THE PHYSIOLOGY OF THE HEART 135 



excitatory wave of the heart arises in nervous tissue within the heart, 

 and is conducted by that tissue. Those holding the nr^ojjejiic doctrine 

 state that the wave has its origin in heart muscle itself, and is pro- 

 pagalecLby muscle. In the end, it seems likely to be shown that 

 the pacemaker of the heart is a kind of tissue half nerve, half 

 jnuscle. 



Of the two views, the neurogenic is the older. As the heart beats 

 in the body after all the nerves passing to the heart are cut, or outside 

 the body when properly fed, it was clear to the older observers that 

 the nervous centre originating the heart impulse could not be in the 

 brain, and therefore it was supposed to be in the ganglion cells of the 

 heart. The fact that a ligature tied tightly round the sinu-auricular 

 groove brings the auricles and ventricles of the frog's heart to a stand- 

 still was thought to indicate that the chief group of ganglion cells 

 concerned in the origin of the excitatory wave was situated in that 

 region. A second ligature applied to the A.-V. groove (the second 

 Stannius' ligature) causes the ventricle to beat again, and the neuro- 

 genist ascribed this to the fact that the ganglia in this region 

 are stimulated by such a ligature. Why one ligature should destroy 

 the action of ganglion cells and a second similar ligature excite their 

 action is not apparent; but it is said that, if the A.-V. ganglia be ex- 

 tirpated, this second ligature is ineffective in starting the ventricle. 

 Other evidence cited as supporting the neurogenic theory is the ex- 

 periment of thrusting a needle into the inter ventricular septum of 

 the mammalian heart. It is said that, if the needle be inserted on 

 the left side of the lower end of the upper third of the septum, it 

 produces, instead of a beat of the ventricles, a condition known as 

 " fibrillation." 



Recent experiments upon the heart of the horseshos crab, L : mulus, 

 afford clear evidence of the neurogenic origin of the excitatory wave 

 in this invertebrate heart. The heart consists of a tube 10 to 15 

 centimetres long, divided into segments by the successive origin of 

 the arteries. When the heart beats, all the segments appear to 

 contract simultaneously, although probably a rapid wave of con- 

 traction passes. There are three nervous strands one median and 

 two lateral which run along the outer surface of the heart and 

 anastomose freely. The median strand contains ganglion cells, and 

 one especially large ganglion. It is easy to separate this strand from 

 the heart without injury to the muscle. Its entire removal causes 

 cessation of the whole heart-beat, while removal of a portion causes 

 stoppage in the corresponding segment of the heart. 



As regards the nervous conduction of the excitatory wave, the 

 chief points advocated in its favour are these: 



1. It has been asserted that a different rhythm in auricles and 

 ventricles (allorhythmia) can be set up by cutting a nerve running 

 from auricle to ventricle. This is unconfirmed. 



2. For a long time no muscular connection was known to exist 

 between auricles and ventricles in the mammalian heart. This piece 

 of evidence is negatived by the discovery of the A.-V. bundle, but 



