PROPERTIES OF HEART MUSCLE 183 



strength of the stimulus. This fact is often referred to as the " all or none" 

 law of cardiac contraction. The crustacean heart (lobster) forms an excep- 

 tion to this rule, since it behaves to stimuli of different strength exactly like 

 skeletal muscle. 



Another peculiarity of heart muscle is that in both cold-blooded animals 

 and in Mammalia it is inexcitable during its contraction up to the maximum 

 of shortening i. e., all stimuli which fall upon it during this (" refractory ") 

 period are entirely without effect (Marey). Only after the maximum short- 

 ening has been reached does the heart muscle become excitable again. A 

 stimulus applied then calls forth an extra contraction, which is greater the 

 later in diastole it falls. After this extra contraction a longer ("compensa- 

 tory "} pause usually occurs, and the first contraction following the pause is 



FIG. 65. Direct stimulation of the isolated heart of the cat while beating, after Langeiidorff 

 (to be read from left to right; systole represented by the downward stroke). Stimulation 

 at the beginning of systole (a) produces no effect. Stimulation at any time during diastole 

 (6) gives an extra contraction. Following this are seen the compensatory pause and con- 

 siderable augmentation in the strength of the next systole. 



considerably augmented. The smaller the extra contraction, the longer is 

 the subsequent pause and vice versa. After such an interference in the regu- 

 lar rhythm of the beats, there is therefore a compensation by which both the 

 frequency and the amount of work done by the heart are conserved (cf. 

 Fig. 65). 



The compensatory pause appears only in those portions of the heart which 

 beat in consequence of a stimulus communicated to them from other portions; 

 it is not seen therefore in a tracing from the venous sinus of the frog. In 

 explanation of this difference it is supposed that the excitation of the venous 

 sinus is continuous, but that it sends a discontinuous stimulus to the other 

 chambers of the heart. When an extra contraction has been induced in the 

 ventricle, one of these regular discontinuous stimuli from the venous sinus would 

 fall at a time when the ventricle is refractory, hence would produce no effect. 

 The ventricle must therefore wait until the next regular stimulus, and thus we 

 get the compensatory pause. It is quite different with the sinus: as soon as 

 the extra contraction has reached its maximum, the constant stimulus again 

 becomes effective and produces the next systole without an intervening pause. 



The inability of the heart muscle to receive stimuli which fall on it during 

 systole, is the reason why with rapidly repeated shocks it cannot be thrown 



