998 PHYSIOLOGY 



At certain intervals or at certain stages of the disease, the fibres of the bundle may 

 present only a partial block, so that the ventricle responds once to every second con- 

 traction of the auricle. The existence of this disease is shown at once on the electro- 

 cardiogram by the dissociation of the normal relation between the auricular and 

 ventricular variations. It may be also shown by a study of the venous pulse (Fig. 444). 



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 stimulated 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 stimu- 

 lus 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 contracts 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, so that 

 increasing the strength of the stimulus merely increases the number of fibres 

 thrown into contraction. According to this view therefore a maximal con- 

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

 heart muscle all the muscle fibres are function ally continuous, so that a stimu- 

 lus, 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 con- 

 tractile elements of the tissue. 



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

 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 contractions 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. 

 a curve in which the second contraction is superposed on and rises higher 



