The =internal epicondyle= (Epicondylus medialis) is a rounded prominence on the internal surface of the internal condyle, to which the internal lateral ligament and the adductor muscle are attached. The corresponding =external epicondyle= (Epicondylus lateralis) is less distinct; it presents a mark where the lateral ligament is attached, below and behind which there is a depression (Fossa musculi poplitei) in which the popliteus muscle arises. Between the external condyle and trochlea is the =extensor fossa= (Fossa extensoria), in which the tendon of origin of the anterior extensor and peroneus tertius is attached. =Development.=—The shaft and the distal end each ossify from one center, but the proximal end has two centers, one of which is for the great trochanter. The edge of the external trochanter also has a separate center. The proximal end fuses with the shaft at three to three and a half years, the distal at about three and a half years. [Illustration: FIG. 67.—DISTAL EXTREMITY OF RIGHT FEMUR OF HORSE, END VIEW. ] THE TIBIA The =tibia= is a long bone which extends obliquely downward and backward from the stifle to the hock. It articulates above with the femur, below with the tarsus, and externally with the fibula. It possesses a shaft and two extremities. The =shaft= or =body= (Corpus tibiæ), large and three-sided above, becomes smaller and flattened in the sagittal direction below, but widens a little at the distal end. It presents for notice three surfaces and three borders. The =internal surface= (Facies medialis) is broad above, where it furnishes insertion to the internal lateral ligament and the sartorius and gracilis muscles; below this it is convex and subcutaneous. The =external surface= (Facies lateralis) is smooth and somewhat spiral. It is wide and concave in its upper part, below which it becomes narrower and slightly convex, and winds gradually to the front of the bone; near the distal end it widens a little, becomes flat, and faces forward. The =posterior surface= (Facies posterior) is flattened, and is divided into two parts by the rough =popliteal line=, which runs obliquely from the upper part of the external border to the middle of the internal border. The triangular area above the line is occupied by the popliteus muscle, while the area below is marked by rough lines (Lineæ musculares) to which the deep flexor muscle of the digit is attached; the lines fade out below, where the surface is smooth and flat. The =nutrient foramen= is situated on or near the popliteal line. The =anterior border= is very prominent in its upper third, forming the =tibial crest= (Crista tibiæ); below it is reduced to a rough line, which ends at a small elevation near the distal end of the bone. The =internal border= (Margo medialis) is rounded in its upper half, to which the popliteus muscle is attached, and a tubercle is found on this part. The lower part is a rough line on well-marked bones. The