150 



ANATOMY AND PHYSIOLOGY OF 



exterior. Its other side, on the contrary, is 

 rongh, having numerous ceHular llocculcnt 

 appendages, by wliich it is united to the 

 parts it invests ; and so close and firm are 

 these adhesions, that to cleanly detach it, 

 in the recent subject, is a very diiiicult and 

 tedious dissection. 



The pleura is a reflected membrane ; by 

 which is meant, one that not only lines the 

 cavity in which the viscera lie enclosed, but 

 by duplicature, or vv^hat in anatomical lan- 

 guage is called reflection, gives a partial or 

 complete covering to the contained organs 

 themselves. It is evident, therefore, that 

 such a membrane admits of division into 

 two portions — a lining- or parietal, and a 

 reflected portion ; and these, with regard to 

 the pleura, have, for the sake of more de- 

 finite description, received the names of 

 pleura costalis and pleura pulmonalis. They 

 are both, however, continuous at all points, 

 are precisely similar in structure and func- 

 tion, and, in fact, are still but one and the 

 same plem'a. 



Mediastinum. — There is yet a third por- 

 tion of this membrane to which a distinct 

 appellation has been given, and that is the 

 mediastinum, the membranous partition be- 

 tween the cavities or sides of the thorax ; 

 it differs from both the others in being com- 

 posed of two layers, which are derived from 

 the two pleurae of the opposite sides. 

 If we conceive the pleurae of the two sides 

 of the thorax to be perfect sacs or bags, 

 with flattened sides turned inwardly, and 

 closely applied and united together, in such 

 a manner that the double membrane formed 

 by their union extends through the middle 

 of the chest, from the dorsal vertebrte to 

 the sternum, we shall at once have a toler- 

 ably correct idea of the formation as well 

 as situation of the mediastinum. 



Structure. — The pleura, from the nature 

 of its secretion, is one of those included 

 in the list of serous membranes to which it 

 has been demonstrated also to be similar in 

 its intimate organization. Like them, it 

 presents a shining secreting surface, of a 

 whitish aspect, and considerable transpa- 

 rency, and is composed of little else than 



condensed cellular substance, whose texture 

 is penetrated by blood-vessels, absorbents, 

 and nerves : by long maceration in water, 

 indeed, it may be entirely resolved into cel- 

 lular substance. In most parts it is ex- 

 tremely thin, and by no means tough ; but 

 it is not so in all, for that portion which 

 faces the diaphragm is much denser and 

 stronger than the pulmonary or costal di- 

 vision of it. 



Organization. — The arteries of the 

 pleura, which come from the adjacent parts, 

 are in the natural state exceeding small, ad- 

 mitting only the colorless parts of the blood 

 — a circumstance that accounts for its pel- 

 lucidity ; under inflammation, however, 

 they contain red blood, and such is the ex- 

 planation of that arborescent vascularity 

 upon the sides of the thorax in horses that 

 die of pnuemonia ; than which state noth- 

 ing can better demonstrate the comparative 

 number and distribution of these blood-ves- 

 sels. The majority of them terminate in 

 exhalent orifices, from which is continually 

 poured, upon the contiguous surfaces of the 

 smooth interior of the membrane, a serous 

 fluid, in the form of steam or vapor, which 

 may at any time be rendered visible by 

 opening the chest of an animal recently 

 dead. The absorbents of this membrane 

 are very numerous ; and, though their ex- 

 treme exflity prevents us from demonstrat- 

 ing them in a state of health, yet may they 

 often be seen in considerable numbers in 

 horses that die of dropsy of the chest ; we 

 have also abundant proofs of their exist- 

 ence from various phenomena that occur 

 in the diseases of the part. We know, for 

 instance, that these vessels take up the 

 serous fluid effused in hydrothorax, for they 

 have been found full of it after death ; and it 

 is a fact that no longer admits of doubt, that 

 blood, extravasated into the chest, is absorbed 

 by the mouths of these minute vessels. 



The nerves of the pleura are too small 

 to be traced by dissection ; but, though it 

 is not possessed of much sensibility in a 

 healthy state, we know, at least we presume 

 from analogy, that it is highly sensitive in 

 the diseased; for few diseases are more 



