378 RESPIRATION. 



It is to be borne in mind, in the first place, that the sources of 

 carbon dioxid in the animal body are numerous. The oxygen 

 which is absorbed at any given time does not immediately appear 

 in the carbon dioxid given off ; it may be absorbed and enter into 

 combinations, which may retain it for a considerable time ; so that 

 at any given time the amount of oxygen absorbed may be 

 greater than that given off in the carbon dioxid, or vice versa. 



Then, too, more CO 2 is formed in proportion to the amount of 

 oxygen absorbed by the decomposition of some substances than 

 others. Thus when carbohydrates constitute the diet the amount 

 of oxygen which they contain is enough to satisfy their hydrogen, 

 but fats and proteids need more, and in the formation of water 

 they use up oxygen ; from this it follows that more oxygen is 

 absorbed during an animal than during a vegetable diet. When 

 the amount of carbon dioxid given off equals the amount of oxygen 

 absorbed, the respiratory quotient is 1. The quotient will be higher 

 in herbivora, where it is from 0.9 to 1.0, than in carnivora, where 

 it is from 0.75 to 0.8. It is interesting to note that when an her- 

 bivorous animal is fasting that is, at a time when it is taking in 

 no food, but is living on its own tissues, and is therefore for the 

 time being a carnivorous animal the quotient is that of the car- 

 nivora, 0.75. 



In observations upon man it is found that before feeding the 

 quotient is 0.84 to 0.89 ; when meat or fat is given, 0.76 ; with 

 potatoes, 0.93 ; and with glucose, 1.03. 



The respiratory quotient is higher in adults than in children ; 

 during the day than at night ; during wakefulness than during 

 sleep ; during activity than during rest. 



Ventilation.- It is manifest that if at each inspiration 

 oxygen is extracted from the air, in the course of time the amount 

 of this gas will be so reduced as to make its want seriously felt. 

 It is necessary, therefore, in order to keep the amount of oxygen 

 up to the standard, that some provision should be made to supply 

 it. Besides the removal of the oxygen, the air is still further 

 rendered unsuited for respiratory purposes by the carbon dioxid, 

 and especially by the organic matter thrown off by the expired 

 air ; the oxygen being still further diminished by stoves and lights, 

 and the air being vitiated by the products of combustion. Another 

 and no less important source of vitiation of the air is the organic 

 matter thrown off from the skin, particularly in those of uncleanly 

 habits. Decayed teeth and foul mouths add to the contamination. 

 To supply oxygen and to remove these impurities are the objects 

 of ventilation. 



A common test to determine whether the air of an enclosed space 

 contains sufficient oxygen for respiratory purposes is to see if a 

 candle will burn in it. This test is used to determine whether the 

 air in vaults or in excavations is fit for respiration. A candle will 

 not burn if the air contains only 17 volumes per cent, of oxygen ; 



