118. The lower extremity of the os femoris, being thicker than the rest of the bone, and protected from the action of external bodies by a thinner covering of soft parts, is yet better secured from fractures than the other parts, for the following reasons: 1st, because counter-strokes, so frequently the cause of fractures of the body and neck of the bone, can affect this part but rarely: 2dly, because the os femoris, being more moveable at a distance from the centre of its motions, yields more easily to whatever strokes and impressions it there receives: 3dly, because motion, when distributed through a greater bulk of matter, has less power to destroy its continuity. § XVIII. OF THE VARIETIES AND THE CAUSES. 119. The fractures which occur in the lower extremity of the os femoris, are of two kinds very different from each other. Sometimes situated above the condyls, they only separate these from the body of the bone: at other times, affecting the condyls themselves, they extend into the very joint. My attention shall at present be confined exclusively to the latter kind, as the other may, in almost every respect, be classed with the fractures which have been already considered. Most authors have neglected to treat of fractures of this kind, under a distinct head, from a persuasion, that, owing to their communication with the joint, they ought to be ranked among complicated fractures, which are known to require a mode of treatment very different from that employed in such as are simple. But I shall presently show what regard ought to be paid to this ancient opinion. 120. The division presents itself, in general, under two different forms: 1st, running obliquely from above downwards, and from within outwards or from without inwards, it may separate a greater or smaller portion of one of the condyls from the rest of the bone: 2dly, these two bony protuberances may be divided from each other by a longitudinal fracture, meeting another transverse or oblique fracture, which by either passing through the whole thickness of the bone, separates both condyls from it, or extending only half way through it, separates but one of them. The fracture is single in the first case, but double in the second. The latter occurs in practice more frequently than the former. Both are usually produced directly, that is, by the immediate action of external bodies. Yet the following fact seems to evince that the accident may, possibly at least, arise from a counter stroke.