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Separate centres are developed in connexion with the epicondyles; that for the lateral appears about the twelfth year, and, rapidly coalescing with the centres for the capitulum and trochlea, forms an epiphysis, which unites with the body about the sixteenth or seventeenth year. The centre for the medial epicondyle appears about the fifth year; it forms a separate epiphysis, which unites with the body about eighteen or nineteen. These two epiphyses at the distal end of the bone are separated by a down-growth of the shaft, which lies between the medial epicondyle and the trochlea, and forms part of the base and medial side of the latter process.
The epicondylic process when present is developed from the diaphysis, and has been observed to be already well ossified by the third year. ("Proc, Anat. Soc." Journ. Anat. and Physiol. 1898.)
Of the two bones of the forearm, the ulna, which is placed medially, is the longer. It consists of a large proximal extremity supporting the olecranon and the coronoid process; a body or shaft tapering distally; and a small rounded. distal end called the head.
Proximal Extremity.-The olecranon lies in line with the body. Its dorsal surface, more or less triangular in form, is smooth and subcutaneous and covered by a bursa. Its proximal aspect, which forms with the posterior surface a nearly rectangular projection-the tip of the elbowfurnishes a surface for the insertion of the tendon of the triceps brachii muscle, together with a smooth area which is overlain by the same tendon, but separated from it by a bursal sac. To the volar (anterior) crescentic border of this process are attached the fibres of the posterior part of the capsule and a portion of the ulnar collateral ligament of the elbow-joint. The volar (anterior) surface is articular, and enters into the formation of the semilunar notch.
The processus coronoideus (coronoid process) is a bracket-like process, which juts forwards from the volar and proximal part of the shaft, and is fused with the olecranon proximally. By its proximal surface it enters into the formation of the semilunar notch, whilst its volar aspect, which is separated from its proximal side by a sharp irregular margin, slopes distally and dorsally to become confluent with the volar surface of the body. Of triangular shape, this area, which is rough and tubercular, terminates inferiorly in an oval elevated tubercle (tuberositas ulnæ), into which the tendon of the brachialis muscle is inserted. Of the lateral margins of the coronoid process, the medial is usually the better defined. Proximally, where it joins the proximal border, there is generally a salient tubercle, to which one of the heads of origin of the flexor digi
torum sublimis muscle is attached, whilst distal to this point the medial border furnishes origins for the pronator teres, and occasionally for the flexor pollicis
longus muscles, from above downwards. The smooth medial surface of the coronoid process merges with the olecranon dorsally, and with the medial surface of the body distally.
The incisura semilunaris (O.T. greater sigmoid cavity), for articulation with the trochlea of the humerus, is a semicircular notch, the proximal part of which is formed by the volar surface of the olecranon, whilst distally it is completed by the proximal surface of the coronoid process. Constricted towards its deepest part by the notching of its borders, the articular surface is occasionally crossed by a narrow impression which serves to define the olecranon proximally from the coronoid distally. The articular area is divided into a medial portion, slightly concave transversely, and a lateral part, transversely convex to a slight degree, by a longitudinal smooth ridge which extends from the most prominent part of the border of the olecranon proximally to the most outstanding point of the coronoid process distally. The margins of the semilunar notch are sharp and well defined, and serve, with the exception of the area occupied by the radial notch, for the attachment of the capsule of the elbow-joint.
The radial notch (O.T. lesser sigmoid cavity), placed on the radial side of the coronoid process, is an oblong articular surface for the reception of the head of the radius. It encroaches on the distal and lateral part of the semilunar notch, so as to narrow it considerably. Separated from it by a rectangular curved edge, it displays a surface which is plane proximodistally, and concave from before backwards. Its volar extremity is narrower and more pointed than its dorsal, and becomes confluent with the anterior edge of the coronoid process, at which point the annular ligament, which retains the head of the radius in position, is attached in front. Its dorsal border, wider and more outstanding, lies in line, and is continuous with the interosseous margin of the shaft. Dorsal to this border, the annular ligament is attached posteriorly.
The body of the ulna (corpus ulnæ), which is nearly straight, or but slightly curved, is stout and thick proximally, gradually tapering towards its distal extremity. It may be divided into three surfaces, a volar (O.T. anterior), a dorsal or posterior, and a medial, by three well-defined borders, an interosseous crest, a dorsal margin, which latter is
subcutaneous throughout its whole length, and a volar margin (O.T. anterior border).
The crista interossea (interosseous crest) is crisp and sharp in the proximal three-fourths of the body, but becomes faint and ill-defined in the distal fourth. To this, with the exception only of the part which forms the dorsal boundary of the hollow in which the tuberosity of the radius is disposed when the two bones are articulated, is attached the interosseous membrane which connects the two bones of the forearm. The dorsal margin, of sinuous outline, curving laterally above, and slightly medially below, is continuous proximally with the triangular subcutaneous area on the back of the olecranon, being formed by the confluence of the borders which bound that surface; well marked above, it becomes faint and more rounded below, but may be traced distally to the dorsal surface of the base of the styloid process. To this border is attached an aponeurosis common to the flexor carpi ulnaris, extensor carpi ulnaris, and flexor digitorum profundus muscles. A noteworthy feature in connexion with this part of the body is the fact that it is subcutaneous, and can easily be felt beneath the skin throughout its whole length.
FLEXOR DIGITORUM SUBLIMIS
FLEXOR POLLICIS LONGUS
FLEXOR DIGITORUM SUBLIMIS
The volar or anterior surface corresponds to the front and medial side of the body. It is described as consisting of two surfaces, a volar and a medial, which are separated by a rounded volar margin, which extends from the tuberosity proximally towards the styloid process distally. The prominence of this ridge varies in different bones, being well marked in bones of a pronounced type, but corresponding merely to the rounding of the surfaces in poorly developed specimens. The volar aspect of the bone affords an extensive origin to the flexor digitorum profundus muscle, which clothes its volar and medial surfaces in its proximal threefourths, reaching as far back as the dorsal border, and extending proximally as high as the medial side of the olecranon process. Immediately distal to the radial notch there is a hollow triangular area, limited dorsally by the proximal part of the interosseous crest, and defined in front by an oblique line which extends distally and backwards from the lateral margin of the coronoid process. In this hollow the tuberosity of the radius rests when the forearm is in the prone position, and to its floor are attached the fibres of origin of the supinator muscle. The distal fourth of the body is crossed by the fibres of the pronator quadratus muscle, which derives its origin from a more or less well-defined crest, which winds spirally distally and backwards towards the volar surface of the root of the styloid process, and is continuous proximally with the volar margin.
FIG. 204.-VOLAR ASPECT OF BONES or
ATTACHMENTS MAPPED OUT.
The dorsal surface of the body lies between the dorsal margin and the interosseous crest. At its proximal part it is placed behind the semilunar and radial notches, extending on to the lateral side of the olecranon. Here an area corresponding to the proximal third of the length of the bone is marked off distally by an oblique ridge which leaves the interosseous crest about an inch or more distal to the dorsal edge of the radial notch. Into this somewhat triangular surface the fibres of the anconæus are inserted. Distal to this the
posterior surface is subdivided by a faint longitudinal ridge, the bone betwee which and the interosseous crest furnishes origins for the abductor pollicis longus, extensor pollicis longus, and extensor indicis proprius muscles, in order proximodistally. The surface of bone between the dorsal margin and the afore-mentioned longitudinal line is smooth and overlain by the extensor carpi ulnaris muscle, which, however, does not arise from it.
The distal extremity of the ulna presents a rounded head (capitulum ulnæ), from which, on its medial and dorsal aspect, there projects distally a cylindrical pointed process called the styloid process. To the extremity of this latter is attached the ulnar collateral ligament of the carpus, whilst on the volar aspect it has connected with it the antero-medial portion of the capsule of the wrist-joint. The antero-lateral half of the circumference of the head is furnished with a smooth narrow convex articular surface, which fits into the ulnar notch of the radius. Its distal surface, flat and semilunar in shape, and separated from the root of the styloid process by a well-marked groove, rests on the upper surface of the triangular articular disc of the wrist, the apex of which is attached to the groove just mentioned. The margins of the head, to the volar side and dorsal to the radial articular surface, have attached to them the volar and dorsal distal radioulnar ligaments. The dorsal and medial surface of the styloid process is channelled by a groove which separates it from the dorsal surface of the head, and extends proximally some little way upon the dorsal surface of the distal end of the body. In this is lodged the tendon of the extensor carpi ulnaris muscle. The proportionate length of the ulna to the body height is as 1 is to 6.26-6.66.
Nutrient Foramina.-A foramen, having a proximal direction, for the nutrient artery of the body opens on the volar surface of the bone from two to three inches distal to the tuberosity. Vascular canals of large size are seen proximal and dorsal to the radial notch, just dorsal to the notched lateral border of the semilunar notch. At the distal end of the bone similar openings are seen in the groove between the styloid process and the distal articular surface of the head.
Fuses with shaft about 16 years
Connexions.-The ulna articulates proximally with the trochlea of the humerus. On the lateral side it is in contact with the radius at both proximal and distal ends, the proximal radioulnar articulation being formed by the head of the radius and the radial notch of the ulna, the distal radio-ulnar joint comprising the head of the ulna, which fits into the ulnar notch of the radius. Between these two joints the bodies of the bones are united by the interosseous membrane. The distal surface of the head of the ulna does not articulate with the carpus, but rests on the proximal surface of the interposed articular disc. The ulna is superficial throughout its entire extent. Proximally the olecranon process can be readily recognised, particularly when the elbow is bent, as in this position the olecranon is withdrawn from the olecranon fossa of the humerus in which it rests when the joint is extended. Distal to this the subcutaneous triangular area on the back of the olecranon can be easily determined, and from it the posterior border of the bone can readily be traced along the line of the "ulnar furrow" to the styloid process. With the hand supine this latter process can be felt to the medial side and slightly behind the wrist. When the hand is pronated, the distal end of the radius rolls round the distal extremity of the ulna, and the antero-lateral surface of the head of the latter bone now forms a wellmarked projection on the dorsum of the wrist in line with the cleft between the little and ring fingers.
Appears early in 2nd month of fœtal life
Appears about 10 years
Ossification.-The ulna is ossified from one primary and two or more secondary centres. The centre for the body appears early in the second month of foetal life. At birth the body and a considerable part of the proximal extremity, including the coronoid process, are ossified, as well as part of the distal extremity. The olecranon and the distal surface of the head and the styloid process are cartilaginous. About ten years of age a secondary centre appears in the cartilage at the proximal end of the bone, and
Appears about 6 years
Fuses with shaft 20-23 years
forms an epiphysis which unites with the body about sixteen. In this connexion Fawcett (Proc. Anat. Soc. Great Britain and Ireland, 1904, p. xxvii) has described the occurrence
of two ossific centres in the olecranon. One, the more volar, the "beak centre," enters into the formation of the proximal end of the articular surface of the semilunar notch, the other centre, not in any way forming it. A scale-like centre appears in the cartilage of the head about the sixth year, from which the distal surface of that part of the bone is developed, and by the extension of which the styloid process is also ossified; this epiphysis does not unite with the shaft till the twentieth or twenty-third year. Independent centres for the styloid process and for the extreme edge of the olecranon have also been described. The student may here be warned that the epiphysial line between the shaft and proximal or olecranon epiphysis does not correspond to the constricted part of the semilunar notch, but lies considerably proximal to it.
Proximal Extremity. -The capitulum (head) is disc-shaped and provided
with a shallow concave surface (fovea capituli radii) proximally for articulation with the capitulum of the humerus. The circumference of the head (circumferentia articularis) is smooth and is embraced by the annular ligament. On the medial side it is usually much broader, and displays an articular surface, plane in the proximo-distal direction,
Ext. poll. brevis
Ext. carpi rad.
GROOVE FOR EXT. Ext. dig. commun.
The radius, or lateral bone of the forearm, is shorter than the ulna, with which it is united on the medial side. Proximally it articulates with the humerus, and distally it supports the carpus. It consists of a head, a neck, a tuberosity, a body, and an expanded distal extremity. The body is narrow proximally, but increases in all its diameters distally.
Ext. poll, STYLOID PROCESS