CHAPTER XXVIT 

 EXTERNAL RESPIRATION 



" The body is sustained by three kinds of nutriment, food, drink, air (nuevixaTa), 

 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 imtriment " 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 o5 Mechanism. 



The lung mechanism may be considered as an clastic bag with 

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

 sides. When the sides are pulled outwards the box increases in 

 capacity and the air is sucked into the bag to keep the pressure 

 constant. When, however, the force which drew the sides out- 



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