THE HEART. 239 



minute blood-vessels ramify. They proceed from the heart, through the 

 medium of the pulmonary 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 pul- 

 monary 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 circulated through the 

 frame. 



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

 posed resemblance to the ear of a dog; and two below, termed ventricles 

 occupying the substance 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 contributes to their mutual 

 strength, and both circulations are carried on at the same time. 



The first is the arterial circulation. No function can be discharged life 



cannot exist, without the presence of arterial blood. The left ventricle that 

 contains it contracts, and by the power of that contraction, aided by other 

 means, which the limits of our work will not permit us to describe, the blood 

 is driven through the whole arterial circulation — the capillary vessels and the 

 veins — and returns again to the heart, but to the right ventricle. The other 

 division of this viscus is likewise employed in circulating the blood thus 

 conveyed to it, but 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 poisonous fluid. Ere it can 

 again convey the principle of nutrition, or give to each organ that impulse or 

 stimulus which enables it to discharge its function, it must be materially 

 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 filled with the air which 

 has descended through the bronchial tubes in the act of inspiration. This 

 delicate membrane permits some of the principles of the air to permeate it. 

 The oxygen of the atmosphere attracts and combines with a portion of the 

 superabundant carbon of this blood, and the expired air is poisoned with 

 carbonic acid gas. Some of the constituents of the blood attract a portion 

 of the oxygen of the air, and obtain their distinguishing character and properties 

 as arterial blood, and being thus revivified, it passes on over the membrane of 

 the lobes, unites into small and then larger vessels, and at length pours its full 

 stream of arterial blood into the left auricle, thence to ascend into the ventricle, 

 and to be diffused over the frame. 



DISEASES OF THE HEART. 



It may be readily supposed that an organ so complicated is subject to disease. 

 It is so to a fearful extent ; and it sympathises with the maladies of every other 

 part. Until lately, however, this subject has been shamefully neglected, and 



