AND THEIR FUNCTIONS. 149 



quadrupeds, the parts of the lung are separated so much 

 that there seem to be two lungs, connected to a single 

 trachea.* 



The fact that the lungs communicate with a single 

 trachea caused Aristotle to speak of the lung, not the lungs, 

 of an animal. This mode of describing the lungs is some 

 what similar to his method of describing the double gills of 

 some fishes. His description of the external appearance of 

 the human lungs is not satisfactory. The lungs, in Man, 

 are distinctly separate, and the left lung has two and the 

 right lung three lobes. 



He says that the hollow parts of the lung comprise 

 cartilaginous passages in the lung substance, such passages 

 contracting to a point, and from them are perforations 

 through all parts of the lung, and these perforations, by 

 branching, become smaller and smaller.! 



This description reads accurately enough when compared 

 with the appearance presented by the branches, within the 

 lungs, of the bronchial passages of an ox or sheep, such 

 branches having been cut longitudinally. The word used 

 in the Greek text to denote the branches is diaphyseis, 

 which indicates that they are in the substance of the lungs. 



In a series of very important passages, Aristotle says that 

 blood-vessels extend from the heart to the lung and branch 

 in the same way as the trachea, closely following its branches 

 throughout the whole lung. Between the branches of the 

 blood-vessels and those of the trachea, he says, there is no 

 common duct, but by reason of their contiguity the blood 

 vessels receive air and transmit it to the heart, for one 

 of the blood-vessels leads to the left and the other to the 

 right chamber of the heart. He also says that the dis 

 position of the blood-vessels and ramifications of the 

 bronchial tubes is such that no part of the lung can be 

 detected in which an air passage exists without a small 

 blood- vessel. I 



These passages are interesting because they foreshadow 

 a conception of that interchange of gases, between the 

 blood and the air within the lungs, which is an important 

 effect of the process of respiration. Aristotle believed that 

 air passed, in some way, from the small air passages into 

 the closely adjacent branches of the pulmonary blood-vessels, 

 and that these branches transmitted it to the heart. He 



* H. A. i. c. 13, s. 7. f Ibid. i. c. 13, s. 7. 



I Ibid. i. c. 14, s. 3, iii. c. 3, 8. 4. 



