1. The os calcis, being a short and thick bone, has such a power of resistance, that it is but seldom fractured. Such an accident does, however, sometimes occur, and may arise from two causes, 1st, the action of external bodies, which is rare: 2dly, the contraction of the gastrocnemii and soleus muscles, from which it almost always proceeds. Thus, the rotula is more frequently broken by the action of the extensor muscles, than by blows received on the bone from without: there is, however, this difference between the effects of muscular contraction in these two cases, namely, that in the former, the rupture of the tendo Achillis is common, and the fracture of the os calcis very rare; whereas in the latter, on the contrary, the rotula is oftentimes broken, while the tendon of the extensors remains almost always sound. This phenomenon is explained by the difference between the thickness of the two bones, between the length of the two tendons, and between the power of the causes. 2. Be the cause of the fracture what it may, it generally occurs in that portion of the os calcis, called its great tuberosity, which projects behind the astragulus, which corresponds above and below to a large quantity of cellular membrane, inwardly to the great groove of the bone, externally to some ligamentous attachments, and behind to the insertion of the tendo Achillis. 3. It is known, 1st, by an evident inequality under the heel: 2dly, by an elevation, sensible to the touch, of the posterior fragment above its usual level: 3dly, by an almost entire inability either to stand or walk: 4thly, by severe pain being the inevitable consequence of moving the foot: 5thly, by its being practicable to increase the displacement by flexing, and to diminish it by extending the foot: 6thly, by the facility with which the posterior fragment may be moved in every direction by taking hold of it with one hand, and steadying the foot with the other: 7thly, by a swelling more or less considerable, which frequently appears around the divided surfaces. 4. The ancients gave in general an unfavourable prognosis respecting this kind of fracture. Hippocrates was apprehensive of some injury being done to the surrounding parts. Pare considered the case mortal, on account of the laceration of numerous vessels which are connected with the bone. Most of the moderns adopt these principles, not for the foregoing reasons, but on account of the vicinity of the accident to the joint of the foot. The practice of Desault cannot throw much light on this subject, as he never had more than one or two such cases of fracture under his care; but the analogy of other fractures, situated in the neighbourhood of joints and even extending into them, induces us to believe, that, if properly treated, fractures of the os calcis will terminate as favourably as those of other bones. § II.