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 lmed, 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 coolig 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 controul 
over animal temperature. As it influences respiration *, 
* Phil. Trans, 1811, p. 36;—and 1812, p. 378. 
