The walls of the auricles are much thinner than those of the ventricles. The wall of the left ventricle is much thicker than that of the right ventricle; but no such difference is perceptible between the two auricles (Figs. II and 12, I and 3).

9. In fact, as we shall see, the ventricles have more work to do than the auricles, and the left ventricle more to do than the right. Hence the ventricles have more muscular substance than the auricles, and the left ventricle than the right; and it is this excess of muscular substance which gives rise to the excess of thickness observed in the left ventricle.

The muscular fibres of the heart are not smooth, nucleated bands, like those of the vessels, but are bundles of transversely-striped fibres, and resemble those of the chief muscles of the body, except that they have no sheath, or sarcolemma, such as we shall find to exist in the latter.

Almost the whole mass of the heart is made up of these muscular fibres, which have a very remarkable and complex arrangement. There is, however, an internal membranous and epithelial lining, called the endocardium; and at the junction between the auricles and ventricles, the apertures of communication between their cavities, called the auriculo-ventricular• apertures, are strengthened by fibrous rings. To these rings the moveable partitions, or valves, between the auricles and ventricles, the arrangement of which must next be considered, are attached.

10. There are three of these partitions attached to the circumference of the right auriculo-ventricular aperture, and two to that of the left (Figs. 11, 12, 13, 14, tv, m v). Each is a broad, thin, but very tough and strong triangular fold of the endocardium, attached by its base, which joins on to its fellow, to the auriculo-ventricular fibrous ring; and hanging with its point downwards into the ventricular cavity. On the right side there are, therefore, three of these broad, pointed membranes, whence the whole apparatus is called the tricuspid valve. On the left side, there are but two, which, when detached from all their connexions but the auriculo-ventricular ring, look something like a bishop's mitre, and hence bear the name of the mitral valve.

FIG. 12.-LEFT SIDE OF THE HEART OF A SHEEP (LAID OPEN). 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 connection 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; Aot, 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).

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 chorda 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 tendinea 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, s.v.; Figs. 13 and 14, Ao. P.A.), which are similar to those of the veins. But as 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 filled, the valves may be made to come together at once by gently squeezing the ventricles. In like manner, if the

base of the aorta, or pulmonary artery, be cut out of the
heart, so as not to injure the semilunar valves, water
poured into the upper ends of the vessel will cause its
valves to close tightly, and allow nothing to flow out after
the first moment.

Thus the arrangement of the auriculo-ventricular valves
is such, that any fluid contained in the chambers of the
heart can be made to pass through the auriculo-ventricular
apertures in only one direction: that is to say, from the

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 tendineæ.

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.

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.

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

L.A.V. left auriculo-ventricular orifice, with m.v. 1 and 2, flaps of mitral valve; c, 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.

II. 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 go on pulsating for a longer or shorter time,