354 THE PROPERTIES OF STRIPED MUSCLE. 



so doing lift a weight which is attached to it, the excitation is said 



to take place under isotonic conditions. 



In the former case the form of the muscle remains nearly the same 



as it was before it was stimulated, although, owing to a certain amount 



of internal displacement, consequent on un- 

 equal contraction of different parts, it is not 

 precisely the same. 1 The muscle which was 

 before relaxed now becomes tense. It thus 

 acquires properties resembling those of a spiral 

 steel spring of which the ends are attached 

 to two fixed points, at a distance from each 

 other which considerably exceeds the length 

 of the spring when left to itself. 



According to the simplest theory of muscular 

 7&. contraction, the work done by a muscle in passing 



from the relaxed to the contracted state, and 

 thereby lifting a weight, is equal to the work 

 done by the same weight on the same muscle in 

 extending it from its length when so shortened to 

 its length when relaxed. Experiment, however, 

 ' _, r shows that it is only under exceptional conditions 



that this equality is realised. It is on these con- 

 ditions, therefore, that the efficiency of muscular 

 action depends. For, just as a stretched spring 

 cannot do more external work when released than 

 was required to stretch it, but may do less, so a 

 \s'mf. loaded muscle, when excited, lifts its load with 



an efficiency which may equal but cannot exceed 

 that which would be exercised on the excited 

 muscle by the load in extending it to the same 

 length. The relation between the two quantities, 

 i.e. between the amount of work done on the 

 excited muscle in extending it, and the amount 

 of external work which can be done by the same 

 muscle in lifting a weight or overcoming a resist- 

 ance, has been investigated experimentally under 

 a variety of conditions by Fick, who embodied 

 the results in a small book on the physiology 

 FIG. 189. The double ad- of muscle some seventeen years ago. 2 As it is of 

 ductor preparation, am, great importance that the subject should be treated 

 sm. seraimembranosus ; . , T -, . ,-, j- 



g g', gracilis of right and experimentally, I propose to begin the discussion 

 left legs respectively of a of it by giving an account of a form of experiment 

 anaesculenta;ipr,iscliio- in which the amount of actual work done is equal 

 pubic ridge ; *, t, head of to that estimated, 

 tibia-fibula of right and 



left side respectively. The amount of work which must be 



done on an excited muscle in order to 



stretch it to its "natural" length when not excited, is, as in the 

 case of a spiral spring or an indiarubber band, the product obtained 

 by multiplying half the increment of length by the weight which is 

 required to extend it. With regard to muscle, this statement is not 

 absolutely exact ; for when an excited muscle is stretched, it does not 



^ee Blix, Skandin. Arch. f. PhysioL, Leipzig, Bd. vi. S. 245. 



2 Fick, "Mechanische Arbeit und Warrae-entwickelung bei der Muskelthatigkeit," 

 Leipzig, 1882. 



