The Phalanges

The First Phalanx

The first phalanx (Phalanx prima) is a long bone, situated between the large metacarpal bone above and the second phalanx below. It is directed obliquely downward and forward, forming an angle of 50 to 55 degrees with the horizontal plane in well-formed limbs. It consists of a shaft and two extremities.

The shaft (Corpus) is wider and much thicker above than below, and presents two surfaces and two borders. The anterior or dorsal surface is convex from side to side and smooth. The posterior or volar surface is flattened, and bears a triangular rough area, bounded laterally by ridges which begin at the tuberosities above and converge below; this area furnishes attachment to the inferior sesamoidean ligaments. The borders, internal and external, are rounded and have a rough area or a tubercle on their middle parts.

The proximal extremity (Basis) is relatively large. It bears an articular surface adapted to the distal end of the large metacarpal bone, consisting of two glenoid cavities separated by a sagittal groove; the inner cavity is a little larger than the outer one. The posterior angles are formed by buttress-like tuberosities for ligamentous attachment. The anterior surface has a slight elevation for the attachment of the lateral extensor tendon.

The distal extremity is smaller, especially in its antero-posterior diameter. It presents a trochlea for articulation with the second phalanx, consisting of a shallow central groove and two lateral convex areas or condyles; the inner area is a little the larger. On either side, just above the margin of the articular surface, is a depression surmounted by a tubercle, to both of which the lateral ligament is attached. Behind the tubercle is a distinct facet to which the superficial flexor tendon is attached.

Development. The first phalanx ossifies from three centers. The distal end unites with the shaft before birth, the proximal end early in the first year.

The first phalanx contains a small medullary canal in the middle of the shaft. It may be remarked that the bone is twisted slightly; when placed volar surface down on the table, it touches the latter by three points only, the proximal tuberosities and the internal condyle.

The Second Phalanx

The second phalanx (Phalanx secunda) is situated between the first and third phalanges, its direction corresponding to that of the first phalanx. It is flattened from before backward, and its width is greater than its height. It may be described as possessing four surfaces.

The upper or proximal surface presents two glenoid cavities separated by a low ridge, and articulates with the first phalanx. The middle of the anterior border is elevated and roughened in front for the attachment of the anterior or common extensor tendon. The posterior border is thick and overhanging; in the fresh state its middle part is covered with cartilage, over which the deep flexor tendon passes. On either side there is an eminence, to which the lateral ligament and the superficial flexor tendon are attached.

The inferior or distal surface is trochlear, and articulates with the third phalanx and third sesamoid bone. It resembles somewhat the trochlea of the first phalanx, but is more extensive and encroaches more on the anterior and posterior surfaces.

The anterior or dorsal surface is convex from side to side and smooth in its middle; on its lower part are lateral rough depressions, surmounted by tuberosities, to both of which ligaments are attached.

The posterior or volar surface is smooth, flattened, and slopes obliquely downward and forward. The borders which separate the anterior and posterior surfaces are concave from above downward, rounded from before backward.

Development.—The second phalanx ossifies like the first, but the proximal end unites with the shaft two or three months earlier.

The Third Phalanx

The third or ungual phalanx (Phalanx tertia) is entirely inclosed by the hoof, to which it conforms in a general way. It presents for examination three surfaces, three borders, and two angles or wings.

The articular surface (Facies articularis) faces upward and backward, and is chiefly adapted to the distal surface of the second phalanx, but a narrow flattened area along the posterior border articulates with the third sesamoid. The anterior or coronary border forms a central eminence, the extensor (or pyramidal) process (Processus extensorius), to the front of which the anterior extensor tendon is attached. On either side is a depression for the attachment of the lateral ligament.

The dorsal or wall surface (Facies dorsalis) slopes downward and forward. The angle of inclination on the ground plane is about 45 to 50 degrees in front. Laterally the height diminishes, and the slope becomes steeper, especially on the inner side. From side to side the curvature is almost semicircular. The surface is rough and porous, resembling pumice stone somewhat. It is perforated by numerous foramina of various sizes; a series of larger ones is situated on or near the lower border. On either side the dorsal (or preplantar) groove (Sulcus dorsalis) passes forward from the wing and ends at one of the larger foramina. In the fresh state this surface is covered by the matrix of the wall of the hoof. The inferior or distal border is thin, sharp, and irregularly notched; there is commonly a wider notch in front.

The volar or inferior surface (Facies volaris) is arched, and divided into two unequal parts by a curved rough line, the semilunar crest (Crista semilunaris). The larger anterior area is crescent-shaped, concave, and comparatively smooth; it corresponds to the sole of the hoof, and may be termed the sole surface. The posterior part is much smaller, and is semilunar; it is related to the deep flexor tendon, and is hence called the flexor or tendon surface (Facies flexoria). It presents a central prominent rough area, on either side of which is the volar (or plantar) foramen (Foramen volare), to which the volar (or plantar) groove (Sulcus volaris) conducts from the wing. The foramina lead into the semilunar canal within the bone, from which small canals lead to some of the foramina of the wall surface. The deep flexor tendon is inserted into the semilunar crest and the central rough area behind it.

The volar grooves and foramina transmit the terminations of the digital arteries into the semilunar canal, where they meet and form a terminal arch, from which branches pass through canals in the bone and emerge through the foramina on the wall surface.

The angles or wings (Anguli) are prismatic masses which project backward on either side; the inner one is usually the shorter. Each is divided into upper and lower parts by a notch, or is perforated by a foramen which leads to the vascular groove on the side of the wall surface. The upper border carries the lateral cartilage.

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.