THE HEART-BEAT IN ITS PHYSIOLOGICAL RELATIONS 143 



the node itself, and the sino-auricular node is by some authors 

 denominated the pace-maker of the heart, the tissue which sets the 

 pace for the rest of the organ and gives the time to auricles and 

 ventricles alike. The experimental results, however, are by no 

 means harmonious, some observers finding that destruction of the 

 region of the node causes no change in the rate of the heart-beat, 

 others that the beat is permanently slowed. But even were we in 

 a position to sharply delimit a given region of the heart as the point 

 at which the strong tendency to contraction inherent in the cardiac 

 tissue as a whole first breaks into an actual beat, this would scarcely 

 enable us to decide offhand where the cause of the automatism 

 resides, in the muscular tissue or in the intrinsic nervous apparatus, 

 because in nearly all animals hitherto investigated the muscular 

 tissue, ganglion-cells, and nerve-fibres are inseparably intermingled. 

 In Limulus, however, the horseshoe or king crab, the cardiac 

 ganglion-cells are collected in a nerve-cord running longitudinally in 

 the median line along the dorsal surface of the segmented heart, and 



Fig. 63. The Heart and the Heart Nerves of Limulus: Dorsal View (Carlson). (The 

 heart is figured one-half the natural size of a large specimen.) aa, Anterior 

 artery; la, lateral arteries; In, lateral nerves; mnc, median nerve-cord; os, ostia. 



sending off at intervals branches to two lateral cords, and also 

 branches which enter the heart muscle directly (Fig. 63). When 

 the median nerve-cord is removed, as can be done without injuring 

 the muscle, the heart ceases for ever to beat spontaneously. It 

 still contracts when directly stimulated, mechanically or electrically, 

 but the contraction never outlasts the stimulation. The automatic 

 power therefore resides in the nerve-cord alone, and not in the 

 muscle. The same is true of the rhythmical power, for excitation 

 of the nerves that pass from the median cord to the muscle produces, 

 ' not a rhythmical series of beats in the resting, and an acceleration 

 of the rhythm in the pulsating heart, but a tetanus closely resembling 

 that produced in skeletal muscle on stimulation of a motor nerve ' 

 (Carlson). Conduction and co-ordination are also effected in this 

 heart through the nervous mechanism, and essentially through the 

 median nerve-cord; for section of the longitudinal nerves in any 

 segment of the heart abolishes the co-ordination of the two ends of 

 the heart on either side of the lesion, while division of the muscle 

 in any segment does not affect the co-ordination. It is not per- 

 missible to transfer these results wholesale to higher hearts, and 



