1062 PHYSIOLOGY 



corresponding part of the heart is warmed. When the ventricle alone 

 is warmed the beats become larger, but the rhythm is unaltered. On 

 lowering the loop' so as to warm the sinus, the rhythm of the whole 

 heart is quickened, but the size of the ventricular beats is unaffected. 

 The [different rhythmic power of these parts of the heart is appa- 

 rently connected with the histological characters of the muscle fibres 

 at each part. The lowly differentiated sinus cell has well-marked 

 rhythmic power and a quick rhythm of beat, but is not able to exert 

 such force in its contraction. The more highly differentiated ventricle 

 cell has only a slight rhythmic power, but beats forcibly and is a good 



servant of the sinus. 



. 



THE PROPAGATION OF THE WAVE OF CONTRACTION 

 The normal contraction started in the sinus venosus is propagated 

 to the auricles, thence to the ventricle, and thence to the bulbus 

 aortse. Between the contractions of each of these cavities there is a 

 slight pause, whereas the contraction spreads so rapidly over each 

 cavity that all parts, say of the auricles or ventricle, appear to con- 

 tract simultaneously. It is obvious that the excitatory wave might be 

 propagated through the heart from one muscle-cell to. another, or by 

 means of nerve fibres, which would excite the muscular tissue of each 

 cavity to contract. 



The distinct pause which intervenes between the contractions of 

 auricles and ventricle was long regarded as evidence for the nervous 

 character of the contraction, and as showing the operation of a nerve- 

 centre in the co-ordination of the contractions of different cavities. 

 A contraction wave may, however, be started at any part of the heart 

 and may travel from this to all other parts. Thus, although the normal 

 direction of the contractions is from sinus to ventricle, it is possible, 

 by stimulating the apex of the ventricle, to excite contractions in the 

 reverse order, viz. from ventricle to sinus. Such a fact is at variance 

 with all our present knowledge of excitation of motor nerves. Excita- 

 tion of the nerve going to the sartorius, or any part of the nerve, may 

 excite contractions of all the fibres of which the muscle is composed. 

 On the other hand, excitation of a part of the muscle which is free from 

 nerve fibres causes a contraction which is limited to the muscle fibres 

 directly excited and does not extend to the nerves. If motor nerves 

 arose from the hypothetical motor ganglion of the heart and passed 

 to the ventricular muscle, one would not expect that contraction of 

 the ventricular muscle could excite these nerves and so cause the 

 propagation of a- wave of contraction in the reverse direction. 



That the propagation cannot be due to any nerve -trunks running 

 from sinus to ventricle is shown by various experiments of Engelmann 

 and Gaskell. Thus, if the auricle is slit up by a series of interdigitating 



