3 io TEXT-BOOK OF PHYSIOLOGY 



the heart's action, the appearance of a refractory condition during the 

 systole and its gradual disappearance during the diastole, as well as other 

 phenomena, are readily explained by the foregoing hypothesis. 



The cause of the dissociation of the energy-yielding material is, however, 

 a subject of discussion. According to Howell it is not necessary to assume 

 the presence of any cause other than the extreme instability of the organic 

 compound in question. According to Engelmann, Langendorff and others, 

 the dissociation is not spontaneous but is the result of the action of a specific 

 stimulus, an "inner stimulus," arising within the muscle elements themselves 

 through metabolic processes; and so long as these processes are chemically 

 and physically conditioned by blood or tissue fluids containing the inorganic 

 salts, so long will this stimulus be produced. As to the nature of this stimu- 

 lus, whether chemic, electric or enzymic, nothing definite can be stated at 

 present. 



The Response of the Heart to the Action of an Artificial Stimulus. 

 The heart of the frog as well as of some other animals may be brought to a 

 standstill by. the ligation of the tissues between the sinus venosus and the 

 auricle, a procedure first introduced by Stannius and now known as the first 

 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, and no matter what the 

 strength of the current may be, if it is sufficiently strong to call forth a 

 contraction, the contraction at once reaches its maximal value. Any in- 

 crease 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, accord- 

 ing 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 facts. 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 the next excitation pro- 

 cess can call forth a ventricular contraction there must be a re-accumula- 

 tion of energy-yielding material, and a restoration of the irritability. 

 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 



