THE CIRCULATION 239 



the same differentiation into distinct groups. Nevertheless 

 they are abundant round the mouths of the great veins, 

 round the edges of the inter- auricular septum, and round 

 the auriculo-ventricular groove. 



While there is no evidence that the nervous structures 

 play an important part in starting or keeping up the con- 

 tractions, there is evidence that they exercise a checking or 

 controlling action. 



If the region between the sinus and auricles in the frog's 

 heart is stimulated by the interrupted current from an 

 induction coil, the heart is slowed or stopped. (Practical 

 Physiology, Chap. IX.) 



If atropine is first applied electric stimulation is without 

 result. (Practical Physiology, Chap. IX.) 



These experiments seem to indicate that there is in the 

 heart a checking mechanism which may be stimulated by 

 electricity, and which is paralysed by atropine. 



4:th. Connections of the Heart with the Central Nervous 

 System. In the frog a branch from the vagus connects the 

 central nervous system with the heart. When the branch is 

 cut no effect is produced, showing that it is not constantly 

 in action ; but when the lower end is stimulated, the heart is 

 generally slowed or brought to a standstill. Sometimes the 

 effect is not marked. The reason for this is that the cardiac 

 branch of the vagus in the frog is really a double nerve 

 derived in part from the spinal accessory and in part from 

 fibres Avhich reach the vagus from the superior thoracic sym- 

 pathetic ganglion. If the spinal accessory is stimulated, the 

 heart is always slowed ; and if the sympathetic fibres are 

 stimulated, it is quickened. Generally stimulation of the 

 cardiac branch containing these two sets of fibres simply 

 gives the result of stimulating the former, but sometimes 

 the stimulation of the latter masks this effect. (Practical 

 Physiology, Chap. IX.) 



In the mammal three sets of nerve fibres pass to the 

 heart : 



1st. Superior cardiac branch of the vagus starts from near 

 the origin of the superior laryngeal nerve, and passes to the 

 heart to end in the endocardium (Fig. 118, S.C.). 



