EXTRINSIC CARDIAC NERVES. 281 



separated from the animal. 3. When cut off from the sinus the 

 beat of the rest of the heart becomes weak, uncertain, and 

 changes its rhythm. 4. When the sinus venosus is suddenly 

 separated by ligature from the auricles and ventricle, both the 

 latter cease to beat, while the motions of the sinus continue. 

 However, if a slight stimulus, such as the touch of a needle, be 

 now applied to the auriculo-ventricular margin, it suffices to give 

 rise to a series of rhythmical contractions. Or if the ventricle 

 be now separated from the auricles by incision, the former com- 

 mences to beat rhythmically, while the auricles commonly remain 

 motionless. These latter observations (which are known as the 

 experiments of Stannius) have been explained in various ways, 

 the most probable of which seems to be the following. When 

 cut off from the influence of the sinus veuosus, the heart fails to 

 contract spontaneously, because it has lost the initiatory stimulus 

 which habitually arrived from the sinus. When the ventricle is 

 cut away from the auricles, the act of incision is sufficient stimulus 

 to set going its rhythmical contractions. 



Although we cannot adequately explain the relationships borne 

 by the different sets of ganglia in the frog's heart to one another, 

 there seems no doubt that the following conclusions may be ac- 

 cepted as proved, and are, moreover, in all probability, applicable 

 to the hearts of mammals: That nerve centres exist in the muscle 

 tissue of the heart, some of which are capable of originating 

 stimuli for this rhythmically contracting muscle. That there 

 exist other ganglionic groups which help to regulate and distribute 

 the stimuli in a sequence throughout the several cavities. 



EXTRINSIC CARDIAC NERVES. 



The intrinsic nerve mechanism of the heart just described is 

 under the immediate control of the great nervous centres through 

 the medium of fibres passing along the vagus and sympathetic 

 nerves from the medulla oblongata. 



Some of these fibres check the action of the intrinsic ganglia, 

 and cause the heart to beat more slowly ; hence they are called 

 inhibitory. Others cause it to beat more quickly, and are called 

 acceleratory. 

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