144 ANATOMY AND PHYSIOLOGY. 



ward, 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. 



This is beautiful mechanism ; but there is another arrangement 

 which surpasses mere mechanism, however beautiful. It has been 

 shown that one edge of the membrane that forms the valves is firmly 

 adherent to 'the wall of the ventricle, while the other edge, when 

 not in action, appears to lie loosely in the ventricle. 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. By these ten- 

 dinous threads, as by so many strings, the membrane is firmly held 

 in its proper position ; 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 mem- 

 brane stretches the tendinous threads attached to it ; the stretching 

 of its tendinous threads stretches the fleshy column ; by this disten- 

 sion 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 w r hich 

 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. 



The contraction of the heart is the power that moves the blood ; 

 and this contraction generates a force which is adequate to impel it 

 through the circle. 



It has been shown that the different chambers of the heart have 



