HISTORICAL INTRODUCTION 477 



all the oxygen absorbed by an animal reappeared combined with 

 carbon as carbon dioxide. Lavoisier maintained, in the joint 

 memoir which he and Sequin published in the year 1790, that 

 the seat of this combustion was in the lungs, but in earlier works 

 he had admitted that it might be in other parts of the body. The 

 view of Lavoisier and Sequin was contested by Lagrange, who 

 argued that if all the heat of the body were produced in the lungs, 

 the temperature of that organ would be raised to such a degree 

 as to destroy its vitality. For many years this objection was 

 held to be fatal to Lavoisier's theory, until Berthelot by a careful 

 calculation showed that, granting all the heat to be produced 

 in the lungs, the temperature of that organ would be raised only 

 a minute fraction of a degree, owing to the great volume of air and 

 blood contained therein, and the rapidity of the circulation, 

 whereby the heat would be distributed all over the body. It 

 is indeed in the tissues that respiration occurs. This has been 

 proved by the researches of Spallanzani, W. F. Edwards, Paul 

 Bert, Pfliiger and his pupils, and will be considered in detail in 

 later portions of these lectures. Although the brilliant and 

 laborious work of these observers has shown that the real seat 

 of respiration is in the tissues, we are still far from possessing a 

 satisfactory knowledge of respiratory exchange. The evidence 

 upon some essential points is so conflicting that it is difficult, 

 or even impossible, at present to draw definite conclusions. 

 We know that respiration is not a simple combination of oxygen 

 with carbon or hydrogen, but we cannot, with our present 

 knowledge, follow the sequence of events, the beginning of which 

 is the absorption of oxygen, the end the discharge of carbon 

 dioxide. 



Definition of Respiration. Respiration may be defined as the 

 exchange of gases between the tissues and the surrounding media ; 

 oxygen is absorbed by the tissues and carbon dioxide is dis- 

 charged. A brief consideration of comparative physiology shows 

 that this definition embraces the process of respiration in all forms 

 of life. The organism composed of a single cell is the seat of such 

 a gaseous interchange, and the more complex animal built up of 

 numerous cells breathes in a similar manner, although there may 

 be a system of tracheae, or air-tubes, as in insects, or internal 

 media, the blood and lymph, as in vertebrates, to facilitate the 

 exchange between the component cells and the external medium. 



