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.
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.
Three kinds of feathers are found, viz. (a) pennae including quills and coverts, (b) down feathers or plumulae, and (c) filoplumes which are rudimentary feathers. The structure of the different kinds of feathers is described on pp. 303-306.
Sometimes a fourth class of feathers, the semiplumae, is recognised. They have the stems of pennae, and the downy barbs and barbules of plumulae.