150 AEISTOTLE'S ANHOMCEOMEEIA 



does not explain how the air passes into the blood-vessels, 

 but says that there are no ducts or vessels connecting the 

 air passages and the blood-vessels. It may be that he be- 

 lieved in the existence of minute apertures in the passages 

 and vessels, which allowed air to pass, but were too small to 

 allow blood to pass through them. 



His views on this part of the subject of respiration are 

 difficult to understand. Some writers have concluded that 

 Aristotle held that some of the blood-vessels contained air 

 and not blood. It may be confidently asserted that his 

 genuine works prove conclusively that, according to him, 

 blood flowed through what are now called arteries as well 

 as through those now called veins. When referring to both 

 kinds of blood-vessels, he often speaks of the blood in them. 

 The erroneous view that some of the vessels contained air 

 rather than blood was held, not by Aristotle, but by many 

 of his followers, as will be shown later. 



Few physiologists, according to Aristotle, had discussed 

 the subject of respiration before his time.* Among others, 

 he mentions Empedocles, who believed that some of the 

 blood-vessels were only partially filled with blood, and com- 

 municated with the external air through passages so small 

 that they allowed air to pass, but not blood, and Aristotle 

 states that Empedocles tried to explain the phenomena of 

 respiration by asserting that the blood moved to and fro in 

 these blood-vessels, causing the external air to be alternately 

 drawn into and expelled from them through the very small 

 passages and through the mouth and nostrils, t The very 

 small passages, too small to allow blood but large enough to 

 allow air to pass through, were referred to by writers on the 

 blood-vessels and respiration for many centuries after the 

 time of Empedocles. It has already been suggested that 

 Aristotle believed in their existence in the walls of the air 

 passages and blood-vessels in the lungs, and, in H. A. iii. 

 c. 3, s. 3, he says that all the chambers of the heart com- 

 municate by passages with the lung, but this is not evident, 

 except in one chamber, because of the smallness of the 

 passages. This does not prove that he believed in the 

 existence of passages as small as those mentioned by 

 Empedocles, but it is the clearest statement I can find on 

 this subject. 



After Aristotle's time, Erasistratus and many others held 

 that some of the blood-vessels, especially the arteries, con- 

 •^= De Bespir. c. i. f Ibid. c. 7. 



