TEMPERATURE SENSE. 291 



from that of touch, and has its own nerve fibers and nerve 

 endings. 



Since heat and cold are only differences in the degree of 

 heat, we would expect both of these kinds of impressions to 

 be received through one set of nerves. There seems, how- 

 ever, to be good evidence of two sets of nerve fibers, one for 

 heat and the other for cold. In the common experience of 

 the foot " going to sleep " by pressure on the sciatic nerve, or 

 the arm from compression of the brachial nerve, the skin may 

 be found, at a certain stage, to be only slightly sensitive to 

 warmth, while distinctly sensitive to cold. In some diseases 

 of the spinal cord the skin may be affected by warmth but 

 not by cold. The sensations of cold and pressure seem to be 

 usually lost or retained together, while those of warmth and 

 pain have a similar connection. But more accurate results 

 are obtained by touching the skin with a blunt metal pencil, 

 warmed or cooled. If this be applied at regular close inter- 

 vals it is found that some places feel the warm point, while 

 others feel the cold. In this way the skin may be mapped 

 out into " warm spots " (warmth-perceiving spots), and " cold 

 spots " (cold-perceiving spots), and still other areas seem not 

 sensitive to temperature. Heat or cold, if applied directly to 

 a nerve trunk, does not rouse sensations of temperature, but, 

 if strong enough, produces pain. If the elbow be dipped into 

 water at the freezing-point, a sensation not of cold but of pain 

 is caused, and is felt in the hand. Heat and cold are not felt 

 in the digestive tube except at or near the openings. If very 

 hot liquid be swallowed, it may cause pain in the gullet and 

 stomach. If a considerable quantity of warm liquid be taken, 

 it may give a feeling of warmth from its effect on the skin of 

 the abdomen, by conduction of heat outward. As with other 

 senses, a sudden change in the degree of the stimulus is more 

 certain to rouse sensation than a gradual change. 



