i o 2 THE INVOL UNTAR Y NER VO US S YSTEM 



the remarkable regularity of such rhythmic contractions of 

 striated muscle. Undoubtedly some muscles are much more 

 easily excited to rhythmic contraction than others; and I have 

 put forward the view that the more primitive forms of muscle 

 possess a greater rhythmic power than the more highly de- 

 veloped forms, in which rapidity of contraction has been gained 

 at the expense of this rhythmic power. 



In the case of the vertebrate heart, I have pointed out that it 

 arose as a vascular tube surrounded by a layer of muscles. The 

 tube became bulged in two places to form the large cavities 

 of auricle and ventricle, and the musculature of these bulged por- 

 tions contracted more rapidly so as to form a more and more effi- 

 cient force pump. The more unmodified portions of the original 

 tube are represented by the sinus, the canalis auricularis between 

 auricles and ventricles, which ultimately becomes the junction be- 

 tween the auricles and ventricle at the auriculo-ventricular groove, 

 and the muscles round the aorta which form the bulbus arteriosus 

 in the frog and the conus arteriosus in the Elasmobranch fishes. 

 Further the rhythmical power of the different parts of the heart 

 was greatest in those parts where the original musculature had 

 not been modified, i.e., sinus, auriculo-ventricular junction and 

 conus arteriosus. Especially striking was the behaviour of the 

 conus arteriosus of the Elasmobranch fishes, which, like the bulbus 

 arteriosus of the frog, as pointed out by Engelmann, possesses a 

 very high degree of rhythmic power. 



This evidence combines to show that in any consideration 

 of the meaning of the heart beat, both in warm and cold-blooded 

 animals, the rhythmic power of the muscular tissues must be 

 taken into account. 



With respect to the sequence of the contractions of dif- 

 ferent parts of the heart, my experiments on the tortoise showed 

 not only that the sequence was not dependent upon the integrity 

 of the main nerves, which pass from the sinus to the auricles and 

 ventricles, but that a contraction wave travelled over the auricular 

 tissue to the ventricle, and, when it reached the ventricle, the ven- 

 tricular muscle contracted. Further, a block could be made in 

 the series of contractions which reached the ventricle by reducing 

 the size of and damaging the bridge of auricular tissue, over 

 which the contraction wave had to pass ; according to the 

 amount of reduction and damage to the tissue at the bridge, 



