AMOUNT OF CARBONIC ACID EXHALED. 585 



ment of good health, only exhaled at the rate of 132'4 grains per hour (Andral and Ga- 

 varret). 



e. State of Health or Disease. Upon this very important cause of variation, few accurate 

 researches have yet been made. The per cent age of carbonic acid in the expired air has 

 been found to be unusually great in the Exanthemata, and in chronic skiu diseases (Mac- 

 gregor) ; and it has been stated to be diminished in typhus (Malcolm). Thus, the average 

 proportion in health being about 3'96 per cent. (Prout), it has been seen at 8 per cent, in 

 confluent small-pox, at 5 percent, in measles, and at 7-2 per cent, in a severe case of icthyosis 

 which terminated fatally; whilst in Typhus the per centage has been found to range from 

 1'IS to 2'50. But these statements do not indicate the total quantity exhaled in each case. 

 The remarkable increase of the exhalation in cases of Chlorosis, has been already noticed; 

 in four cases recorded by Hannover, the hourly expiration was 123-6, 118-6, 116-9, and 106-3 

 grains, the absolute quantity diminishing as the respirations increased in rapidity. In chronic 

 diseases of the respiratory organs, as might be anticipated, the amount of Carbonic acid ex- 

 haled undergoes a sensible diminution (Nysten and Hannover). Further researches are 

 much needed on this subject; but, for obvious reasons, they cannot be readily made in severe 

 forms of disease. 



f. Muscular Exertion or Repose. The effect of bodily exercise, in moderation, is to produce 

 a considerable increase in fhe amount of carbonic acid exhaled, both during its continuance, 

 and for some little time subsequently to its cessation. According to the observations of Vie- 

 rordt, the increase amounts to one-third of the quantity exhaled during rest; and it lasts for 

 more than an hour afterwards, being manifested in the greater quantity of air respired, and 

 in the larger per centage of carbonic acid contained in it. If the exercise be prolonged, 

 however, so as to occasion fatigue, it is succeeded by a diminished exhalation. The connec- 

 tion between muscular exertion and the exhalation of carbonic acid, is most remarkably 

 shown in Insects; in which animals we may witness the rapid transition betvveen the op- 

 posite conditions of extreme muscular exertion, and tranquil repose; and in which the effects 

 of these upon the respiratory process are not masked by that exhalation of carbonic acid, 

 which is required in warm-blooded animals simply for the maintenance of a fixed tempera- 

 ture. Thus a Humble-Bee has been found to produce one-third of a cubic inch of carbonic 

 acid, in the course of a single hour, during which its whole body was in a state of constant 

 movement, from the excitement resulting from its capture; and yet, during the whole twenty- 

 four hours of the succeeding day, -which it passed in a state of comparative rest, the quantity 

 of carbonic acid generated by it was absolutely less. 



g. Sleep 07- Watchfulness. The amount of carbonic acid exhaled during sleep is considera- 

 bly less than that set free in the waking state. This is particularly shown by the experi- 

 ments of Scharling; who confined the subjects of them in an air-tight chamber, within 

 which they could sleep, take their meals, &c. Thus in one case, the hourly exhalation sank 

 from 160 to 100, in another from 194-7 to 122-3, and in another from 99 to 75-1. The cause 

 of this result is partly to be sought in the cessation of all muscular exertion (save that con- 

 cerned in the maintenance of the respiration) ; and partly in the diminution in the dissipa- 

 tion of the heat of the body itself. 



h. State of the Digestive Process. It is well established, that the exhalation of carbonic acid 

 is greatly increased by eating, and that it is diminished by fasting. Thus Prof. Scharling 

 states the hourly exhalation to have increased in one instance from 145 to 190, after break- 

 fast and a walk; in another from 140 to 177 after breakfast alone; and in another from 111'9 

 to 188-9, after dinner. It is remarkable that alcoholic drinks have a tendency to diminish the 

 exhalation of carbonic acid, especially when taken into an empty stomach; and strong tea is 

 said to have the same effect (Prout, Vierordt). The quantity is also increased by exhilarating 

 emotions, and decreased by depressing affections of the mind (Prout). 



t. Period of the Day. Independently of these variations, which have their source in the 

 condition of the individual, there appears to be a slight tendency to increase in the quantity 

 of carbonic acid exhaled during the early part of the day, and a steady decrease during the 

 afternoon; so that, in the evening the quantity is decidedly less than in the morning. It is 

 very difficult to separate the effects of this influence, however, from those of the causes pre- 

 viously adverted to. 



768. The aeration of the blood may take place, not only by means of the 

 Lungs, but also through the medium of the Cutaneous surface. In some of 

 the lower tribes of animals, indeed, this is a very important part of their 

 respiratory process : and even in some Vertebrata, the cutaneous respiration 

 is capable of supporting life for a considerable time. This is especially the 

 case in the Batrachia, whose skin is soft, thin, and moist ; and the effect is 

 here the greater, since the blood which circulates through the system is, from 



