98 HABIT AND INTELLIGENCE. [cHAP. 



and during Another proof that animals have a stock of vital energy 

 ■ to draw on for conversion into heat when needed, is 

 afforded by the fact that starving animals are able for some 

 days to produce heat at a greater rate than the chemical 

 actions that go on in the body could supply.^ For some 

 days the temperature is kept very nearly up to its normal 

 level, and afterwards it falls very rapidly ; and the imme- 

 diate cause of death is the depression of temperature. It 

 is a familiar fact that starving animals lose substance, and 

 it is certain that the oxidation of the substance that dis- 

 appears helps to keep up the temperature ; but it now 

 appears that there is more heat produced than this will 

 account for ; and the excess can be, I think, due to no 

 cause except the transformation of vital energy. 



The vital energy in the body is of course not fixed but 

 fluctuating in quantity; it is expended by muscular 

 exertion, and restored again during rest, and especially 

 during sleep.^ Abundance of vital energy is probably the 



probably a case of the same kind. He put a bottle containing lecclies 

 into a freezing mixture ; a thermometer among the leeches sank to 31°; 

 it afterwards rose to 32°, and the leeches froze. (Carpenter's Comparative 

 Physiology, p. 848.) The thermometer rising when the leeches froze, 

 seems to show that their vital energy must have been transformed into 

 heat. 



1 " It has been experimentally found that in the ordinary life of an adult 

 mammal, the quantity of food necessary to keep the body in its normal 

 condition, is nearly twice that which would be required to supply the waste 

 of the organism, as measured by the total amount of excreta when food is 

 withheld." (Dr. Carpenter, Quarterly Journal of Science, vol. i. p. 265.) 



It follows from this, that animals from which food is withheld are able to 

 keep up their temperature, as starving animals do imtil they are near 

 death, at an expenditure of little more than half the material (as shown by 

 the amount of the waste of the body) that is consumed in weU-fed animals. 

 I do not see any possible way of doing this, except by di'awing on their 

 stock of vital energy for conversion into heat. 



1 presume that in the experiments referred to by Dr. Carpenter the 

 " excreta " include the carbon that passes away by the lungs. Otherwise 

 the experiments prove nothing at all, at least on the present subject. 



2 It is a general instinct among animals to keep themselves from 

 external cold during sleep ; and this may be in order to prevent the energy 

 that they need for storing up in the muscular system, from being expended 

 in conversion into animal heat, and employed in merely keeping up the 

 temperature of the body. 



