260 THE HUMAN BODY. 



this case the distension of the muscle or some chemical con- 

 stituent of the liquid acts as a stimulus; but in no other 

 muscle do we find blood-supply or mere stretching act as a 

 stimulus, and if they are to be assumed as so acting in this 

 case their action is uniform, while the resulting contractions 

 are interrupted and rhythmic: moreover, they are co-ordi- 

 nated; they are not irregular twitches first of one bundle of 

 the myocardiac fibres and then of another, but duly combined, 

 so as by their mutual action to empty the cavity they surround. 

 The evidence thus obtained as to the possession of some auto- 

 matic and some co-ordinative properties by the frog's cardiac 

 muscle is strengthened by experiments on the hearts of tor- 

 toises and terrapins. In those animals the apical portions of 

 the ventricle are devoid of nerve-cells, yet narrow strips of 

 them hung up and slightly loaded will usually begin to beat 

 after a time. If they do not, all that is necessary is to stimu- 

 late them rhythmically for a short time; then on ceasing the 

 stimulation the rhythmic contractions continue. Here, no 

 doubt, the loading is a favoring condition, but so it is for the 

 activity of ordinary muscles, on which, nevertheless, it does 

 not act as a stimulus. 



The conclusion to which we are led is that the muscle- 

 cells of the frog's -heart have retained to some extent those 

 automatic and co-ordinating faculties of undifferentiated 

 protoplasm which the more highly evolved fibre of skeletal 

 muscle -has lost. We find in the presence of certain of the 

 nerve- cells of the heart a highly favorable condition for the 

 exhibition of those powers: the nerve-elements perhaps influ- 

 ence the nutrition, perhaps in some other mode affect the 

 molecular structure of the muscle-cells connected with them 

 so as to favor spontaneous contraction, but, like stretching 

 the isolated strip of ventricle, they merely bring about a state 

 of things promoting the exercise of powers inherent in the 

 cardiac muscle tissue itself. 



The evidence as to the automaticity of the muscle of the 

 mammalian heart is not quite as full as in the case of the frog. 

 In it also there are collections of ganglion-cells where the 

 great veins join the auricles and near the base of the ventri- 

 cles; but there are others in the apical region of the ventricles, 

 so it is not possible to examine an isolated apex free from 

 ganglion-cells as it is in the frog. The musculature of the 

 auricles is prolonged for some little way on the ends of the 



