88 FORM AND ACTION. 



knee, for the same reason that the veterinarian would look upon 

 the knee of a man as his stifle. One grand difference, however, 

 between these structures is, that, in man the femoral bone stands 

 perpendicularly upon the tibia, whereas, in the quadruped the 

 bones are placed at a right angle, almost, in regard to each other : 

 a circumstance from which we may infer that the patella was not 

 added for the purpose of making the joint complete so much as 

 for the grand object of serving as a pulley and a lever to the 

 muscles engaged in the important business of extending the thigh 

 under the body and aiding in progression. The biped — man — is 

 enabled to maintain his erect posture with comparative ease, or at 

 little expense of muscular action, by means, principally, of large 

 and powerful muscles inserted into his knee-pan : were the knees 

 not kept straightened the stability and strength of the standing 

 posture would be lost: when from weakness, or any other cause, 

 the extensor muscles lose part of their power, so that the legs 

 cannot be completely straightened, we know how insecure the 

 standing is, to say nothing of the awkwardness and infirmness it 

 occasions in progression. Even after a man has had fracture of 

 one of his knee-pans, and the fractured divisions of bone have 

 united — as they commonly do through the interposition of liga- 

 mentous substance — the increased length of the pulley and conse- 

 quent diminished effect resulting from the contractions of the ex- 

 tensor muscles, occasions halting in the walk, and detracts from 

 the stability of the standing posture. To the quadruped these 

 observations are not altogether strictly applicable. Standing, as 

 he does, upon four legs, and these being so placed that the body is 

 mechanically supported by them, after the manner of a stool or 

 form upon its four supporters, but very little muscular action is 

 necessary to keep him standing ; and although the muscles affixed 

 to the patella contribute to this function, yet is that office com- 

 paratively trifling to the one they perform in the work of pro- 

 gression. When the hind limbs, through the agency of the flexor 

 muscles, have been raised or flexed to their utmost, then do the 

 extensor muscles come into play, projecting the limbs underneath 

 the body, and pointing the toes forward, in order that they may 

 become fixed points upon the ground, and serve a* fulcra in the 

 working of the machine onward. 



