more normally whenever, 

 because of a lack of muscular exercise, the irritability of the muscles and 

 their tonic balancing back and forth is increased. This condition is all 

 too familiar to persons of sedentary habit. They feel " fidgety," and 

 yet nothing is necessary to relieve the unpleasant feeling but vigorous 

 muscular exercise with its accompanying aeration of the tissues and the 

 removal of accumulated metabolic substances from the muscles and the 

 nerve-centers. It is one of the functions of these essential afferent end- 

 organs then to incite the individual to a normal amount of physical 

 exercise. Thus, indirectly, one maintains the normal metabolism in the 

 muscles (about half the mass of the body) and, by increasing the flow of 

 lymph, invigorates and stimulates all the essential organs of the body. 



VISION. 



Markedly in contrast in some respects with the kinesthetic sense is 

 the queenly sense of vision, of experiencing the flooding light of day. 

 The organs of the muscle-sense and its congeners are minute, simple, 

 and hidden. They send inward a host, one may almost say figuratively 

 a mass, of impulses, most of them singly quite "unfelt," to direct the 

 body's motions. . The eyes, on the contrary, are large and complicated 

 end-organs, stars of "the human face divine" which somehow (we can 

 never imagine how) set going into the brain nerve-impulses which give 

 rise to that flooding and overwhelming sensation we call light. Vision 

 at first seems easily " queen of the senses," and, by its very nature, to 

 many the larger part of consciousness. 



The organ of vision, the eye, is too elaborate an instrument for detailed 

 description here, and consequently we shall run over the structure of 

 only those portions of the organ which are immediately essential to its 

 various functions as the end-organs of certain afferent nerves and brain- 



