164 ESCAPE OF CARBONIC ACID FROM THE BLOOD. 



If an aqueous film, not more than three eighths of a millionth of an inch 

 in thickness, can completely disturb the law of diffusion by the condens- 

 ing action it exerts on carbonic acid and oxygen, what may be expected 

 from the moist walls of the air-cells and pulmonary artery, which con- 

 jointly must be more than a thousand times as thick ? 



From these complications, it is not possible to assign any definite ratio 

 as expressing the gaseous exchange between the interior of the cells and 

 the blood, for, so far from this being a case of exchange between two gas- 

 es without any obstruction intervening, the condition under which alone 

 the law of diffusion applies, the nitrogen is doubtless in a state of solu- 

 tion in the blood, the steam in the liquid condition of water ; and re- 

 specting the carbonic acid, nothing certain is known whether it be in so- 

 lution or chemically combined. Perhaps it is united with soda in the 

 blood as a bi-carbonate. From this latter substance hydrogen gas will 

 expel one half of its carbonic acid, and in like manner a stream of hy- 

 drogen gas passed through blood deprived of its fibrin removes carbonic 

 acid. Upon such principles it has been supposed that atmospheric oxy- 

 gen removes carbonic acid from the blood during respiration, just as would 

 a stream of hydrogen rernove half the acid from a solution of bi-carbon- 

 ate of soda. 



The generation of carbonic acid in the system is commonly localized 

 Place of the ^y referring it to the soft tissues. But, though doubtless 

 generation of much originates in this way, as is illustrated by the case of 

 Cld ' insects, in which the air is carried directly to the parenchyma 

 of the organs without the intervention of any proper oxidizing blood, 

 there can be no doubt that in man, as in all the higher tribes, a very 

 large proportion is generated in the blood itself. If there were no other 

 reason to bring us to this conclusion, it would be sufficient to recall that 

 ultimate oxidation by no means occurs at 6nce, but that the various 

 wasted products pass from stage to stage in their retrograde career. 

 Thus, between the syntonin of muscular fibre and the urea of the urine, 

 many steps or stages intervene, and that much of these changes is ac- 

 complished in the blood itself is demonstrated by what occurs in the 

 use of excesses of starch, albumen, or gelatine in the food. Such sub- 

 stances, finding access through the absorbents in a modified form, but not 

 wanted for the repair of any part, are dismissed without ever entering 

 into the composition of any organ, by the lungs or the kidneys as prod- 

 ucts of oxidation or derivatives thereof. 



The act of respiration in man is therefore accomplished in the follow- 

 Generai state- ing way. The air, introduced by atmospheric pressure, 

 process of res- brought into play by the action of the diaphragm and other 

 piration. respiratory muscles, fills the nasal passages, the trachea, and 



larger ramifications of the bronchial tubes. Between it and the gas 



