The first phalanges are distinguished by their greater length; but chiefly by the shape of their upper (metacarpal) ends, which do not form strictly hinge-joints, but have concave oval surfaces, with the long diameters transverse, adapted for lateral movement as well as flexion on the heads of the metacarpal bones. In accordance with this lateral movement, we observe, on each side, a tubercle for the advantageous insertion of the interosseous muscles which produce it. With this exception, all the other joints of the fingers are strictly hinge-joints.

The second phalanges are recognised by the shape of their upper ends, which have two surfaces, with an intervening ridge, so as to form a hinge with the first phalanges. They have also tubercles behind for the insertion of the extensor tendons.

The last or ungual phalanges are the shortest. Their ends expand into a horse-shoe shape, smooth on one surface for the support of the nails, and rough on the other for the support of the pulp of the fingers.

It may, perhaps, be interesting to some persons to know that the "middle digit" is the most constant of all the digits in the vertebrate series. Few are aware that the bones forming the three joints of this finger answer to those called "great pastern bone," "little pastern bone," and coffin bone in the horse; and that the nail in this finger represents the hoof.

Each phalanx has two centres of ossification: one for the shaft and the lower end; the other for the upper end, which remains an epiphysis till about the twentieth year.

SESAMOID BONES.

These little bones are so called from their resemblance in size and shape to the grain "sesamum." They are met with in the substance of tendons in the neighbourhood of joints, -the "patella," or "knee-pan," being the best example. Their use is to increase the leverage of the tendons. The thumb has two of these bones beneath its metacarpal joint, to increase the leverage of the "flexor brevis pollicis." We rarely find any for the fingers.

Of all animals, the mole has the most remarkable apparatus of "sesamoid" bones. Its prodigiously strong digging feet are richly provided with them, in order to increase the leverage of the brachial muscles, which enable the animal, as it were, to swim through the earth.

Here we must remark, that in consequence of the flexors and extensors of the wrist being inserted below the second row of the carpal bones, they necessarily act on the "intercarpal" as well as on the "radio-carpal" joint. Thus, a greater amount of motion is provided for the wrist than it otherwise could have had with safety; for if so free a motion had been given to one joint, the angle of flexion must have been very great, and the ligaments must have been looser than would have been consistent with the security of the joint.

I. Base of first phalanx.

O. Back of ulna.

Extensor secundi internodii pollicis {1. Base of third phalanx.