370 PHYSIOLOGY CHAP. 



The need of breathing is, as a fact of common experience, im- 

 perative; we begin to breathe when we begin to live, we cease 

 to breathe when we die. 



Aristotle (354 B.C.) remarked that all mammals, including the 

 whales which live in water, breathe air, and that fishes, molluscs, 

 and crustaceans breathe the water in which they live. Both air and 

 water serve to refrigerate them, i.e. to temper the innate heat. He 

 notices that the warmer animals breathe more intensely, and 

 explains this on the supposition that they had a greater need of 

 refrigeration a confusion of effect and cause. Animals in closed 

 vessels perished, according to Aristotle, because they warm their 

 environment, and can no longer cool themselves by respiration. 



Herophilus and Erasistratus, the leaders of the Alexandrian 

 School (300 B.C.), had a more physiological notion of respiratory 

 phenomena. They described a systole and a diastole of the lungs 

 (expiratory and inspiratory movements), which permitted the 

 pneuma to penetrate into the arteries, whence it was conducted to 

 the different parts of the body in order to vivify, that is, to warm 

 them. It was, however, Galen (see Chap. VI. 2) who first grasped 

 the chemical function of respiration, since he assumed that the 

 vital spirit was absorbed in the pulmonary diastole, while the 

 fuliginous vapours were expelled along with the water vapour in 

 the pulmonary systole. 



With Galen, therefore, as we shall see, begins the experimental 

 study of the mechanics of respiration. His ideas prevailed 

 unchanged during the Middle Ages. Servetus' book, Eestitutio 

 Christianismi, was the last echo stifled in the flames kindled by 

 the Catholics of Vienna and the Calvinists of Geneva. 



Leonardo da Vinci's conception of the respiratory function 

 (1452-1519) was far superior to Galen's in accuracy and lucidity. 

 Leonardo was one of the most universal geniuses the human race 

 has ever seen, inasmuch as he combined with the eminent gifts of 

 an artist the experimental instinct and divining power of the man 

 of science, in the most modern sense of the word. In the scientific 

 aphorisms published after his death there is a brilliant study on 

 the nature of a candle flame, which is a complex physical and 

 chemical problem. In this study, among other admirable observa- 

 tions, he affirms that " the flame first disposes of the material that 

 is to nourish it (i.e. reduces to the gaseous state the combustible 

 matter of the candle), and then feeds itself on the same . . . 

 where the air is not fitted to maintain the flame (i.e. where the 

 air has been consumed by the flame), no flame can live, neither 

 any terrestrial nor aerial animal . . . where flame cannot live, no 

 animal that breathes can sustain existence" (Codice Atlantico 

 folio 170, fasc. XXIII. p. 963). 



These words, besides being an inspired conception of the analogy 

 between the phenomena of combustion and of respiration, convey 



