18 THE EXTERIOR OF THE HORSE. 



The economy offers numerous examples of this variety. All the flexor mus- 

 cles act as levers of this kind. The large psoas muscle flexes the femur by this 

 mechanism. The point of support is at the coxo-femoral articulation, the resist- 

 ance (the weight of the member) is applied at the femoro-tibial articulation, and 

 the power is exerted at the internal trochanter. The flexors of the leg, meta- 

 tarsus, arm, forearm, metacarpus, phalanges, head, spinal column, etc., act upon 

 levers of the third kind. We are right in saying that this lever is that of flexion 

 in the same manner as that of the first kind is the lever of extension. 



In the animal mechanism all the levers are not distributed in the same 

 number. The manner of articulation of the bones and the function of the parts 

 demand here the use of the inter-fixed lever, there that of the inter-puissant 

 lever, and elsewhere that of the inter-resisting. 



We have, however, a right to ask why the organism utilizes two levers of speed, 

 the first and the third class, since one of them may become that of force. We 

 have seen, indeed, that the bones which act as levers of the first kind when the 

 member is raised become the second kind during support, by the simple dis- 

 placement of the fixed point and of the resistance. It cannot be thus for those 

 which act as levers of the third kind. In other words, the lever of flexion can- 

 not be of the first kind ; it belongs especially to the third. For, were it otherwise, 

 we should find in the sinus of the articular angles apophyses analogous to the ole- 

 cranon, the calcaneum, the trochanter, the sesamoids, or the patella, peculiarities 

 destined to substitute the lever of the first class for that of the third. It is not 

 difficult to see that the movement of flexion, already sufficiently limited by the 

 interposition of the muscular masses occupying the articular sinus, would be 

 almost impossible. The presence of these eminences on the opposite side of the 

 locomotory angles does not cause any inconvenience, for the extension is never 

 complete, and, were it so, it would place the two segments in prolongation with 

 each other, which would not limit the normal movement. In flexion it is different ; 

 the concavity of the angles should be free in order that certain points of the movable 

 segment do not meet too soon the fixed segment and therefore limit its displacement. 



In all the preceding cases we have supposed the muscular action isolated in 

 order to analyze it better. It is, however, never thus. Every muscle which con- 

 tracts to displace a bone is aided in its role by the contraction of one or more 

 neighboring muscles. These latter have the effect of fixing the one of the two 

 segments which should not be moved. Not one of the pieces of the machine is 

 arranged in an immovable manner, since all are agencies to produce movement. 

 It is then important, in order to avoid the discharge of force, that certain ones 

 among them be immovable, and it is on this account that the contraction of a 

 muscle is always assisted by that of a congener. This fact, most of the time diffi- 

 cult to determine, becomes most evident at the time of the production of the effort. 



Finally, we remark that, if the organism frequently employ the lever, the 

 simplest of machines, in the functions of its locomotive apparatus, this animal 

 lever differs much from the ordinary one and, with still greater reason, from 

 the mathematical. If we apply to it the laws of the latter, we simply arrive 

 at a sufficient approximation of its action without seeking to establish any abso- 

 lute principle. 



With M. Mignon, 1 we think that in the animal lever the point of support is 



* Mignon, M6canique animate, in Recueil de mSdecine vt6rinaire, 1841, p. 67. 



