246 



ANIMAL MECHANICS. 



done = F (1x981) X S {!) = 981 ergs, or units of work, in absolute 

 measure. 



The pull of gravity on the unit of mass is often taken to represent 

 the unit of force, and in these gravitation units — 



W=MxS. 



If 



the 

 of 

 the 

 the 

 the 



Thus, if 10 lb. be lifted 10 ft., we have the work of 100 foot-lbs 

 10 grms. be lifted 10 cms., we have 100 grin. -cms. of work. 



While the force exerted by a muscle depends upon its cross-section 

 and not upon its length, the work done will be proportioned not only to 

 the cross-section but also to the length, inasmuch as the space through 



which the force acts 

 is one of the de- 

 terminants of the 

 work done, and the 

 work done will 

 therefore bear a 

 proportion to the 

 whole mass of 

 muscle. Many 

 the muscles of 

 body, such as 

 long head of 

 biceps, pass over 

 two joints ; and one 

 advantage of its 

 length is the fact 

 that the work done 

 by such a muscle is 

 greater than can be 

 accomplished by a 

 shorter one of the 

 same girth, 

 mass which 

 muscle has 

 modifies in an im- 

 portant manner the 

 amount of work 

 which it will per- 

 form. As the 

 muscle is gradually 



The 

 a given 

 to lift 



Fig 



, 131. — Represents the curves of a resting and contracting 

 muscle, when loaded with 0, 10, 20, etc. grms. On 

 loading a muscle with 10 grms. and stimulating it, the 

 weight will be raised from c to d, and the work done will 

 he represented by the area abed. The areas QRST and 

 XYRQ are other areas representing work done (see text). 



loaded, the distance through which the loads are raised diminishes, as we 

 have already seen ; but the product of this diminished space and the 

 increased weight becomes greater and greater. After a time a maximum 

 is reached, and after this the work done falls gradually to zero, at 

 which point S = 0. 



We may represent the work done by an area. Thus, if in Fig. 131 the 

 muscle lifts a weight of 10 grms. through 10 mm., the muscle performs 

 100 grin. -mm. of work, which is represented by the area abed. 



The work may be increased, as Fick lias shown, 1 by any mechanical device 

 which diminishes the load while, a muscle is contracting. It will be under- 

 stood by consulting Fig. 131, that, on loading the muscle with 40 grms., it 



1 Hermann's "Handbuch," Bd. i. S. 241. 



