1074 PHYSIOLOGY 



THE PHYSIOLOGICAL PROPERTIES OF THE CARDIAC 



MUSCLE 



THE RESPONSE OF HEART-MUSCLE TO DIRECT EXCITATION 



When a skeletal muscle is directly stimulated with induction 

 shocks of varying strength, within narrow limits the height of the 

 contraction is proportional to the strength of the stimulus. If the 

 frog's ventricle, rendered motionless by a Stannius ligature, be stimu- 

 lated with a single induction shock, if it responds at all it will respond 

 with a maximal contraction, no change in the extent of the contraction 

 being obtainable, however the stimulus may be increased. There is 

 thus no proportionality in the heart between strength of stimulus and 

 height of contraction. The heart, if it contracts at all, always con- 

 tracts to its utmost, the height of the contraction being dependent, 

 not on the strength of stimulus, but on other conditions affecting the 

 muscle at the time of its response. 



Although much stress has been laid on this supposed difference 

 between heart-muscle and voluntary muscle, a renewed investigation 

 of the response of the latter to graded stimuli by Gotch and by Keith 

 Lucas tends to show that the distinction is not so fundamental. 

 According to these observers the fact that the response to a minimal 

 stimulus in skeletal muscle is smaller than the response to a maximal 

 stimulus is simply owing to the fact that in the former case only a 

 small proportion of the muscle fibres is active and that increasing the 

 strength of the stimulus merely increases the number of fibres thrown 

 into contraction. According to this view therefore a maximal contrac- 

 tion of skeletal muscle would be one involving all the fibres. In the 

 heart-muscle all the muscle fibres are functionally continuous, so that 

 a stimulus, if it excites at all, must excite all the fibres, and every 

 contraction must be analogous to the maximal contraction of a skeletal 

 muscle. The existence of the ' all or none ' law in any contractile tissue 

 would be therefore dependent on the existence of functional continuity 

 between all the contractile elements of the tissue. 



In the retractor penis of the dog it is possible to get graded con- 

 tractions with graded strength of stimuli, and in this case it is easy to 

 observe that with increasing strength of stimulus a greater extent of 

 the muscle is thrown into the contractile state. Closely connected 

 with this manner of response is the fact that in heart-muscle, under 

 normal circumstances, it is not possible to get summation of contrac- 

 tions by putting in a stimulus, however strong, before the muscle has 

 returned to rest. If, however, the propagation of the first contraction 

 throughout the heart-muscle be retarded or prevented by a partial 

 death of the tissue, or by stimulus of the vagus nerve, it is possible, as 

 Frank has shown, to obtain an apparent summation of two stimuli, i.e. 



