THE CAUSATION OF THE HEART-BEAT 949 



leads the excitatory process to many different parts of the ventricle 

 almost simultaneously. 



There is no doubt that the ventricular systole is comparable with a 

 simple muscular twitch and cannot be regarded as the summation of several 

 contractions. Since the excitatory process extends in the form of a wave 

 not only to all parts of the same cavity but to all parts of the heart, it is 

 evident that the musculature of the heart is to be compared, not with skeletal 

 muscle composed of many fibres, but to a single muscle fibre in which all 

 parts are in functional continuity. 



THE BEAT OF THE MAMMALIAN HEART 



The mammalian heart, like the heart of cold-blooded animals, will beat 

 for some time after it has been cut out of the body, and a perfectly rhythmic 

 activity may be maintained for hours by feeding the heart from the coronary 

 arteries either with defibrinated blood or with oxygenated Ringer's solution, 

 with or without the addition of glucose. 



Ganglion-cells are found in the mammalian heart around the openings of the great 

 veins, along the border of the interauricular septum, in the groove between auricles 

 and ventricles, and in the basal parts of the ventricles. 



The ventricles of mammals are endowed with a greater rhythmic power 

 than the corresponding cavities in the frog and tortoise. It is possible to 

 sever or crush all the nervous and muscular connections between auricles and 

 ventricles without destroying their mechanical connection by means of fibrous 

 tissue. Such a procedure does not, even for a moment, stop the contractions 

 of the ventricles, which go on beating at a rhythm which is independent of and 

 slower than that of the auricles. Porter has shown that a mere fragment 

 of the ventricular wall, perfectly free from ganglion-cells, may maintain 

 rhythmic contractions for some hours if fed by an artificial circulation through 

 a branch of the coronary artery. We may therefore conclude that in the 

 mammalian, as in the amphibian, heart the cause of the rhythm is to be 

 sought in the properties of the muscle fibres themselves, and that every part 

 of the heart-muscle possesses the power of rhythmic activity, the normal 

 sequence of the beats being determined by the greater frequency of the 

 natural rhythm of the venous end of the heart. 



In the mammalian, as in the amphibian, heart the excitatory condition 

 started at one point in the muscle spreads through the muscle in all directions, 

 and the process of conduction of excitation seems to be independent of nerve 

 fibres. The excitatory process may be conducted not only in the ordinary 

 direction from auricles to ventricles, but also from ventricles to auricles. 

 If the ventricles be excited at a rhythm of higher frequency than the natural 

 beat which is starting at the venous end of the heart, we may obtain a 

 reversed rhythm in which the order of the beats is ventricles auricles. 

 It is difficult to conceive of an arrangement of neurons which would propa- 

 gate impulses impartially from auricles to ventricles or from ventricles to 

 auricles. Such a condition would seem to be in contradiction to the law 

 of forward direction which obtains throughout the nervous system. On the 



