THE CIRCULATION OF THE BLOOD. 301 



Stannius ligature. Under such circumstances the heart may be made to 

 contract by stimulating it with the single induced current. With each 

 passage of the current the heart contracts. Contrary to what is observed 

 in skeletal muscles, the heart contraction, if it occurs at all, at once reaches 

 its maximal value. Any increase in the strength of the stimulus above the 

 threshold value has no greater effect on the extent or force of the contraction 

 than the minimal stimulus. A conclusion which may be drawn from this 

 fact, according to Engelmann is as follows: By reason of the fact that the 

 heart contracts at its maximal value to the action of any strength of stimulus, 

 under given conditions, there is always ensured a more or less complete 

 emptying of the ventricular contents and a uniform discharge of blood into 

 the arteries, which would not be the case if the extent of the contraction 

 varied with the strength of the stimulus; and there are reasons for believing 

 that the normal stimulus for the contraction varies within wide limits above 

 the threshold value both in normal and abnormal conditions of the heart. 

 The changes in the extent or force of the contraction are the result, not of 

 changes in the intensity of the stimulus, but of changes in the heart-muscle, 

 caused by variations in mechanical resistances. 



The periodicity of the heart's action or its rhythm may also be elucidated 

 by the foregoing fact. There are reasons for believing that at the time of 

 the contraction practically all of the available energy-yielding material is 

 completely utilized, after which the heart relaxes and remains at rest in the 

 diastolic condition for a given period; and before a second excitation wave 

 can be developed and pass from the sinus over the heart there must be a 

 re-accumulation of energy-yielding material, and a restoration of the irrita- 

 bility. This is accomplished during the diastole. By virtue of this fact the 

 heart cannot act otherwise than in a periodic manner. 



Inasmuch as there is a conversion of all of the potential energy into 

 kinetic energy during the systole, there is of necessity, a lowering of the 

 irritability, and to so great an extent is this the case that the heart will not 

 respond to the action of a second stimulus either physiologic or artificial 

 during the systolic period. This non-responsiveness of the heart may be 

 shown by throwing into it a second stimulus at any moment during the systole. 

 Whatever the moment or whatever the strength of the stimulus may be the 

 extent of the contraction remains the same. During the systolic period the 

 heart is said, therefore, to be refractory or non-responsive to a second 

 stimulus. If, however, a second stimulus of average strength be thrown 

 into the ventricle at any moment during the relaxation, a second contraction 

 will be developed, which is known as the extra systole. 



The Extra Systole. The extent of this extra systole will be propor- 

 tional to the time at which the stimulus is thrown into the ventricle as it 

 passes from the beginning to the end of its relaxation. Whatever the extent of 

 the extra systole, its height is no greater than that of the first systole (Fig. 

 139) . For this reason it is believed a tetanic contraction cannot be developed. 

 If the stimulus be thrown into the heart just as the relaxation is completed, 

 the extra systole attains the same height as the preceding systole. In passing 

 from the beginning to the end of the relaxation and into the diastolic or 

 resting period, it has been found that the extra systole can be evoked by a 

 stimulus which is steadily decreased in intensity. It is evident from this 



