Excretion. 59 



sweat is being excreted continuously. Formation of 

 sweat depends on the activity of the gland cells, there- 

 fore increase of the blood supply to the skin would mean 

 increased sweating. More water and more solids are 

 excreted by the skin than by the lungs. The trotting 

 horseman, as the result of experience, seems to appreci- 

 ate the work of the sweat glands more fully than any 

 other class of stockmen, as is evidenced by the thorough 

 attention given to their charges when training or racing, 

 in any case thorough grooming at regular and frequent 

 intervals is necessary to prevent dried sweat and dead 

 skin scales from blocking the exit (pores of the skin) of 

 the sweat glands. An experiment conducted to show 

 how necessary to the good health of the animal open 

 pores are, consisted of the varnishing of the entire body 

 of the animal — death resulted. 



D. Respired Air. This form of excretion borders so 

 closely on the inspiration (taking in of fresh air — one 

 form of food) that the description of the organs perform- 

 ing this work will be discussed separately. 



Breathing (respiration) consists of the taking in of 

 pure air, charged with oxygen (the life-giving gas), and 

 the exchange of that oxygen for carbon di-oxide (a 

 poisonous gas), the exchange taking place in the lungs. 

 The blood is brought into intimate relation with the air 

 in the lungs, therefore we find that as a result of such 

 contact the blood becomes charged with oxygen, thicker 

 in a sense, of a deeper red color, and discharges carbon 

 di-oxide and heat, the latter the result of the exchange 

 of the one gas for the other. Here then we have in a 

 nutshell the reasons for ventilation : (1) to remove im- 



