The =lateral borders= converge to the apex below, and each forms a prominence or angle at the base. The inner angle and the adjacent part of the border give attachment to the fibro-cartilage of the patella, which increases the articular surface. The external border is rounded and its angle is less prominent. The =base= (Basis patellæ) faces upward and backward, and is convex transversely, concave from before backward. The =apex= forms a blunt point directed downward. =Development.=—The patella develops as a sesamoid bone from a single center in a cartilaginous deposit in the tendon of the quadriceps femoris muscle. THE TARSUS The =tarsus= or =hock= of the horse usually comprises six short bones (Ossa tarsi), but exceptionally seven are present. THE TIBIAL TARSAL BONE The =tibial tarsal= bone (Os tarsi tibiale, astragalus, or talus) is the inner bone of the proximal row. It is extremely irregular in form, but may be considered as offering six surfaces for description. [Illustration: FIG. 73.—RIGHT PATELLA OF HORSE, ANTERIOR VIEW. ] [Illustration: FIG. 74.—RIGHT PATELLA OF HORSE, POSTERIOR VIEW. ] The =superior= and the =anterior= or =dorsal surface= are continuous, and form a =trochlea= for articulation with the distal end of the tibia. The trochlea consists of two oblique ridges with a deep groove between them, which curve spirally forward, downward, and outward. There is usually a shallow =synovial fossa= in the groove. The =inferior surface= is convex from before backward, and most of it articulates with the central tarsal; externally it has an oblique facet for the fourth tarsal, and a non-articular groove cuts into the surface to its middle. The =posterior= or =plantar surface= is oblique and extremely irregular; it presents four facets for articulation with the fibular tarsal bone; the facets are separated by rough excavated areas, and the largest fossa (Sulcus tali) forms with a corresponding one on the fibular tarsal a cavity termed the =sinus tarsi=. The =internal surface= bears on its lower part a large tuberosity, and on its upper part a small one for the attachment of the internal lateral ligament. The =external surface= is smaller than the internal, and is marked by a wide rough fossa in which the external ligament is attached. THE FIBULAR TARSAL BONE [Illustration: FIG. 75.—RIGHT TARSUS AND UPPER PART OF METATARSUS OF HORSE, INTERNAL VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ] [Illustration: FIG. 76.—RIGHT TARSUS AND UPPER PART OF METATARSUS OF HORSE, POSTERIOR (PLANTAR) VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ] The =fibular tarsal= bone (Os tarsi fibulare, calcaneum, os calcis) is the largest bone of the hock. It is elongated, flattened from side to side, and forms a lever for the muscles which extend the hock joint. It consists of a body and an inner process, the sustentaculum tali.