THE PHYSIOLOGY OF THE JJLOOl). 
516 
little carbonic acid gas and watery vapour. Oxygen is an 
essential supporter of animal life, but inhaled alone is too stimu- 
lating, producing exhaustion, and even death. 
Nitrogen appears to be a diluent for the oxygen, as but little 
change takes place in it during respiration, and if inhaled alone 
is quite inert; but that an alteration takes place in its quantity 
in different seasons, and under different circumstances, has been 
proved by experiment. Dr. Edwards, in one of his experiments, 
where the animal was confined in a mixture of oxygen and hydro- 
gen gases, found that nitrogen was given off even more than 
equal to the bulk of the animal, and the hydrogen was absorbed : 
this was proved also by the experiments of Prevost and Dumas. 
Dr. E. also found it was influenced by the seasons. (i In 
summer, and till October, an increase of nitrogen was detected in 
the expired air, varying in quantity ; and from theclose of October 
to the spring, there was a sensible diminution of it.” 
The presence of oxygen is not necessary for the disengagement 
of carbonic acid gas in respiration. Animals have been confined 
in hydrogen gas, and carbonic acid has been given off. In one 
instance particularly, when Dr. Edwards was experimenting on 
a frog, and the flanks were squeezed so as to force all the air out 
of the lungs before introducing the animal into the gas, carbonic 
acid was given off, proving direct exhalation. 
The generally received opinion is, that oxygen is taken up into 
the blood by direct absorption, and carbonic acid given off by 
direct exhalation. The quantity of oxygen absorbed differs with 
the species of animal as well as the age. In some it is not more 
than half the carbonic acid given off ; in others, about two-thirds ; 
and in others equal ; but, as the mean, there is quite as much 
oxygen absorbed as carbonic acid gas given off. 
From the experiments of Dr. Prout it appears that the quantity 
of carbonic acid emitted is- variable at particular periods of the 
day, and in particular states of the system : it is more abundant 
during the day than the night; from daybreak it increases 
gradually until noon, from which time it gradually decreases 
until night; and during the night it seems to remain uniform. It 
is reduced in quantity by any debilitating cause, such as low 
diet, depressing passions, &c. The expired air, according to the 
experiments of Allen and Pepys, contains from six to eight per 
cent, of carbonic acid gas. Dr. Apjohn, of Dublin, observed 
that it contained only from three to six per cent. 
In addition to these changes, there is an increased quantity of 
watery vapour. Whence is this ? Dr. Prout thinks it is derived 
from the weak albumen of the chyle in its change into the strong- 
albumen of arterial blood. Others, that the venous blood con- 
