844 SPECIAL PHYSIOLOGY. 



shown in the proportionally larger amount of oxygen absorbed, and 

 of carbonic acid exhaled, in that sex. In the male also, the increase 

 due to age, continues progressively up to the thirtieth year, at which 

 period it is stationary ; whereas in the female, the gradual increase 

 stops at the age of puberty, and the quantity remains stationary until 

 about forty, when it once more increases for a time, before the dimi- 

 nution dependent on old age begins. The smaller absolute quantity 

 of carbonic acid exhaled in childhood, is, nevertheless, very large, in 

 proportion to the weight of the body, in accordance with the high 

 activity of the nutritive function, and with the large consumption of 

 food at that period. The size of the body, in different adults, pro- 

 duces a correspondent result on the total quantity of carbonic acid 

 exhaled. The development of the muscular system, however, pro- 

 duces a greater effect than that which depends on the mere height or 

 weight of the body, or on the dimensions of the thorax. 



Exercise, as might be expected, increases the quantity of carbonic 

 acid exhaled, not only whilst it is being taken, but also for a short 

 time afterwards. The increase may equal one-third of the amount 

 exhaled during rest, and this may continue for one hour after the ces- 

 sation of exertion. This result depends both on a greater quantity 

 of air being breathed, and on an increased percentage of carbonic 

 acid in the expired air. (Yierordt.) Other observations show even a 

 greater relative increase, for in walking two and three miles per hour, 

 the quantities, 18.1 grs. and 25.8 grs., were about two or two and a 

 half times as great as the normal amount in the sitting posture : at the 

 tread-wheel, the quantity fluctuated between 42.9 grs. and 48.6 grs., 

 that is, from about four and a half to five times as great, the pulse and 

 the respiration being, of course, greatly accelerated. (E. Smith.) Pro- 

 longed exertion producing fatigue, diminishes the exhalation. Much 

 less carbonic acid is exhaled during the night than in the day. During 

 sleep, the amount given off is considerably diminished, in correspond- 

 ence with the more superficial and slower character of the respiratory 

 movements of the chest, with the cessation of the ordinary actions of 

 the muscular and nervous tissues and of the usual metamorphoses of 

 the respiratory food, and with the smaller loss and production of heat. 

 In experiments performed in air-tight chambers, the diminution per 

 hour in sleep, was about one-third of the normal quantity. (Scharling.) 

 According to other estimates, the quantity exhaled, in a given time, 

 during profound sleep, is about one-half that of the average quantity 

 in the same time during the day. 



The period of the day influences the quantity of carbonic acid ex- 

 haled, quite independently of the condition of sleep or wakefulness. 

 The ratio in a like time of the night and day, being as 1 to 1.25 

 (Scharling), or as 1 to 1.8. (E. Smith.) Taking the whole day of 24 

 hours, the smallest quantity is exhaled in the middle of the night, and 

 the largest in the middle of the day ; a slight increase occurs at sun- 

 rise, and a prolonged and constant diminution after 9 o'clock in the 

 evening. (E. Smith.) The difficulty of resisting the effects of severe 

 cold, between midnight and sunrise, is well known. A seasonal influ- 

 ence on the products of respiration has also been noticed : the maxi- 



