The heart is a hollow organ made of muscle, whose fibres resemble, in some respects, those of voluntary muscle, described on p. 111, but differ from them in being beyond the control of the will, that is, involuntary. It is situated in the chest, between the right and left lungs, which partly cover it. Fig. 133 shows its posi tion, and Plate XVI should be studied.
In shape the heart resembles a cone, the base of which is directed upwards. It lies, however, obliquely in the chest, so that the base is not only directed upwards, but also backwards and to the right side, while the point of the cone is downwards, forwards, and to the left side. The heart lies more to the left than to the right, but, as may be seen from the figure, it yet ex tends slightly beyond the middle line to the right. When the lungs are fully expanded only a small part of the front of the heart is exposed. One may easily map out on the chest of a healthy man the position occupied by the hearts Let a straight line be drawn, in ink, across the chest on a level with the upper border of the cartilage of the third rib (see Fig. 20, p. CI), let a second line be drawn across at the level of the junction of the breast-bone and xiphoid car tilage (q of Fig. 20). These lines indicate the extreme limits which the heart reaches upwards and downwards in health. The extent to which the heart reaches on each side is marked by drawing one upright line at a distance of inch to the right of the middle of the breast bone, and a second upright line about 3i inches to the left of the middle of the breast-bone. The position of the apex is obtained by marking the spot where the heart is felt beating against the wall of the chest (x in Fig. 133), which is usually about 1 inch below, and a little to the right of, the left nipple in the space between the fifth and sixth ribs. With the aid of this mark, and within the lines, the outline of the heart may bo drawn, and thus a good idea obtained of its position.
The average size of the heart is 4 inches long, 3} inches broad, and 2i inches thick. It is usually smaller in women than in men. It
is roughly measured in individuals by the size of the closed fist.
The coats of the heart. The organ is sus pended in the chest by the great vessels con nected with it at the base, to be considered later; but it does not hang free. It is surrounded by a membranous bag called the pericardium. The bag is really a double one, consisting of two layers, one within the other, the inner one being closely adherent to the heart, and being separated from the outer one by a slight space, in which there is usually a small quantity of serous fluid—the pericardial fluid. The peri cardium passes over the roots of the great ves sels at the base of the heart, and it is here that the inner layer becomes continuous with the outer layer. A good idea of the pericardium will be obtained if one takes two thin paper bags, of which one is slightly smaller than the other, so that one may be contained within the other, both being fully distended. Now slightly fold back the edge of the mouth of the inner bag and gum it all round to the edge of the mouth of the outer one. There is now made a double bag with an inner and an outer layer, and a small space between them, completely shut off from the outside. Suppose the closed fist to be just large enough to fill the inner bag, it will represent the heart, to which the inner layer of the pericardium is adherent. The wrist will represent the great vessels passing off from the heart, around which the neck of the double bag extends. Part of the pericardium towards the apex is adherent, on the outside, to the dia phragm below—the muscular partition which separates the chest from the belly. The peri cardium forms thus an outer coat or covering for the heart. Within it is the proper structure of the heart, the muscular structure, supplied, like all other muscles, with blood-vessels, nerves, lymphatics, &c. The muscular fibres are dis posed in several layers, which appear to be spirally arranged with reference to one another. The inner lining is called the endocardium, and is very delicate.