104 



THE SKELETON OF THE HORSE 



face. 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 dorsal surface. 



The angles or ^^dngs (Anguli) are prismatic masses which project backward on 

 either side; the medial 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 dorsal 

 groove.^ The proximal border carries the cartilage. 



The cartilages of the third phalanx (Cartilagines phalangis tertise)^ are rhom- 

 boid curved plates which surmount the angles 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 proximal border is sinuous 



and thin; the distal 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 for- 

 amina for the passage of veins. 



It will be noted that the size and form 

 of the angles vary much in different specimens. 

 In the new-born foal the angle is a small, 

 pointed projection. Later the process of ossi- 

 fication invades the lower part of the car- 

 tilage to a varying extent. In some cases the 

 greater part of the cartilage is ossified — a con- 

 dition commonly termed " sidebone. " In the 

 j'oung subject the cartilage is hyaline, but later 

 it changes to the fibrous type. 



Development. — The ossification of 

 the third phalanx is peculiar. While 

 the proximal articular part is still car- 

 tilaginous, 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 dorsal sur- 

 face; these are not canals for nutrient 

 vessels of the bone, but transmit 

 arteries to the corium of the hoof. Thick layers of compact substance are found 

 at the articular and flexor surfaces and the extensor process, i. c, at the points of 

 greatest pressure and traction. 



Fig, S3 



Phalaxge.? of 

 New-born Foal; Dor- 

 sal View. 

 Cartilages of third phalanx 

 removed. 



Fig. 84. — Phalanges and 

 Distal Sesamoid of 

 New-born Foal; Vo- 

 lar View. 



Cartilages of third phalanx 

 removed. 



The SESAMom Bones 



The two proximal sesamoids (Ossa sesamoidea phalangis primae) 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 articular surface (Facies articularis) conforms to the corresponding part of the 



1 The upper and lower divisions of the angle are sometimes termed the basilar and retrossal 

 processes respectively. 



2 These are usually called the lateral cartilages, but' this designation could not be retained. 

 They ha\'e the same relation to the third phalanx that the cartilage of the scapula has to the latter 

 bone, and are here named in similar fashion. 



