THE LUNGS — THE HEART. 181 



of a serous one is generally isolated. It was to limit the progress of disease that tlu8 

 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 sym- 

 pathy 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 

 ihe 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 ])ropagation of sympathetic irritation ; but the lungs 

 or the bronchial tubes that ramify through them are continuous with the mucous mem- 

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

 continuity 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 mem- 

 brane, it is probable that, instead of being shared between tiiem, the one will be trans- 

 ferred 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 com- 

 paratively little bulk tlie blood, and other fluids scarcely transformed into blood, or 

 soon separated from it, which traverse the whole of the frame. They consist of count- 

 less 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 loind. 



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

 blood-vessels ramify. They proceed from the heart, through the medium of \he puU 

 monary artery — they follow all the subdivisions of the bronchial tubes — they ramify 

 upon the membrane of these multitudinous lobules, and at length return to the heart, 

 through the medium of the pulmonary veins, the character of the blood which they 

 contain being essentially changed. The mechanism of this, and the effect produced 

 must be briefly considered. 



THE HEART. 



The heart is placed between a doubling of the pleura — the mediastinum ; by means 

 of which it is supported in its natural situation, and all dangerous friction between 

 these important organs is avoided. It is also surrounded by a membrane or bag of 

 its own, called the pericardium, whose office is of a similar nature. By means of 

 the heart, the blood is circnlated through the frame. 



It is composed of four cavities — two above, called auricles, from their supposed 

 resemblance to the ear of a dog; and two below, termed ventricles, occupying the sub- 

 stance of the heart. In point of fact, there are two hearts — the one on the left side 

 impelling the blood through the frame, the other on the right side conveying it through 

 16 



