MUSCULAR CONTRACTILITY. 131 



striking effect, so long as the nerve retains its own vital properties. 

 In the former case, the contraction is named idio-muscular ; in the 

 latter, neuro-muscular. The nerve itself is said to conduct the stimu- 

 lus to the muscle ; but, as we shall hereafter see, the nerve is probably 

 excited by the stimulus, a certain change takes place in it, and this 

 change is propagated along the nerve. Even in the so-called direct 

 stimulation of a muscle, the nerves contained within it may be con- 

 cerned, so as to constitute that also a case of indirect stimulation. 



Certain chemical stimuli, such as alkaline solutions, act equally 

 well, if applied either to the muscle or to the nerve ; some, such as 

 alcohol, creasote, and lactic acid, act almost solely through the nerves; 

 whilst others, as sulphate of copper and ammonia, operate powerfully 

 and almost exclusively on the muscle, but hardly at all through the 

 nerve. Over-stimulation, as, for example, repeated electrical and 

 powerful discharges, temporarily destroys the contractility. So like- 

 wise portions of muscle subjected to extreme weight lose their con- 

 tractility. Moderate, but numerous and rapid, electric shocks pro- 

 duce a state of continuous contraction, known as tetanus ; in the frog, 

 at least 15 shocks per second are necessary to tetanize the muscles ; 

 with about 100 shocks per second the tetanus ceases, but it is again 

 induced by increasing the strength of the current. A uniform con- 

 tinuous current does not maintain the original amount of contraction, 

 the muscle gradually lengthening again somewhat. A muscle ex- 

 hausted by long-continued stimulation, recovers its contractility after 

 sufficient rest ; this is also true of muscles recently separated from the 

 body. But in atrophied muscles, in which the transverse striae are 

 destroyed, and the whole fibre filled with fat particles (fatty degener- 

 ation), there is no contractile power remaining. 



The phenomena which characterize muscular contraction, have been 

 chiefly studied in the striped muscular fibres of animal life. The act 

 of contraction usually begins at either end of a fibre, but often at one 

 or more intermediate spots. When a single fibre undergoes contrac- 

 tion, a slightly darker spot first appears at some point of its border; 

 this spreads across the whole diameter of the muscle, and, on careful 

 examination, it is seen that the transverse striae become finer, and are 

 drawn closer together, becoming twice or even four times as close as 

 in the relaxed fibre; this action, the limits of which are well defined, 

 then continues to spread each way through the fibre, by a wave-like 

 progression, parts of the fibre becoming contracted, whilst other parts 

 are assuming a relaxed form. The changes thus described, and the 

 accompanying approximation of the transverse striae, appear to be 

 due to a corresponding shortening of all the component fibrils of a 

 single fibre, and the general result is a shortening of the whole fibre, 

 which, at the same time, becomes thicker in its contracting portions. 

 Supposing the ends of the fibre to be free, it still continues soft and 

 flexible; but when muscular fibres, as in a perfect muscle, are attached 

 at their two ends, then their substance becomes firm and hard, as may 

 be felt during the contraction of a living muscle in the arm. This 

 increased firmness is due to increased tension of the fibres, and does 

 not imply, as might at first be supposed, any important condensation 



