CH. XXI.] CARDIAC RHYTHM AND CONDUCTION 255 



neurogenic. There are still a few physiologists who maintain the 

 older neurogenic theory, but these are mainly those whose chief work 

 has been performed on the hearts of invertebrate animals, and it is 

 quite possible that the mechanism there is a different one. But so 

 far as the vertebrate heart is concerned, the myogenic theory is now 

 held, because (1) the foetal heart manifests rhythm long before any 

 nerves reach it ; (2) the apex of the ventricle of such animals as frogs 

 and tortoises can be made to beat rhythmically by perfusing it 

 with suitable fluids at high pressure ; and this part of the heart has 

 few nerves and no ganglion cells ; and (3) the rate of conduction of 

 the peristaltic wave is slow, and corresponds to the rate of muscular 

 rather than of nervous conduction. 



The older observers, who first made the striking observation that 

 a heart will continue to beat after its removal from the body for a 

 considerable period, and who at the same time were imbued with 

 the neurogenic theory, naturally placed the seat of rhythm in the 

 intracardiac ganglia. They were not at the time aware of the general 

 arrangements of autonomic nerves, and therefore did not recognise 

 that the ganglia were terminal cell-stations on the course of the 

 nerves which reach the heart via vagus and sympathetic. 



The intracardiac nerves have been chiefly studied in the frog ; in 

 this animal the two vago-sympathetic nerves terminate in various 

 groups of ganglion-cells ; of these the most important are Remalts 

 ganglion, situated at the junction of the sinus with the right auricle; 

 and Bidder's ganglion, at the junction of the auricles and ventricle. 

 A third collection of ganglion cells (von Bezold's ganglion) is situated 

 in the interauricular septum. From the ganglion - cells, post- 

 ganglionic fibres spread out over the walls of the sinus, auricles, and 

 upper part of the ventricle. Kemak's ganglion used to be called the 

 local inhibitory centre of the heart ; it is really the chief cell-station 

 of the inhibitory fibres, and stimulation of the heart at the sino- 

 auricular junction is the most certain way of obtaining stoppage of 

 the heart. Bidder's ganglion was called the local accelerator centre 

 for a corresponding reason. 



The intracardiac ganglia have been examined in a few other cold- 

 blooded animals (for instance the tortoise), but any precise knowledge 

 of their arrangement and position in the mammalian and human 

 heart is unfortunately lacking. 



Conduction in the Heart. As already stated, the slow rate of 

 propagation of the wave points to the link being muscular rather 

 than nervous, and histology lends support to this view, the muscular 

 fibres being connected to each other by intercellular bridges of proto- 

 plasm (see p. 71). An experimental proof of the same view is the 

 following: if a strip of the heart wall is taken and a number of 

 cuts going nearly completely across it, be made first from one side, 



