287. But the chambers, alternately with forcible contraction, perform the action of forcible dilatation. This movement of dilatation is effected by the reaction of the elasticity of the tendinous matter on which the muscular fibres are supported (267). This highly elastic substance, by the contraction of the fibres, is brought into a state of extreme tension. The contraction of the fibres ceasing, that moment the tense tendon recoils with a force exactly proportionate to the degree of tension into which it had been brought. Thus the very agent that is employed forcibly to close the chamber is made the main instrument of securing its instantaneous re-opening. A vital energy is appointed to accomplish what is indispensable, and what nothing else can effect, the origination of a motive power; a physical agent is conjoined to perform the easier task to which it is competent; and the two powers, the vital and the physical, work in harmony, each acting alternately, and each, with undeviating regularity and unfailing energy, fulfilling its appropriate office. 288. When the chambers of the heart which open into each other, and which as freely communicate with the great vessels that enter and proceed from them, are forcibly closed, and the blood they contain is projected from them, how is one uniform forward direction given to the current? Why, when the right ventricle contracts, is the blood not sent back into the right auricle, as well as forward into the pulmonary artery? There is but one mode of preventing such an event, which is to place a flood-gate between the two chambers; and there a flood-gate is placed, and that flood-gate is the valve. As long as the blood proceeds onwards in the direct course of the circulation, it presses this membrane close to the side of the heart, and thereby prevents it from occasioning any impediment to the current. When, on the contrary, the blood is forced backwards, and attempts to re-enter the auricle, being of course driven in all directions, some of it passes between the wall of the ventricle and the valve. The moment it is in this situation it raises up the valve, carries it over the mouth of the passage, and shuts up the channel. There cannot be a more perfect flood-gate.