2 18 THE CONTRACTION OF CARDIAC MUSCLE. 



muscarine prevented any such action of the nerve, and am therefore 

 inclined to believe that in this case also the motor effect cannot be 

 produced, unless some portion of the heart (in this case perhaps the 

 large veins) is still beating. 



Is it then possible to demonstrate any action of the augmentor nerve upon 

 the quiescent muscular tissue of the heart? The evidence appears clearly 

 to show that the augmentor fibres in the frog and toad supply the muscular 

 fibres of all parts of the heart sinus, auricles, and ventricle and cause such 

 changes in that tissue that the rhythmical tissue of the sinus beats more quickly 

 and more strongly, and the contractions of the auricle and ventricle increase in 

 force, while yet they are unable to produce any visible effect in any part of the 

 heart which is quiescent. Can we by any means isolate the ventricle, and yet 

 leave the augmentor nerves to the ventricle intact, so as to test their action on 

 the quiescent ventricle ? This can be done either by causing a block between 

 sinus and auricles, by means of the application of muscarine to the sinus, or 

 by clamping between auricles and ventricle. In both cases the ventricle remains 

 still, and in both cases stimulation of the augmentor nerve is able to force the 

 block, and thus produce a series of ventricular contractions. These contractions 

 are increased in force, showing that the augmentor fibres to the ventricle are 

 intact. Increase the muscarine very carefully, or slightly tighten the clamp, 

 and it is then seen that the block cannot be forced, and so the ventricle 

 still remains quiescent when the nerve is stimulated, and no visible effect 

 is produced. However, upon testing this quiescent muscle by means of 

 the galvanometer in the same way as already described in the case of the vagus 

 nerve, a distinct variation in the muscle current was seen upon each stimula- 

 tion of the nerve, of the opposite character to that seen upon vagus stimulation 

 in the case of the tortoise auricle. 1 Give still more muscarine, or clamp still 

 tighter, and then stimulation of the nerve is absolutely without effect, showing 

 that the previous effect was due to a true nervous action. 



This experiment shows that the augmentor nerve causes an electrical 

 change in the muscular tissue, of the opposite kind to that caused by the 

 inhibitory nerve, and it is worthy of note that the process to which this 

 increase of negativity is due is like the corresponding positive change, a 

 slow slight one, very different indeed from the rapid excessive process 

 which initiates a contraction. The contrast between this slow motor 

 effect of the augmentor nerve and the rapid movement of the galvano- 

 meter needle in the same direction when a contraction takes place, 

 is most striking. The nature of the electrical change in the case of both 

 nerves seems to me to point to the conclusion that the change which 

 occurs in the quiescent muscle, upon stimulation of the inhibitory nerve, 

 is of the nature of a relaxation or diminution of tone, while the change 

 which occurs upon stimulation of the augmentor nerve is of the nature 

 of an increase of tone. In neither case have I succeeded in actually 

 measuring any difference in length of the muscle, coincidently with 

 the electrical change, so that the alteration of tone must be slight. 



It seems to me that an alteration of tone in the two directions, due 

 to an activity emanating from the ganglion cells of the two sets of 

 nerves, is in accordance with the known facts of the function of the 

 efferent vagrant ganglia, and the nerves emanating from them, in other 

 cases, e.g. the vasomotor and viscero-motor ganglia and nerves. 



The augmentor nerves, like the inhibitory, appear to affect the different 

 parts of the heart to a different degree in different animals, and apparently 

 1 Journ. Physiol., Cambridge and London, 1887, vol. viii. p. 404, PI. XIII. 



