The only fold or duplicature made by each pleum that is comparable to the mesenteries or omenta formed by the peritoneum is that called the broad ligament of the lung (ligyunentum latum pulmonis). It is a fold carried down wards and backwards from the root of the lung. It may be described as having four edges, the upper one of which is attached along the lower aspect of the root of the lung; the outer one is attached to the inner aspect of the lung from its root to its lower border; the inner one is attached to the mediastinum from the root of the lung downwards and backwards to the cesophageal opening in the diaphragm ; the remaining. edge is free and directed outwards, downwards, and backwards. Its inner or rnediastinal attachment is by far the longest, so that its figure is four-sided, with one corner extremely drawn out or prolonged.
There are frequently found, especially about the pericardium, numerous pyriform masses of. fat covered with pleura, like appendices epiploicm.
The outer surface of the pleura is but loosely adherent to the ribs and intercostal muscles; it is more firmly connected with the diaphragm and pericardium, and still more firmly with the lungs. The adhesion of the two pleurm in the mediastinum is extremely loose in the human subject, large quantities of areolar tissue and frequently fat being interposed; so that in many subjects they can hardly be said to come into immediate contact at all.
The pleura covering the ribs and that forming the mediastinum is strengthened by a fibrous layer, but that covering the lungs is destitute of such fibres, and consequently extremely thin and delicate. The terms pleura pulinonalis, pleura costalis, and pleura diaphragmatic-a, are applied respectively to those parts of the pleurm which are connected with the lungs, the ribs, and the diaphragm : and these expressions are frequently found extremely convenient.
From the extreme frequency of abnormal adhesions of the apposed surfaces of the pleura it appears that this serous membrane is un usually liable to inflammation, which liability may be due to its being unusually exposed to external circumstances through its extreme proximity to the air in the lungs.
The pleum is peculiar to the class Mammalia. In Birds the lungs are adherent to the thoracic parietes; and in Reptiles, there being no dis tinction of thorax and abdomen, they are in vested by the peritoneum. To this, however, there is an exception in the Crocodilians, in which reptiles a rudimentary diaphragm exists. The pleurm of these animals are disposed around the outer, anterior, and posterior, but not the inner aspect of each lung ; so that the lung seems to be adherent to the mediastinum.
( Simon Rood Pittard.)