520 MATER i A MEDIC A AND THERAPEUTICS. 

 CHAPTER XIV. 



THE BODY HEAT, AND ITS REGULATION! THE SKIN. 



I. PHYSIOLOGICAL RELATIONS. 



HEAT is produced in every act of vital energy ; is distributed 

 throughout the body; and is finally lost in the surrounding 

 medium. In so-called " cold-blooded " animals, the vital heat 

 is lost as rapidly as it is produced ; in " warm-blooded " 

 animals the heat produced does not escape until a certain 

 amount has accumulated within the system. Thereupon loss 

 sets in, and exactly balances the production, whilst the 

 accumulated store remains constant, and is known as the " body 

 heat," amounting, in man, to 98'4 degrees. 



So wide is the range, so sudden are the changes, of the 

 external temperature to which man is exposed, and so variable 

 the amount of heat produced in the system at different moments, 

 that in the course of its evolution the body has come to possess 

 a complex and sensitive nervous mechanism, by which its 

 temperature is controlled. This mechanism consists of govern- 

 ing centres, afferent nerves from impressionable parts, and 

 efferent nerves to active organs. The afferent thermal nerves, 

 originating in the skin, and possibly in other parts of the body, 

 such as the mucous membranes and viscera, carry impressions 

 of temperature (heat and cold) to the brain and cord. There 

 these impressions are specially received by three of the great 

 centres, viz. the cerebrum, where they become sensations of 

 temperature ; the sweat centres in the cord and medulla ; and 

 the metabolic or trophic centres, the centres of nutrition, in the 

 brain (Ppons) and cord. They also fall into the vaso- 

 motor, cardiac, respiratory, and possibly the renal and other 

 visceral centres. Efferent impulses from the sweat centres proceed 

 to the sudoriparous glands, which they stimulate or depress, as 

 the case may be ; from the metabolic centres they are directed 

 to the various sources of heat production the muscles, glands, 

 etc., which they depress or stimulate. At the same time, the 

 circxilation through the skin is modified, as well as the blood 

 pressure generally, the respiration, renal secretion, and pro- 

 bably every other bodily function in some degree. 



Thus, when the temperature of the air rises, the regulative 

 mechanism comes into action, and two great effects are pro- 

 duced: (1) there is increased loss of heat by the perspiration, by 

 cooling of the blood in the dilated cutaneous vessels, and by 

 cooling of the blood in the lungs ; and (2) there is diminished 

 production of heat in the muscles, glands, etc. The same effect 



