510 NUTRITION. 



else, bodily or mental, ingestion of food, or by diminished external temperature, the gen- 

 eration of heat in the body is correspondingly augmented ; and, on the other hand, it is 

 diminished by conditions which physiologically decrease the absorption of oxygen and the 

 exhalation of carbonic acid. The relations of animal heat to the general process of nutri- 

 tion are most intimate. Any condition that increases the activity of nutrition and of 

 disassimilation, or even any thing that increases disassimilation alone, will increase the 

 production of heat. The reverse of this proposition is equally true. 



Influence of Exercise, etc., upon the Heat of the Body. The influence of muscular 

 activity upon animal heat is interesting in connection with the theories of calorification, 

 from the fact that the muscular system constitutes the greatest part of the organism ; 

 and a muscle taken from a living animal is not only capable of contraction upon the 

 application of a stimulus, but it will perform for a time certain acts of nutrition and 

 disassimilation, such as the appropriation of oxygen and the exhalation of carbonic 

 acid. 



The most complete repose of the muscular system is observed during sleep, when 

 hardly any of the muscles are brought into action, except those concerned in tranquil 

 respiration. There is always a notable diminution in the general temperature at this 

 time. In the diurnal variations in the heat of the body, the minimum is always during 

 the night ; and, as we have already seen, this is not entirely dependent upon sleep, for a 

 depression in temperature is always observed at that time, even when sleep is avoided. 



It is a matter of common observation, that one of the most efficient methods of resist- 

 ing the depressing influence of cold is to constantly exercise the muscles ; and it is well 

 known that, after long exposure to intense cold, the tendency to sleep, which becomes 

 almost irresistible, if yielded to, is followed by a very rapid loss of heat and almost cer- 

 tain death. In some animals, the amount of increase in the temperature during muscular 

 activity is very great, and this is notably marked in the class of insects. In the experi- 

 ments of Newport, upon bees and other insects, a difference of about 27 was noted 

 between the conditions of complete repose and great muscular activity. These facts are 

 interesting, as showing the very great elevation of temperature that can be produced in 

 the lower order of beings during violent excitement; but, in man, the differences, although 

 distinct, are never very considerable, for the reason that violent muscular exertion is gen- 

 erally attended with greatly-increased action of the skin, which keeps the heat of the body 

 within very restricted limits. In the experiments of Newport, the loss of heat from the 

 surface was arrested by confining the insects in small glass bottles. 



The elevation in temperature that attends muscular action is produced directly in the 

 substance of the muscle. This important fact was settled by the very interesting and 

 ingenious experiments of Becquerel and Breschet. Introducing a thermo-electric needle 

 into the biceps of a man who used the arm in sawing wood for five minutes, these physi- 

 ologists noted an elevation of temperature of one degree centigrade (nearly two degrees 

 Fahr.). The production of heat in the muscular tissue was even more strikingly illus- 

 trated by Matteucci, in experiments with portions of muscle from the frog. Not only did 

 he observe absorption of oxygen and exhalation of carbonic acid after the muscle had 

 been removed from the body of the animal, but he noted an elevation in temperature of 

 about one degree Fahr., following contractions artificially excited. 



Observations upon the influence of mental exertion on the temperature of the body 

 have not been so numerous, but they are, apparently, no less exact in their results. Dr. 

 Davy was the first to make any extended experiments upon this point, and he noted a 

 slight but constant elevation during " excited and sustained attention." The same line 

 of observation has been followed by Prof. Lombard, who employed much more exact 

 methods of investigation. Prof. Lombard noted an elevation of temperature in the head 

 during mental exertion of various kinds, but it was slight, the highest rise not exceeding 

 one-twentieth of a degree. It is stated, also, by Burdach, that the temperature of the body 



