54 THE HEART 



possibilities. The excitatory wave, for instance, may conceivably 

 arise in nerve and be conducted by nerve. This is the neurogenic 

 theory. Or the excitatory wave may arise in the heart muscle 

 and be conducted by muscle. This is the myogenic theory. These 

 certainly seem more probable than either (1) that the excitatory 

 wave may arise in nerve and be conducted by muscle ; or (2) 

 that it may arise in muscle and be conducted by nerve. Even 

 here the possibilities do not cease, for the co-ordination of the 

 movements of the different chambers of the heart is essentially 

 a complicated yet all-important process, so that it may well 

 be that, arise the excitatory wave how it may, both muscle and 

 nerve may be called into play as conducting agents in order that 

 the proper sequence of contraction of the different chambers of the 

 heart may be assured. 



Taking the first possibility, that the excitatory wave may arise 

 in nervous tissue and be conducted by that tissue, let us consider 

 the evidence for and against it. This view gained sway in the 

 middle of the last century chiefly perhaps because it offered a 

 satisfactory explanation of the great discovery by the brothers 

 Weber of the action of the vagus nerve upon the heart. At that 

 time such an inhibitory influence could only be explained on the 

 analogy of nervous influence of centres situated in the central 

 nervous system, such as the respiratory centre of the medulla and 

 the motor centres of the spinal cord. The ganglion cells, there- 

 fore, were thought to be the central apparatus of the heart, 

 sending out a continuous stimulus to the heart muscle. Different 

 afferent impulses to this central apparatus modified the stimulus 

 in different ways. The very plausibility of the view immediately 

 accorded it a place as a theory without any very substantial 

 evidence. Its chief support is the experiment of Stannius, now 

 known as the Stannius ligature experiment. If in the frog's heart a 

 ligature be placed around the sino-auricular groove and tied tightly, 

 the auricle and ventricle of the heart are reduced to a standstill ; 

 in consequence, it is asserted, of the cutting off of the influence 

 of Remak's ganglia in this neighbourhood. The sinus, however, 

 continues to beat. If now a ligature be placed round the 

 A-V groove and tied, the ventricle again starts to beat, owing, 

 it is said, to the stimulation of Bidder's ganglia in this region. 

 Bidder's ganglia are therefore looked upon as a subsidiary 

 centre normally under the control of Remak's at the sinus. In 



