312 PHYSIOLOGY CHAP. 



shown that the conduction of excitability and co-ordination of the 

 cardiac movements of Limulus depend on the median ganglion 

 cord. When this is removed the beats cease entirely, while the 

 intact heart, when exposed, may continue to beat regularly for 

 many hours. On dividing the cord at any point, the beats become 

 a-synchronous in the several segments, and continue so for an 

 indefinite time. When, on the other hand, the muscle is divided 

 transversely at any point along the heart, while the ganglion cord 

 is spared, there is no longer any appreciable disturbance of co- 

 ordination in the pulsations. If a single node of the ganglion 

 cord is extirpated, the heart ceases to beat in the corresponding 

 muscular segment, while it continues to beat in the rest. 



The ganglion cord of Limulus is the centre not only for the 

 automatic activity of the heart, but for its reflex activity as well. 

 It receives the moderator and accelerator nerves, which modify the 

 cardiac beats acting not directly on the muscle cells, but reflexly or 

 through the ganglion. When this has been excised, and the beats 

 of the heart have ceased, rhythmical activity is not restored on 

 exciting the lateral nerves; tetanic contractions are, however, 

 obtained. 



Lastly, certain cardiac poisons (atropine, nicotine) act on the 

 Limulus heart as on that of vertebrates, paralysing the activity of 

 the inhibitory nerve fibres which reach it from without. 



There can be no doubt as to the accuracy of Carlson's experi- 

 ments, or the theoretical conclusions which result from them. As 

 the supporters of the myogenic theory emphasise particularly the 

 automatic rhythmical activity of the embryonic heart and that of 

 certain invertebrates in which nervous elements are wanting ; so 

 the supporters of the neurogenic theory may invoke the Limulus 

 heart as direct evidence and proof that its automatic and renex 

 excitability, the conduction and the co-ordination of excitations, 

 depend exclusively on the ganglion cells which it contains. 



Without venturing on any general conclusion, and assuming 

 that the results obtained from Limulus are applicable to the heart 

 of vertebrates also, it is only fair to admit unhesitatingly that the 

 neurogenic theory seems more probable since Carlson's discovery 

 than the myogenic. Its definite and unconditional acceptance for 

 the heart of every animal could only be possible if, with better 

 methods of research, it were discovered that both the embryonic 

 heart of mammals, and the heart of all invertebrates, possess 

 ganglionic elements that have so far evaded detection. 



VI. The elements which constitute the vertebrate myocar- 

 dium are not perfectly comparable with those of either striated 

 or smooth muscle. They are neither composed of fibres, nor of 

 fusiform fibro-cells ; but consist of nucleated cells of prismatic 

 form, which usually bifurcate into two broad short processes, and 

 exhibit a rather dark striation. Each cell is joined at its 



