THE PLEUKA. 2C1 



we have described, changed. It is no longer able to support life : it is 

 possessed of a poisonous principle, and that principle is a superabundance 

 of a substance called carbon, which must be got rid of,_ before the blood 

 can again be usefully employed. There is an ingredient in the atmospheric 

 air called oxygen, which has a strong attraction for this carbon, and which 

 will unite with it wherever it finds it. The chest enlarges by the action 

 of the diaphragm, and the intercostal and other muscles, as we have 

 narrated, and the lungs expanding with the chest, in order to fill up the 

 vacuum which would otherwise exist between them and the sides of the 

 chest, these cells enlarge, and a kind of vacuum is formed in each of them, 

 and the air rushes down and fills them, and being divided from the venous 

 and poisoned blood by these membranes alone, it is enabled to act upon 

 the blood, the oxygen combines Avith the carbon to form carbonic acid, 

 and thus purifies it, and renders it arterial blood, and fit for the purposes 

 of life. This being accomplished, the chest contracts, the lungs are 

 pressed into smaller compass, and a portion of the air impregnated with 

 carbonic acid, and rendered poisonous in its turn, is pressed out. Presently 

 the chest expands again, and the lungs expand with it, and fresh, pure air 

 is admitted, which is shortly pressed out again, empoisoned by the carbon 

 of the blood : and these alternate expansions and contractions constitute 

 the act of breathing. 



THE PLEURA. 



The walls of the chest are lined, and the lungs are covered, by a smooth 

 olistening membrane, the pleura. It is a serous membrane, so called from 

 the nature of its exhalation, in distinction from the mucous secretion yielded 

 by the membrane of the air-passages. The serous membrane generally 

 invests the most important organs, and always those that are essentially 

 connected with life, and lines all the enclosed cavities of the body ; while the 

 mucous membrane hues the interior of those cavities which have external 

 openings. The pleura is the investing membrane of the lungs, and a mucous 

 membrane the lining one of the bronchial tubes. 



Among the circumstances principally to be noticed, with regard to the 

 pleura, is the polish of its internal surface. The glistening appearance of the 

 lungs, and of the inside of the chest, is to be attributed to the membrane by 

 whi'cli they are covered, and by means of which the motion of the various 

 organs is freer and less dangerous. Although the lungs, and the bony walls 

 which contain them, are in constant approximation with each other, both in 

 expiration and inspiration, yet in the frequently hurried and violent motion 

 of the animal, and, in fact, in every act of expiration and inspiration, of 

 dilatation and contraction, much and injurious friction would ensue if the 

 surfaces did not glide freely over each other by means of the peculiar 

 polish of this membrane. 



Every serous membrane has innumerable exhalent vessels upon its 

 sui'face, from which a certain quantity of fluid is poured out. In hfe and 

 during health it exists in the chest only as a kind of dew, just sufficient 

 to lubricate the sm^faces. Wlien the chest is opened soon after death, we 

 recognise it in the steam that arises, and in the few drops of fluid, which, 

 beins: 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 surfiice of the body. If very Httle 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 powei' of the 



