92 THE SKELETON OF THE HORSE 



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 primse) are 

 situated behind the distal end of the large metacarpal bone, and are closely at- 

 tached 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 1 )y a layer of carti- 

 lage 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 tertise) is 

 shuttle-shaped, and is situated behind the junction of the second and third phal- 

 anges. Its long axis is transverse, and it possesses two surfaces, two borders, and 



two extremities. The articular surface 



... , (Facies articularis) faces upward and 



Articular sur- ) . . „ , . 



face for second forward; it consists 01 a central emi- 

 phalanx nence, flanked by concave areas, and 



Articular ^^^^Mce£' '^'^ articulates with the distal end of the 



/^'^ww^'r^^ssaK. r, • , 7 7 7 second lohalanx. The flexor or tendon 



W y'.j:-:i,f>*:rriy^ Distal border /t^. n ■ \ ■ i- , 



— -^« 'i^a » ^" "^ surface (Facies liexoria) is directed 



Fig. 59.— Third Sesamoid or Navicular Bone of downward and backward. It reSCm- 

 Horse. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ii,i ,-i c -c i. 



Dies the articular surface m 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 promi- 

 nent edge. Tlic extremities are blunt-pointed. 

 Development. — It ossifies from a single center. 



The Bones of the Pelvic Limb 



The pelvic girdle consists of the ossa coxae, which unite ventrally at the sym- 

 physis pelvis, and articulate with the sacrum dorsally. 



OS COX^ 

 The OS coxae (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. 



