200 THE CIRCULATION OF THE BLOOD 



strength of the stimulus will give contractions of increasing amplitude until 

 the maximum contraction is reached. In the case of the heart-beats this is 

 not so, since the minimal stimulus which has any effect is followed by the maxi- 

 mum contraction; in other words, the weakest effectual stimulus brings out as 

 great a contraction as the strongest. If a contraction is induced earlier than 

 it would automatically occur, then the succeeding pause is longer; i.e., there 

 is a compensatory pause. Also the contraction induced is smaller and the 

 one following the compensatory pause is proportionately larger. This 

 observation can easily be demonstrated on the heart strip, see figure 174, 

 or on the whole ventricle of the frog, which was originally used by 

 Bowditch. 



Nerve influence, nutrition, temperature, etc., will of course affect the 

 extent of the contractions, but under a given set of conditions it is held that 

 the contractions which occur are maximal for the particular set of nutri- 

 tive and other conditions. This is more readily understood when taken in 

 connection with the fact that when a contraction originates in a cardiac 

 cell it is conducted throughout the continuity of all the cells of the mass. 



Theories of the Heart-beat. The cause of the rhythmic power of 

 the heart as a whole has been the subject of much discussion and experi- 

 mental observation. Two leading hypotheses have given inspiration to 

 investigators, and now one, now the other theory has attracted followers 

 as new facts have been discovered. These are known as the neurogenic 

 theory and the myogenic theory, respectively, though neither is proven 

 correct. 



The heart has long been known to have the power of rhythmic contrac- 

 tions after removal from the body and after all connection with the central 

 nervous system has been destroyed. 



The heart can be taken entirely away from the body of an animal 

 and kept beating rhythmically with ease. This is true for many inverte- 

 brates and for all vertebrates examined including fishes, frogs, turtles, 

 snakes, birds, and numerous mammals including man himself. It is 

 only necessary to supply the heart through its local circulatory vessels 

 with the proper nutritive fluid well aerated with oxygen and at the 

 normal temperature of the animal from which the heart is taken. The 

 question long debated is this. What initiates . these wonderfully persis- 

 tent and regularly repeated contractions? 



If the frog's or terrapin's heart is removed from the body entire, it will 

 continue to contract for many hours and even days, and the contractions 

 have no apparent difference from the contractions of the heart before 

 removal. The contractions will take place, as we have mentioned, without 

 the presence of blood or other fluid within its chambers. Not only is this 

 the case, but the auricles and ventricle may be cut off from the sinus, 

 and all parts continue to pulsate. Further, the auricles may be divided 



