SIGNS OF ACTIVITY IN MUSCLE AND NERVE 437 



After these preliminary remarks we can proceed 

 with the discussion of the effect of load on the work of 

 a muscle. We can say in general that the height of 

 the contraction is less the greater the load. But this 

 rule cannot stand without qualification. For under 

 the isotonic arrangement we find the height less with 

 a very light load than it is with one somewhat heavier 

 (v. Frey), and under the purely auxotonic arrange- 

 ment the height increases with the load up to a fairly 

 high primary tension. Moreover, even if the height 

 of the contraction does decrease as the load increases, 

 it does so much more slowly than the load increases ; so 

 that up to a certain limit the work done (product of 

 the load by the height of contraction) is greater, the 

 greater the load (E. F. Weber). 



Again, an increase in the tension of a muscle dur- I 



ing its contraction has a decidedly favorable effect on 

 its performance. Under some circumstances the con- 

 tractions against a stiff spring are just as high or even 

 higher than isotonic contractions obtained with a pri- iPSBnHwHSP 

 mary tension of the same amount (Santesson) ; and ^^^^^^^^^ 

 projectile contractions are sometimes higher than iso- ^!^S^^^^& 

 tonic contractions with the same primary tension (cf. '^^ff^^^^i J 

 Fig. 173). Finally, cases have been recorded where 

 auxotonic contractions which begin with the same ten- 

 sion are higher with a strong spring than with a weak 

 one. We can say, therefore, that within certain limits 

 the work done by a muscle is increased both by a higher 

 primary tension and by an increase in the tension dur- 

 ing the contraction. 



In close connection with this comes the additional 

 fact that under the isometric arrangement the increase 

 in tension takes place much more rapidly than does 

 the shortening under the so-called isotonic arrange- 

 ment; or, in other words, its length remaining the 



same, the muscle reaches its maximum tension much ff^l^S^^^^l > 

 earlier than it reaches its maximum shortening when ^^j^^Sa^SS^ 

 the tension remains the same (Fick, Fig. 176). 



A muscle appears therefore to have the power of 

 regulating the amount of work done under a given 

 stimulus, according to the requirements of the case. 

 We must forego a complete theoretical discussion of 

 these facts here; but we would direct attention to the 

 significance of the red fibers in this connection. They 

 are, as it appears, the most important source of the 

 additional work done as the result of an increased 

 tension. Thus we find that the secondary lift due to 

 these fibers, in contractions against a tense spring in- 



