588 PHYSIOLOGY OP RESPIRATION. 



pose of supplying the tissues with oxygen and of removing the 

 carbon dioxid. 



The true understanding of the object of the act of respiration we 

 owe to Lavoisier, the discoverer of oxygen. In his paper published 

 in 1777, entitled "Experiments on the Respiration of Animals and 

 on the Changes which the Air Undergoes in Passing through the 

 Lungs," he laid the foundations of our present knowledge, and 

 in subsequent work he developed a conception of the nature of 

 physiological oxidations which has dominated the physiological 

 theories of nutrition up to the present time. The discovery of the 

 physiological meaning of respiration and the function of the lungs 

 constitutes the most interesting part of the history of physiology. 

 All the great physiologists of past ages contributed their part to 

 the story, and as we look back we can count distinctly the different 

 steps made toward the truth as we understand it to-day. The 

 history of this subject is not only most instructive in demonstrating 

 the triumphant although slow progress of scientific investigation, 

 but it illustrates well also the intimate interrelations of physiology 

 with the sister sciences of chemistry and physics and the great value 

 of the experimental method. The theory of respiration held in each 

 century was formulated to explain, as far as possible, the facts that 

 were known, and as we look back from our vantage point it is most 

 impressive to realize how well-known phenomena, imperfectly 

 understood, were apparently explained by theories which we now 

 know to be incorrect. Without doubt, many of the explanations 

 accepted to-day will in later times be found to rest upon a similar 

 incomplete knowledge. Each generation must do the best it can 

 with the knowledge of its times. 



The history of respiration, the successive steps in its progress may 

 be summarized in a few words. Aristotle thought that the main 

 function of respiration is to regulate the heat of the body, which was 

 supposed to be produced in the heart; hence the increased respira- 

 tions after muscular exercise when the body-heat is increased. At 

 the same time he believed, with the philosophers of his times, that 

 the body receives something from the air that is necessary to life, a 

 subtle something that he designated as the " pneuma." Praxagoras 

 taught that blood is contained only in the veins, and that the ar- 

 teries are filled with a gaseous substance, the "pneuma" derived 

 from the air, an unfortunate error that prevailed in medicine for 

 several centuries. The two celebrated anatomists and physiologists 

 of the Alexandrian school, Herophilus and Erasistratus, distin- 

 guished two kinds of pneuma, the vital spirits, which are made or 

 extracted from the air in the lungs and whose production consti- 

 tutes the chief function of respiration, and the animal spirits, elabo- 

 rated in the brain from the vital spirits and responsible for the 



