The chambers of the heart. It has been said that the heart is a hollow organ, but the cavity is not single. A complete muscular par tition divides it into two, so that there is a right and a left side, between which there is no direct communication except in the child before birth.
That partition or septum is indicated on the outside of the heart by a groove passing from the base to the apex, along which large blood vessels run (Fig. 1;;4). This partition, as the groove indicates, keeps to the right side of the apex, so that the whole of the apex is, by this division, on the left side. The two cavities thus formed are each divided by a partition into au upper and a lower chamber. The partitions are not permanent, however, for they consist of flap valves, which, when closed, completely separate the upper and lower chambers, but which are capable of opening so that the cham bers become continuous, just as folding-doors between two rooms, if closed, make them two separate rooms, but if open, make them practi cally one room. The heart has thus four cham bers, two on each side. Fig.135 shows them very well. It represents a heart cut open length ways. From L the upright partition is seen pass ing, in a slightly irregular course, to at, and it is plainly shown how the apex is kept to the left. The cross partition is in the direction from E to and the four chambers are marked c, D, A, and B. Now the two upper chambers are called auricles (Latin auricula, the outer ear), from their supposed external resemblance to the ear (see Figs. 134 and 135, c and n). There are, therefore, the right and left auricles. The lower chambers are called ventricles (Latin venter, the belly), for they form the chief portion of the muscular substance of the organ, and there is a right and a left ventricle (Figs. 134 and 135, A and a). There is a great difference between the walls of the auricles and those of the ventricles, the former being thin and soft, the latter thick and strong and very muscular; especially is the increased thickness evident in the walls of the left ventricle. This difference is connected with time greater Lniount of work thrown on the ven tricles and especially the left, as will be seen when the action of the heart is considered (p. 302).
The valves of the heart are most important structures. They consist of folds of the inner lining— endocardium —strengthened by fibres of connective and elastic tissue. One is situated in the narrow part between the right auricle and ventricle. It is called the tricuspid valve, because it consists of three cusps or flaps. In Fig. 135s points to one flap. When the flaps are stretched across they completely close the communication between the upper and lower chamber. On the left side the valve in the narrow part between left auricle and ventricle consists of only two flaps, and is called the mitral valve (Fig. 135, r), because of its sup posed resemblance to a bishop's mitre. When the valve is closed the flaps meet in the middle line and block the opening between the two chambers. When not stretched across, the flaps of both valves are hanging downwards into the lower chambers, and the passages between the auricles and ventricles are open so that the upper and lower chambers of each side are continuous. The passage on the right aide, guarded by the tricuspid valve, is called the right auriculo ventricular opening, that ou the left side, guarded by the mitral valve, is called the left auriculo-ventricular opening. ,From the right ventricle a large vessel passes to the lungs— the pulmonary artery,—and from the left ven tricle a large vessel goes off—the aorta. The openings of these vessels into the ventricles are guarded by valves—semi-lunar valves. Each semi-lunar valve has three flaps, and each flap is half-moon shaped, hence the term semilunar (semi, half, and luna, the moon). These valves, when shut, cut off the communication, on the right side between the pulmonary artery and the right ventricle, and on the left side between the aorta and the left ventricle. Most cases of heart-disease consist of some affection of one or other of these valves ; and anything that interferes with their efficient opening or closing may produce most profound changes in every organ of the body. How this is possible will be perceived if the action of the valves is rightly understood.