THE THIRD PHALANX 91 



tenor 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 liga- 

 ment. 



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 nu- 

 merous 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 l)y 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 b}- 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 wail 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 carti- 

 lage. 



The lateral cartilages (Cartilagines ungulse) 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 



^ The upper and lower divisions of the wing are sometimes termed the basilar and retrossal 

 processes respectively. 



