You all know there is a fluid in the body which we call blood, and that the object of this fluid is to nourish the tissues of the body; you can see, therefore, that it is necessary that this fluid should be conducted in some way or other to all parts of the body, and this is done by means of what we call the Circulatory Apparatus, which consists of a pump and pipes. The pump is the heart, and the pipes ase the blood-vessels. A person's heart is about the size of his closed fist. It is conical in shape (or heart-shaped if you like), having therefore a base and an apex. It is situated in the thorax or chest, between the lungs, rather more on the left side than on the right. Its base is turned upwards towards the right, and its apex is downwards towards the left. It is almost entirely surrounded by a membrane something like those bags of membrane described as being inside the true joints, a closed bag which nearly surrounds the heart, although the heart is entirely outside of it.

I want you to understand that, because most of the internal organs of the body have bags like this surrounding them. The bag just referred to, because it is round the heart, is called the pericardium; the lower part of this bag is attached to the diaphragm.

The heart has two sides, with a cavity on each side, and the two sides do not communicate with one another directly; they axe completely separated by a partition that passes down from the base to the apex, and is called the septum. The walls of the heart and this partition are all made of muscle, but it is a muscle over which we have no control by means of the will, and it is therefore, unlike the muscles of the limbs, an involuntary muscle.

The walls, then, and the partition, are all made of muscular tissue, and the partition divides the cavity of the heart completely into two, one on the right side and one on the left; each of these is subdivided into two cavities, which do communicate with one another. The upper cavities are called auricles, because they have little appendages, which look like ears, belonging to them. Two auricles then-the right auricle and the left auricle. The lower cavities, which communicate with the upper ones, are called ventricles-the right ventricle and the left ventricle.

Between these cavities, on each side, there are flaps of membrane, between the right auricle and the right ventricle, and between the left auricle and the left ventricle-flaps of membrane which are continuations of the membrane which lines the interior of these cavities- just as there is a membrane outside the heart, so there is a membrane lining the interior. This is called the endocardium.

The flaps of membrane we call valves. On the right side, between the right auricle and the right ventricle, there are three flaps, and the valve on that side goes by the name of the tricuspid, or three-pointed valve.

On the left side there are only two such flaps of membrane, and they go by the name of the bicuspid valve, sometimes called the mitral valve, because it is similar in shape to a bishop's mitre. These flaps hang downwards into the ventricles, and are fastened by means of fine tendinous cords to little muscular projections from the walls of the ventricles.

Into the auricles, on each side, there open large bloodvessels. Into the right auricle two large blood-vessels open. They are called large veins. I will tell you why by and by. Into the left auricle four veins open, and at the openings of these vessels into the auricles there are no valves,-they open straight into the cavities of the auricles.

From each ventricle there rises a large vessel, which in each case is called an artery. The vessel that rises from the left ventricle is called the aorta, or great artery of the body. The vessel that rises from the right ventricle is the artery which goes to the lungs, and is called the pulmonary artery for that reason.

At the beginning of both these vessels there are three little pouches of membrane placed round the vessel* The interior of these pouches looks up into the vessel, and they are called the semilunar (half-moon-shaped) valves, from their shape.

We will now pass on to the consideration of the action of the heart, and of the tubes that are connected with it. Suppose we begin with the left ventricle. When the left ventricle is fall of blood the muscular walls of that ventricle contract and squeeze the blood that is in it against the flaps of membrane that hang down from the auricle, forming the bicuspid or mitral valve, which are so tied down that they cannot be turned inside out, and the pressure of blood on those flaps closes them like a floodgate, so that the blood cannot go from the ventricle into the auricle; but it is forced also against the little pouches of membrane or valves at the beginning of the great artery of the body, which is called the aorta; so that the pressure of blood on the under side presses them against the walls of the aorta, and as the ventricle contracts it squeezes the blood past those little valves into the aorta. This is a strong tube, with very elastic walls. In its walls there is a great deal of what we call elastic tissue. It is full of blood already, and when the ventricle squeezes three ounces more into it, of course the walls of this elastic tube are bulged out. As soon as the ventricle has emptied itself it finishes contracting, and its walls begin to relax; then the elastic walls of the aorta recoil upon the blood and squeeze it; and as when any pressure is exerted upon a fluid, it presses equally in all directions, so the blood is pressed both ways, and is forced back against these little flaps of membrane at the beginning of the artery, pressing them all against one another, so that the force of the blood backwards towards the heart by the recoil of the elastic walls of the artery closes those little valves; the blood then cannot go back into the heart, and so the pressure caused by the recoil of the artery drives it forwards into another part of the artery and bulges that, and then that in its turn recoils and presses upon the blood, but as there is sufficient blood behind it to resist the pressure, it must go forward, and it continually goes forward like a wave.