RESPIRATORY SYSTEM. 231 



giving ofTin their passage numerous other smaller tubes of simi- 

 lar structure ; but, as we prosecute our dissection of them, we 

 shall find that, in growing smaller, they partake less and less of 

 the nature of cartilage, and tliat the extreme ramifications are 

 not only entirely membranous in their composition, but of so 

 fine a texture as to be perfectly transparent. It will be remem- 

 bered here, that, in speaking of the trachea, a membranous lin- 

 ing to it was described of the mucous kind, which, it was ob- 

 served, thence passed into the bronchial vessels : now, it is of 

 the continuation of this membrane in an attenuated state that 

 the minute air-tubes appear entirely to consist ; at the extremity 

 of every one of which the membrane is prolonged into a kind of 

 blind bag or cul-de-sac, to which the name of air-cell has been 

 given. 



From the arborescent ramification and peculiar mode of ter- 

 mination of the bronchial tubes, some anatomists have compared 

 them, and the cells at their extremities, to a bunch of grapes — 

 supposing the stalks to represent the ramifications of the former, 

 and the grapes connected with them the air-cells ; others have 

 described them as having a resemblance to a honeycomb : and 

 so far as the knife, with the aid of glasses, can develope their 

 intimate structure, the first is an apt comparison, insomuch as it 

 relates to the disposition of their cells ; the last, insomuch as it 

 conveys an idea of their ready inter-communication. For, 

 though they do not communicate but through the ramifications 

 of the bronchial tubes, this is a medium of intercourse at once so 

 general and free, that numbers of them are inflated at the same 

 time by impelling air into any one of the larger branches : with 

 the parenchymatous substance, however, they have no communi- 

 cation whatever*. 



The bloodvessels that enter into the composition of the lungs 

 are denominated the pulmonary. The pulmonary artery, having 

 taken its origin from the right ventricle of the heart, winds up- 

 ward to the root of the left lung, and there divides into the right 

 and left pulmonary arteries, which divisions enter their corres- 

 pondent lungs. The ramifications of these vessels (which differ 

 from other arteries in having no anastomotic communications one 



* If the substance of the luna^s be Ulcerated or rent asunder, the surface 

 will be found to present a lohulated aspect. Introduce a blow-pipe into 

 one of these lohuli, and all the other lobules — the entire lung- — may be in- 

 flated from this one ; shewing- the free communication existing between 

 them. The same may be effected by injecting quicksilver. You may do 

 the same with the interstitial substance : but in this case you do not fill 

 the lobules. In fine, the lungs with their cells resemble a sponge ; only 

 that the connecting tissue has no communication with the sponge. ■ 



