228 RESPIRATORY SYSTEM. 



Like them, it presents a shining secreting surface, of a whitish 

 aspect, and considerable transparency, and is composed of httle 

 else than condensed cellular substance, whose texture is pene- 

 trated by bloodvessels, absorbents, and nerves : by long mace- 

 ration in water, indeed, it may be entirely resolved into cellular 

 substance. In most parts it is extremely 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 division 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 colourless parts of the blood — a circumstance 

 that accounts for its pellucidity ; under inflammation, however, 

 they contain red blood, and such is the explanation of that ar- 

 borescent vascularity upon the sides of the thorax in horses 

 that die of pneumonia ; than which state nothing can better de- 

 monstrate the comparative number and distribution of these blood- 

 vessels. The majority of them terminate in exhalent orifices, 

 from which is continually poured, upon the contiguous surfaces ol 

 the smooth interior of the membrane, a serous fluid, in the form 

 of steam or vapour, which may at any time be rendered visible 

 by opening the chest of an animal recently dead. The absorbenls 

 of this membrane are very numerous ; and though their extreme 

 exility prevents us from demonstrating 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 existence 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 dissec- 

 tion ; 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 

 acutely painful in the human subject than pleurisy, and we have 

 every reason to believe that horses suffer much from the same 

 malady. 



Secretion. — It has been observed that the exhalents of the 

 pleura secrete a serous fluid, which is emitted, in the form of an 

 exhalation, or vapour, into the cavity of the thorax ; and that it 

 may be rendered visible at any time, if an animal, recently dead, 

 be opened while yet warm ; or, if an opening be made into the 

 chest of a live animal : in either case, a whitish steam will be 



