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spinal cord, or of these two parts of the nervous centres together. 

 These and other phenomena of a similar kind are due to excitement 

 or shock, and their effects are often more or less transitory. The 

 sympathetic nervous centres, and the cervical parts of the spinal cord 

 with which they are connected, also influence the movements of the 

 heart, as is shown by experiments, and by the effects of blows on the 

 abdomen, or of other injury or disease (p. 307). But if the injury, 

 either to the cerebro-spinal or sympathetic system, be gradually in- 

 flicted, the heart's movements will continue, even although the brain 

 and spinal cord be removed, particularly if artificial respiration be per- 

 formed. From these facts, and especially from the circumstance that 

 the rhythmic movements continue after the removal of the heart, it is 

 evident that the regulating agent of these movements is not in the 

 great nervous centres, but somewhere in or upon the heart itself. It 

 is now admitted, indeed, that the numerous sympathetic ganglia con- 

 nected with the nerves upon the heart, are the sources of the stimulus 

 or force which excites the rhythmic contractions of its muscular fibres. 

 In the hearts of the frog and tortoise, these ganglia are chiefly found 

 near the junction of the auricles and ventricles, in the neighborhood 

 of the auriculo-ventricular openings. If the heart of one of these 

 animals be removed from the body, and be divided longitudinally into 

 its right and left halves, the auricle and ventricle in each half will 

 still continue to contract and dilate rhythmically; if, however, the 

 heart be divided transversely, below the base of the ventricles, so that 

 a larger or smaller portion of the ventricles is detached from the rest 

 of the heart, the auricles and the base of the ventricles which are con- 

 nected, continue to contract rhythmically. But the separated piece 

 of the ventricles no longer does so, although isolated and spreading 

 contractions may still be excited in it, by the application of a me- 

 chanical or other stimulus. By yet further sections, the regulating 

 agent of the rhythmic action is shown to be confined to the immediate 

 neighborhood of the auriculo-ventricular orifices, or to the line of 

 junction between the auricles and ventricles, in which part the chief 

 ganglia are found. The synchronous combination of the auricular and 

 ventricular motions on the two sides, may be due to connections be- 

 tween the several ganglia. These local cardiac ganglia must be re- 

 garded as nervous centres, which originate a co-ordinate and rhythmi- 

 cally exerted energy, stimulating the muscular fibres of the auricles 

 and ventricles to perform their characteristic movements, in regular 

 and periodic succession. It has been suggested that these nervous 

 centres exert or discharge such energy rhythmically, or at periodic 

 intervals, owing to a periodicity in their nutritive processes, by which 

 they alternately accumulate and discharge the nerve-force necessary 

 to excite the muscular substance of the heart. (Paget.) Why this 

 periodicity of nutrition occurs, is still unexplained. 



It has been supposed that the condition of the blood distributed to 

 the substance of the heart itself may, in some way, determine its 

 rhythmic actions, either the presence of dark venous blood in its capil- 

 laries directly stimulating the muscular contractility (Brown-Sequard), 

 or the absence of oxygen acting in a similar manner. (Radcliff'e.) 



