1. Deficiency And Excess Of Formation

The pleural sacs are altogether absent when there is an entire deficiency of the respiratory apparatus, as in cases of acephalia, the thoracic cavity being then filled with a dense fibro-cellular tissue. A partial deficiency occurs when, in consequence of the absence of the diaphragm, the pleura and peritoneum form one continuous membrane.

An excessive development occurs in the form of duplication of the pleura, except in cases of double monstrosity, when the thoracic cavity is more or less doubled, or the two lungs are deposited in a large common cavity. It is, however, extremely rare, especially as contrasted with the frequent occurrence of congenital duplications of the peritoneum; and on this account, as well as in consequence of its relations, I would direct attention to a hitherto unnoticed persistent duplication of the right pleural sac. It forms, at the obtuse apex of the pleura, a fold, hanging from above downwards, and from without inwards, including the arch of the vena azygos, and lying in a supernumerary fissure, which divides the upper lobe into two parts.

2. Anomalies In The Size And Form Of The Pleural Sacs

The size and volume of the pleural sacs are proportional to the congenital or acquired volume of the lungs; thus, for large lungs there are wide and spacious pleurae, while for small lungs the pleural sacs are less capacious. When the lungs become enlarged, as in emphysema, they also enlarge; when the lungs from any morbid process, become atrophied, they also diminish in a corresponding degree. Moreover the pleural cavities may undergo various degrees of enlargement by the accumulation within them of gases or liquids, and may be diminished in various ways by an increased size of the peritoneum or pericardium, by adventitious products in the niediastina, or by malformation of the bony case of the thorax.

The form of the pleural sacs is regulated by that of the osseous thorax, and we must refer our readers to the remarks on the anomalies in the form of that cavity.

3. Diseases Of Texture. A. Hyperoemia Of The Pleura

A continuous increased flow of blood to this membrane gives rise, according to its extent, either to local or general opacity, and to hypertrophy; and ultimately to the development of an anomalous cartilaginous and osseous substance in its texture, and in that of the subjacent areolar tissue.

On the other hand, congestion may cause increased secretion, which, in accordance with its character and the general state of the blood, may terminate in a temporary or permanent accumulation of various quantities and kinds of serous fluid, constituting hydropspleurse or hydrothorax. It is very seldom that it leads to hemorrhage of the pleura or haemothorax.