2 1 6 THE CONTRA CTION OF CA RDIA C MUSCLE. 



the vagus upon the force of the ventricular contractions of the avian heart ; it 

 would undoubtedly be a very interesting subject of research. So also, in the 

 case of Elasmobranchii, I cannot find anything definite ; it does not appear 

 to be known whether they behave like Amphibia or Teleosts. It would 

 also be interesting to investigate the lung-breathing fishes, Protopterus, etc., 

 which I should expect to behave like Amphibia. 



It is perfectly conceivable, as Roy l has pointed out, that the strength of 

 the auricular contractions is the most important factor in the beat of the heart, 

 and that, therefore, it is not so necessary that the force of the ventricular 

 contractions should be regulated to the same extent as the auricular ; but still 

 it is difficult to believe that in the evolution of the heart one part of its 

 muscular structure was supplied with nerves, and not the other ; we must, it 

 seems to me, imagine that in the protovertebrate, the whole of the muscular 

 tissue of the longitudinal venous sinuses was supplied with the inhibitory 

 fibres, and that, as the different groups evolved themselves with the formation 

 of auricles and ventricles, the inhibitory fibres still supplied all parts of the 

 heart. It is possible that the difference in the behaviour of the ventricles in 

 the two groups is due to a difference in the evolution of the striated muscular 

 tissue of the ventricle, and that it still remains to be found out what change 

 the inhibitory fibres do produce in the ventricular muscle of such an animal as 

 the tortoise. It has certainly seemed to me difficult to believe in the absence 

 of any diminution of tone in the muscular tissue of the ventricle upon stimu- 

 lation of the vagus, when one sees the enormous increase in the volume of the 

 whole heart when the nerve is stimulated. Still, on the other hand, it is not 

 easy to understand how a nerve can produce a muscular relaxation without any 

 accompanying diminution in the force of the contractions. AVe must wait for 

 further investigations of the action of the cardiac nerves on the tortoise 

 ventricle before it is possible to decide this question. 



2. The action of the augmentor nerves. — The primary action of 

 the augmentor nerves is in every respect exactly the opposite of that 

 of the inhibitory. The evidence both in cold-blooded and warm-blooded 

 animals shows that the rate of rhythm is increased, that the force of the 

 contractions is increased, and that the conductivity is increased. Here, 

 just as in the case of the inhibitory nerve, it is difficult to make any 

 positive statement as to any increase in tonic contraction, apart, of 

 course, from the diminished relaxation that of necessity accompanies an 

 increased rate of contraction in a normally beating heart. Of these 

 different phenomena I have already illustrated the alteration of the rate 

 and force of the contractions in the case of the toad or frog, and it may 

 be noticed that the augmentor nerve takes a very much longer time than 

 the inhibitory nerve to reach its maximum effect, whether that effect 

 consists of an alteration of rate or of contraction force ; it is partly 

 for this reason that, in the conjoint stimulation of the two nerves, the 

 action of the inhibitory nerve is always most apparent at first, and that 



ventricle in the mammalian heart, though to a less extent than those of the auricle. 

 When a small dose of atropine is given, then it frequently occurs that the contractions of 

 the ventricle are slightly larger during stimulation of the nerve, provided that such 

 stimulation has caused some slowing of rate. He finds, therefore, that the mammalian 

 ventricle hehaves like the amphibian, but the atropinised ventricle behaves like the 

 chehiniiin. In the case of the pigeon he states expressly, and illustrates his statement by 

 tracings (Fig 1, Taf. II.. and Figs. 1 and 8, Taf. IV.), that the ventricle of its heart reacts 

 normally without atropine to vagus stimulation in the same manner as the atropinised 

 mammalian ventricle, i.e. its contractions are not diminished but increased in size if the 

 rate is dintim'shed : that, in fact, the avian ventricle behaves to vagus stimulation like the 

 chelonian and reptilian and m>t like the amphibian ventricle. 

 1 Joiirn. Physiol., Cambridge and London, 1878, vol. i. p. 452. 



