CHANGES IN THE BLOOD DURING RESPIRATION. 247 



direct union of the inspired oxygen with the carbon of the blood 

 in the pulmonary vessels. It was found afterward, however, that 

 this could not be the case ; since carbonic acid exists already formed 

 in the blood, previously to its entrance into the lungs. It was then 

 imagined that the oxidation of carbon, and the consequent produc- 

 tion of carbonic acid, took place in the capillaries of the general 

 circulation, since it could not be shown to take place in the lungs, 

 nor between the lungs and the capillaries. The truth is, however, 

 that no direct evidence exists of such a direct oxidation taking 

 place anywhere. The formation of carbonic acid, as it is now 

 understood, takes place in three different modes : 1st, in the lungs ; 

 2d, in the blood ; and 3d, in the tissues. 



First, in the lungs. There exists in the pulmonary tissue a pecu- 

 liar acid substance, first described by Yerdeil 1 under the name of 

 "pneumic" or "pulmonic" acid. It is a crystallizable body, soluble 

 in water, which is produced in the substance of the pulmonary 

 tissue by transformation of some of its other ingredients, in the 

 same manner as sugar is produced in the tissue of the liver. It is 

 on account of the presence of this substance that the fresh tissue of 

 the lung has usually an acid reaction to test-paper, and that it has 

 also the property, which has been noticed by several observers, of 

 decomposing the metallic cyanides, with the production of hydro- 

 cyanic acid ; a property not possessed by sections of areolar tissue, 

 the internal surface of the skin, &c. &c. When the blood, there- 

 fore, comes in contact with the pulmonary tissue, which is 

 permeated everywhere by pneumic acid in a soluble form, its 

 alkaline carbonates and bicarbonates, if any be present, are decom- 

 posed with the production on the one hand of the pneumates of 

 soda and potassa, and on the other of free carbonic acid, which is 

 exhaled. M. Bernard has found 3 that if a solution of bicarbonate 

 of soda be rapidly injected into the jugular vein of a rabbit, it 

 becomes decomposed in the lungs with so rapid a development of 

 carbonic acid, that the gas accumulates in the pulmonary tissue, 

 and even in the pulmonary vessels and the cavities of the heart, to 

 such an extent as to cause immediate death by stoppage of the 

 circulation. In the normal condition, however, the carbonates and 

 bicarbonates of the blood arrive so slowly at the lungs that as fast 

 as they are decomposed there, the carbonic acid is readily exhaled 

 by expiration, and produces no deleterious effect on the circulation. 



1 Robin and Verdeil, op. cit., vol. ii. p. 460. 



2 Arclm-es Gen. de Med., xvi. 222. 



