THE RECEPTORS 857 



only with the obvious contact of some object with the foot, i. e., if it 

 results from a combination of the sense of touch and sensations of other 

 quality, the sensation as a whole is referred to the external object, and 

 we speak of the object being cold, hard, etc. When components of such 

 a sensation occur without the obvious contact of some object, the sensa- 

 tion is referred to the part of the body in which the stimulation arises, 

 and we speak of the foot being cold or in pain. Similarly we have learned 

 to associate sensations resulting from stimuli arising in certain sense 

 organs with objects at a distance from the body. We have learned for 

 example to associate sensations which arise from stimuli falling upon 

 particular parts of the retina with the particular regions in space from 

 which light may come which can stimulate that particular retinal area. 

 Unconscious of the mechanism of the optical system, or of the arrange- 

 ment of the afferent paths of vision, we have simply learned that cer- 

 tain visual sensations can always be attributed to objects occupying a 

 certain position in space, and we can consequently assert with assurance 

 that the upper part of a picture is blue. The local sign of sensation is con- 

 sequently largely a matter of experience or learning. 



Reflex acts also bear an accurate local sign, although they may be carried 

 out by parts of the nervous system apparently devoid of consciousness or 

 of the ability to learn. Thus a frog, from which the brain has been re- 

 moved will raise its hind leg and attempt to sweep away an irritating ob- 

 ject placed on any part of the body, directing its foot to the point of stimula- 

 tion with the utmost accuracy. A spinal dog will carry out a scratch reflex 

 which differs considerably in its objective point, depending on the part of 

 the back to which the stimulus is applied. How the mechanism has come 

 about which enables stimulation of receptors in one part of the body to 

 bring about these reflex movements of an appropriate sort is a fundamental 

 problem in evolution which we cannot take up. 



The basis of the recognition of quality and local sign in sensation is 

 of the utmost importance in interpreting the sensory manifestation of 

 disease in the central nervous system. It rests on the fact that sensation 

 of definite quality and local sign depends on the arrival at a certain sta- 

 tion in the brain of impulses which are set up ordinarily in a group of 

 sense organs specialized for the reception of one particular type of 

 stimulus and located in a definite region of the body. Loss of sensation 

 may result from the interruption of the path of conduction anywhere 

 between, and including, the receptor and the sensory center in the brain. 



Sensations of definite quality may be set up by stimulation of the 

 nerve fibers at any point along this path, or of the sensory center itself. 

 One must exercise great care before accepting the location to which the 

 patient attributes the source of sensation, for he has only the experience 

 of healthy life to guide him. Consequently he may be misguided into 



