448 . RHYTHMIC CONTRACTION. 



It is probable that one cause of cessation of muscular contraction in 

 Effect of accu- any one point of a fibre is the momentary accumulation of 

 mulation or wa sted material, as might be illustrated in a coarse manner 

 rial. by the difficulty of causing a fire to continue burning when 



the ashes are permitted to accumulate, and the necessity of their removal 

 before the combustion can go on. Two separate events have to occur 

 before a fibril that has been in contraction is ready to contract again : 

 these are the removal of the oxidized products, and the renovation of the 

 interior of the cells. The two probably go on coincidently, the veins 

 taking one part of the duty, and the arterial capillaries the other. 



In non-striated muscular fibre, in which the supply of blood-vessels is 

 Peculiarity in much less copious, there is a possibility for a lateral propa- 

 S^onltrilted g ati on of effect, because of the possibility of the lateral prop- 

 fibre, agation of the heat, either supplied directly from the nerve 

 tubule or arising from the oxidation going on. The sluggishness of its 

 first contraction, the longer continuance, the propagation from fibre to 

 fibre laterally until the effect wears out or is re-enforced by some new 

 stimulus, might almost seem to be the necessary result of the imperfect 

 supply of arterial blood, the sluggish removal of the products of waste, 

 and the more perfect opportunity for the diffusion of heat. This doc- 

 trine therefore meets with a very happy illustration in the phenomenon 

 displayed by the contraction of the two kinds of fibre. 



It may still farther illustrate these views to examine that other variety 

 Rhythmic con- of contraction, rhythmic in its nature, which is exhibited, for 

 tractions. example, by the heart, of which it may be said that the fibres 

 show a simultaneous contraction alternating with periods of repose, con- 

 traction and relaxation succeeding each other at definite intervals. If, 

 as we have just said, the cessation of contractility arises from the mo- 

 mentary accumulation of products of waste, and the capacity for its re- 

 newal is due to restoration of the original state by nutrition, rhythmic ac- 

 tion may follow as the consequence of an arrangement of muscular fibrils 

 with an adjusted supply of arterial and venous capillaries. An original 

 excitation producing a contraction can not act in a permanent way, for 

 the result of that contraction is an accumulation of wasted material which 

 must be removed. It may require but a moment for the removal to take 

 place to a sufficient extent to enable the original disturbance to act once 

 more, and be checked in its action again. Whatever value there may 

 be in such explanations as these, they undoubtedly gather a deep inter- 

 est from thus enabling us to comprehend that it is possible to resolve 

 such mysterious phenomena as rhythmic periodicities into the results of 

 ordinary mechanical laws. 



But the question returns upon us. Admitting the descriptions that 

 have now been given to be a true representation of the facts, and also of 



