THE HORSE. 



163 



enter their correspondent lungs. The rami- 

 fications of these vessels (which differ from 

 other arteries in having no anastomotic 

 communications one with another) accom- 

 pany those of the bronchial tubes, and, 

 like them, divide and subdivide, grow 

 smaller and augment in number, as they 

 approach the air-cells ; upon the internal * 

 surfaces of wliich they become capillary, 

 and assuma a texture of correspondent 

 thinness and pellucidity with the cells them- 

 selves. 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 color is thereby effected in it, and we 

 have now an opportunity of seeing in what 

 manner 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 

 interposed ; so that, whatever this influence 

 be, it must take effect 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 con- 

 sider the incalculable number of air-cells 

 they must contain, the globular surface of 

 every one of which is furnished with an ex- 

 pansion of pulmonary vessels, we shall feel 

 more surprise and admiration at the extreme 

 division and diffusion of this fluid in order 

 to receive the necessary 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-cells, arise the pul- 

 monary 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 ter- 



* Some say, "upon the external surfaces." 

 20 



minate 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) re- 

 quire 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 capil- 

 laries upon the air-cells. 



Orga7iization. — Besides the pulmonary 

 blood-vessels, there are two others, named 

 bronchial arteries. They come off, by one 

 trunk, from the posterior aorta, and each 

 of them enters a division of the lungs, in 

 the substance of which it branches forth, 

 and takes the course of the bronchial tubes. 

 These tubes they supply, as well as the coats 

 of the pulmonary vessels, and the paren- 

 chyma of the lungs, with blood : in fact, 

 they may be regarded as the nutrient ves- 

 sels of these organs. It has been, however, 

 and still remains, a subject of dispute, 

 whether these vessels do exclusively nourish 

 the substance of the lungs or not ; some 

 say that they do ; while others assert that 

 they are assisted in this function by the pul- 

 monary artery, with some of the branches 

 of which they anastomose. The latter 

 opinion certainly does not appear to be sup- 

 ported by facts of much weight; on the 

 conti-ary, the blood which the pulmonary 

 arteries contain is dark-colored, and unfit 

 for the nutriment of any organ ; and as for 

 anastomosis, we have no demonstrative 

 proof of its existence. The bronchial veins 

 end in one trunk, which returns the blood 

 into the vena azygos. 



The nerves of the lungs are derived prin- 

 cipally from a large plexus within the chest, 

 constituted of the par vagum and sympa- 

 thetic. They enter the pulmonary structure 

 in company with the bronchial tubes and 

 blood-vessels, and continue their course 

 with them, to be dispersed upon the bron- 

 chial membrane and parietes of the air-cells. 



