236 HANDBOOK OF PHYSIOLOGY. 



In connection with the rhythmic contraction of muscle, it is netes- 

 sary to allude briefly to what is known as Stannius' experiment. 

 This experiment consisted originally of applying a tight ligature to the 

 heart between, the sinus and the right auricle, the effect of which is to stop 

 the beat of tke heart below r the ligature,, while the sinus and the veins 

 leading into it continue to beat. If a second ligature be applied at the 

 junction of the auricles and ventricle, the ventricle may begin to beat 

 slowly, while the auricles continue quiescent. In both cases the quies- 

 cent parts of the heart may be made to give single contractions in 

 response to mechanical stimulation. A considerable amount of discussion 

 has arisen as to the explanation of these phenomena. It was suggested 

 that the action of the ligature is to stimulate some inhibitory nervous 

 mechanism in the sinus, whereby the auricles and ventricle can no 

 longer continue to contract, but this suggestion must certainly be given 

 up if the present theory as to the functions of the nerve ganglia be cor- 

 rect. It may be that the effect of Stannius' ligature is simply an exam- 

 ple of what has been called by Gaskcll blocking. The explanation of 

 this term is as follows : it appears that under normal conditions the 

 wave of contraction in the heart starts at the sinus and travels down- 

 ward over the auricles to the ventricle, the irritability of the muscle 

 and the power of rhythmic contractility being greatest in the sinus, less 

 in the auricles and still less in the ventricle, while under ordinary con- 

 ditions the apical portion of the ventricle exhibits very slight irritability 

 and still less power of spontaneous contraction. Thus it may be sup- 

 posed that the wave of contraction beginning at the sinus is more or 

 less blocked by a ring of muscle of lower irritability at its junction with 

 the auricles, and again the wave in the auricles is similarly delayed in 

 its passage over to the ventricle by a ring of lesser irritability, and thus 

 the wave of contraction starting at the sinus is broken as it were both 

 at the auricles and at the ventricle. By an arrangement of ligatures, or 

 better, of a system of clamps, one part of the heart may be isolated from 

 the other portion, and the contraction when stimulated by an induction 

 shock may be made to stop in the portion of the heart-muscle in which 

 it begins. It is not unlikely that the contraction of one portion of the 

 heart acts as a stimulus to the next portion, and that the sinus contrac- 

 tion generally begins first, since the sinus is the most irritable to stimuli, 

 and possesses the power of rhythmic contractility to the most highly 

 developed degree. It must not be thought, however, that the w 7 ave of 

 contraction is incapable of passing over the heart in any other direction 

 than from the sinus downward; it has been shown that by application 

 of appropriate stimuli at appropriate instants, the natural sequence of 

 beats may be reversed, and the contraction starting at the arterial part 

 of the ventricle may pass upward to the auricles and then to the sinus 

 in order. 



