202 THE CIRCULATION OF THE BLOOD 



times two cells are said to exist in the same envelope, constituting the twin 

 cells of Dogiel. The cells are large, and have very large round nuclei and 

 nucleoli, figure 175. The neurogenic theory assumes that the periodic 

 discharge of motor nerve impulses takes place from these neurones thus 

 stimulating the musculature of the heart to rhythmic contractions. The 

 stimuli start at the region of the sinus and are conducted over the heart 

 in orderly sequence, but their origin in nerves is questioned. 



In the myogenic theory ,the heart's rhythmical contractions are 

 explained as due to the inherent property of the cardiac muscle itself. 

 Most convincing facts in support of this theory have been arrived at by a 

 study of cardiac muscle, as such, and by studies on the whole heart, 

 particularly by Gaskell's method of blocking. The term blocking is 

 explained as follows: It will be remembered that under normal con- 

 ditions the wave of the contractions in the heart starts at the sinus and 

 travels down over the auricles to the ventricles. The irritability of the 

 muscle and its power of rhythmic contractions is greatest in the sinus, less 

 in the auricles, and least in the ventricles. By an arrangement of liga- 

 tures or by a system of clamping, one part of the heart may be more or less 

 isolated from any other portion. With such a clamp the contraction 

 waves can be more or less completely interrupted in their passage from the 

 sinus end of the heart past the clamp toward the ventricular end. If the 

 clamp is complete, so as to interrupt the physiological continuity between 

 the parts, then any contractions in the apical portion will be entirely 

 independent of those in the sino-auricular portion. If the blocking is 

 partial only, then the ventricular end of the heart ordinarily contracts 

 in unison with the sino-auricular, although its rate may be as i to 2, 

 i to 3, etc. In other words, only every second or every third sino-auricu- 

 lar contraction will be able to pass the block to the ventricle. 



The effects of blocking are due to the interruption of muscle continuity 

 rather than to nerve continuity. This is beautifully demonstrated by an 

 experiment of zigzag cutting of the ventricle in the terrapin, since it cannot 

 be supposed that any nerves would pass through the ventricular mass by 

 such a zigzag course. In this experiment the contraction wave passes 

 down over the muscle and around the end of the cuts until it reaches the 

 apex. The apex muscle contracts in sequence with the auricle and 

 successive pieces of the ventricle. If the zigzag cuts are made almost 

 complete so as to reduce to a minimum the muscular tissue which bridges 

 from one cut to the next, then it happens that occasional contractions will 

 be unable to pass and the apex contracts after its preceding piece in the 

 ratio of i to 2, or i to 3, etc., as described above Thus, division of 

 the muscle has the same effect as partial clamping in the same position. 

 These facts all point to a greater power of rhythmicity in the cardiac 

 tissue nearer the venous end of the heart. This difference of rhythmicity 



