THE PROPERTIES OF MUSCULAR TISSUE. 137 



done by a muscle varies with its form. The thicker the mus- 

 cle, that is the greater the number of fibres present in a sec- 

 tion made across the long axes of the fasciculi, the greater 

 the load that can be lifted or the other resistance that can be 

 overcome. On the other hand, the extent through which a 

 muscle can move a weight increases with the length of its 

 fasciculi. A muscle a foot in length can contract more than 

 a muscle six inches long, and so would move a bone through 

 a greater distance, provided the resistance were not too 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 together. 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 fibres inserted obliquely into it. 

 The muscle (gastrocnemius) in the calf of the leg for instance 

 (Fig. 53, 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) hi section can just lift 2800 

 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 

 between elasticity and contractility. Elasticity is a physical 

 property 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 contraction. 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 



