THE SECRETION OF SWEAT 201 



and there is an increased flow of blood through these 

 capillaries. At the same time the cells of the glands Ijegin to 

 pour out an increased quantity of fluid, in other words they 

 begin to secrete. The first of the above two results is 

 brought about by a lessening of the vaso-constrictor 

 imprdses which had previously been keeping the arteries 

 constricted (see p. 68). But what, on the other hand, is 

 the cause of the simultaneously increased activity of the 

 sweat-glands ? Do they simply secrete faster because of 

 the increased supply of blood brought to them ? Or is it 

 because their cells are urged on to greater activity by 

 special nervous impulses sent to them ? The latter is the 

 real explanation of the increased activity of the cells, as 

 shown by the following facts. 



It is possible to obtain an increased secretion of sweat 

 by the stimulation of nerves in parts of an animal's body 

 from which the blood supply has been previously cut off. 

 Again, certain drugs may lead to sweating without at the 

 same time producing any vascular changes, and the same 

 efiect is often observed in sweating which results from 

 mental emotions and in the "cold sweats" of a disease 

 such as phthisis. The nerves which can thus make the 

 cells of the sweat-glands become more active may be called 

 secretory nerves. They appear to be connected with 

 a centre in the central nervous system, and by this means 

 sweating may be brought about reflexly, as when placing 

 mustard in the mouth causes the face to sweat. The 

 possibility of such reflex stimulation of the sweat-glands 

 acquires an extraordinary importance, as we shall see 

 when we come to consider the means by which the tem- 

 perature of the body is regulated (p. 203). 



11. Animal Heat : its Production and Distribution. 

 —It has l^een seen that heat is being constantly given 

 off from the skin and from the air-passages ; and every- 

 thing that passes from the body carries away with it, 

 in like manner, a certain quantity of heat. Further- 

 more, the surface of the body is much more exposed to 



