PECULIAR SECRETIONS. , 383 



whales alone are destitute of hair. But, besides the pro- 

 tection yielded by the extraordinary thickness of the skin, 

 and the layer of fat (likewise a bad conductor) with which 

 its interior is lined, these animals reside constantly in the 

 water, and are in a great measure protected by the uni- 

 form temperature of that element. The seals, bears, and 

 walruses, on the other hand, which seek their food in the 

 same seas, are, nevertheless, obliged to come to land to 

 sleep and bring forth their young, and consequently have 

 a coating of hair to protect them from the varying tem- 

 perature of the atmosphere in which they occasionally so- 

 journ. In the mammalia, the quantity of hair on the body 

 is, in general, proportional to the cold of the climate in 

 which they reside. The same law does not seem to pre- 

 vail to the same extent, in regard to the quantity of fea- 

 thers on the birds of different climates. 



The influence which the muscular system exercises over 

 the heat of the body, is of great extent. When we exert 

 ourselves in speaking, walking, or running, our animal heat 

 is kept at its natural standard, even when the body is ex- 

 posed to a great degree of cold. On the other hand, if, 

 while the body is exposed to the cooling influence' of a low 

 temperature, we remain at rest, our heat is speedily dimi- 

 nished, and we are aroused to action by the painful sensa- 

 tions of cold. During sleep, our temperature sinks a little, 

 obviously, in consequence of our state of rest. But to 

 guard against the prejudicial effects which might follow, we 

 observe all animals, before going to sleep, retire to such 

 places, or assume such positions, as are best calculated to 

 protect them from the cooling influence of external objects. 



The nervous system likewise exercises a great cohtrout 

 over animal temperature. As it influences respiration*,. 



Phil. Trans. 1811, p. 36;- and 1812, p. 3T8. 



