THE MEANING OF THE HEART-BEAT. 175 



groups of ganglion cells along the line where this junction- wall passes into the 

 auriculo-ventricular ring along, that is, the side of the ring furthest removed 

 from the aorta, which may be called the basal part of the ring. Thence a 

 plexus of nerve fibres, also containing ganglia, though much more sparingly 

 than the basal portion, passes round the auriculo-ventricular ring. From this 

 ganglionic nervous ring, fibres ramify over both the ventricle and the auricles, 

 in each case accompanied by ganglion cells for a short distance. 



We see, then, that the ganglion cells in the sinus and sino-auricular junction 

 correspond to Eemak's ganglia in the frog. The cells in the junction-wall 

 between the two auricles correspond to those on the inter-auricular nerves in 

 the septum between the auricles of the frog, and the cells of the auriculo- 

 ventricular ring correspond to Bidder's ganglia in the frog. 



In the tortoise, as in the frog, the Stannius experiment succeeds ; we 

 see here, however, very plainly, what had been already observed by 

 Stannius l in the frog, that the standstill after removal of the sinus is 

 not permanent ; the auricle and ventricle commence after a time to beat 

 again, at first slowly, then gradually quicker and quicker, until at last a 

 good steady rate of rhythm is observed. This rhythm of development is' 

 characteristic. If the cut is anywhere between sinus and ventricle, the 

 same thing may be observed ; the difference being that the primary stand- 

 still is longer the nearer to the ventricle the cut is made ; and the rate of 

 rhythm finally obtained is liable to be less the nearer to the ventricle. 

 In all cases the first effect of the section is to cause one or more contrac- 

 tions followed by standstill. 



When the cut is made close against the ventricle in the auriculo- 

 ventricular groove, i.e. corresponds to the second Stannius ligature, the 

 ventricle beats rapidly for a time, gradually slowing more and more, 

 until at last it stands still ; then after a time it begins to beat again, at 

 first very slowly, steadily gaining in rate until it has reached a steady 

 uniformity. 



We see, in fact, that the ligature or section in the auriculo-ventricular 

 groove produces two different rhythmical phenomena in the ventricle ; 

 on the one hand, a series of rhythmical contractions take place 

 immediately upon the section or ligature, which are characterised by the 

 peculiarity that the rate of the contractions is quickest at the commence- 

 ment, and then gradually becomes slower and slower, until a standstill 

 occurs ; this may be called a rhythm of excitation. On the other hand, 

 in contradistinction to this rhythm of excitation, a subsequent more 

 permanent series of rhythmical contractions, which may be called the 

 rhythm of development, occurs under favourable conditions in the 

 ventricle, after the rhythm of excitation is over. 



This rhythm of development, like the similar rhythm in the auricle 

 after the first Stannius ligature, or section, is characterised by its slow- 

 rate at the commencement, and its gradual development to a steady 

 rate, the rapidity of which depends upon the rhythmical modulus of 

 the tissue and the conditions of nutrition, temperature, etc. If, then, 

 we can remove this rhythm of excitation, the effect of the ligature at 

 the auriculo-ventricular groove would be no different from that of the 

 ligature at the junction of sinus and auricles, or across any part of 

 the auricles. This can be done in the frog by means of a clamp with 

 a fine screw adjustment, for it is then possible, by very slowly tightening 

 the clamp in the auriculo-ventricular groove, to bring the ventricle to a 

 1 Stannius, Arch. f. Anat., Physiol. u. wissensch. Med., 1852, S. 81. 



