The Thigh. 329 



To extend the knee, and, when acting from below, to 

 support the femur on the tibia. 



Since the shaft of the femur is directed somewhat in- 

 wards, because of the separation, above, by the pelvis, the 

 quadriceps extensor, excepting that part formed by the 

 vastus internus, would tend, when contracting, to displace 

 outwards the patella, and, this, it would do, were it not 

 for the direction and insertion of the fibres of the vastus 

 internus, which arise lower down on the thigh than the 

 vastus externus (the lower level of the muscular fibres of 

 the latter being opposite a point about one inch above the 

 external condyle, while the lower level of the former, i.e., 

 the vastus internus, is opposite a point about half an inch 

 below the upper level of the internal condyle), and, not 

 only is its origin lower down, but its insertion is, also, 

 since it is inserted into the upper half, whereas, the vas- 

 tus externus is inserted into the upper third of the corre- 

 sponding border. It results, therefore, that, when the 

 quadriceps extensor contracts, the inward traction of one 

 of its constituent parts, viz., the vastus internus, counter- 

 acts the tendency to outward displacement of the patella, 

 the result of the action of the rest of the muscle. 



Nerve Associations. In connection with the 

 nerve supply of the above muscles it is interesting to note 

 that the adductor muscles, the gracilis and the obturator 

 externus, which all adduct the thigh, are supplied by the 

 third and fourth lumbar nerves through the obturator, 

 and that the pectineus, which also adducts the thigh re- 

 ceives a branch from the obturator and accessory obtura- 

 tor. But the chief action of the pectineus is to flex the 

 limb like the psoas, iliacus, sartorius and the rectus mus- 

 cles, and, like them, it is supplied by branches from the 

 anterior crural. Thus, muscles having similar actions 

 are controlled by the same nerves. This is seen in the 



