Mastication, Digestion atid Assimilation. 23 



39. Respiration. — We have seen that the blood is being con- 

 stantly reinlbrced with nutriment. At the same time it is taking 

 up the waste matter of the body which must be removed. This 

 is accomplished in part by the lungs, into which the blood is 

 forced from the heart. By a mechanical movement of the ribs the 

 lungs are expanded and air is drawn into them. Kespiration varies 

 greatly in different animals, and at different ages for the same 

 animal, as shown in the following table: 



Respirations per minute. 



In young horse 10 — 12 



Adult horse 9 — 20 



Young ox 18 — 20 



Adult ox 15 — 18 



Lamb 16 — 17 



Sheep 13 — 16 



While in the lungs, the blood takes up oxygen from the inspired 

 air, and the exjjired air carries with it carbonic acid and water 

 eliminated by the blood. The difference between the air as it 

 passes into the lungs and as it comes from them is shown in the 

 following table: 



Oxygen, Nitrogen. Carbonic acid. 



Inspired air contains 20. 81 79. 15 .04 



Expii-ed air contains 16.033 79.557 4.38 



The expired air therefore contains from 4 to 5 per cent, less 

 oxygen and that much more carbonic acid than the inspired air. 

 Considerable moisture also escapes with the expired air, and a 

 small amount of other products in the form of organic matter. 

 The amount of watery vapor given off by a man is estimated at 

 1. 5 pounds per day. 



40. Excretion from the kidneys. — A large portion of the water 

 taken into the body is filtered from the blood as it passes through 

 the kidneys, from which organs it is conveyed to the bladder. 

 The amount of water excreted through the kidneys is dependent 

 upon the quantity drank and that in the food. 



Boussingault found the composition of the urine of a horse 

 fed oats, clover and gravss, a cow fed hay and potatoes, and a pig 

 fed cooked potatoes, to be as follows: 



