SPLANCHNOLOGY. 183. 
contained in it. 275 parts by weight of carbonic acid contain 75 parts of 
carbon and 200 of oxygen. In one hour 10.572 grammes of oxygen are 
consumed, that is, 38.764 grammes of carbonic acid are generated. This 
amount varies greatly, however, with age, sex, and external conditions. 
In males the carbonic acid exhaled regularly increases from eight to thirty 
years of age; from thirty to forty it is stationary ; and from fifty to extreme 
age it goes on diminishing. It is greater in males than in females at all 
ages beyond eight years. In females, nearly the same proportionate increase 
goes on to the time of puberty, when the quantity abruptly ceases to 
increase, and remains stationary as long as menstruation continues, again to 
increase when this ceases. The more robust the individual, the greater is 
the amount of carbonic acid given off. 
Although the volume of the nitrogen taken into the lungs remains nearly 
the same, yet there is a constant absorption and exhalation of this gas. 
The amount is somewhat greater in summer than in winter. There is 
usually some organic matter contained in exhaled air, either chemically or 
mechanically, as sulphuric acid is reddened by being breathed through for 
a considerable time. 
The reaction which takes place between the air and the blood is one of 
simple endosmose and exosmose. If the blood come to the lungs charged 
with carbonic acid, and is exposed in their cells to the influence of atimo- 
spheric air, which is a mixture of oxygen and nitrogen, the carbonic acid 
will pass out, to be replaced by oxygen and nitrogen. The quantity of 
oxygen which enters will be much greater than that of nitrogen, owing to 
the superior facility with which oxygen passes through porous membranes. 
If the venous blood contain nitrogen as well as carbonic acid, it also will 
pass out, to be replaced by oxygen. The relative amount of these several 
interchanges of gases will be subject to continued variation from secondary 
causes. The combination of oxygen with carbon, to form carbonic acid, 
takes place not only in the lungs, but throughout the whole system, and this 
combustion is attended with the evolution of a considerable amount of heat. 
To this is mainly to be ascribed the animal heat of the body, which in man 
amounts at a mean to about 100° F. 
VII. ORGANS OF DIGESTION. 
(SPLANCHNOLOGY.) F 
The daily wear and tear of life, involving the destruction of life in indi- 
vidual atoms of the body, the loss of matter in the secretions and excretions, 
and the necessity of supplying fuel to keep up the temperature of the body, 
all require the constant introduction of fresh organic matter to supply the 
waste. A wise Providence has imparted to us certain sensations, known as 
hunger and thirst, by means of which we are enabled to ascertain when 
solid and liquid food is required. The various alimentary substances are 
taken into the mouth and masticated by the teeth, then carried to the 
889 
