136 PHYSIOLOGY FOR DENTAL STUDENTS. 



thermo-regulator, which, being activated by the temperature of 

 the rooms, acts on the furnace so as to raise or lower its rate of 

 combustion. 



In the animal body the thermo-regulator is the nervous sys- 

 tem. Whenever the temperature of the blood changes from the 

 normal, a nerve centre called the thcrmoycnic becomes acted on 

 with the result that it transmits impulses to the muscles, which, 

 by increasing or diminishing their tone (see p. 253), cause a 

 greater or a less heat-production. But the centre does more than 

 the thermo-regulator of a house, for it controls the agencies of 

 heat-loss. Thus, when the blood temperature tends to rise, the 

 thermogenic centre causes more heat to be lost from the skin and 

 lungs in the following ways: (1) It acts on the blood vessels of 

 the skin, causing them to dilate so that more blood is brought to 

 the surface of the body to be cooled off. (2) It excites the sweat 

 glands, so that more heat has to be utilized to evaporate the sweat. 

 (3) -It quickens the respirations, so that more air has to be 

 warmed and saturated with moisture. The degree to which these 

 cooling processes are used varies in different animals. Thus in 

 the dog, since there are no sweat glands over the surface of the 

 body (they are confined to the pads of the paws), increase in the 

 respiration is the chief method of cooling, hence the panting 

 on warm days. 



In the case of man, civilization has stepped in to assist the 

 reflex control of heat loss, as by the choice of clothing and the 

 artificial heating of rooms. Desirable though this voluntary 

 control of heat-loss from the body may be, there can be little 

 doubt that it is often overdone to the detriment of good health. 

 Living in overheated rooms during the cooler months of the 

 year suppresses to a very low degree the heat loss from the body 

 and thereby lowers the tone and heat production of the muscular 

 system. The food is thereby incompletely metabolized and is 

 stored away as fat ; the superficial capillaries are constricted and 

 the skin becomes bloodless. But it is not looks alone that suffer, 

 but health, as well, for, by having so little to do, the heat-regulat- 

 ing mechanism gets out of gear so that when it is required to 

 act, as when the person goes outside, it may not do so promptly 



