There are various things to notice connected with the flow of blood through arteries, veins, and capillaries, and associated with the differ ences in the structure of these vessels.
The Circulation in the Arteries.--It has been pointed out that the large arteries are specially elastic. The effect of this is not diffi cult to understand. Take the aorta as an ex ample. It is filled with blood, but by the con traction of the ventricle an additional quantity is thrown into it. Sufficient room cannot at once be made by the onward flow of that which already occupies the vessel, and so part of the vessel distends sufficiently to accommodate it. As soon as the contraction of the ventricle is over, the force distending the artery is dimin ished, and, by their own elasticity, the walls of the vessel recoil, that is, they return to their natural degree of distension. In doing this the walls press on the blood contained by them, and tend to drive the blood both backwards and forwards. The blood is prevented passing back wards by the closure of the aortic valve, and it must all pass on into the next part of the artery, which in turn distends, being already full, to receive it. The same process is here repeated ; the distended wall speedily recoils and passes the blood on to a succeeding part, which, also, first dilates and then recoils on the blood within it. Thus the contraction of the heart is not the only force in driving on the blood. It is aided by the elastic recoil of the arterial walls suc ceeding it, and the blood is forced onwards in waves.
The Pulse is thus explained. If a finger be laid on a part of the body where an artery approaches the surface a throbbing movement will be felt, and the finger, if lightly applied, or, better still, a piece of straw laid over the place, will rise and fall in a regular manner, and the beat, thus produced, will be perceptibly in time with the contraction of the heart. This move ment is the result of the alternate distension of the vessel by the wave of blood, and the recoil of the walls by their elasticity. It does not occur at the same instant as the contraction of the ventricle, because it takes a little time for the wave to pass from the heart to the place where the pulse is felt, and it is, therefore, a little later at the ankle than at the wrist Wherever an artery approaches the surface the pulse may be felt. The wrist is the most con
venient place, but it is easily found at the temple and the inner side of the ankle. Various influences affecting the characters of the pulse are noted on p. 40.
The jerky movement of the blood in an artery is seen if the vessel be cut. The blood does not issue in a regular stream, but in spurts, corresponding to beats of the heart. This en ables one to ascertain readily whether a wounded blood-vessel is an artery or a vein, for the flow of blood from the latter is continuous. This is of importance, because if an artery be cut we know the blood is flowing from the heart to the distant parts of the body. If the loss of blood is to be prevented, the vessel must be closed nearer to the heart than the wound, to prevent new supplies of blood coining on to the wound. This is done by pressing with the fingers or a pad on the heart side of the wound. If, on the contrary, it is a vein that is wounded, we know the blood is flowing in it towards the heart from the distant part, and pressure must be exerted on the side of the wound farthest from the heart.
While elasticity is the feature of the large arteries, the contractility of small arteries is their characteristic. If the muscular fibres which surround a vessel contract, the channel will be narrowed and less blood will be permitted to flow through it. If the muscular fibres relax, the force of the blood within the vessel—the blood pressure — will cause the channel to ' enlarge by dilating the tube, and more blood will be permitted to flow along. Thus the dia meter of the vessel may be regulated by the muscular contraction, and the quantity of blood proceeding to a tissue or organ will be thereby affected. The contraction of the muscular fibres is controlled by the nervous system, and so the nervous system regulates and controls the blood supply to the various regions of the body. How this is effected is explained on p. 312.