AIR 



41 



the air in the lungs loses four or five per cent, of its 

 oxygen, and gains three or four per cent, of carbonic acid, 

 and these figures with slight differences appear to hold good 

 for most domestic animals. 



An average inspiration in the horse is 250 cubic inches ; 

 if the composition of the ingoing and outgoing air be known,' 

 it is easy to calculate— once the volume of the air respired 

 is ascertained— how much oxygen an animal uses up, and 

 how much carbonic acid it produces in twenty-four hours. 

 In the horse this has been fixed at 84 cubic feet of oxygen 

 absorbed, and 72 cubic feet of carbonic acid produced in 

 twenty-four hours, during a state of repose, but there is 

 considerable variation, and the question is not susceptible 

 of the accuracy these figures might imply. 



During work both the absorption of oxygen and excretion 

 of carbonic acid are very greatly increased, but this question 

 does not seriously affect the question of ventilation, as the 

 excess of both of these gases rapidly falls on the animal's 

 return from work to the stable. 



The proportion of oxygen absorbed to carbonic acid pro- 

 duced is not the same in all animals ; in the herbivora 

 owing to their diet being rich in starch there may at times 

 be nearly as much carbonic acid produced as oxygen 

 absorbed, while in omnivora and especially carnivora, 

 there is a marked increase in the oxygen absorbed over 

 the carbonic acid produced. 



In the herbivora there are also other gases which tend to 

 produce air impurity, there is little doubt that marsh gas 

 and hj'drogen are given off by the lungs in both horses and 

 cattle ; the amount is small, viz. under 200 cubic inches in 

 twenty-four hours. These are derived from the intestinal 

 canal, being absorbed by the bloodvessels and carried to 

 the lungs. The gases formed in the intestinal canal are a 

 considerable source of air impurity, some of these are most 

 offensive owing to the putrefactive changes in the large 

 intestine, carbonic acid and sulphuretted hydrogen pre- 

 dominate, marsh gas, nitrogen and hydrogen also exist in 

 considerable quantity. The nature of the diet influences 



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