THE LUNGS ^HEAET. 155 



THE LUNGS. 



The lungs form two distinct bodies, the right somewhat largei 

 than the left, and are divided from each other by the duplicature 

 of the pleura, which has been already described — the mediasti- 

 num. 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 per- 

 fectly to occupy and fill the chest. 



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

 raerahle irregularly formed compartments, to which anatomists 

 have given the name of lobules, or little lobes. They are distinct 

 &om each other, and impervious. On close examination, they 

 can be subdivided almost without end. Th^e is no communica- 

 tion between them, or if perchance such communication exists, it 

 '•onstitutes the disease known by the name of broken loind. 



On the delicate membrane of which these cells are composed, 

 irmumerable minute blood-vessels ramify. They proceed from the 

 heart, through the medium of the pulmonary artery- — they fol- 

 low 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 — ^termed 

 the mediastinum ; by means of which it is supported in its nat- 

 ural situation, and all dangerous friction between these important 

 organs is avoided. It is also surroimded 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 cavitiesi — two above, called auricles, 

 from their supposed 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, unit.'d in the 

 manner in which they are, their junction contributes to their "iD 



