THE PLEURA. 237 



steam that arises, and in the few drops of fluid, which, being condensed, are 

 found at the lowest part of the chest. 



The quantity, however, which is exhaled from all the serous membranes, 

 must be very great. It is perhaps equal or superior to that which is yielded by 

 the vessels on the surface of the body. If very little is found in ordinary cases, 

 it is because the absorbents are as numerous and as active as the exhalents, and, 

 during health, that which is poured out by the one is taken up by the other ; 

 but in circumstances of disease, either when the exhalents are stimulated to 

 undue action, or the power of the absorbents is diminished, the fluid rapidly 

 and greatly accumulates. Thus we have hydrothorax or dropsy of the chest, 

 as one of the consequences of inflammation of the chest ; and the same disturbed 

 balance of action will produce similar effusion in other cavities. 



The extensibility of membrane generally is nowhere more strikingly dis- 

 played than in the serous membranes, and particularly in that under considera- 

 tion. How different the bulk of the lungs before the act of inspiration has 

 commenced, and after it has been completed, and especially in the laborious 

 respiration of disease or rapid exertion ! In either state of the lungs the pleura 

 is perfectly fitted to that which it envelopes. 



The pleura, like other serous membranes, is possessed of very little sensibility. 

 Few nerves from the sensitive column of the spinal chord reach it. Acute 

 feeling would render these membranes generally, and this membrane in particu- 

 lar, unfit for the function they have to discharge. It has too much motion, 

 even during sleep ; and far too forcible friction with the parietes of the thorax 

 in morbid or hurried respiration, to render it convenient or useful for it to pos- 

 sess much sensation. Some of those anatomists, whose experiments on the 

 living animal do no credit to their humanity, have given most singular proof of 

 the insensibility, not only of these serous membranes, but of the organs which 

 they invest. Bichat frequently examined the spleen of dogs. He detached it 

 from some of its adhesions, and left it protruding from the wound in the abdo- 

 men, in order " to study the phenomena ;" and he saw " them tearing off that 

 organ, and eating it, and thus feeding upon their own substance." In some 

 experiments, in which part of their intestines were left out, he observed them, 

 as soon as they had the opportunity, tear to pieces their own viscera without 

 any visible pain. 



Although it may be advantageous that these important organs shall be thus 

 devoid of sensibility when in health, in order that we may be unconscious of 

 their action and motion, and that they may be rendered perfectly independent 

 of the will, yet it is equally needful that, by the feeling of pain, we should be 

 warned of the existence of any dangerous disease : and thence it happens that 

 this membrane, and also the organ which it invests, acquire under inflammation 

 the highest degree of sensibility. The countenance of the horse labouring 

 under pleurisy or pneumonia will sufficiently indicate a state of suffering ; and 

 the spasmed bend of his neck, and his long and anxious and intense gaze upon 

 his side, tell us that that suffering is extreme. 



Nature, however, is wise and benevolent even here. It is not of every morbid 

 affection, or morbid change, that the animal is conscious. If a mucous mem- 

 brane is diseased, he is rendered painfully aware of that, for neither respiration 

 nor digestion could be perfectly carried on while there was any considerable 

 lesion of it ; but, on the other hand, we find tubercles in the parenchyma of the 

 lungs, or induration or hepatization of their substance, or extensive adhesions, 

 of which there were few or no indications during life. 



The pleura adheres intimately to the ribs and to the substance of the lungs , 

 yet it is a very singular connexion. It is not a continuance of the same organ- 

 isation; it is not an interchange of vessels. The organ and its membrane, 



