232 RESPIRATORY SYSTEM. 



with another) accompany those of tlie bronchial tubes, and, like 

 them, divide and subdivide, grow smaller and augment in num- 

 ber as they approach the air-cells ; upon the internal* surfaces 

 of which they become capillary, and assume a texture of corres- 

 pondent thinness and pellucidity with the cells themselves. 

 Through these minute vessels every particle of blood is impelled 

 every time it is circulated over the system, as was stated when 

 on the blood : a remarkable change of colour is thereby effected 

 in it, and we have now an opportunity of seeing in what man- 

 ner this fluid is exposed to the influence of atmospheric air for 

 the purpose. It is evident that no immediate contact can hap- 

 pen between the air and the blood, for the thin, transparent side 

 of the vessel, if not that of the air-cell likewise, must ever be inter- 

 posed ; so that whatever this influence be, it must take eftect 

 through one or other or both of these membranes. We might 

 conceive, indeed, that such minute vessels could not transmit 

 through them such a body of fluid as the blood ; but when we 

 look at the volume of the lungs, and consider the incalculable 

 number of air-cells they must contain, the globular surface of 

 every one of which is furnished with an expansion of pulmonary 

 vessels, we shall feel more surprise and admiration at the extreme 

 division and diff"usion of this fluid in order to receive the neces- 

 sary change, than that such a prodigious number of capillaries 

 should be equal, in their united caliber, to the pulmonary artery 

 itself. 



From the extremities of the arteries, upon the surface of the air- 

 cell, arise the pulmonary veins. These, by repeated union with 

 one another, form themselves, first, into visible branches, which 

 subsequently become branches of larger size, until at length they 

 end in eight pulmonary venous trunks, which proceed to, and by 

 four openings terminate in, the left auricle of the heart. The 

 ramifications of these veins, unlike the generality of others, are 

 not more numerous than those of their correspondent arteries : and 

 the reason for this is obvious ; for, here, one set of vessels are 

 not more subject to compression than the other, nor does the 

 heart (which is so proximate to them) require any such aid as an 

 additional number of veins affords to carry on the circulation. 

 The pulmonary veins have only to convey the blood back to the 

 heart, after it has received its due change within the capillaries 

 upon the air-cells. 



Organization. — Besides the pulmonary bloodvessels, there are 

 two others, named the bronchial arteries. They come off, by one 

 trunk, from the posterior aorta, and each of them enters a division 



* Some say, " upon the external surfaces." 



