289. This is beautiful mechanism; but there is another arrangement which surpasses mere mechanism, however beautiful. It has been shown (260) that one edge of the membrane that forms the valve is firmly adherent to the wall of the ventricle, while the other edge, when not in action, appears to lie loosely in the ventricle (fig. CXVI. 10). Were this edge really loose the refluent current would carry it back completely into the auricle, and so counteract its action as a valve; but it is attached to the tendinous threads proceeding from the fleshy columns that stand along the wall of the ventricle (fig. CXVI. 12). By these tendinous threads, as by so many strings, the membrane is firmly held in its proper position (fig. CXVI. 10, 12); and the refluent current cannot carry it into the auricle. Thus far the arrangement is mechanical. But each of these fleshy columns is a muscle, exerting a proper muscular action. Among the stimulants which excite the contractility of the muscular fibre, one of the most powerful is distension. The refluent current distends the membrane; the distension of the membrane stretches the tendinous threads attached to it; the stretching of its tendinous threads stretches the fleshy column; by this distension of the column it is excited to contraction; by the contraction of the column its thread is shortened; by the shortening of the thread the valve is tightened, and that in the exact degree in which the thread is shortened. So, the greater the impetus of the refluent blood, the greater the distension of the membrane; and the greater the distension of the membrane, the greater the excitement of the fleshy column; the greater the energy with which it is stimulated to act, the greater, therefore, the security that the valve will be held just in the position that is required, with exactly the force that is needed. Here, then, is a flood-gate not only well constructed as far as regards the mechanical arrangement, but so endowed as to be able to act with additional force whenever additional force is requisite; to put forth on every occasion, as the occasion arises, just the degree of strength required, and no more.