368 SPECIAL PHYSIOLOGY. 



The Sense of Temperature. 



By means of a peculiar modification of sensation, we appreciate those 

 interrnolecular motions, which cause changes of temperature in the skin, 

 and thus arrive at notions of the temperature of external objects, 

 whether these affect us by actual contact and conduction, or, without 

 contact, by radiation. 



Impressions of heat or cold, or thermal sensations, can only be com- 

 municated to the extremities of the nerves of the skin or adjacent parts 

 of the mucous surfaces ; that is, through some recipient sensory sur- 

 face ; for it is impossible to excite such impressions, by acting directly 

 on the very nerves which ultimately transmit them. For example : 

 on the raw surfaces left after destruction of the whole thickness of the 

 skin in burns, the sense of temperature is lost, heat or cold, applied 

 to such surfaces, merely producing pain. The skin over the ulnar 

 nerve, behind the elbow, does not exhibit greater sensibility to moder- 

 ate differences in temperature, than other parts of the body ; but when 

 the degree of heat or cold passes certain limits, pain is the only sen- 

 sation experienced ; a mixture of ice and water applied over this nerve, 

 causes intense pain in a few seconds. In the same manner, the con- 

 tact of frozen quicksilver, or solid carbonic acid, with the skin, causes 

 a painful sensation similar to that produced by touching red-hot iron. 



Thermal sensations are excited by bodies, the temperature of which 

 ranges between 50 and 117 Fahr. ; above or below those points, 

 objects no longer excite the feeling of heat or cold, but cause a sensation 

 of pain. Water at a temperature of about 130 no longer feels warm, 

 but imparts a slight burning sensation; in the same manner, the feel- 

 ing of cold is no longer experienced a few degrees above the freezing 

 point, painful sensations being then produced. 



Sensations of heat or cold are not absolute, but are relative to the 

 temperature of the part of the body acted upon. Hence, objects appear 

 warm or cold, in proportion to the temperature of the body at the time 

 of contact, imparting the sensation of warmth or heat, when their 

 temperature is higher than that of the body, and the feeling of cold, 

 when it is lower. The temperature of the hand is a few degrees lower 

 than that of parts nearer to the centre of the body ; hence, when 

 placed in the armpit, it feels cold, whereas the axilla appears warm to 

 the hand. So long as the temperature of the skin remains constant, 

 thermal sensations in it are very slight, or altogether absent, for the 

 various temperatures of the skin of the cheeks, hands, feet, and other 

 parts, do not usually excite in us sensations of temperature. When 

 the amount of heat given off, or taken up, in a stated time, is propor- 

 tionally great, the sensation of heat or cold is persistent ; for sensa- 

 tions of temperature are experienced, not only during the immediate 

 changes of temperature in the skin, but also during the passage of a 

 certain quantity of heat through it. 



The experiments of Weber show that the sense of temperature is 

 much modified, according to the extent of surface of the body exposed 

 to the impression, the greater the extent of surface exposed, the more 



