The inferior or carpal extremity of the radius is the largest part of the bone ; it is irregularly quadrilateral in form. Its inferior surface forms an articular excavation, the outline of which is triangular, the apex being external and the base internal ; this surface is divided into two by a slightly prominent line which passes from before backwards ; the outer of these two portions retains the triangular form, and is articulated with the scaphoid bone of the carpus; the internal is quadrilateral, and articulated with the lunar bone. At its inner margin, this surface is continuous with a slightly excavated articular facet on the ulnar side of the inferior extremity of the hone, which is articulated with the convex surface on the cor responding part of the ulna. The inferior ex tremity of the radius presents, at its outer part, a pyramidal process projecting downwards and slightly outwards; this is the styloid process, which by its apex gives attachment to the ex ternal lateral ligament of the wrist joint. The anterior margin of the inferior extremity is slightly concave from side to side ; it gives at tachment to the anterior ligament of the wrist joint, and the tendons of the flexor muscles of the fingers pass over it into the palm of the hand. On the posterior margin of this extremity we observe two grooves: the internal one, wide and very superficial, lodges the tendons of the com mon extensor of the fingers and the indicator ; the external, deeper and oblique, lodges the extensor tertii intcrnodii pollicis. Externally we notice likewise two superficial grooves, of which the posterior lodges the radial extensors of the wrist, and the anterior is traversed by the extensores primi et secundi internodii pollicis.
Structure.—The central canal extends up wards into the neck of the bone; it is cylin drical at the extremities, and prismatic in the centre. Both extremities are composed of can cellated structure.
Developement of the bones of the fore-arm.— Both bones appear about the same time, and if not synchronously with the humerus, at least a very little later. With both bones the ossifi cation begins on the shafts, which are very early completed ; the ossific point of the shaft of the radius is said, by Beclard and Cruveil bier, to begin some days before that of the ulna. In the radius the inferior extremity begins to ossify before the superior, about the end of the second year. The ossification of the superior extremity begins between the seventh and ninth year; it is united to the shaft about the twelfth year, whilst the inferior extremity, whose ossification begins earlier, is not united till the eighteenth or twentieth year. The progress of the ossification of the ulna is very similar. The inferior extremity developed by a single point of ossification begins first about the sixth year. A little later the olecra non begins to ossify; the eoronoid is formed by an extension of ossification from the shaft. The union of the superior extremity of the ulna with the shaft takes place about the fifteenth or sixteenth year ; that of the inferior about the eighteenth or twentieth.
It is important to observe that the articula tion of the radius with the ulna, in the manner in which it is effected in man, has reference to the motions of the hand. Pronation and supi nation of the hand are effected by the rotation of the head of the radius within the coronary ligament and on the lesser sigmoid cavity of the ulna. The hand is so connected with the radius that it follows the motions of that bone; when, therefore, the radius rotates in such a direction that its inferior part crosses the ulna, the posterior edge is directed outwards, and its anterior surface inwards and backwards; the palm of the hand is turned backwards and the dorsum forwards; the forearm and hand are then said to be in pronation. On the contrary, when the rotation is such that the ulna and ra dius are placed on the same plane, the dorsum of the hand is directed backwards and the palm forwards; this is supination.
In the lower animals we never find this mode of articulation of the radius with the ulna, unless there be also present the motions of su pination and pronation of the hand. In such animals, evidence of the existence of these motions is afforded by certain points in the conformation of the radius and ulna them selves, such as the peculiar form of the head of the radius, and the concave articular sur face on the ulnar side of its lower extremity, as well as the lesser sigmoid cavity of the ulna, and the convexity on the radial side of the head of the same bone. This is found in many of the Carnivora, but chiefly in the Quadru mana.
In the Ruminants and Solipeds the radius and ulna are consolidated together so as to form one bone; they can, however, be distin guished at the humeral end, where the latter bone is conspicuous by its elongated olecranon, which not only affords insertion to the extensor muscles of the arm, but also increases the secu rity of the elbow-joint. The radius, which is the principal bone of the fore-arm, is so arti culated with the humerus as to admit of free flexion and extension, but it is fixed in the state of pronation. In many of the other Nam Inaba the radius and ulna are distinct through out, but do not admit of the rotation of the one on the other; this is the case in Rodentia, many Carnivora, Pachydermata, Edentata, In sectivora, and Cetacea. In the Sloth, how ever, among the Edentata, the motions of pro nation and supination are conspicuous, and the olecranon is imperfectly developed ; on the contrary, in the Edentata proper, as the Arma dillo, Megatherium, &c. these motions do not exist, and the olecranon is very much deve loped. In the Cheiroptera the radius is the principal hone of the fore-arm, the ulna being developed only as to its humeral extremity consisting sometimes of little more than its olecranon ; and in some, as the Vespertilio yam pyrus, the olecranon exists in the form of a pa tella, connected with the upper extremity of the ulna.