524 STRUCTURE OF THE LUNGS. 



They are covered by the proper coat of the lungs, which is 

 extremely delicate, and closely connected to the general cover- 

 ing derived from the pleurae. Between the lobuli, where they 

 are in contact with each other, there is a portion of common 

 cellular substance, which is easily distinguished through the 

 membrane covering the lungs. This is very distinct from the 

 cellular structure which coramunicates with the ramifications 

 of the bronchia, and contains air ; for it has no communica- 

 tion with the air, unless the proper coat of the lungs be rup- 

 tured. If a pipe be introduced by a puncture of the external 

 coat of the lungs, and this interstitial cellular membrane be 

 inflated, it will compress the lobuli. This cellular membrane 

 is always free from adipose matter : it may be easily examined 

 in the lungs of the bullock. 



Upon the membranes which compose the air-cells, the pul- 

 monary artery and vein ramify most minutely ; and it seems 

 to have been proved by the united labors of chemists and 

 physiologists, that the great object of respiration is to effect a 

 chemical process between the atmospheric air, when taken into 

 the air-cells, and the blood which circulates in these vessels. 



In addition to the blood-vessels which thus pass through the 

 substance of the lungs, there are several smaller arteries deno- 

 minated Bronchial, which arise either from the upper inter- 

 costal, or from the aorta itself; they pass upon the bronchia, 

 and are distributed to the substance of the lungs. The veins 

 which correspond with these arteries terminate ultimately in 

 the vena azygos. 



The nerves of the lungs are small in proportion to the bulk 

 ^ of these organs. They are derived principally from the par 

 vagurn and the great sympathetic nerves. 



They form one plexus on the front, and another on the 

 posterior surface of the bronchia, along which they are con- 

 ducted to the minutest subdivision of the latter in the substance 

 of the lungs. 



The elasticity of the air-cells of the lungs and of the ramifi- 

 cations of the bronchia which lead to them, is apparent in their 



