The Appendicular Endoskeleton

The limbs of all vertebrated animals make their appearance as buds on each side of the body. In all but fishes, these buds become divided by constrictions into three segments. Of these, the proximal is called brachium in the fore-limbs, femur in the hind; the middle is antebrachium, or crus; the distal is manus, or pes. Each of these divisions has its proper skeleton, composed of cartilage and bone. The proximal division, normally, contains only one bone, os humeri, or humerus, in the bracbium, and OS femoris, or femur, in the thigh; the middle, two bones, side by side, radius and ulna, or tibia and fibula; the distal, many bones, so disposed as to form not more than five longitudinal series, except in the Ichthyosauria, where marginal bones are added, and some of the digits bifurcate.

The skeletal elements of the manus and pes are divisible into a proximal set, constituting the carpus or tarsus; and a distal set, the digits, of which there are normally five, articulated with the distal bones of the carpus and tarsus. Each digit has a proximal basi-digital (metacarpal or metatarsal) bone, upon which follows a linear series of phalanges. It is convenient always to count the digits in the same way, commencing from the radial or tibial side. Thus, the thumb is the first digit of the hand in man; and the great-toe the first digit of the foot. Adopting this system, the digits may be represented by the numbers i, ii, iii, iv, v.

There is reason to believe that, when least modified, the carpus and the tarsus are composed of skeletal elements which are alike in number and in arrangement. One of these, primitively situated in the centre of the carpus or tarsus, is termed the centrale; on the distal side of this are five carpalia, or tarsalia, which articulate with the several metacarpal or metatarsal bones; while, on its proximal side, are three bones-one radiale or tibiale, articulating with the radius or tibia; one ulnare or fibulare, with the ulna or fibula; and one intermedium, situated between the foregoing. Carpal and tarsal bones, or cartilages, thus disposed are to be met with in some Amphibia and Chelonia (Fig. 11), but, commonly, the typical arrangement is disturbed by the suppression of some of these elements, or their coalescence with one another. Thus, in the carpus of man, the radiale, intermedium, and ulnare are represented by the scaphoides, lunare, and cuneiforme respectively. The pisiforme is a sesamoid bono developed in the tendon of the flexor carpi udnaris, which has nothing to do with the primitive carpus. The centrale is not represented in a distinct shape, having probably coalesced with one of the other elements of the carpus. The fourth and fifth carpalia have coalesced, and form the single uniforme. In the tarsus of man, the astragalus represents the coalesced tibiale and intermedium; the calcaneum the fibulare. The naviculare is the centrale. Like the corresponding bones in the carpus, the fourth and fifth tarsalia have coalesced to form the cuboides.