86 THE HOESE AND ITS DISEASES. 



Nature of the Heart. 



The heart is placed between a daublin of the pleura, 

 the medeastenum 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 bay of its own, called the pericardium, 

 whose office is of a similar nature. By means of the 

 heart the blood is circulated 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 

 the pulmonary system, but united in the manner in 

 which they are, their junction constitutes theii' natural 

 strength, and both circulations are carried on at the 

 same time — the first is the arterial circulation. JN"© 

 function can be discharged — life cannot exist without 

 the presence of the arterial blood — the left ventricle 

 that contains it, contracts, and by the power of that 

 contraction, aided by other means, which the limits of 

 this work will not permit me to describe, the blood is 

 driven through the whole arterial circulation, the ca- 

 pillary vessels, and the veins, and returns again to the 

 heart. The right ventricle (the other division of this 

 viscus) is likewise employed in circulating the blood 

 thus conveyed to it, but it is not the same fluid which 

 was contained in the left ventricle — it has gradually 

 lost its vital power ; as it has passed along, it has 

 changed from red to black, and from a vital to a poison- 

 ous fluid, ere it can convey the principle of nutrition, 

 or give to each organ that impulse, or stimulus, which 

 enables it to discharge its functions, it must be mate- 

 rially changed. When the right ventricle contracts, and 

 the blood is driven into the lungs, it passes over the 

 gossamer membrane, of which the lobules of the lungs 

 have been described as consisting ; these lobules being 



