two chambers, an upper chamber and a lower chamber; so that altogether we have four chambers,--two upper chambers, one on each side, marked _R.A._ and _L.A._, these are called the =right and left auricles=; and two lower chambers, one on each side, marked _R.V._ and _L.V._, these are called the =right and left ventricles=. The right auricle, _R.A._, opens in the direction of the arrow into the right ventricle, _R.V._, the opening being guarded, as we shall see, by a valve. The left auricle, _L.A._, opens into the left ventricle, _L.V._, the opening being likewise guarded by a valve; but you have to go quite a roundabout way to get from either the right auricle or ventricle to the left auricle or ventricle. Let us see how we can get round the figure. Suppose we begin with the two tubes marked _V.C.S._ and _V.C.I._, the walls of which are drawn with thin lines. These both open into the right auricle. They are the vena cava superior and inferior, which you have just made out in the sheep’s heart. From the right auricle you pass easily into the right ventricle; thence, following the arrow, the way is straight into the tube marked _P.A._ This is the pulmonary artery, the outside of which you saw in the sheep’s heart (Fig. 5, _P.A._) Travelling along this pulmonary artery, you come to the lungs, and after passing through branches not represented in the figure, picking your way through arteries which continually get smaller and smaller, you find yourself at last in the capillaries of the lungs. Squeezing your way through these, you come out into veins, and gradually advancing through larger and larger veins, you, still following the arrow, find yourself in one of four large veins (only one of them is represented in the diagram) which land you in the left auricle. From the left auricle it is but a jump into the left ventricle. From the left ventricle the way is open, as indicated by the arrow, into the tube marked _Ao_. This is intended to represent the aorta, which you have already seen in the sheep’s heart (Fig. 5, _Ao_). It is here drawn for simplicity’s sake as dividing into two branches, but you have already been told, and must bear in mind, that it does not in reality divide in this way, but gives off a good many branches of various sizes. However, taking the figure as it stands, suppose we travel along _A_^{2}. Following the arrow, and shooting through arteries which continually get smaller and smaller, we come at last to capillaries somewhere, in the skin or in some muscle, or in a bone, or in the brain, or almost anywhere, in fact, in the upper part of the body. Out of the capillaries we pass into veins, which, joining together and so forming larger and larger trunks, bring us at last to the point from which we started, the superior vena cava, _V.C.S._ If we had taken the other road, _A_^{2}, we should have passed through capillaries somewhere in the lower part of the body instead of the