RESPIRATION 131 



minutes, but if a current of air be passed through the intestine after 

 closure of the trachea it will live 61 per cent, longer. In fish, intestinal 

 respiration is probably as important as dermal respiration is to amphi- 

 bians. 



In the frog, at least, the mucous membrane of the mouth and pharynx 

 is an important respiratory organ, and in a species of salamander more 

 respiratory exchange takes place in these localities than is conducted by 

 the skin on the whole surface of the body. 



The Quantity and Quality of Air Required for Respiration. In order that 

 respiration may be conducted normally and with an expenditure of only 

 a minimum amount of energy, it is necessary that the air to be breathed 

 should be pure within certain physiological limits that is, that it should 

 contain enough oxygen and not too much carbon dioxide. The atmos- 

 sphere being for all respiratory purposes a boundless reservoir of oxygen 

 and an infinite absorber of carbon dioxide, the most natural way of 

 maintaining the requirements inside our dwellings and assembly rooms 

 is to let in fresh air in the required amount. As it enters this necessarily 

 drives out an equal amount of air already present, but more or less viti- 

 ated. This is the simple-enough principle of ventilation. The problems 

 then are to determine how much air per hour a person needs for perfect 

 breathing, and then to so provide this quantity of air from the atmos- 

 phere under all its varying conditions that the supply, without draughts 

 of a harmful intensity, may be certain, economical, and constant. The 

 former part of this problem has been solved satisfactorily ; the latter part 

 in everyday life is not so easy. 



The ratio of required oxygen to carbon dioxide cast out is so fairly 

 constant that it is customary to measure the purity of a given mass of air 

 to be breathed by the percentage of carbon dioxide it contains. Each 

 adult man or woman is found to expire about 0.6 cubic foot of carbon 

 dioxide on the average every hour. In a room of ten feet cube this 

 would make the proportion of carbon dioxide at the end of the hour 

 0.06 per cent., and this proportion is taken as the proper maximum of 

 vitiation by this gas. It is an index also of the oxygen required. In 

 ordinary dwellings, on average days, if there be but one person to this 

 space namely, to every thousand cubic feet ventilation will take care 

 of itself through the cracks about the windows and the doors, draughts 

 in the chimneys, and by the opening and closing of the doors. When, 

 however, more than one person breathes from this thousand cubic feet 

 of air the problem is complicated, and chiefly by the fact that in 

 winter a draught of air at once cools off a room and imposes on the 

 occupants, at least indirectly, some slight risk of a catarrhal inflam- 

 mation of the nose or chest, of neuralgia, etc. in short of "taking 

 cold." 



The ideal mode of ventilation undoubtedly is by means of an open- 

 grate fire in every room, and modern dwellings of the better class are fast 

 meeting this ideal. Even if no fire be burning in the grate, there is a 

 draught of considerable proportions constantly passing up the chimney. 



