480 



with other organs, with which, but for their connexion through the 

 nervous system, it could only sympathize more slowly through the 

 medium of the blood. 



But, now, it will not have escaped you to question, why are 

 these nervous centres rhythmic in their action? granting all that 

 has been said, why is it that these nervous centres accumulate and 

 discharge nerve-force, as it would seem, not only spontaneously, but 

 at time-regulated intervals? To put on them, rather than on the 

 heart's muscles, the work of rhythm, is only to put the real difficulty 

 of the matter a step further back. 



This is very true ; and I will add, that they who hold that the 

 rhythmic, like the ordinary, action of muscles, cannot ensue without 

 stimulus, are in the like predicament, at a step still further back ; 

 for how, it may be asked, is the stimulus (of the heart suppose) 

 applied or generated at regular intervals ? And the difficulty is not 

 in the case of the heart alone, but in that of all the rhythmic mus- 

 cular movements. For why, it may be asked, does the medulla 

 oblongata rhythmically excite the respiratory muscles into action? 

 and if it be answered that it is itself stimulated by venous blood, or by 

 impressions on the pneumogastric and other centripetal nerves, then 

 how does the venous blood, or any other substance, thus rhythmically 

 and not constantly stimulate ? Constant stimulus, or constant produc- 

 tion of an excess of nervous or muscular force, might produce constant 

 muscular action, whether continuous or disorderly, but could not 

 alone maintain a regularly interrupted action, in which the lengths 

 of time of action and of inaction are in definite proportion. 



This shifting of the question as to the cause of rhythmic motion 

 suggests that we should enter on a larger inquiry, and take notice of 

 whatever organic processes are performed with rhythm; and this 

 seems the more necessary, while we consider that rhythmic action is 

 not tied to any particular structure, whether muscular or nervous, 

 nor employed in only one or two purposes, as blood-movement or 

 breathing ; nor even limited to the animal kingdom : so that, really, 

 the peculiarity we have to study is not one of force, or not one of 

 force alone, but one of time as an element in the organic processes. 

 No explanation of the rhythmic action of the heart, therefore, would 

 be sufficient, which did not involve or appear consistent with some 

 general law to which we may refer all other rhythmic organic pro- 



