Part 21

FIG. 48.—LEFT RADIUS AND ULNA OF HORSE, EXTERNAL VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

FIG. 49.—LEFT RADIUS AND ULNA OF HORSE, POSTERIOR VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

The =distal extremity= is also compressed from before backward. It presents the =carpal articular surface= (Facies articularis carpea) which consists of three parts. The inner facet is the largest, is quadrilateral, concavo-convex from before backward, and articulates with the radial carpal bone (or scaphoid); the middle one is somewhat similar in form but smaller, and articulates with the intermediate carpal bone (or semilunar); the outer facet is smaller, is convex, and articulates below with the ulnar carpal (or cuneiform) and behind with the accessory carpal (or pisiform). The anterior surface presents three =grooves=, separated by ridges. The middle one is vertical and gives passage to the tendon of the extensor carpi radialis; the outer one is similar and contains the tendon of the anterior extensor of the digit; the inner one is small and oblique and lodges the tendon of the extensor carpi obliquus. The posterior aspect is crossed by a rough ridge, below which are three depressions. On either side is a tuberosity (Tuberculum ligamenti) to which the lateral ligament is attached. The outer one is marked by a small vertical groove for the passage of the lateral extensor tendon.

=Development.=—The radius ossifies from four centers, viz., one each for the shaft, the two extremities, and the outer part of the distal end; the last is morphologically the distal end of the ulna which has fused with the radius, and the line of fusion is often indicated by a distinct groove on the carpal articular surface. The proximal extremity unites with the shaft at about one and a half years, the distal end at about three and a half years.

FIG. 50.—UPPER HALF OF RADIUS AND ULNA OF HORSE, INTERNAL VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

THE ULNA

The =ulna= of the horse is a reduced long bone situated behind the radius, with which it is partially fused in the adult.

The =shaft= (Corpus ulnæ) is three-sided and tapers to a point below. The =anterior surface= (Facies dorsalis) is applied to the posterior surface of the radius, and below the interosseous space the two bones are fused in the adult. The surface which enters into the formation of the space is smooth and usually presents a small nutrient foramen, directed upward. Above the space it is rough and is attached to the radius by an interosseous ligament which is usually permanent. The =internal surface= (Facies medialis) is smooth and slightly concave. The =external surface= (Facies lateralis) is flattened. The =internal= and =external borders= are thin and sharp, except at the interosseous space. The =posterior border= is slightly concave in its length and is rounded. The lower end is pointed and is usually a little below the middle of the radius. It is commonly continued by a fibrous cord to the distal external tuberosity of the radius, but this band may be replaced in part or entirely by bone.

The =proximal extremity= is the major part of the bone. It projects upward and somewhat backward behind the lower end of the humerus, and forms a lever arm for the extensor muscles of the elbow. The =internal surface= is concave and smooth. The =external surface= is convex and is roughened above. The =anterior border= bears on its middle a pointed projection, the =processus anconæus= or “beak,” which overhangs the =semilunar notch= or =sigmoid cavity= (Incisura semilunaris). The latter is triangular in outline, concave from above downward, and articulates with the humerus; in the middle of its lower part is an extensive synovial fossa. Just below the notch are two convex =facets= which articulate with those on the posterior aspect of the proximal end of the radius. The =posterior border= is nearly straight, and is thick and rounded. The free end or summit is a rough tuberosity, the =olecranon=, which gives attachment to the triceps brachii and other muscles.

The primitive =distal extremity= has, as previously stated, fused with the radius.

=Development.=—The ulna ossifies from three centers, of which one is for the main part of the bone, one for the olecranon, and one for the distal end. The cartilaginous embryonic ulna extends the entire length of the forearm. The lower part of the shaft is usually reduced to a small fibrous band or may disappear entirely; in some cases a variable remnant of it ossifies. The distal extremity fuses early with the radius. The olecranon unites with the rest of the bone at three to three and a half years. A medullary canal appears to occur constantly in the adult—contrary to the statements of some authors.

THE CARPUS

The =carpus= of the horse consists of seven or eight bones (Ossa carpi) arranged in two rows, proximal or antibrachial, and distal or metacarpal. The (abbreviated) names and relative positions of the bones of the left carpus as seen from in front are indicated below.

Proximal Row: Radial Intermediate Ulnar Accessory

Distal Row: First Second Third Fourth

THE RADIAL CARPAL BONE

The =radial carpal= bone (Os carpi radiale, scaphoid) is the largest bone of the upper row; it is somewhat compressed laterally, and is clearly six-sided. The =superior= or =proximal surface= is convex in front, concave behind, and articulates with the inner facet on the distal end of the radius. The =inferior= or =distal surface= is also convex in front and concave behind; it articulates with the second and third carpal bones. The =external surface= bears upper and lower facets on its anterior part for articulation with the intermediate; between and behind these it is excavated and rough. The =anterior= or =dorsal surface= is rough and slightly convex. The =internal surface= and the =posterior= or =volar surface= are rough and tuberculate.

FIG. 51.—SAGITTAL SECTION OF UPPER PART OF RADIUS AND ULNA OF HORSE. Cm, Medullary cavity of ulna. ]

THE INTERMEDIATE CARPAL BONE

The =intermediate carpal= bone (Os carpi intermedium, semilunar, lunar) is somewhat wedge-shaped, wider in front than behind. The =superior= or =proximal surface= is saddle-shaped, and articulates with the middle facet on the distal end of the radius. The =inferior= or =distal surface= is smaller, convex in front, concave behind, and articulates with the third and fourth carpal bones. The =internal surface= has upper and lower facets for articulation with the radial carpal, and between these it is excavated and rough. The =external surface= is similar to the preceding and articulates with the ulnar carpal. The =anterior= or =dorsal surface= is rough and slightly convex. The =posterior= or =volar surface= bears a tuberosity on its lower part.

THE ULNAR CARPAL BONE

The =ulnar carpal= bone (Os carpi ulnare, cuneiform) is the smallest and most irregular bone of the upper row. The =superior= or =proximal surface= is concave, and fits the lower part of the outer facet on the distal end of the radius. The =inferior= or =distal surface= is oblique and undulating for articulation with the fourth carpal bone. The =internal surface= has upper and lower facets for articulation with the intermediate. The =anterior= or =dorsal= and =external surfaces= are continuous, convex, and rough. The =posterior= or =volar surface= is oblique, and bears a concave facet for articulation with the accessory carpal bone; below this is a tubercle.

FIG. 52.—LEFT CARPAL BONES OF HORSE, WITH DISTAL END OF RADIUS AND PROXIMAL END OF METACARPUS; INTERNAL VIEW. ]

FIG. 53.—LEFT CARPAL BONES OF HORSE, WITH ADJACENT ENDS OF RADIUS AND METACARPUS; EXTERNAL VIEW.

Ca, Accessory carpal bone; Cr, radial carpal; Ci, intermediate carpal; Cu, ulnar carpal; C1–4, first to fourth carpals; Mc. II, III, IV, metacarpal bones; 1, groove for tendon of extensor carpi obliquus; 2, groove for lateral extensor tendon; 3, groove for tendon of flexor carpi externus; 4, metacarpal tuberosity. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

THE ACCESSORY CARPAL BONE

The =accessory carpal= bone (Os carpi accessorium, pisiform) is situated behind the ulnar carpal bone and the outer part of the distal end of the ulna. It is discoid and presents for description two surfaces and a circumference. The =internal surface= is concave and forms the outer wall of the carpal groove. The =external surface= is convex and rough; a smooth groove for the outer tendon of the flexor carpi externus crosses its anterior part obliquely downward and slightly forward. The =anterior border= bears two facets; the upper one is concave and articulates with the back of the outer facet on the distal end of the radius; the lower one is convex and articulates with the ulnar carpal bone. The remainder of the circumference is rounded and rough.

The accessory does not directly bear weight, and may be regarded as a sesamoid bone interposed in the course of the tendons of the middle and external flexors of the carpus, which it enables to act at a mechanical advantage. The posterior border furnishes attachment to the transverse carpal ligament, which completes the carpal canal for the flexors of the digit.