438 MANUAL OF PHYSIOLOGY. 



to the surface vessels. (d) And commonly an increase in the 

 amount of sweat poured out on the surface, affording opportunity 

 for greater surface evaporation. As an example of these points may 

 be mentioned active muscular exercise, which daily experience 

 shows us is always accompanied by quick breathing, rapid heart's 

 action, and a moist skin. The increased production of heat in fever 

 gives rise to the same results, with the exception of the secretion 

 of the sweat ; the want of the secretion (probably owing to the 

 toxic inhibition of the special nerve mechanisms of the glands) is 

 a deficiency in the heat-regulating arrangements, which has much 

 to do with the abnormally high temperature of the disease. 



When a lesser quantity of heat is produced, owing to inactivity 

 of the heat-producing tissues, the reverse of these events takes 

 place, namely, the respiration and heart's action are slow, the skin 

 is pale and dry, so that little heat can escape. 



COMPENSATION FOR EXTERNAL VARIATIONS OF TEMPERATURE. 



When the temperature of the air rises much above the average, 

 the escape of heat is correspondingly hindered ; and when the 

 general body temperature begins to rise by this retention of calo- 

 ric, we have the sequence of events detailed in the last paragraph 

 as being caused by excessive production of heat. But before the 

 blood can become warmer by the influence of the increased exter- 

 nal temperature, the warm air, by stimulating the skin, brings 

 about certain changes independent of the body temperature which 

 satisfactorily check the tendency to an abnormal rise. This can 

 be shown by the local application of external heat, by means of 

 which (a) a rush of blood to the skin, and (6) copious sweat secre- 

 tion may be induced in a part. This is brought about by impulses 

 sent directly from the skin to the centres regulating the vasomo- 

 tor and secretory mechanisms, and thus causing vascular dilata- 

 tion and secretive activity. If only a part be warmed, only a 

 local effort is made to cool that part, and this has but little influ- 

 ence on the general body temperature. 



When, however, the atmosphere becomes very warm, all the 

 cutaneous vessels dilate simultaneously, and the escape of heat is 

 greatly increased ; while, at the same time, so much blood being 



