DISEASES OF THE HEART AND BLOOD VESSELS. 79 



voluntary, because its action contraction is not controlled by the 

 will; but it is well to remark that it is a variety of striated or striped 

 muscle, and striped muscle, with but few exceptions, of which this is 

 one, belongs to the class known as voluntary muscle. 



The nutrition of the heart is derived from the blood distributed to its 

 muscular tissue by the coronary arteries (and their branches), which 

 arc the only vessels given off by the common aorta before it divides 

 into the posterior and anterior aortas. The branches of the coronary 

 arteries are the vessels which occupy the furrows on the external sur- 

 face of the heart. The venous blood from the structure of the heart is 

 emptied into the right auricle by the coronary vein. The lymphatics 

 of the heart accompany the course of the coronary arteries and empty 

 into the lymphatic glands near the base of the heart. The nervous 

 supply of the heart is from the cardiac plexus, which is derived from 

 the pneumogastric and sympathetic nerves. 



The cavities of the heart are lined by a kind of serous membrane 

 called the endocardium. It is very thin and closely adherent and forms 

 the internal surface. The endocardium may be considered as continued 

 into the veins and the arteries, forming their internal lining. In the 

 cavities of the right side the endocardium has a reddish tint, which is 

 deeper in the ventricle; in the cavities of the left side the tint is yel- 

 lowish. The walls of the ventricles are thicker than those of the auri- 

 cles, and the walls of the left ventricle are much thicker than those of 

 the right. 



The heart is enveloped by a fibrous sac (or bag) called the pericar- 

 dium, wliich assumes much of the general shape of the outer surface of 

 the heart. The internal surface of the pericardium is smooth and 

 glistening like the external surface of the heart itself. These smooth 

 surfaces are opposed one to the other, and are in fact the serous mem- 

 brane of the pericardium; they are kept moist by the serum which 

 exudes from their surface to prevent the serious consequence of friction 

 to the surface of the heart. In health no appreciable quantity of fluid 

 collects in the sac, but in some cases of disease, and in instances of old 

 ft, serum accumulates within the pericardium to a greater or !:-;> 

 extent. 



The heart is the principal organ of the circulatory apparatus, and its 

 function is to assure the movement of the blood by the regular contrac- 

 tion of its walls which force the blood into the vessels called arteries. 

 The auricles may IMJ considered as the reservoirs or receivers of the 

 blood, and the v.-nn !<!.> as the puuips, therefore the function of the 

 heart, resembles the action of a force pump. During the interval be- 

 tween contractions, the heart being in momentary repose, the blood 

 pours into the auricles from the veins; the auriculo- ventricular orifices 

 being widely open, the ventricles also receive blood; the aurirles con- 

 tract and the ventricles are filled; contraction of the ventricles follows; 

 the auriculo- vi'iitr.'cular valves arc forced up by the pressure of the 



