CITAP. ii.] THE CONTRACTILE TISSUES. 57 



but, nevertheless, is in the main the result of intrinsic processes 

 going on in the muscle, the reverse of those leading to the shorten- 

 ing. The return of the muscle to its elongated condition, is 

 not a mere passive stretching, after the causes leading to the 

 shortening have passed away; it like the shortening itself is a 

 manifestation of activity. And hence we find that the complete- 

 ness of the relaxation is dependent on the complex changes which 

 we speak of as the nutrition of the muscle. Thus in their natural 

 position in the living body, muscles, owing to their vigorous nu- 

 trition, assisted by the fact that their anatomical disposition keeps 

 them always on the stretch, return completely to their original 

 length, after even powerful and prolonged contractions. In a 

 muscle out of the body, on the other hand, even when loaded, re- 

 peated successive contractions frequently result in the failure to 

 achieve complete relaxation becoming very conspicuous ; and the 

 tetanus curves, Figs. 6 and 7, shew very strikingly this shortcoming, 

 which is often spoken of as the 'contraction remainder/ 



We may speak of the relaxation as the result of an elastic 

 reaction, but only in the sense that the elastic qualities of the 

 muscle, at any moment, are the expression of deep-seated and con- 

 tinually varying molecular changes going on in the muscular sub- 

 stance. And in this connection attention may be called to a peculiar 

 physical character of contracting muscle. Living muscle at rest 

 is very extensible, but when stretched returns after the extending 

 cause has been removed, rapidly and completely to its former 

 length. In physical language muscle is spoken of as possessing 

 slight but perfect elasticity. It might be imagined that during a 

 contraction this extensibility would be diminished in order that none 

 of the resistance which the muscle had to overcome, no part of the 

 weight for instance which had to be lifted, should be employed in 

 stretching the muscle itself and thus lead to an apparent waste of 

 energy. On the contrary we find that during a contraction there 

 is an increase of extensibility ; thus if a muscle at rest be loaded 

 with a given weight, say 50 grammes, and its extension observed, 

 and be then while unloaded thrown into tetanus, and the load 

 applied during the tetanus, the extension in the second case will 

 be distinctly greater than in the first. During the contraction 

 there is so to speak a greater mobility of the muscular molecules, 

 and though this greater mobility may have its advantages, the 

 loaded muscle has in contracting to overcome its own increased 

 tendency to lengthen on extension before it can produce any effect 

 on the weight which it has to lift. 



The elasticity and extensibility of the muscular substance is how- 

 ever a complicated and difficult subject, and it will be sufficient to 

 reassert that it is essentially a vital property, being dependent, like 

 the irritability of the muscular substance, on certain nutritive factors. 

 As the muscular substance becomes weary with too much work or 

 impoverished by scanty nutrition, its elasticity suffers pari passu 



