NERVOUS SYSTEM IN GENERAL. 265 



Perhaps the most practical definition of sensation that we 

 can give is that it is the interpretation that the cells of 

 the gray matter of the brain give to the nerve impulses 

 that come from without. This will apply to ordinary sen- 

 sations. 



Subjective Sensations. But sensations may be subjec- 

 tive ; that is, they may exist without any corresponding 

 external exciting cause. For some unexplained reason the 

 cells of the brain are active, and their activity, however 

 caused, constitutes what we call a sensation. Certain 

 drugs, such as hashish, may excite an unusual degree of 

 cerebral activity. Here the action is roused through af- 

 ferent nerves, but through unusual channels ; that is, the 

 subject sees, but not through the nerves of sight. Many 

 hallucinations are explainable to a certain degree ; others 

 we cannot account for. 



The Relative Nature of Sensations. If one hand be 

 held in a basin of hot water and the other in a basin of 

 cold water, and then the two be suddenly plunged into 

 a third basin containing tepid water, a sensation of cold will 

 be received from the hand that was in the hot water, while 

 the hand from the cold water will feel heat. Sensations 

 depend on comparison and contrast. After listening to 

 low sounds, a sudden loud noise is painful ; and after hear- 

 ing loud noises, it is difficult to detect slight sounds. We 

 hardly notice the gradual fading of the light at sunset. 

 And the nose does not usually detect the slow fouling of 

 the air in a room ; but let one come in from the fresh out- 

 side air, and the contrast is striking. A constant current 

 of electricity usually causes a muscular contraction at the 

 time the current enters the muscle and at the time when 

 the current is stopped, that is, at the "making" and the 



