CHAPTER XXVI 

 EXTERNAL RESPIRATION 



" The body is sustained by three kinds of nutriment, food, drink, air (irvtvfj.a.Ta), 

 of which the last is by far the most important." 



HIPPOCRATES. 



FEW of the mechanical arrangements of the body lend themselves 

 better to popular descriptive writing than the lungs, and fewer 

 still have given rise to more misconception of the actual means 

 employed in the performance of their function. From the 

 earliest times of which written records exist, one of the most 

 important and yet most mysterious problems of physiology has 

 been the part played by the lungs. The regular inhalation of air 

 and its regular exhalation was recognised by all as essential to life. 

 Prolonged stoppage of either caused death, and death was accom- 

 panied by cessation of breathing. Hippocrates, following Hindu 

 philosophers, maintained that " aerial nutriment " was " the chief 

 support of animal life " (Cicero). Aristotle denied this and 

 considered that the function of respiration was to cool the heart. 

 The followers of Hippocrates, noticing that the arteries and veins 

 differed in structure, suggested that they might differ also in 

 function. It was further observed that the arteries of a dead man 

 were empty although the veins were full. Hence they argued that 

 the arteries were channels for air and not for blood (Erasistratus, 

 circa 294 B.C.). That these philosophers had a glimmering of the 

 truth may be adduced from Galen's writings, e.g. "The air which is 

 drawn outwards from the rough arteries (trachea and bronchial 

 tubes) receives its first elaboration in the flesh of the lungs but 

 afterwards in the heart and arteries." It is our business at 

 present to consider the first step in this sequence, viz., the passage 

 of the respiratory gases between lungs and atmosphere. 



Principle of Mechanism. 



The lung mechanism may be considered as an elastic bag with 

 one opening, the whole suspended in an air-tight box with movable 



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