THE LEVER AND MUSCULAR MECHANISM. 15 



parallelism is a hinderance to the execution of the initial movement ; hence the 

 utility of the eminences which the bones of the members present, more particu- 

 larly in the vicinity of their extremities. These projections, u})on which the 

 muscles are inflected or attached, have the effect of removing them from the 

 bones and increasing the intensity of their action. Such is the role of the great 

 sesamoids, the supra-carpal bone, the olecranon, the coracoid process, the calca- 

 neum, the anterior tuberosity of the tibia, the patella, the trochanter, etc. Such 

 is the advantage presented by the enlarged extremities of the bones of the 

 members, and such the result of the inclination of the different bones upon 

 each other. These diverse dispositions retain the advantages of parallelism, 

 whilst avoiding the manifest inconveniences at the beginning of the muscular 

 action. 



Again, the study of the lever teaches us that the paths described by the lever- 

 arms are in direct ratio to their length, since they describe circumferences which 

 are to each other as their radii. If, consequently, one of the forces act upon an 

 arm shorter than another, the arm of the latter will traverse a path much more 

 considerable.' 



In organisms it is very remarkable to find that the lever-arm of the muscles 

 is usually very weak, especially when the latter are charged with the production 

 of speed. As Lecoq * judiciously remarks, power acts in this case with much 

 less intensity, but it may become much greater through the multiplicity of the 

 muscular fibres, which, having only to produce a contraction slightly marked, 

 can be disposed obliquely and be much more numerous in the muscle. On the 

 other hand, if the muscles have their insertion very far from the point of sup- 

 port, their contraction, by removing them from this point, deprives the member 

 of its slender form. 



From what we have said above, a propos of the inclination of the forces 

 upon their lever-arms, it is easy to determine the force utilized at each instant 

 of the muscular contraction. It is clearly seen that, according as a muscle is at 

 the beginning or at the end of its contraction, there is a tendency to an approx- 

 imation or a separation of the articular surfaces. 



It is said that a muscle is at its moment when its traction is exercised per- 

 pendicularly to the displaced bone ; but a great many muscles cease their action 

 before reaching that position ; they often only commence the movement which 

 is terminated by the intervention of other muscles. This takes place in the 

 flexors of the metacarpus, for example, whose parallelism to the lever-arm is 

 almost complete ; if the flexors of the phalanges did not initiate the flexion of 

 the metacarpus, these muscles could perhaps not produce it by themselves. 



In all the instances which we have just considered the mobile bone repre- 

 sents a lever upon which we always find the three fundamental points : the point 

 of support and the points of application of the power and of the resistance. 



The point of support is nearly always situated on a level with the articula- 

 tion with the fixed bone ; this is also the centre of movement. The power is 

 always applied at the mobile insertion of the motor muscle. As to the resistance, 

 it is situated upon the lever, at that variable point where the weight or the 

 obstacles to the displacement of the mobile bone act, whatever they may be. 



1 See, for further details, G. Colin, Physiologie compar6e, t. i. p. 396, 3e 6d. 

 • F. Lecoq, Recueil de m^deciue v6t6rinaire, 1843, p. 493. 



