238 THE PLEURA. 



although so closely connected for a particular purpose, yet in very many cases, 

 and where it would least of all be suspected, have little or no sympathy with 

 each other. Inflammation of the lungs will sometimes exist, and will run on 

 to ulceration, while the pleura will be very little affected : and, much oftener, 

 the pleura will be the seat of inflammation and will be attended by increased 

 exhalation to such an extent as to suffocate the animal, and yet the lungs will 

 exhibit little other morbid appearance than that of mere compression. The 

 disease of a mucous membrane spreads to other parts that of a serous one is 

 generally isolated. It was to limit the progress of disease that this difference 

 of structure between the organ and its membrane was contrived. 



The investing membrane of the lungs and that of the heart are in continual 

 contact with each other, but they are as distinct and unconnected, as if they 

 were placed in different parts of the frame. Is there no meaning in this ? 



It is to preserve the perfect independence of organs equally important, yet 

 altogether different in structure and function to oppose an insuperable barrier 

 to hurtful sympathy between them, and especially to cut off the communication 

 of disease. 



Perhaps a little light begins to be thrown on a circumstance of which we have 

 occasional painful experience. While we may administer physic, or mild 

 aperients at least, in pleurisy, not only with little danger, but with manifest 

 advantage, we may just as well give a dose of poison as a physic-ball to a horse 

 labouring under pneumonia. The pleura is connected with the lungs, and with 

 the lungs alone, and the organisation is so different, that there is very little 

 sympathy between them. A physic-ball may, therefore, act as a counter- 

 irritant, or as giving a new determination to the vital current, without the pro- 

 pagation of sympathetic irritation ; but the lungs or the bronchial tubes that 

 ramify through them are continuous with the mucous membranes of the 

 digestive as well as all the respiratory passages ; and on account of the conti- 

 nuity and similarity of organisation, there is much sympathy between them. 

 If there is irritation excited at the same time in two different portions of the 

 same membrane, it is probable that, instead of being shared between them, the 

 one will be transferred to the other will increase or double the other, and act 

 with fearful and fatal violence. 



THE LUNGS. 



The lungs are the seat of a peculiar circulation. They convey through their 

 comparatively little bulk the blood, and other fluids scarcely transformed into 

 blood, or soon separated from it, which traverse the whole of the frame. They 

 consist of countless ramifications of air-tubes and blood-vessels connected together 

 by intervening cellular substance. 



They form two distinct bodies, the right somewhat larger than the left, and 

 are divided from each other by the duplicature of the pleura, which has been 

 already described the mediastinum. Each lung has the same structure, and 

 properties, and uses. Each of them is subdivided, the right lobe consisting of 

 three lobes, and the left of two. The intention of these divisions is probably to 

 adapt the substance of the lungs to the form of the cavity in which they are 

 placed, and to enable them more perfectly to occupy and fill the chest. 



If one of these lobes is cut into, it is found to consist of innumerable 

 irregularly formed compartments, to which anatomists have given the name of 

 lobules, or little lobes. They are distinct from each other, and impervious. 

 On close examination, they can be subdivided almost without end. There is 

 no communication between them, or if perchance such communication exists, 

 it constitutes the disease known by the name of broken wind. 



On the delicate membrane of which these cells are composed, innumerable 



