ANIMAL HEAT. 5^3 



the various theories of calorification. As a rule, when the respiratory activity is physio- 

 logically increased, as it is by exercise, bodily or mental, ingestion of food, or by dimin- 

 ished external temperature, the generation of heat in the body is correspondingly aug- 

 mented ; 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 nutrition 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. In pathology, the heat of the body may be 

 increased by a deficient action of the skin in keeping down the temperature, without any 

 increase in the activity of calorification. 



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

 activity upon animal heat is peculiarly interesting in connection with the theories of calori- 

 fication, from the fact that the muscular system constitutes the greatest part of the organ- 

 ism ; and, as has repeatedly been shown by experiment, 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 of the acts of nutrition and disassimilation, such as the appro- 

 priation of oxygen and the generation and 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. This fact has been observed by all who have studied the question experimentally. 

 In the diurnal variations in the temperature 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 constantly observed at that time, even when sleep is avoided. 



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

 resisting 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 indulged in, is followed by a very rapid loss of heat and 

 almost certain death. It is not necessary to cite the accounts of travellers and others in 

 support of these facts. 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 experiments 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 differ- 

 ences, although distinct, are never very considerable, for the reason that violent muscular 

 exertion is generally 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 effects of active exercise, as in fast walking or riding, were very well observed 

 by Dr. Davy. He found a constant elevation in the general temperature (taken under 

 the tongue), amounting to between one and two degrees ; but the most marked effects 

 were observed in the extremities, especially when they were cold before taking the 

 exercise. 



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 

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