something like a bishop's mitre, and hence bear the name of the mitral valve.

The edges and apices of the valves are not completely free and loose. On the contrary, a number of fine, but strong, tendinous cords, called chorda tendineæ, connect them with some column-like elevations of the fleshy substance of the walls of the ventricle, which are termed papillary muscles (Figs. 11 and 12, pp); similar columnlike elevations of the walls of the ventricles, but having no chordæ tendineæ attached to them, are called columnæ

carnea.

It follows, from this arrangement, that the valves oppose no obstacle to the passage of fluid from the auricles to the ventricles; but if any should be forced the other way, it will at once get between the valve and the wall of the heart, and drive the valve backwards and upwards. Partly because they soon meet in the middle and oppose one another's action, and partly because the chorda tendineæ hold their edges and prevent them from going back too far, the valves, thus forced back, give rise to the formation of a complete transverse partition between the ventricle and the auricle, through which no fluid can pass.

Where the aorta opens into the left ventricle and where the pulmonary artery opens into the right ventricle another valvular apparatus is placed, consisting in each case of three pouch-like valves called the semilunar valves (Fig. 11, §.v.; Figs. 13 and 14, A0. P.A.), which are similar to those of the veins. Since they are placed on the same level and meet in the middle line, they completely stop the passage when any fluid is forced along the artery towards the heart. On the other hand, these valves flap back and allow any fluid to pass from the heart into the artery, with the utmost readiness.

The action of the auriculo-ventricular valves may be demonstrated with great ease on a sheep's heart, in which the aorta and pulmonary artery have been tied and the greater part of the auricles cut away, by pouring water into the ventricles through the auriculo-ventricular aperture. The tricuspid and mitral valves then usually become closed by the upward pressure of the water which gets behind them. Or, if the ventricles be nearly

P.V. pulmonary veins opening into the left auricle by four openings, as shown by the styles; a, a style passed from auricle into ventricle through the auriculo-ventricular orifice; b, a style passed into the coronary vein, which, though it has no connexion with the left auricle, is, from its position, necessarily cut across in thus laying open the auricle.

M.V. the two flaps of the mitral valve (drawn somewhat diagrammatically); pp, papillary muscles, belonging as before to the part of the ventricle cut away; c, a style passed from ventricle in Ao. aorta; Ao1. branch of aorta (see Fig. 8, A'o'); P.A. pulmonary artery; S.V.C. superior vena cava. 1, wall of ventricle cut across; 2, wall of auricle cut away around auriculoventricular orifice; 3, other portions of auricular wall cut across; 4, mass of fat around base of ventricle (see Fig. 8, 2).

PA

R.A.V. right auriculo-ventricular orifice surrounded by the three flaps,
t.v. 1, t.v. 2, t.v. 3, of the tricuspid valve; these are stretched by weights
attached to the chorda tendinea.

L.A. V. left auriculo-ventricular orifice surrounded in same way by the two
flaps, m.v. 1, m.v. 2, of mitral valve; P.A. the orifice of pulmonary artery,
the semilunar valves having met and closed together; Ao. the orifice of the
aorta with its semilunar valves. The shaded portion, leading from R.A.V.
to P.A., represents the funnel seen in Fig. 11.

heart can be made to pass through the auriculo-ventricular apertures in one direction only: that is to say, from the auricles to the ventricles. On the other hand, the arrangement of the semilunar valves is such that the fluid contents of the ventricles pass easily into the aorta and pulmonary artery, while none can be made to travel the other way from the arterial trunks to the ventricles.

FIG. 14. THE ORIFICES OF THE HEART SEEN FROM ABOVE, THE AURICLES AND GREAT VESSELS BEING CUT AWAY.

P.A. pulmonary artery, with its semilunar valves; Ao. aorta, do. R.A.V. right auriculo-ventricular orifice with the three flaps (1.v. 1, 2, 3) of tricuspid valve.

L.A.V. left auriculo-ventricular orifice, with m.v. 1 and 2, flaps of mitral valve; b, style passed into coronary vein. On the left part of L.A. V., the section of the auricle is carried through the auricular appendage; hence the toothed appearance due to the portions in relief cut across.

11. Like all other muscular tissues, the substance of the heart is contractile: but, unlike most muscles, the heart contains within itself a something which causes its different parts to contract in a definite succession and at regular intervals.

If the heart of a living animal be removed from the body, it will, though in most cases for a very short time only, go on pulsating much as it did while in the body. And careful attention to these pulsations will show that they consist of :—(1) A simultaneous contraction of the walls of both auricles. (2) Immediately following this, a simultaneous contraction of the walls of both ventricles. (3) Then comes a pause, or state of rest; after which the auricles and ventricles contract again in the same order as before, and their contractions are followed by the same pause as before.

If the auricular contraction be represented by A, the ventricular by V, and the pauses by the series of actions will be as follows: AV-; A°V ̃·; A° V -; &c. Thus, the contraction of the heart is rhythmical, two short contractions of its upper and lower halves respectively being followed by a pause of the whole, which occupies nearly as much time as the two contractions.

The state of contraction of the ventricle or auricle is called its systole; the state of relaxation, during which it undergoes dilatation, its diastole.

12. Having now acquired a notion of the arrangement of the different pipes and reservoirs of the circulatory system, of the position of the valves, and of the rhythmical contractions of the heart, it will be easy to comprehend what must happen if, when the whole apparatus is full of blood, the first step in the pulsation of the heart occurs and the auricles contract.

By this action each auricle tends to squeeze the fluid which it contains out of itself in two directions-the one towards the great veins, the other towards the ventricles; and the direction which the blood, as a whole, will take, will depend upon the relative resistance offered to it in these two directions. Towards the great veins it is resisted by the mass of the blood contained in the veins. Towards the ventricles, on the contrary, there is no resistance worth mentioning, inasmuch as the valves are open, the walls of the ventricles, in their uncontracted state, are flaccid and easily distended, and the entire pressure of the arterial blood is taken off by the semilunar valves, which are necessarily closed. The return of blood into