478 



have rhythmic motion, though still capable of repeated single actions 

 when artificially stimulated. 



Such nervous centres may exist among the numerous ganglia in 

 the heart, and the indications of their position in the frog's heart 

 given by the experiments, agree very well with the dissections by 

 Bidder and Bosenberger. There is, indeed, nothing in the structure 

 or aspect of the ganglia presumed to be rhythmic, which can make 

 us sure that they can act as nervous centres of peculiar force, much 

 less (if it were possible) that they must act rhythmically. But the 

 assumption of such rhythmic nervous centres is justified, not only by 

 its sufficiency as an hypothesis, and by its accordance with the ex- 

 istence of ganglia in the appropriate places in hearts, but by the 

 analogy of the only other example of naturally rhythmic muscular 

 movements in our own or any other mammalian economy, I mean 

 the respiratory movements. These are certainly determined in their 

 rhythm by the medulla oblongata. It is true, they are greatly 

 influenced as to their rate and force, though not as to their very ex- 

 istence, by external conditions, and are amenable in a measure to the 

 power of the will ; and the rate and force of their rhythm are much 

 more dependent, than are those of the heart's, on the conditions of 

 distant parts. Nevertheless, essentially and habitually, the medulla 

 oblongata is not only the nervous centre, but the rhythmic nervous 

 centre, of the respiratory movements. When a certain portion of it 

 is destroyed, the proper respiratory rhythm ceases, and nothing can 

 renew it, whatever stimulus or variety of external conditions be 

 present. Any of the respiratory muscles may be again stimulated 

 to contraction ; some of them will again, and for a few times, contract 

 repeatedly and at regular intervals, imitating the old rhythm* ; but 

 there is no longer any combination or timely succession of the move- 

 ments of the many respiratory muscles, as for their proper purpose. 



* Such surviving rhythmic movements of the respiratory muscles may, perhaps, 

 be ascribed to the continuance of the rhythmic nutrition, of which mention is 

 made in a later part of the lecture, and which, though not the primary cause of 

 their rhythmic action, must coexist with it ; but other than the respiratory 

 muscles may thus for a time act rhythmically when dying, or after their nerves 

 are divided. See especially the observations of M. Brown Sequard in his ' Experi- 

 mental Researches.' The facts may be very difficult to explain, but are not 

 inconsistent with what I offer as the explanation of the rhythmic action of the 

 heart. 



