1064 



PHYSIOLOGY 



the block becoming complete. On moistening the bridge again every 

 contraction may be seen to pass. 



By the methylene-blue method it is possible to demonstrate a close 

 network of non-medullated fibres surrounding all the muscle-cells of 

 the heart. It is obvious that the experiment just quoted would not 

 exclude the possibility of propagation occurring through such a nerve 



network. The properties of the network would 

 have to differ from those of any of the nerve - 

 tissues with which we are acquainted ; whereas 

 we know that under certain circumstances im- 

 pulses may be transmitted from fibre to fibre 

 even in striated muscle, and such a mode of pro- 

 pagation is the most obvious explanation of the 

 phenomena observed in the heart. 



If the auricles be soaked for some time in 

 distilled water they enter into a condition of 

 what is known as water-rigor (Wasserstarre). In 

 this condition they are incapable of contracting, 

 but can still propagate the wave from sinus to 

 ventricle. This experiment has been regarded 

 as a demonstration of the part taken by nerve 

 fibres in the propagation of the wave, but such an explanation is not 

 necessary, since a similar condition of water-rigor in a voluntary 

 muscle fibre has been shown to allow the passage of an excitatory 



PIG. 428. Contraction 

 of auricles and ventri- 

 cles of tortoise heart. 

 The auricula-ventricu- 

 lar groove has been 

 clamped so as to pro- 

 duce a partial block 

 allowing only every 

 second contraction to 

 pass. (GASKELL.) 



la 



FIG. 429. Heart of Limulus from dorsal surface. (CARLSON.) 

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



wave through the affected part to the normal portion of the muscle, 

 which then responds by a contraction. 



A series of interesting researches by Carlson on the mechanism of the heart- 

 beat in the king-crab Limulus have been thought to throw light on the vexed 

 question of the automatism of the vertebrate heart. 



In Limulus the heart forms a segmented tube of ordinary striated muscular 

 fibres. In large specimens the tube may be from 10 to 15 cm. long and 2 to 

 2| cm. broad. Like the hearts of most other invertebrates and of all vertebrates, 

 it has a local system of ganglion-cells, but so sittiated 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. 429. 



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 



