THE MUSCLE AS A MACHINE. 243 



elongated. In this case the force of the muscle exactly balances gravity, 

 and may be measured by it. We may say that the force of a particular 

 muscle per square centimetre of its cross-section is a weight of 3 kilos. 

 If we wish to express this in units of force, or dynes, we must multiply 

 the grammes or units of the mass, namely 3000, by 981, the acceleration 

 due to gravity. 



Force = mass x acceleration. 



In this case force = 3000 x 981 = 2,943,000 dynes. 



The dynamometer may be used with advantage to gauge absolute 

 muscular force. It is a spring in some form or another, whose distor- 

 tions under the action of gravity have been previously ascertained, as by 

 suspending weights from it and recording its elongations. If the spring 

 be now pulled upon by a muscle, and elongated an inch, an elongation 

 previously produced by gravity acting upon a mass of 3 kilos, suspended 

 from it, we conclude that the force of the muscle is equal to 3 kilos., 

 or 2,943,000 dynes. 



The absolute muscular force is not the same for the muscles of all animals. 

 Thus Roseiithal l found that per square centimetre of cross section the gastro- 

 cnemii of a frog can just raise 3 kilos., while in warm-blooded animals the same 

 sectional area will raise about twice as much. 2 It is probable that the 

 muscular force is not the same for all muscles of the body, and it is sub- 

 ject to physiological and pathological variation in one and the same muscle. 

 Exercise is said to increase it, and the greater power of the right arm. over 

 the left is sometimes instanced as an example of this increase. It is, however, 

 to be borne in mind that the girth of an exercised muscle, e.g. that of the 

 right arm, increases, so that in this case it is not clear that per unit area 

 the muscular force is increased. Disuse and degenerative changes greatly 

 diminish the muscular force, and fatigue operates in a similar direction. 



It had long been known that many of the lower animals, insects for 

 instance, are, for their size, remarkably strong. 3 Thus, while a horse can 

 barely drag its own weight, many insects can drag a weight over sixty times their 

 body-weight. But the comparison between the force of the pull of different 

 animals, when this is expressed in terms of the body-weight, yields no data 

 by which we may compare their absolute muscular forces. The reason of this 

 is, that whereas in ascending a scale of size the body-weight increases as the 

 cube, the cross-section of the muscles, on which the absolute force depends, 

 increases only as the square. If, then, the absolute muscle force be the same 

 throughout the whole animal kingdom, the larger animal would be able to lift 

 or drag a smaller proportion of his body-weight than the smaller one ; indeed, 

 the insect who can drag sixty times its own body-weight may have muscles 

 which are, per cross-section, weaker than those of a horse which cannot drag 

 its body-weight. 



When a muscle is stretched to or beyond its full physiological 

 length, 4 it is able to exert more power in raising a weight ; its absolute 

 force is greater. As it shortens, its force becomes less and less. 5 This 

 is clear from Fig. 130, in which is represented the curve of extension 

 of a contracted muscle, the abscissae being weights applied, and the 



1 Compt. rend. Acad. d. sc., Paris, 1867, tome Ixiv. p. 1143. 



2 Haughton, " Principles of Animal Mechanism," 2nd edition, London, 1873 ; Knorz, 

 Diss., Marburg, 1865 ; Henke, Ztschr.f. rat. Med., 1865, Bd. xxiv. S. 247. 



3 Plateau, Bull. Acad. roy. d. sc. de Belg., Brux., 1865, p. 732. 



4 The full physiological length is the greatest length it ever assumes during life. 



5 Schwann, Mailer's " Handbuch d. Phys.," 1837, Bd. ii. S. 59. 



