LECTURES ON HORSES. 303 
become fixed points upon the ground, and serve as fulcra in the 
working of the machine onward. 
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 power and 
facility of action to 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 it acting on 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- 
ening to throw the limb forward, and the continuance of the same 
act it is, enforced by additional powers, which propels the machine 
in progression. A well-formed stifle is, therefore, a point of some 
importance, and the best-formed joint of this description is that 
which, from the sharpness of the angle between the femoral and 
