The inferior or carpal extremity of the ulna is very small ; it forms a slightly rounded head ; on its posterior and internal part is a small process, projecting vertically downwards and ending in a point, to which the internal lateral ligament of the wrist-joint is attached : this process is the styloid process ; external to this is a depression or pit, into which is inserted the triangular cartilage of the wrist joint, and external to this depression is the rounded head, which is smooth on its inferior surface, covered with cartilage in the recent state; the triangular cartilage glides upon this surface. On the outer side of the head is an articular convexity which articulates with a concave surface on the inner side of the carpal extremity of the radius. On the posterior surface of the head, imme diately external to the styloid process, there is a slight channel, in which is lodged the tendon of the extensor carpi ulnaris.
Structure. — The olecranon and coronoid processes are completely cellular in structure, excepting the external cortex of compact tissue. The inferior extremity of the ulna is likewise cellular, but the shaft is mainly composed of compact tissue, hollowed by a medullary canal, which commences a little below the coronoid process, and terminates just above the inferior extremity.
Radius, (Germ. die Speiche,) so called from its being compared to the spoke of a wheel ; it is the shorter of the two bones of the forearm ; its Proportion to the ulna being as 11 to 12.
The superior extremity or head of the radius is a cylindrical head excavated on its superior surface so as to form a superficial cavity, eavitas glenoidca, which is articulated with the external condyle Of the humerus. The circumference of this head consists of a deep lip, of bone present ing a smooth surface covered by cartilage in the recent state, the depth of which, measured verti cally, is greatest on the inner side, so as there to form an oval convex articular facet which is adapted to the lesser sigmoid cavity of the ulna; the remainder of the circumference is embraced by the annular ligament of the radius. The head of the radius is connected to the shaft by a short and cylindrical neck, which passes obliquely downwards and inwards; the neck of the radius is limited inferiorly and on the ulnar side by a rounded tubercular process, into the internal posterior and rough part of which the biceps mus cle is inserted, the biczpital tuberosity or tubercle of the radius; the anterior part of this tubercle, over which the tendon of the biceps glides, is smooth. For about an inch below this process the bone retains the cylindrical form, being here embraced by the inferior fibres of the su pinator brevis muscle; but below this the bone becomes distinctly prismatic in its form, and begins to expand to its inferior or carpal extre mity. We here describe three surfaces as in
the ulna : the anterior is inclined inwards, its aspect is forwards and inwards ; about its middle this surface is slightly hollowed from above downwards; at the junction of its middle and inferior third it is convex, and in its inferior third, where it attains its greatest lateral expan sion, it is concave again. At the superior third of the bone we notice on this surface the nutri tious foramen, the canal following the same direction as that of the ulna, namely upwards. The muscles attached to the anterior surface of the radius are the flexor pollicis proprius, con nected with the two superior thirds of the bone, and the pronator quadratus occupying the in ferior third. The posterior surface of the radius is likewise inclined, and looks backwards and inwards, very narrow in its whole extent, but broadest at its inferior extremity, convex in its superior and inferior thirds, and slightly con cave from above downwards in its middle third. This last portion of the bone affords attachment to the two inferior extensor muscles of the thumb ; the superior third is embraced by the supinator brevis, and the inferior third has applied to it the tendon of the common extensor of the fingers, the indicator, and the extensor tertii internodii pollicis. The external surface is convex in its whole extent, and like the others expands inferiorly; about its middle we observe a rough surface, which gives insertion to the pronator quadratus; in its upper portion the surface is embraced by the supinator brevis, and inferiorly the radial extensors of the wrist are applied to it.
Of the three edges which separate these sur faces, the internal is sharp, and extends from about an inch below the bicipital tuberosity to about the same distance above the carpal extre mity of the radius; at this latter point the edge seems to bifurcate and form a plane triangular surface above the inferior extremity of the ra dius. This edge gives attachment in its entire extent to the interosseous ligament. The an terior edge is rounded ; it distinctly originates from the bicipital tuberosity, and terminates at the outer side of the carpal extremity of the radius in front of the styloid process. The su pinator brevis, the proper flexor of the thumb, and the flexor sublimis of the fingers, have attachments to this edge above, and below the pronator quadratus and supinator longus are inserted into it. The posterior edge is very imperfectly defined, being distinct only in its middle.