2(50 THE BROXCHIAL TUBES.— THE LUXGS. 



small plates of cartilage reach a considerable way down tlie divisions of 

 thebronclii, and the last ring has a central trianguhir projection, which 

 covers and defends the bifurcation of the trachea. 



THE BEONCHIAL TUBES. 



The windpipe has been traced through its course down the neck into 

 the chest. It is there continued through the mediastinum to the base of 

 the heart, and then divided into two tubes . corresponding with the two 

 di\4sions of the lungs — the Bronchial Tubes — the right of wliich is rather 

 the largest. These trunks enter deeply into the substance of the lungs. 

 They presently subdivide, and the subdivision is continued in every direc- 

 tion, until branches from the trachea penetrate every assignal)le portion 

 and part of the lungs. They are still air-passages, cai-rying on this fluid 

 to its destination, for the accomplishment of a vital purpose. 



They also continue exposed to pressure ; but it is pressure of a new 

 kind, a pressure alternately supplied and removed. The lungs in which 

 they are embedded alternately contract and expand ; and these tubes must 

 contract and expand likewise. Embedded in the lungs, the cartilaginous 

 ring of the bronchi remains, but it is divided into five or six segments con- 

 nected with each other. The lungs being compressed, the segments over- 

 lap each other, and fold up and occupy little space ; but the principle of 

 elasticity is still at work ; and as the pressure is removed, they start again, 

 and resume their previotis form and calibre. It is a beautiful contrivance, 

 and exquisitely adapted to the situation in which these tubes are placed, 

 and the functions they have to discharge. 



THE LUNGS. 



The lungs are the seat of a peculiar circulation. They convey through 

 their comparatively small bulk the blood, and other fluids scarcely trans- 

 formed into blood, or soon separated from it, which traverse the whole 

 of the frame. They consist of countless ramifications of air-tubes and 

 blood-vessels connected together by intervening cellular substance. 



They form two distinct bodies, the right somewhat larger than the left, 

 and are divided from each other by the duphcature of the pleura, which 

 has been already described— the mediastinum. Each lung has the same 

 structure, and properties, and uses. Each of them is subdivided, the right 

 lobe consisting of three lobes, and the left of two. The intention of these 

 divisions is probably to adapt the substance of the lungs to the form of the 

 cavity in which they are placed, and to enable them more perfectly to 

 occupy and fill the chest. 



If one of these lobes is cut into, it is found to consist of innumerable 

 irregularly formed compartments, to which anatomists have given the 

 name of Mules, or httle lobes. They are distinct from each other, and 

 impervious. On close examination, they can be subdivided almost without 

 end. There is no communication between them, or if perchance such 

 communication exists, it constitutes the disease known by the name of 

 broken wind. 



On the dehcate membrane of which these cells are composed, innumer- 

 able minute blood-vessels ramify. They proceed from the heart, through 

 the medium of the jjulmonary artery — they follow all the subdivisions of 

 the bronchial tubes — they ramify upon the membrane of these multitu- 

 dinous lobules, and at length return to the heart, through the medium of 

 the pulmonary veins, the blood, the character of which has been essentially 

 changed. The office of the lungs may be very shortly stated. The blood 

 passing thi-ough the capillaries of the body and contributing to the 

 nourishment of the frame, and furnishing all the secretions, becomes, as 



