320 



ftrs and extensors, the limbs would still be in a state of flexion, if the 

 fibres of the former being longer, they made the parts move through a 

 greater space. 



*tf we examine the different parts of the body, the articulations of the 

 limbs, and especially of the knee, the knowledge of which is of the high- 

 est importance in understanding the theory of standing, it will be seen 

 that the flexor muscles exceed the extensors, in the number and length 

 of their fleshy fibres. If we compare the biceps cruris, the semi tendi- 

 nosus, the serni membranosus,the rcctus internus, the sartorius, the ge- 

 melli, the plantaris, and the popliteus, which all concur in the flexion of 

 the leg, to the triceps cruris and to the rectus, which extend the leg, we 

 shall readily understand that the fibres of these last are much shorter, and 

 in smaller number. Those of the sartorius and rectus internus are the 

 longest of all the muscles employed in voluntary motion ; the fibres of 

 the posterior muscles of the limb are not inferior in length to the fibres 

 of the muscles at the fore part. 



Besides, the flexor muscles are inserted into the bones which they are 

 to move farther from their centre of motion. In fact, if the insertion of 

 the semi membranosus is situated nearly at the same height, the sartorius, 

 the rectus internus, the semi tendinosus, the biceps, and the popliteus, 

 are inserted lower than the extensors of the leg. But this difference is 

 particularly observable in the plantaris and gemelli, which terminate at 

 the greatest possible distance from the centre of motion, and which act 

 with a very long lever*; lastly most of these muscles depart much more 

 than the extensors, from a parallel direction to the bones of the leg. We 

 all know the curved line of the course of the sartorius, of the rectus in- 

 ternus, and semi tendinosus, by which the angle of their insertion becomes 

 more favourable. 



The flexor muscles which, on their being first called into action, are 

 nearly parallel to the levers which they are to move, tend to become 

 perpendicular to them, in proportion as the motion of flexon is carried on. 

 Thus, the brachialis, the biceps brachii, and the supinator longus, the 

 main line of direction of which is nearly parallel to that of the bones of 

 the fore arm, when the flexion of this limb commences, become oblique, 

 then perpendicular to this bone, and at last form with it the angle most 

 favourable their action. The same applies to the flexors of the leg; the 

 angle of their insertion becomes greater the more it bends on the thigh. 

 The extensors, on the contrary, are in the most favourable state for ac- 

 tion, at the moment when their contraction begins, in proportion as the ex- 

 tension goes on, they have a tendency to become parallel to the levers 

 which they sei in motion; their action even ceases before the parallelism 

 is complete, at the elbow by the resistance of the olecranon, and at the 

 knee by the numerous ligaments and by the tendons situated towards the 

 posterior part of the articulation. 



The flexor muscles have, therefore, fibres of greater length and more 

 numerous than those of the extensors. They are inserted into the bones, 



* We may, in this respect, compare the gemelli to the supinator longus, the use of 

 which is not limited, as was shown by Heister, to the supination of the hand ; but which 

 is, likewise, a flexor of the fore arm, and acts the more powerfully, as its inferior insertion 

 is at a greater distance from the elbow joint, and as its fibres are the longest of all those, 

 of the muscles of the upper extremity. Copland, 



