[ 345 ] 



XIX. Observations on the Theory of Respiration. By William Stevens, M.D. D.C.L. 

 Fellow of the Royal College of Physicians in Copenhagen, Fellow of the Royal 

 College of Surgeons in London, ^c. S^c. Communicated hy W. T. Brande, Esq. 

 V.P.R.S. 



Received April 30,— Read May 14, 1835. 



A HE cause of the dark colour of the blood in the venous circulation has long been 

 a subject of discussion ; but even at the present moment the question has not been 

 satisfactorily decided. 



It is universally admitted that the expired air contains carbonic acid, but it is still 

 doubtful in what part of the system this acid is formed. Lavoisier maintained that 

 carbon was the cause of the dark colour of the venous blood, and that the acid was 

 formed in the pulmonary organs, by the combination of that carbon with the oxygen 

 of the air. At one period this theory was generally adopted, though the evidence in 

 its favour is almost entirely hypothetical ; for hitherto there has not been even one 

 well-conducted experiment which proves the existence of any form of free carbon in 

 the venous blood. 



Another class of physiologists maintain that the carbonic acid is formed, not in 

 the lungs, but in the general round of the great circulation ; in proof of which 

 some experimentalists have asserted that they had obtained carbonic acid from 

 venous blood ; but others, of equal respectability, who have repeated the same expe- 

 riments, deny the existence of this acid in the venous current. The air-pump has 

 hitherto been almost exclusively used for the purpose of deciding this question ; but 

 the positive proofs which have been brought forward by the one class of physiolo- 

 gists have been so completely contradicted by the negative proofs of the other, that 

 a great majority remains still in favour of the old theory. In fact, Tiedemann and 

 Gmelin, the latest writers on this subject, are decidedly of opinion that venous 

 blood does not contain carbonic acid. As this is a very important question, the fol- 

 lowing experiments were made in reference to it. 



1. A glass vessel containing a small quantity of warm and fluid venous blood was 

 put under the receiver of an air-pump. In proportion as the air was exhausted, a 

 number of globules appeared to escape from the blood, which were at first small, but 

 in proportion as the air was removed they became larger in size. 



2. A small quantity of venous blood, contained in a glass vessel, was covered with 

 a layer of barytic water. This was put under the receiver of an air-pump. When the 



2 y2 



