Excretion. 65 



else. Perspiration (sweat) is spoken of as sensible or 

 insensible, in the former is visible; in normal conditions 

 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 

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