The =tibio-tarsal articulation= (Articulatio talo-cruralis) is a typical ginglymus formed by the trochlea of the tibial tarsal bone (astragalus or talus) and the corresponding surface of the distal end of the tibia. The ridges and grooves of these surfaces are directed obliquely forward and outward at an angle of about 12° to 15°, with a sagittal plane. The trochlear surface is about twice as extensive as that on the tibia, and its ridges have a spiral curvature. The other articulations are arthrodia, which have joint surfaces and ligaments of such a nature as to allow only a minimal amount of gliding motion. As in the case of the carpal joints, it is convenient to describe first the common capsule and ligaments, which are the more important practically, and then to consider very briefly the special ligaments. The fibrous part of the =joint capsule= is attached around the margin of the tibial articular surface above and the metatarsal surfaces below; it is also attached in part to the free surface of the bones which it covers, and blends with the lateral ligaments. Its anterior part (anterior ligament) is rather thin; in distention of the capsule, as in “bog-spavin,” its antero-internal part, which is not bound down by the tendons passing over the joint, forms a fluctuating swelling over the inner ridge of the trochlea. The posterior part (posterior and tarso-metatarsal ligaments) is very thick, and is intimately attached to the tarsal bones. Its superficial face is in part cartilaginous, and forms a smooth surface for the perforans tendon. Superiorly, it pouches upward behind the distal end of the tibia for a distance of about two inches (ca. 5 cm.). Interiorly, it is continued downward, forming the =subtarsal= or =check ligament=, which unites with the perforans tendon about the middle of the metatarsus. [Illustration: FIG. 164.—LEFT HOCK JOINT OF HORSE, ANTERIOR VIEW. _21_, Tibia; _22_, trochlea of tibial tarsal bone; _25_, large metatarsal bone; _50_, dorsal or oblique ligament. (After Ellenberger-Baum, Anat. für Künstler.) ] There are four =synovial sacs=: 1. The =tibio-tarsal sac= lubricates the proximal joint, and is much the largest and most important. It is chiefly involved in the swelling produced by excess of fluid in the joint cavity, when the capsule bulges antero-internally and postero-superiorly. 2. The =first intertarsal sac= lines the joints formed by the tibial and fibular tarsal bones above, and the central and fourth tarsals below; it communicates in front with the proximal synovial capsule. 3. The =second intertarsal sac= lubricates the joints formed between the central tarsal and the bones below and on either side. 4. The =tarso-metatarsal sac= lubricates the joints formed between the tarsal and metatarsal bones, those between the proximal ends of the metatarsal bones, and those formed by the third tarsal with the bones on either side.