132 



MOVEMENT Ix\ ANIMALS 



muscle-group in the body is that by which one rises on 

 tiptoe; it is located in the calf of the leg. Direct 

 tests show that the pressure that can be applied on the 

 ball of the foot by the contraction of the calf muscles may 

 amount to as much as 400 pounds in an adult of average 

 strength. So strong a muscle as 

 this must have a great many fibers 

 joined in the pull. If the number 

 which are necessary to exert this 

 force were placed side by side in 

 such fashion that they all pulled 

 directly upon the tendon the 

 calf would be at least twice as 

 bulky as it is. Compactness, to- 

 gether with the necessary strength, 

 is secured in this muscle by having 

 the tendon by which the muscle 

 is attached to the upper leg extend 

 down through the muscle nearly 

 to its tip and having the fibers in- 

 serted on the slant (see Fig. 35). 

 This makes possible the presence 

 in the muscle of a great many more 

 fibers than could otherwise be 

 accommodated in the same bulk. 

 Long Muscles. — There are 

 some instances in the body where 

 movement must be exerted 

 through a considerable distance 

 but need not be especially power- 

 ful. A muscle which makes this 

 kind of movement must be long 

 because a single fiber can shorten by only a very 

 small fraction of an inch, hence to get a total shortening 

 of several inches a great many fibers must be placed 

 end to end. The best example of this sort is the 

 so-called tailor's muscle which is the muscle by which 



Fig. 35. — Diagram 

 showing how a Muscle 

 may 1^ composed of 

 many Fibers without 

 being unduly bulky. 

 The upper tendon extends 

 well down through the 

 muscle. The fil)ers extend 

 slantwise downward to 

 the surface of the muscle 

 where they are attached 

 to a surface tendon which 

 merges l)elow into the 

 Achilles tendon of the heel. 



