THE CAUSATION OF THE HEART BEAT 



989 



Like the hearts of most other invertebrates and of all vertebrates, it has a local system 

 of ganglion cells, but so situated that they can be cut away entirely from the muscular 

 portions of the organ. The arrangement of the cardiac nervous system in Limulus is 

 shown in Fig. 436. 



The ganglion cells are collected chiefly in a dorsal nerve ganglion cord which runs 

 almost the whole length of the heart. From this cord non-medullated nerve fibres pass 

 directly into the substance of the heart, and also send branches to 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 contraction 

 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. 



mnc In la 



FIG. 436. Heart of Limulus from dorsal surface. (CARLSON.) 

 mnc, median nerve cord; In, lateral nerve trunks. 



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 contractions. 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 con- 

 tract, showing that it was being excited by impulses arising in the posterior part of the 



FIG. 437. * Nerve-muscle preparation ' of heart of Limulus consisting of the muscle 

 of the two anterior segments, with the two lateral nerves. (CARLSON.) 



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. 437). 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 originated 

 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 con- 

 firming 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 com- 

 plete tetanus. The question naturally arises how far the heart of Liniulus is to be 



