268 THE PLEUEiE. 



in the alimentary mucous membrane, would appear to be absent here, 

 at least in the normal condition. The mrvcs come from the trunk and 

 recurrent branches of the pneumo-g-astric, and from the sympathetic 

 system. Their mode of termination has not yet been satisfactorily traced. 



The general structure of the bronchi corresponds with that of the 

 trachea in every particular. Their cartilaginous rings, which resemble 

 those of the trachea in being imperfect Ijehind, are, however, shorter 

 and narrower. The number of rings in the right bronchus varies from 

 six to eight, whilst in the left the number is from nine to twelve. 



The bronchi are supplied by the bronchial arteries and veins, and the 

 nerves are from the same source as those of the trachea. 



THE LUNGS AND PLEURA. 



The lungs, placed one on the right and the other on the left of the 

 heart and large vessels, occupy by far the larger part of the cavity of 

 the chest, and during life are always in accurate contact with the in- 

 ternal surface of its wall. Each lung is attached at a comparatively 

 small part of its flattened inner or median surface b}^ a part named the 

 root, and by a thin membranous fold which is continued downwards 

 from it. In other directions the lung is free and its surface is closely 

 covered by a serous membrane, belonging to itself and to the corre- 

 sponding side of the thorax, and named accordingly, the right or left 

 lileura. 



THE PLEURiE. 



The pleurae are serous membranes forming two shut sacs, quite dis- 

 tinct from each other, which line the right and left sides of the thoracic 

 cavity, form by their approximation in the middle line the mediastinal 

 partition, and are reflected each upon the root and over the entire free 

 surface of the corresponding lung (see fig. 1G3). 



Each pleura consists of a visceral ^n^ a parietal portion. The visceral 

 "portion, pleura jndmonalis, coYers the lung; and the parietal portion 

 lines the ribs and intercostal spaces, pleura costalis, covers the upper 

 convex surface of the diaphragm, enters into the formation of the 

 mediastinum, and adheres to the sides of the pericardium. 



The mediastinum, or partition between the two pleural cavities, is 

 formed by the reflection of each pleura from the anterior wall of the 

 chest backwards on the pericardium to the root of the lung, and from 

 the back of the root of the lung to the vertebral column. Its division 

 into anterior, middle, and posterior mediastina, and the position and 

 contents of each, have been already described (p. 239). 



At the root of each lung the visceral and parietal portions of the 

 corresponding pleura are continuous with one another ; and, at the 

 lower border of the root, is a triangular fold of the serous membrane, 

 extending vertically along the inner surface of the lung down to the 

 diaphragm, to which it is attached by its extremity ; this fold is 

 named ligamenfum latum pulmonis. 



The upper part of the pleura, together with the apex of the cor- 

 responding lung, rises into the root of the neck, reaching an inch or 

 even an inch and a half above the first rib, and passes up under cover 

 of the scaleni muscles, — a small slip of which, arising from the trans- 

 verse process of the last cervical vertebra, is described by Sibson as 



