RESPIRATION 119 



of carbon dioxide formed is less than the oxygen absorbed, but 

 there are exceptions, as in hibernating animals where the pro- 

 cesses of oxidation are at their lowest point. In such cases the 

 R.Q. shows the possibility of oxidation occurring without the 

 production of carbon dioxide, and of carbon dioxide being formed 

 without a sufficient absorption of oxygen to account for its 

 production. Respiratory quotients as low as 0*3 have been 

 found in hibernating animals, while during the period of their 

 active life, when laying up or storing fat for winter use, the 

 R.Q. may be greater than unity — even as high as 1*39. Such 

 high quotients are not unknown among non-hibernating animals 

 after a diet rich in carbohydrates, as in the case of fattening 

 animals. 



In these cases the carbon dioxide excreted is in excess of the 

 oxygen absorbed. The carbohydrates are being stored up as 

 fat, which is poor in oxygen ; more oxygen is consequently 

 rendered available for the food for the production of C0 2 , which 

 is split off from the carbohydrate molecule when converted into 

 fat. 



The R.Q. is not altered by muscular work, provided it is not 

 excessive or carried to the point of fatigue (see Table, p. 141). 

 More oxygen is absorbed and more carbon dioxide given off 

 during work, but the ratio is the same as during repose. The 

 inference from this is that the material which is being used up 

 by the muscles both during rest and work is the same. With 

 fatiguing work the R.Q. may fall, and this suggests that the body 

 fat is being drawn upon. 



In a starving animal the R.Q. diminishes, the creature lives 

 on its own tissues, and as the body contains only a trifling amount 

 of carbohydrate, it is on the fats and protein that the animal 

 lives. During starvation the output of carbon dioxide falls 

 off at a greater rate than the consumption of oxygen. 



There are other gases returned from the lungs besides oxygen 

 and carbon dioxide, but very little is known about them. Both 

 hydrogen and marsh gas are given off in the expired air of 

 ruminants, derived from the intestinal canal after having been 

 absorbed by the blood. The amount has been placed at 3 litres 

 (183 cubic inches) in twenty-four hours. The nitrogen of the 

 air is returned unaltered. 



Ventilation of the Lungs. — The lungs of an average horse 

 contain, when fully distended, 42 litres (1 J cubic feet) of air, and 

 at each inspiration 4 litres (250 cubic inches) are drawn in 

 during a condition of repose. As the animal during repose 

 breathes ten times a minute, the whole lung is ventilated about 

 once a minute. 



The column of air in the respiratory passages extends from 



