HEAT REGULATION. 437 



for the perfect maintenance of uniform temperature only within 

 certain limits. When the limits are passed by the rise or fall of 

 the surrounding medium, the preservation, for any greath length 

 of time, of a perfectly uniform body temperature becomes impos- 

 sible. These limits vary very much in different animals, many 

 of which have special coverings protecting them from external 

 influences, and thus retaining their warmth for all their lifetime 

 in a temperature seldom above C. In man the limits vary 

 much, different individuals being differently affected according to 

 many circumstances, e.g., in both extremes of age the limits are 

 narrowed. It would appear that for about 10 C. above and 

 below the body temperature our skin-regulating mechanisms are 

 adequate, but beyond these limits external changes affect our 

 general temperature, and if continued become injurious. Of 

 course by imitating with clothing the natural protection with 

 which some animals are endowed we can aid the normal regulating 

 factors, and bear much greater extremes of temperature with safety 

 or even comfort. 



It surprises many people to hear that their, bodies are always 

 at the same temperature, no matter how hot or cold they feel, 

 but, practically, this is the case, for our sensations of being hot or 

 cold mean simply this ; when we feel hot our cutaneous vessels 

 are full of warm blood, and this communicates to the cutaneous 

 nerve terminals the sensory nerves the sensation of general 

 warmth. On the other hand, when the cutaneous vessels are 

 empty, the sensory nerves are directly affected by the cold of the 

 external air. Since the full or empty state of the vessels of the skin 

 depends generally on the heat or cold of the air, we commonly 

 speak of its being cold and ourselves being cold as synonymous 

 terms. But we can make ourselves warm by violent exercise even 

 on a frosty day, because we generate so much heat by muscular 

 action that the cutaneous vessels have to be dilated in order to 

 get rid of the surplus, and thereby regulate our body temperature, 

 and thus we have the sensation of being warm. Our feelings 

 when we say we are warm or cold simply depend upon our cuta- 

 neous vessels being full or empty of warm blood. 



The local appreciation of differences of temperature will be dis- 

 cussed under the sense of Touch. 



