The =tibio-tarsus= is a thick straight bone nearly twice as long as the femur. Both ends of the bone are considerably expanded. The proximal end bears two slight depressions which articulate with the condyles of the femur, and a third depression which partly lodges the patella. The proximal end of the anterior or extensor surface is drawn out into a very prominent =cnemial crest= which bends over towards the postaxial side of the bone; a slight ridge is continued from it all the way down the shaft. The proximal part of the shaft of the tibio-tarsus bears a roughened ridge with which the fibula is closely connected. The distal end is expanded and rotated outwardly and forms a prominent pulley-like surface which articulates with the tarso-metatarsus. The =fibula= is reduced to the proximal portion only, which is expanded and articulates with a depression behind the external condyle of the femur. The fibula further extends about a third of the way down the shaft of the tibio-tarsus. The =patella= or =knee-cap= is a sesamoid bone due to an ossification in the tendon of the extensor muscles of the leg. The =ankle joint= lies between the proximal and distal tarsals which as previously mentioned fuse respectively with the tibia and metatarsus. The =Pes=. The pes includes four digits, and consists of the tarso-metatarsus and the phalanges. The proximal tarsals which are fused with the tibia also really belong to the pes. The =tarso-metatarsus= is a strong straight bone nearly as long as the femur, and is formed by the fusion of the distal tarsals with the second, third and fourth metatarsals. The proximal end of the bone is expanded and bears two facets for articulation with the tibio-tarsus, and near them on the posterior surface is a large roughened projection. The lines of junction between the several metatarsals are marked along the shaft by slight ridges. At the distal end of the bone the three metatarsals diverge from one another and each bears a prominent convex pulley-like surface. The =first metatarsal= is reduced to the distal end, which tapers to a point proximally, and is attached by ligaments near the distal end of the tarso-metatarsus. The =digits=. Four digits are present, each consisting of a metatarsal (already described) and a certain number of phalanges, the terminal one being in each case clawed. The first digit or =hallux= has two phalanges, the second three, the third four, and the fourth five. FOOTNOTES: [101] See R.S. Wray, _P.Z.S._, 1887, p. 343. [102] Often called the manubrium, but not homologous with the manubrium of the mammalian sternum. [103] Cp. fig. 63. [104] See p. 28. CHAPTER XIX. GENERAL ACCOUNT OF THE SKELETON IN BIRDS. EXOSKELETON. The epidermal exoskeleton of birds is very greatly developed, feathers constituting its most important part.