440 MANUAL OF PHYSIOLOGY. 



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 great length of 

 time, of a perfectly uniform body temperature becomes impossible. 

 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 tempera- 

 ture seldom above C. In man the limits vary much, different 

 individuals being differently affected according to many circum- 

 stances, e. </., 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 tempera- 

 ture, 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 com- 

 monly speak of its being cold and ourselves being cold as synony- 

 mous terms. But we can make ourselves warm by violent exer- 

 cise 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 cutaneous vessels being full or empty of warm blood. 



The local appreciation of differences of temperature will be 

 discussed under the sense of Touch. 



