Now try and find what are marked in the figure as _S.V.C._ and _I.V.C._ You will perhaps have a little difficulty in this; and when you have found them you will understand why. They are the great veins of the body. _S.V.C._ is the =superior vena cava=, to form which all the veins from the head and neck and arms join, the vein in which you were journeying a little while ago. _I.V.C._ is the =inferior vena cava=, made out of all the veins from the trunk and the legs. Being veins, they have thin flabby walls; and their sides fall flat together, so that they seem nothing more than little folds of skin, and it becomes very hard to find the passage inside them. But when you have found the opening into them, you will see that you can stretch them out into quite wide tubes, and that their walls, though very much thinner than those of the aorta, so thin indeed that they are almost transparent, are still after a fashion strong. If you put a penholder or thin rod through either you will find that they both seem to lead right into the middle of the heart. With a little care you can pass a rod up _I.V.C._ and bring the end of it out at the top of _S.V.C._ Of course you will understand that both of these veins have been cut off short. =28.= Before we go on any further with the sheep’s heart, let me tell you something about it, by help of the diagram in Fig. 6, which is meant to represent the whole circulation. You must remember that this figure is a =diagram=, and not a picture; it does not represent the way the blood-vessels are really arranged in your own body. If you had no arms and no legs, and if you only had a few capillaries at the top of your head and at the bottom of your body, it might be more like than it is. In the centre of the figure is the heart. This you will see is completely divided by an upright partition into two halves, a right half and a left half. Each half is further marked off, but not completely divided, into [Illustration: FIG. 6.--_Diagram of the Heart and Vessels, with the Course of the Circulation, viewed from behind so that the proper left of the Observer corresponds with the left side of the Heart in the Diagram._ _L.A._ left auricle; _L.V._ left ventricle; _Ao._ aorta; _A_^{1}. arteries to the upper part of the body; _A_^{2}. arteries to the lower part of the body; _H.A._ hepatic artery, which supplies the liver with part of its blood; _V_^{2}. veins of the upper part of the body; _V_^{2}. veins of the lower part of the body; _V.P._ vena portæ; _H.V._ hepatic vein; _V.C.I._ inferior vena cava; _V.C.S._ superior vena cava; _R.A._ right auricle; _R.V._ right ventricle; _P.A._ pulmonary artery; _Lg._ lung; _P.V._ pulmonary vein; _Lct._ lacteals; _Ly._ lymphatics; _Th.D._ thoracic duct; _Al._ alimentary canal; _Lr._ liver. The arrows indicate the course of the blood, lymph, and chyle. The vessels which contain arterial blood have dark contours, while those which carry venous blood have light contours.]