582 OF RESPIRATION. 



them, it is not possible to ascertain it with accuracy. But the amount which 

 can be expelled by a forcible expiration, after a full inspiration, may be taken 

 as a measure of the comparative " capacity of respiration" in different indi- 

 viduals ; and the researches of Mr. Hutchinson have shown that, in the state 

 of health, this bears a very constant proportion to the height. Thus he found 

 that the average capacity of men of 5 ft. 7 in. is about 224 cubic inches, whilst 

 that of men of 5 ft. 2 in. is about 173 cubic inches, and that of men of 6 feet, 

 about 255 cubic inches. The size of the chest affords no good indication of 

 the capacity of expiration. The results of such examinations are so nearly 

 uniform, that disease may be suspected in any man, who cannot blow out 

 nearly so many cubic inches as the average of those of the same height ; the 

 only exceptions among healthy subjects, being in the case of fat men, whose 

 capacity is always low. It is obvious from these facts, that the amount of air 

 ordinarily respired will vary greatly in different individuals ; and this is doubt- 

 less one source of the discrepancy of the results of the various experiments, 

 which have been made to determine this point. Some of the most recent ex- 

 periments on the subject are those of Mr. Coathupe,* in which the Author has 

 much reason to feel confidence. According to his estimate, about 266^ cubic 

 feet, or 460,224 cubic inches of air, pass through the lungs of a middle-sized 

 man in 24 hours; reckoning the average number of inspirations at 16 per 

 minute, this would give 20 cubic inches as the amount inhaled at each. 



2. Effects of Respiration on the JLir. 



765. We naturally pass from the foregoing inquiries, to those that relate to 

 the alterations in the Air, which are effected by Respiration. These mainly 

 consist in the removal of a portion of the Oxygen, and the substitution of a 

 quantity of Carbonic Acid, rather less in bulk than the Oxygen which has 

 disappeared. The proportion of the air thus changed appears to vary accord- 

 ing to the frequency of the respirations. Thus Vierordt found that, if he only 

 respired six times in a minute, the quantity of Carbonic acid was 5'5 per 

 cent, of the whole air exhaled ; with twelve respirations, it was 4*2 ; with 

 twenty-foiir, it was 3'3; \viihforty-eight, it was 3'0 ; and with ninety-six, it 

 was 2'6 per cent. In some of the experiments of Messrs. Allen and Pepys, 

 it was as much as 8 per cent. Probably about 4 per cent, may be taken as 

 the average, at the ordinary rate of respiration. It appears, however, from the 

 researches of the last-named experimenters, that if the air be already charged 

 in some degree with Carbonic acid, the quantity exhaled is much less ; for 

 when 300 cubic inches of air were respired for three minutes, only 28 cubic 

 inches (9i per cent.) of Carbonic acid were found in it; although the previous 

 rate of its production, when fresh air was taken in at every respiration, was 

 32 cubic inches in a minute. Knowing, then, the necessity of a free excre- 

 tion of carbonic acid, we are led by this fact to perceive the high importance 

 of ventilation; for it is not sufficient for health, that a room should contain the 

 quantity of air requisite for the support of its inhabitants during a given time; 

 since after they have remained in it but a part of that time, the quantity of 

 carbonic acid which its atmosphere will contain, will be large enough to inter- 

 fere greatly with the due aeration of their blood, and will thus cause oppres- 

 sion of the brain and the other morbid affections that result from the accumu- 

 lation of carbonic acid in the circulating fluid. On the other hand, it has been 

 ascertained by the recent experiments of Dr. Bosvvell Reid that, if the carbonic 

 acid be removed as fast as it is formed, an animal may remain in a limited 

 quantity of air, without much inconvenience, until nearly the whole of its 



* Athcnoeum Report of Meeting of die British Association, 1839; p. 707. 



