GREGORY, QUADRUPEDAL LOCOMOTION 277 



it serves partly as a ligament. All muscles in situ are stretched to a 

 certain degree, and thus act as ligaments. 28 "Over extension" of the 

 muscle is prevented by the inextensible connective tissue fibers. 29 Ac- 

 cording to Stillman, 30 the length of the muscles cannot be increased by 

 exercise, otherwise the tension necessary to prompt action would be lost. 



The contractile force is highest when a muscle is stretched to its full 

 "physiological length" (that is the greatest length it ever assumes during 

 life). As shortening takes place, the contractile force becomes less and 

 less (Haycraft). 31 



Contractile force and speed of movement augmented oy "hold and 

 let go" arrangements. — Fick and Helmholtz showed 32 that the greatest 

 force and velocity of contraction are developed when the movement of the 

 muscle is checked during the initial stages and when the resistance is 

 suddenly diminished. 



Amount of contraction. — -The shortening of individual muscles is in 

 general proportional to their length when in repose, but different in- 

 vestigators give somewhat different estimates. "While Weber described 

 a muscle as shortening 70 per cent, of its length, when unweighted, more 

 recent observers incline to put the shortening at 20 to 30 per cent, of its 

 length" (Haycraft). 33 Marey 34 estimates "the mean shortening of a 

 muscle while contracting, when it is not detached from the animal," as 

 "about a third of its length when in repose." Bishop's estimate is one- 

 fourth (Stillman, p. 31). "When the fibers are not parallel, but 

 obliquely set, as in the gastrocnemius, we have a greatly extended trans- 

 verse area of muscular fibers, which act therefore very powerfully, 

 though, on account of their short length, they can exercise their pull but 

 a comparatively short distance" (Haycraft) . 35 



THE "ANGLE OF INSERTION" AND THE PRINCIPLE OF THE PARALLELOGRAM 



OF FORCES 



In Pig. 2 (I), let AC represent a rod free to rotate around the point 

 A in the direction CC; let BD represent a contractile spring fastened 

 at D, inserted on AG at B and forming the angle ABD (a). Assume 

 that the length of BD is proportional to its contractile force ; then from 



28 IUd., p. 245. 

 23 IUd., p. 242. 

 30 Op. cit., p. 32. 

 a Op. cit., p. 242. 



32 Haycraft, op. cit., p. 248. 



33 Op. cit., p. 244. 



3 4 Op. cit., p. 62. 

 33 Op. cit., p. 242. 



