CHAPTER XV 



THE RESPIRATORY EXCHANGE 



Historical Introduction. The necessity of respiration for the 

 support of life must have been one of the earliest facts learnt by 

 primitive man ; the most obvious signs of life, when the voluntary 

 activities of the body are suspended by sleep, are the movements 

 of the chest, and sleep is most easily distinguished from death 

 by the presence of breathing. The influence of this early know- 

 ledge, the association of life with breathing, is shown in the most 

 ancient writings, and survives to the present day in the common 

 phrases of e very-day life. 



Notwithstanding the importance which was rightly attached 

 to the renewal of the air in the lungs, the true nature of breathing 

 remained quite unknown for centuries. About the year 294 B.C. 

 Erasistratus taught that the arteries carried air from the lungs 

 to the various parts of the body ; those vessels contained air and 

 air only, and the blood was carried in other vessels, the veins. 1 

 This view Galen overthrew when he demonstrated by experiments 

 that the arteries contained not air but blood alone. The pulmonary 

 circulation, however, was unknown to him ; he believed that the 

 ventricular septum of the heart was perforated by small pores, 

 and the sole function of respiration was to cool the blood. It 

 is true that he recognised two kinds of blood, the venous and the 

 arterial, but he thought that the veins carried blood to the grosser 

 organs, such as the liver, and the arteries conveyed the purer 

 blood, which had been refined in the left ventricle of the heart, 

 to the more delicate organs, such as the lungs. These views were 

 accepted for centuries after the death of Galen in 198 A.D. 



The circulation of the blood through the lungs was discovered 

 about the year 1553 by Servetus ( 1 ), an ardent theologian, whose 



1 The arteries appear to have been considered by some of the ancients to be 

 ramifications of the wind-pipe, or arteria, called later rj dprrjpia rpaxeia. or 

 77 T/>axeta. ' ' Sanguis per venas in omne corpus diff unditur et spiritus per 

 arterias." Cicero, De Natura Deorum, 2, 55, 138. 



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