THE PROPERTIES OF MUSCULAR TISSUE 97 



great for its strength. But if the shorter muscle had double the 

 thickness, then it could lift twice the weight that the longer 

 muscle could. We find in the Body muscles constructed on both 

 plans; some to have a great range of movement, others to 

 overcome great resistance, besides numerous intermediate forms 

 which cannot be called either long and slender or short and thick ; 

 many short muscles for example are not specially thick, but are 

 short merely because the parts on which they act lie near to- 

 gether. It must be borne in mind, too, that many apparently 

 long muscles are really short stout ones those namely in which 

 a tendon runs down the side or middle of the muscle, and has 

 the fibers inserted obliquely into it. The muscle (gastrocnemius) 

 in the calf of the leg, for instance (Fig. 44, B), is really a short stout 

 muscle, for its working length depends on the length of its fasciculi 

 and these are short and oblique, while its true cross-section is that 

 at right angles to the fasciculi and is considerable. The force 

 with which a muscle can shorten is very great. A frog's muscle 

 of 1 square centimeter (0.39 inch) in section can just lift 2,800 

 grams (98.5 ounces), and a human muscle of the same area more 

 than twice as much. 



Muscular Elasticity. A clear distinction must be made be- 

 tween elasticity and contractility. Elasticity is a physical prop- 

 erty of matter in virtue of which various bodies tend to assume 

 or retain a certain shape, and when removed from it, forcibly to 

 return to it. When a spiral steel spring is stretched it will, if let 

 go, " contract " in a certain sense, by virtue of its elasticity, but 

 such a contraction is clearly quite different from a muscular con- 

 traction. The spring will only contract as a result of previous 

 distortion; it cannot originate a change of form, while the muscle 

 can actively contract or change its shape when a stimulus acts 

 upon it, and that without being previously stretched. It does 

 not merely tend to return to a natural shape from which it has 

 been removed, but it assumes a quite new natural shape, so that 

 physiological contractility is a different thing from mere physical 

 elasticity; the essential difference being that the coiled spring or 

 a stretched band only gives back mechanical work which has 

 already been spent on it, while the muscle originates work inde- 

 pendently of any previous mechanical stretching. In addition 

 to their contractility, however, muscles are highly elastic. If a 



