THE LEVER AND MUSCULAR MECHANISM. 



17 



may be the respective positions of the three points, the lever-arm of power exceeds 

 that of resistance, since PA always remains hypothetically longer than BA. This 

 lever is, then, very truly that 

 of force, but has more disad- 

 vantage concerning the pro- 

 duction of speed, for, the 

 latter being proportional to 

 the lever-arm, the path of the 

 resistance will never be as 

 long as that of the power. 



Several examples are seen 

 in animals. It is well known 

 that while standing the weight 

 of the body tends to close all 

 the articular angles of the 

 members upon each other. Fig. 8. 



There are also observed, on 



the convex side or the summit of the latter, muscles which are opposed to this 

 closing, by acting as levers of the second class. It is thus that the supra-spina- 

 tus, the coraco-radialis, the sub-spinatus, the subscapular, sustain the scapulo- 

 humeral angle. The insertions of these different muscles are all situated a little 

 beyond the articulation, which represents the point of application of the resist- 

 ance to be overcome, whilst the point of support is placed at the articulation of 

 the elbow. It is in the same manner that the extensors of the forearm sustain 

 the humero-radial articulation upon which are transmitted the actions of the 

 weight, that the suspensory ligament of the fetlock maintains this region, that 

 the gastrocnemius and the perforatus muscles of the leg retain the calcaneum, 

 that the middle gluteal muscle prevents the closing up of the coxo-femoral angle. 

 It will be noticed that all these muscles act as levers of the first class wJien the 

 members do not serve as a support, so that the organism has no need of inter- 

 posing new anatomical dispo- 

 sitions in order to develop 

 force. The same levers adapt 

 themselves with the same 

 powers to different conditions 

 in order to produce, according 

 to the necessity of the move- 

 ment, either force or speed. 



Finally, a third case pre- 

 sents itself in the relative 

 situation of the three flmda- 

 mental points of the lever. It 

 is that in which the power 

 is placed between the resist- Fig. 9. 



ance and the point of support. 

 This lever is designated the lever of the third class, or inter-puissant (Fig. 9). 



Here the arm of resistance is always hypothetically longer than that of the 

 power, and the velocity is increased at the expense of the force. Like that of 

 the first class, it can also be called the lever of speed. 



9. 



