RESPIRATION. 145 



tion, there would be just sufficient oxygen to account for the 

 formation of the carbonic acid. But there is an additional 

 amount of oxygen inhaled, rendering the volume of air 

 expired smaller than that which is inspired; and this addi- 

 tional amount must be used for some other purpose than the 

 formation of the carbonic acid escaping by the lungs. A 

 small portion may be used in the formation of the carbonic 

 acid which escapes by the skin, estimated at one -fiftieth 

 of what is exhaled by the lungs; but experiments on the 

 total respiration, both pulmonary and cutaneous, made by 

 placing a man in an air-tight chamber and estimating 

 the carbonic acid evolved, agree with those confined to 

 the pulmonary in. showing that the oxygen given off in 

 twenty-four hours, in form of carbonic acid, is less than what 

 is taken up; and we must therefore suppose that the excess 

 of the oxygen is used in other processes of oxidation, con- 

 verting the hydrogen of organic matters into water, and 

 their sulphur and phosphorus into sulphuric and phosphoric 

 acids. This is in keeping with the observation that the 

 proportion of oxygen absorbed is greater in feeding on 

 animal than on vegetable food; for the carbohydrates, it 

 will be recollected, already contain as much oxygen as would 

 combine with their hydrogen to form water, whereas oils and 

 nitrogenous substances are comparatively deficient in oxygen. 



108. Gases brought into contact one with another tend to 

 diffuse till they form a uniform mixture; and when two 

 gases are separated by a membrane, they pass in opposite 

 directions through it in definite proportions. The first of 

 these laws is, in all probability, of the utmost importance in 

 diffusing the inspired air through the residual and reserve 

 air left in the lungs after the last expiration. But the 

 variability which has been mentioned in the proportion of 

 the inspired oxygen to the expired carbonic acid, affords 

 sufficient proof that it is not by diffusion, as has sometimes 

 been supposed, that the interchange of these gases takes 

 place between the air and the blood. Another objection to 

 the supposition is that the gases appear to be, at least in 

 part, in chemical combination with the blood. 



109. When respiration is obstructed, either mechanically or 

 by deficiency or impurity of air, asphyxia or suffocation 



U K 



