RESPIRATION. 243 



was greater during digestion than in the fasting condition ; in the 

 waking hours than during sleep ; and in a state of activity than in 

 one of repose. It was diminished by fatigue, and by most conditions 

 which interfere with health. 



In man the rate of exhalation also varies according to age, sex, con- 

 stitution, and development. These variations were investigated by 

 Andral and Gavarret, who found them very marked in different indi- 

 viduals, notwithstanding that the experiments were made at the same 

 period of the day, and with the subject as nearly as possible in the 

 same condition. The carbonic acid exhaled per hour in five different 

 persons was as follows : 



QUANTITY OF CARBONIC ACID PEE HOTTE. 



In subject Ko. 1 19,770 cubic centimetres. 



" " "2- 15,888 " " 



" " 3 20,475 ' " 



" " " 4 20,475 " " 



" " "5 26,060 " " 



From eight years up to puberty the quantity of carbonic acid in- 

 creases constantly with the age. Thus a boy of eight years exhales, 

 on the average, 9238 cubic centimetres per hour ; while a boy of fifteen 

 exhales 16,168 cubic centimetres in the same time. Boys exhale during 

 this period more carbonic acid than girls of the same age. In males 

 the quantity of carbonic acid increases until the twenty-fifth or thir- 

 tieth year, when it reaches, on the average, 22,899 cubic centimetres 

 per hour. It then remains stationary for ten or fifteen years ; dimin- 

 ishes slightly from the fortieth to the sixtieth year ; and after sixty 

 shows a marked reduction, falling sometimes as low as 17,000 cubic 

 centimetres. In one superannuated person, 102 years of age, the hourly 

 quantity was less than 11,000 cubic centimetres. 



In women, the increase of carbonic acid ceases at puberty ; its pro- 

 duction then remaining constant until the cessation of menstruation, 

 about the fortieth or forty-fifth year. At that time it increases again 

 until after fifty years, when it subsequently diminishes with the 

 approach of old age, as in men. Pregnancy, occurring at any time 

 in the above period, produces a temporary increase in the quantity of 

 carbonic acid. 



The strength of constitution, and particularly the development of 

 the muscular system, has great influence in this respect. The largest 

 production of carbonic acid observed was in a young man, 26 years of 

 age, of remarkably vigorous and athletic development, who exhaled 

 26,060 cubic centimetres per hour. On the other hand, an unusually 

 large skeleton, or an abundance of adipose tissue, is not accompanied 

 by a corresponding increase in carbonic acid. 



The discharge of carbonic acid is not altogether confined to the 

 lungs, but takes place also, in some measure, by the urine and the per- 



