856 CENTRAL NERVOUS SYSTEM 



produces a sensation, not of warmth, but of cold when applied to a re- 

 ceptor for cold, and of pain when applied to a pain receptor. Pressure 

 applied to the eyeball gives similarly a sensation of light. This is be- 

 cause the quality of a sensation depends on the part of the brain to 

 which the nerve impulses set up by the stimulus are conducted. All 

 the fibers from a given type of receptor group themselves together in 

 the spinal cord or brain stem and lead to a common group of cells, or 

 sensory area, in the brain from which the sensation arises. The im- 

 pulses traveling over these fibers are apparently the same in quality 

 no matter what form of physical or chemical disturbance has set them up. 

 Consequently on reaching the brain centers, no difference can be recog- 

 nized between them, and they all give rise to a common quality of sen- 

 sation. The quality of a sensation depends on two factors: (1) the low 

 threshold of a special group of sense organs for the particular stimulus, 

 and (2) the anatomical arrangement by which the impulses from such 

 a group are conducted to a common region in the brain. From whatever 

 source impulses reach this region the character of the sensation will 

 be the same. These considerations give rise to the law of the specific 

 properties of nerve, which is to the effect that, however excited, each 

 nerve of special sense gives rise to its own peculiar sensation. 



The sensations which are set up by the stimulation of receptors not 

 only have a definite quality, but are recognized as coming from a defi- 

 nite region of space. They are said to have a local sign. Certain sensa- 

 tions are referred to parts of our body, as we recognize when we say 

 our feet are cold, our tooth aches, or our skin itches. Others are referred 

 to objects recognized to be in contact with the body, as when we assert 

 that a bed is hard, or a piece of ice cold. Still others are referred to 

 distant objects, as when we recognize the color of a picture, the sound 

 of a bell, or the odor of a flower. In all these cases the process of ex- 

 citation is located actually in a sense organ within or at the surface of 

 the body, and the phenomenon of sensation is set up somewhere within 

 the brain. The reference of sensation is a psychological phenomenon 

 depending on the past experience of the individual. We have learned, 

 for example, that whenever impulses arrive in certain regions of the 

 brain, giving rise to characteristic sensations, that they have come from 

 stimuli set up in some particular group of receptors located in some par- 

 ticular part of the body and excited by a disturbance which we have 

 discovered can come only from some particular region in space. Thus 

 experience tells us that a certain sensation is associated with an object 

 in contact with the foot. Whenever the afferent paths from the foot 

 are brought into play, the same sensation results, and we learn to asso- 

 ciate the resulting sensation with the foot and refer all such sensations 

 with accuracy to the foot. If the sensation is one commonly associated 



