THE CHEST, ITS CONTENTS AND CONNECTIONS. 47 



every time the diaphragm contracted, and would, in consequence, 

 be liable to considerable injury ; but being surrounded with 

 tendinous substance which possesses no such power of con- 

 traction, all danger of compression is at once removed, without 

 any sacrifice of strength or power in tlie muscle. The dia- 

 phragm, when in a quiescent state, is convex towards the chest ; 

 and when in action, it becomes flat, thus enlarging the cavity of 

 the chest. 



The thorax is everywhere lined internally by a thin serous 

 membrane, which secretes a fluid, by which the surface of the 

 cavity is lubricated, and its contents are enabled to glide upon 

 each other without occasioning any friction or inconvenience. 

 This membrane is called the jtleura, and the portion which 

 lines the chest itself, is designated the pleura costalis, while that 

 which covers the lungs is distinguished as the pleura pulmonalis. 

 This membrane divides the chest into three cavities, one on the 

 right side, containing the right lung, and the other two on the 

 left side, the smaller of which contains the heart, and the larger 

 the left luno;. 



The r'ujlit lung is thus the largest, and consists of three lobes 

 or divisions, whilst the left lung only contains two. These 

 divisions of the chest do not communicate with each other, so 

 that if one cavity is injured, or air is admitted into it, respir- 

 ation can be carried on in the other. 



The wind-pipe, or trunk of the lungs, is composed of carti- 

 lages or cartilaginous rings, joined together by a strong and 

 elastic membrane. The cartilaginous rings are strong and thick 

 in the front part of the windpipe, but gradually become thinner, 

 and terminate in thin slips, or mere membranes, which pass over 

 each otlier, instead of meeting end to end. By this contrivance 

 the windpipe will admit of being compressed considerably, by 

 the ends passing over each other ; but such is the elasticity of 

 the cartilage, that the moment the pressure is removed, the 

 windpipe returns to its original form. The use of the elastic 

 membranes which unite the cartilaginous rings, is to admit of 

 the various motions of the windpipe, especially that of its being 

 elongated ; and so great is their strength, that we never in any 

 accident find it torn, or the cartilaginous rings separated from 

 each other. 



The windpipe, on entering the chest between the first pair of 

 ribs separates into two parts, one going to the right, and the 

 other to the left lung ; from these divisions numerous branches, 

 called the bronchi, proceed to every part of the lungs, dividing 

 and subdividing, and diminishing in their course. Tiiese rami- 

 fications gradually lose their cartilaginous character, and at 

 length terminate in an immense number of diminutive air cells. 



The lungs themselves, therefore, are composed of the air cells, 



