In the class of Birds, the humerus is de veloped as regards the prominence of its mus cular protubemnees, in proportion to the powers of flight. In birds which fly, those eminences are strong and prominent, and the bone itself is proportionally strong; but in those which do not fly, the bone is weak and gene rally short. In the common pigeon, for ex ample, the enlargement of the scapular ex tremity of the humerus, and the developement of the tubercles is very manifest, as well as the strength and thickness of the shaft of the bone.
Structure.—The structure of the humerus is characteristic of that of long bones in general. In a vertical section we observe that the re ticular texture is chiefly accumulated towards the extremities; the shaft being mainly formed of compact tissue. At the upper extremity we notice the mark of union of the epiphysis of the head, which corresponds to the line of the anatomical neck of the bone. The canal, when a transverse section of it is viewed, appears somewhat quadrilateral in form. Its walls are formed of very dense compact tissue.
Developcment.—The ossification of the hu merus begins in its shaft, and that very early, according to Meckel about the second month; the shaft goes on enlarging, but the extremities are still cartilaginous during the whole of in trauterine life, and for the first year after birth. The superior extremity is developed by two points of ossification, one for the head, the other for the great tuberosity ; about the be ginning of the second year the ossification of the head of the bone commences, and from the four-and-twentieth to the thirtieth month the ossification of the great tuberosity begins. According to Bedard, a small ossific point for the lesser tuberosity is visible in the fifth or sixth year ; from the eighth to the ninth year the ossific elements of the head of the hume rus become united and the head is com pleted.
The inferior extremity of the humerus, accord ing to Cruveilhier, begins to ossify later than the superior. The first point of ossification noticed in it is for the external condyle: this appears at the age of two years and a half; at seven years a second point of ossification commences for the epitrochlea; at twelve a third point appears for the internal edge of the trochlea; and at sixteen years a fourth point for the epicondyle. These four points of ossification, Cruveilhier states, are united in the following order : first, in the second year, the two points of the trochlea are united; and, secondly, at sixteen years the trochlea, epicondyle, and the condyle form a a single piece.. The union of the extremities
with the shaft of the bone takes place from the eighteenth to the twentieth year; and all ob servers agree in stating that the union of the inferior extremity with the shaft always pre cedes that of the superior extremity, although the ossification of the latter is prior.
l'orcarm.— The bones of the forearm are the ulna and radius, of which the former con stitutes the second essential element in the elbow-joint, the radius being chiefly an acces sory bone to provide for the wider range of motion of the hand. The ulna therefore is the principal lever of the forearm, and the motions of flexion and extension of that segment of the limb upon the arm depend upon it ; at its superior extremity it forms a very firm hinge joint with the trochlea of the humerus, but in feriorly its connexion with the carpus at the wrist-joint is very slight, and it forms by no means an essential element of that joint. On the other hand, the radius at its inferior ex tremity forms a very important part of the wrist-joint, but at its superior its connection with the elbow-joint is due to its necessary articulation with the outer side of the ulna.
Ulna (xvgiva, cubitus; Fr. os du coude ; Germ. des Ellenbogenbein.f) This bone is situated on the inner side of the forearm. It is the longest and the largest bone of that region, and in the vertical position of the limb it is directed downwards and a little outwards, the obliquity being occasioned by the greater pro jection downwards of the inner lip of the trochlea of the humerus, as already alluded to in describing that bone.
The upper or humeral extremity of the ulna is at once distinguished by its great size from the inferior extremity. It consists of two pro cesses joined to each other at a right angle, and so that that angle opens forwards. One of these processes is vertical, and is continued in the direction of the long axis of the bone, and is little else than a continuation of the shaft ; this is the olccranon: the other is horizontal, anterior to the olecranon, as it were placed upon the superior extremity of the bone, so as to project considerably beyond the plane of its anterior surface : this is the eoronoid process.