THE SKELETAL SYSTEM 39 



is the shortest and is nearly straight, the others are more or less 

 curved. 



The sternum, or breast bone, is situated in the ventral part of 

 the chest wall. It is suspended by means of the ribs. There are 

 seven segments or sternebrse which never undergo complete ossi- 

 fication. The upper surface is triangular in shape, narrow in 

 front. The lateral surfaces present seven articular cavities for 

 the cartilages of the true ribs. The inferior border is convex, 

 keel-like, and may be distinctly felt in the living animal. Both 

 extremities are cartilaginous. 



BONES OF THE APPENDICULAR SKELETON 



The appendicular skeleton comprises the bones of the thoracic 

 and pelvic limbs. 



A. The bones of the thoracic or fore Zmfe, named from above down- 

 ward, are the scapula or shoulder-blade, the humerus or arm, the 

 radius and ulna or forearm, the carpus or knee, the metacarpus or 

 cannon, the first phalanx or large pastern, the second phalanx 

 or small pastern, the third phalanx or coffin bone, the proximal pair 

 of sesamoids, and the distal sesamoid or navicular bone (Fig. 4). 



The scapula is located on the anterolateral surface of the thorax. 

 In the horse it is connected to the axial skeleton by muscles only 

 (clavicle in man and some other animals) . It is directed obliquely 

 downward and forward. The scapula is a flat bone, triangular in 

 shape, presenting for description two surfaces, three borders, and 

 three angles. The external surface is divided into two unequal 

 parts by the spine, a prominent ridge running lengthwise the bone. 

 The narrow fossa in front of the spine is named the supraspinous 

 fossa and the one posterior to the spine the infraspinous fossa. 

 The internal surface is taken up by the subscapular fossa. Above 

 on each side are two rough triangular areas for muscular attach- 

 ment. The superior border is thick and straight in the young and 

 growing animal, but becomes uneven with age; it gives attachment 

 to the scapular cartilage, the unossified part of the fetal scapula. 

 The anterior border is thin and rough in its upper part, concave 

 and smooth below. The posterior border is thick, concave, and 

 rough. There are three angles to the bone, named respectively 

 anterior, posterior, and articular or humeral. The articular angle 

 carries a glenoid cavity for articulation with the head of the 

 humerus. 



