Part 24

The =lateral cartilages= (Cartilagines ungulæ) are rhomboid curved plates, which surmount the wings on either side. They are relatively large and extend above the margin of the hoof sufficiently to be distinctly palpable. The abaxial surface is convex, the axial concave. The upper border is convex and thin; the lower is thicker and is in part attached to the wing. The anterior end is attached by ligament to the side of the second phalanx. The posterior end curves toward its fellow at the heel, and is perforated by numerous foramina for the passage of veins. The central part is mainly hyaline, the periphery mainly fibrous.

It will be noted that the size and form of the wings vary much in different specimens. In the new-born foal the wing is a small, pointed projection. Later the process of ossification invades the lower part of the cartilage to a varying extent. In some cases the greater part of the cartilage is ossified—a condition commonly termed “sidebone.”

=Development.=—The ossification of the terminal phalanx is peculiar. While the proximal articular part is still cartilaginous, a perichondrial cap of bone is formed in relation to the hoof. Later the process extends into the upper part.

=Structure.=—The interior of this bone is channeled by numerous canals for vessels, most of which radiate from the semilunar canal to the wall surface; these are not canals for nutrient vessels of the bone, but transmit arteries to the matrix of the hoof. Thick layers of compact substance are found at the articular and flexor surfaces and the extensor process, i. e., at the points of greatest pressure and traction.

THE SESAMOID BONES

The two =proximal= or =great sesamoids= (Ossa sesamoidea phalangis primæ) are situated behind the distal end of the large metacarpal bone, and are closely attached to the first phalanx by strong ligaments. Each has the form of a three-sided pyramid. The =anterior= or =articular surface= conforms to the corresponding part of the distal end of the large metacarpal bone. The =posterior= or =flexor surface= is flattened and oblique; in the fresh state it is covered by a layer of cartilage which also fills the interval between the opposed borders of the two bones, and forms a smooth groove for the deep flexor tendon. The =abaxial surface= is concave, and gives attachment to part of the suspensory ligament; it is separated from the posterior surface by a rough everted border. The =base= faces downward, and furnishes attachment to the inferior sesamoidean ligaments. The =apex= is directed upward and is rounded.

The =third sesamoid= or =navicular= bone (Os sesamoideum phalangis tertiæ) is shuttle-shaped, and is situated behind the junction of the second and third phalanges. Its long axis is transverse, and it possesses two surfaces, two borders, and two extremities. The =articular surface= (Facies articularis) faces upward and forward; it consists of a central eminence, flanked by concave areas, and articulates with the distal end of the second phalanx. The =flexor= or =tendon surface= (Facies flexoria) is directed downward and backward. It resembles the articular surface in form, but is more extensive and not so smooth. In the fresh state it is coated with cartilage and the deep flexor tendon plays over it. The =proximal border= (Margo liber) is wide and grooved in its middle, narrower and rounded on either side. The =distal border= (Margo ligamenti) bears in front a narrow facet for articulation with the third phalanx. Behind this is a groove, which contains a number of relatively large foramina, and is bounded behind by a prominent edge. The =extremities= are blunt-pointed.

FIG. 59.—THIRD SESAMOID OR NAVICULAR BONE OF HORSE. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

=Development.=—It ossifies from a single center.

THE BONES OF THE PELVIC LIMB

The =pelvic girdle= consists of the ossa coxæ, which unite ventrally at the symphysis pelvis, and articulate with the sacrum dorsally.

OS COXÆ

The =os coxæ= (or os innominatum) forms the skeleton of the hip or haunch, and is the largest of the flat bones. It consists primarily of three parts, the =ilium=, =ischium=, and =pubis=, which meet to form the =acetabulum=, a large cotyloid cavity for articulation with the head of the femur. These parts are fused at about one year of age, but it is convenient to describe them separately.

THE ILIUM

The =ilium= (Os ilium) is the largest of the three parts. It is irregularly triangular and presents two surfaces, three borders, and three angles.

The =gluteal surface= (Facies glutæa) faces upward, backward, and outward. It is wide and concave in front, narrower and convex behind. The wide part is crossed by the curved =gluteal line= (Linea glutæa), which extends from the middle of the inner border toward the external angle. This surface gives attachment to the middle and deep gluteal muscles.

FIG. 60.—OSSA COXARUM OF MARE, DORSAL VIEW.

O.il., Ilium; O.p., pubis; O.is., ischium; A.o.i., wing of ilium; C.o.i., shaft of ilium; Cr.o.i., anterior border (crest) of ilium; 1, obturator foramen; 2, acetabulum; 3, internal angle of ilium; 4, external angle of ilium; 5, gluteal line; 6, psoas tubercle; 7, 8, acetabular and symphyseal branches of pubis; 9, 10, acetabular and symphyseal branches of ischium; 12, lesser sciatic notch; 13, ischial arch; 14, great sciatic notch; 15, symphysis pelvis; 17, ilio-pectineal eminence; 18, anterior borders of pubic bones; 19, posterior gluteal line. (Struska, Anat. d. Haustiere.) ]

The =pelvic surface= (Facies pelvina) faces in the opposite direction; it is convex, and consists of two distinct parts. The inner triangular part (Pars articularis) is roughened for ligamentous attachment, and bears an irregular facet, the =auricular surface= (Facies auricularis), for articulation with the sacrum. The outer quadrilateral part (Pars iliaca) is in general smooth. It is crossed by the =ilio-pectineal line= (Crista iliopectinea), which begins below the auricular surface and is continued on the shaft of the bone to join the anterior border of the pubis. The line is interrupted by furrows for the iliaco-femoral vessels, and below these it bears the =psoas tubercle= (Tuberculum psoadicum), which gives attachment to the psoas minor muscle. The iliacus muscle is attached to the surface external to the ilio-pectineal line.

The =anterior border= or =crest= (Crista iliaca) is concave, thick, and rough.

The =internal border= is deeply concave; its middle part forms the lower boundary of the =great sciatic foramen=, and it is continuous behind with the =superior ischiatic spine=.

The =external border= is concave and in great part rough. Its fore part is crossed by grooves for the ilio-lumbar vessels, which are continued on the pelvic surface. The =nutrient foramen= is usually situated on or near the posterior part of this border.

FIG. 61.—OSSA COXARUM OF MARE, VENTRAL VIEW.

D, Ilium; Sch., pubis; S, ischium; a, anterior border (crest) of ilium; b, internal angle of ilium; c′, external angle of ilium; d, great sciatic notch; e, external border of ilium; f, iliac surface; g, linea arcuata; h, rough ligamentous area; i, auricular surface; k, ilio-pectineal line; l, psoas tubercle; m, transverse branch, and n, symphyseal branch of pubis; o, o′, symphysis pelvis; p, anterior border of pubis (pecten); q, tuberculum pubicum; r, ilio-pectineal eminence; s, tuber ischii; t, ischial arch; u, lesser sciatic notch; v, acetabular branch, and w, symphyseal branch of ischium; x, obturator foramen; y, articular surface of acetabulum; z, fossa acetabuli; 1, groove for ilio-lumbar artery; 2, groove for iliaco-femoral artery; 3, subpubic groove; 4, depression for inner tendon of origin of rectus femoris; 5, rough area for attachment of adductor muscles. (Ellenberger-Baum, Anat. d. Haustiere.) ]

The =internal= or =sacral angle= (Tuber sacrale) curves upward and a little backward close to the first sacral spine, and forms here the highest point of the skeleton. It is somewhat thickened and rough.

The =external= or =coxal angle= (Tuber coxæ) forms the basis of the point of the hip. It is a large quadrangular mass, narrow in its middle, and enlarged at either end, where it bears a pair of tuberosities. It is roughened for muscular attachment.

The =posterior= or =acetabular angle= meets the other two bones at the acetabulum, of which it forms about two-fifths. Its prominent upper border forms part of the =superior ischiatic spine=, which is roughened externally, smooth internally. Two depressions above and in front of the acetabulum give attachment to the tendons of origin of the rectus femoris muscle. This angle is connected with the ala or wide part of the bone by a constricted part, often termed the =shaft=. The latter is of three-sided prismatic form. Its external surface is convex and rough, and gives attachment to the deep gluteus muscle. Its pelvic surface is smooth and is grooved for the obturator vessels and nerves. Its ventral surface is crossed by vascular grooves, below which is a rough area, bounded internally by the psoas tubercle.

THE ISCHIUM