30 COMPARATIVE ANATOMY 



there is a distinct sternum or breast-bone with which the 

 lower ends of the shoulder-girdle articulate. 



In the mid-ventral line of the breast there lies anteriorly a 

 flat circular plate of cartilage called the episternum. Posterior 

 to this is a bony rod, the omosternum. This is succeeded by 

 a pair of cartilaginous strips, the epicoracoids, which belong to 

 the shoulder girdle; and these again are succeeded by the 

 mesosternum, a rod of cartilage ensheathed in bone; and 

 hindmost of all the xiphisternum, a flat plate of cartilage, 

 somewhat heart-shaped, with a bifid apex. 



The shoulder-girdle proper is to be thought of as two half- 

 rings of bone and cartilage which encircle the fore-part of the 

 body, being united below in the sternum but separate above. 

 Each half-ring is divided by an articular cavity, the glenoid, 

 into an upper or scapular and a lower or coracoid moiety. 

 The scapular moiety comprises an upper expanded portion 

 formed of calcified cartilage, the suprascapula, and a lower 

 bony portion, the scapula. In the coracoid moiety we dis- 

 tinguish a coracoid, a stout bony rod, expanded at its two ends 

 and joined to its fellow of the opposite side by the interven- 

 tion of the epicoracoid cartilages already mentioned. In front 

 of the coracoid a rod of cartilage known as the pre-coracoid 

 connects the scapula with the anterior ends of the epicoracoids,' 

 and this rod is ensheathed by a splint-like bone generally 

 described as the clavicle or collar-bone. 



In the fore- limb there is one bone, the humerus, in the 

 upper arm. It has a shaft and two articular surfaces, one at 

 each end. That of the upper or proximal extremity of the 

 bone is called the head, and fits into the glenoid cavity of the 

 shoulder-girdle. The distal extremity has a rounded knob, 

 internal to which is a small flat articulating surface, the 

 trochlea, placed upon a little eminence 'called the internal 

 condyle. There is a similar external condyle on the outer 

 side of the articular process. 



The fore-arm has two bones, the radius and the ulna, but 

 they are firmly and immovably united together throughout 

 their length so as to form one bone, the radio-ulna. The 

 upper end of the double bone has a large concavity for articu- 

 lation with the humerus, beyond which a hook-shaped process, 

 the olecranon, projects from the ulnar component of the bone. 

 At its distal end the radio-ulnar bears two articular surfaces 



