THE METABOLISM OP THE BODY. 447 



appearing as heat, the animal body produces | as work and | as 

 heat, from its incom.e of food and oxygen. 



While it is perfectly clear that it is in the metabolic pro- 

 cesses of the body that we must seek for the final cause of the 

 heat produced, it is incumbent on the physiologist to explain 

 the remarkable fact that the mammalian body maintains, 

 under a changing external temperature and other climatic 

 conditions, and with a varying diet, during rest and labor, a 

 temperature varying within, usually, no more than a fraction 

 of a degree centigrade. This we shall now endeavor to explain 

 in part. 



The Begnlation of Temperature.— It is manifest from the 

 facts adduced that so long as life lasts heat is being of necessity 

 constantly produced. If there were no provision for getting 

 rid of a portion of this heat, it is plain that the body would soon 

 be consumed as effectually as if it were placed in a furnace. 

 We observe, however, that heat is being constantly lost by the 

 breath, by perspiration (insensible), by conduction and radia- 

 tion from the surface of the body, and periodically by the 

 urine and faeces. We have seen that, while heat is being pro- 

 duced in all the tissues and organs of the body, some are es- 

 specially thermogenic, as the glands and muscles. The skin 

 presents an extensive surface, abundantly supplied with blood- 

 vessels, which when dilated may receive a large quantity of 

 blood, and when contracted may necessitate a much larger in- 

 ternal supply, in the splanchnic region especially. It is a mat- 

 ter of common observation that, when an individual exercises,, 

 the skin becomes flushed, and so with the increased production 

 of heat, especially in the muscles (see page 195), there is a pro- 

 vision for unusual escape of the surplus ; at the same time 

 sweat breaks out visibly, or if not, the insensible perspiration 

 is generally increased ; and this accounts for an additional in- 

 crement of loss ; while the lungs do extra work and exhale an 

 increased quantity of aqueous vapor, so that in these various 

 ways the body is cooled. Manifestly there is some sort of rela- 

 tion between the processes of heat production and heat expendi- 

 ture. The vaso-motor, secretory, and respiratory functions are 

 modified. We may suppose that the various co-ordinations 

 effected, chiefly at all events through the central nervous sys- 

 tem, and not by the direct action of the heat upon local nerv- 

 ous mechanisms, or the tissues themselves directly, are re- 

 flexes. 



