STIFLE. 89 



We may, therefore, fairly ascribe three functions or uses to the 

 patella. Firstly, it serves to complete the stifle-joint in front, and 

 to protect it against injury coming in that direction. Secondly, it 

 forms a pulley, playing over the condyloid surface of the femoral 

 bone, and thus greatly facilitating the action of the extensor mus- 

 cles. Thirdly, it may be regarded in the light of a process, though 

 a moveable one, giving the muscles inserted into it the advantage 

 of considerable leverage in their operation upon the thigh : con- 

 sequently, the more prominent the stifle-bone is, the greater the 

 power given to the muscles. Were there no stifle-bone existing, 

 the tendons of the extensor muscles of the thigh would have to 

 play over the bare condyloid cavity of the femur, under the dis- 

 advantage of increase of friction and loss of leverage ; and had 

 the bone been fixed instead of moveable, the projection from the 

 head of the tibia must have been of a lengthy and awkward 

 shape, and, withal, would not have conferred the same poAver and 

 facility of action on the muscles which they possess at present. 

 Perhaps nothing more strikingly demonstrates the utility of the 

 patella than the accident of its dislocation : the bone has no sooner 

 slipped out of its place than all power of extension of the thigh is 

 lost; and the result is, dragging of the toe of the hind leg upon 

 the ground, the animal having no power to advance the limb un- 

 derneath the body. The bone, in being dislocated, has got into a 

 situation in which it is rendered a fixture, and the muscles con- 

 sequently become powerless. The instant, however, the bone is 

 righted, all power and action are restored, the same as if nothing 

 had happened. 



The operation of the muscles implanted into the patella, and 

 through its medium into the tibia or true thigh-bone, is, then, exten- 

 sion of the thigh, and thereby bringing the leg forward underneath 

 the body, preparatory to the effort of progression ; the act of pro- 

 gression itself being, as we shall hereafter discover, mainly effected 

 by the muscles inserted into the hock, assisted, however, by those 

 of the patella. Action in the hind extremity is commenced by a 

 general flexion of the limb — flexion of the femoral bone on the 

 pelvis, elevating the stifle against the body ; flexion of the tibia 

 and hock, raising the foot off the ground, and preparing the limb 

 for projection underneath the body : then comes the act of straight- 



