THE CAUSATION OF THE HEART-BEAT 1065 



two lateral nerve-trunks, by which fibres are distributed to all parts of the 

 heart. 



The heart normally contracts about forty times per minute. Each contrac- 

 tion affects all parts practically simultaneously, though in the dying heart the 

 posterior portions apparently contract slightly before the anterior, and may 

 continue to contract after the anterior end has come to a standstill. 



Division of the muscular tissue leaving the nerve-strands intact does not 

 alter in any way the synchronism of contraction of the two ends of the heart. 

 Division of the nervous cord into two parts, the section being carried between 

 the posterior third and anterior two-thirds, causes complete lack of co-ordination 

 between the two ends ; both ends of the heart continue to contract, but at 

 different rhythms. Extirpation of the nerve-cord abolishes spontaneous contrac- 

 tions. If the anterior half of the dorsal ganglionic cord be excised, all parts of the 

 heart will continue to contract in unison. If now the lateral nerve-trunks be 

 divided, the anterior half of the heart ceases to contract, showing that it was 



FIG. 430. 'Nerve-muscle preparation' of heart of Limulus consisting of 

 the muscle of the two anterior segments, with the two lateral nerves. 

 (CARLSON.) 



being excited by impulses arising in the posterior part of the ganglionic cord. 

 It is possible therefore to make a nerve-muscle preparation of the anterior 

 part of the heart, consisting of the muscle of the first two segments with a longer 

 stretch of the lateral nerves (Fig, 430). Stimulation of the lateral nerves with 

 a single shock causes a single beat of the anterior segments ; tetanising shocks 

 cause a continued contraction of the muscle preparation. 



There seems to be no doubt that in this animal the beat of the heart is origi- 

 nated and co-ordinated by the action of the local ganglionic centres. Moreover 

 Carlson has shown that the inhibitory nerve to the heart acts, not by direct 

 influence on the muscle fibres, but by an inhibition of the automatic activity 

 of the ganglionic cells,, thus confirming for this special case the general view 

 of inhibition long ago put forward by Morat, but not now generally accepted. 



The heart muscle does not show a refractory period, but on direct stimulation 

 with repeated shocks there may be a summation of contractions, which may 

 fuse to a complete tetanus. The question naturally arises how far the heart of 

 Limulus is to be regarded as a special case, or how far we may transfer results 

 gained from experience on this heart to those of other hearts in which a perfect 

 separation between ganglion-cells and muscle fibres is not so easily attainable. 

 Carlson has sought to show the applicability of his results to the explanation 

 of the cardiac mechanism in vertebrates by a series of observations on other 

 invertebrates' hearts, where the muscular and nervous tissues are not so easily 

 dissociable. Such hearts present phenomena very analogous to those of the 

 frog's heart. According to him the phenomenon of the refractory period, the ' all 

 or none ' law of contraction, and the absence of tetanus in the heart of the frog is 

 due, not to the peculiar functions of the muscle fibres, but to the fact that in all 

 our experiments we are affecting muscular and nervous tissues simultaneously. 



