The olecranon, (bAirv, cubit us, memo', caput,) also called processes ancontrus, may be said to begin from the angle of junction of the coro noid process with it ; there the bone appears slightly constricted, for above that point it ex pands. We notice five surfaces upon it. The superior surface is horizontal ; it presents pos teriorly a muscular impression affording inser tion to the triceps extensor, and anteriorly it ends in a remarkable beak, which, in the state of complete extension, is received into the ole cranon cavity of the humerus. The posterior surface is rough with a very obviously trian gular outline; this surface gives insertion to the triceps muscle. The internal surface is also rough, and covered by the fibrous ex pansion from the tendon of the triceps, and at its anterior margin affords insertion to the superior fibres of the internal lateral ligament. The external surface is smooth, and also is covered by the fibrous expansion from the ten don of the triceps. The anterior surface is articular; it presents the appearance of having been covered by articular cartilage ; it is divided by a rounded vertical ridge into two unequal portions, of which the internal is larger than the external. This surface is limited below by a transverse depression, non-articular, in which some fatty matter is deposited in the recent state. The surface is convex from side to side in the centre, and each of its lateral portions is concave; the whole surface is concave from above downwards. In the extended state of the forearm this articular surface of the olecra non is applied to the posterior part of the troehlea of the humerus ; it forms the posterior part of the great sigmoid eavity of the ulna.
The coronoid process is wedge-shaped, at tached by its base to the anterior surface of the ulna, the sharper edge projecting forwards and free. This edge is convex, and sometimes forms a point; it is received into the coronoid cavity of the humerus. On the external sur face of the coronoid process is an oval articular facet, concave from behind forwards, whose long axis is horizontal ; this is the lesser sigmoid cavity, and is articulated with the inner side of the head of the radius ; the internal surface is rough, and has a projecting lip, which affords attachment to the anterior fibres of the internal lateral ligament. The anterior surface is in clined from above downwards and from before backwards, so that its aspect is downwards and forwards ; it is slightly hollowed transversely, I and is rough, the roughness being continued down for a little way in front of the bone, thus forming a rough surface triangular in form, the I base corresponding to the anterior edge of the coronoid process ; this surface affords insertion to the brachireus anticus muscle. The superior surface forms the anterior portion of the great sigmoid cavity ; like the similar surface on the olecranon, it is divided by an obtuse ridge directed from before backwards, into two qual portions ; these portions correspond in shape and size with those already noticed on the olecmnon.
The shaft of the ulna gradually tapers from above downwards; it is triangular in its entire extent, excepting for about an inch above the inferior extremity, where the bone is distinctly cylindrical. On the shaft anatomists commonly describe three surfaces. the anterior surface is broader in the middle than at its extremities; it is slightly concave in the transverse direction in its middle third ; on this surface, at its upper part, we notice the orifice of the nutritious canal, which is directed upwards towards the coro noid and olecranon. By its three superior fourths this surface affords attachments to the flexor digitorum profundus, and by its inferior fourth to the pronator quadratus ; the place of attachment of this latter muscle is limited above by an oblique line which passes from without inwards and from above downwards. The in ternal surface is smooth, and convex in its en tire extent; widest above, it gradually tapers to the inferior extremity. In its inferior fourth it is subcutaneous, and to its three superior fourths is attached the deep flexor muscle of the fingers ; the aspect of this surface is back wards as well as inwards.
The third surface is posterior. The two in ferior thirds of this surface are smooth, the mid dle being flat and the lowest rounded ; here are attached the extensor muscles of the thumb and that of the index finger. In the superior third we distinctly notice two surfaces, easily distinguishable by the difference of aspect ; the internal one, which is continued up on the olecranon process, looks backwards and slightly outwards; to it the ancomeus muscle is attached superiorly, and inferiorly the extensor carpi ulnaris. The external of these two surfaces looks directly outwards, and is separated from that last described by a line which passes ob liquely downwards and inwards ; to this sur face, which commences just below the lesser sigmoid cavity, the supinator brevis is attached, and below it, commences the line of attachment of the extensor muscles already alluded to.
Three edges separate the surfaces above de scribed ; of these the external is at once distin guished by its greater prominence; it is sharp in nearly its two inferior thirds, and superiorly is lost on the surface to which the supinator brevis is attached ; all that part of this edge which is prominent and sharp gives insertion to the intcrosseous ligament. The anterior edge commences just below the coronoid pro cess, and terminates, inclining a little back wards, in front of the styloid process of the ulna: it is rounded and smooth in its entire extent, and has the deep flexor of the fingers and the pronator quadratus inserted into it. The posterior edge commences at the apex of the posterior surface of the oleeranon, and ter minates insensibly towards the inferior fourth of the bone.