692 HUMERUS. 



upper surface is smooth. Its lower presents a ridge for 

 the attachment of the rhomboid or costo-clavicular liga- 

 ment,, and a groove for the subclavian muscle. The sternal 

 portion is convex in front, and concave behind. The hu- 

 meral end is just the reverse. Two-thirds of the anterior 

 margin give origin to the pectoralis major, the rest to the 

 deltoid muscle. The posterior margin has one or more 

 foramina for nutritious vessels. The sternal superior sur- 

 face gives origin to the sterno-cleido-mastoideus. The 

 sternal extremity is triangular, thick, and forms the ar- 

 ticulating surface. Its margin has ligaments attached to 

 it. The acromiol end is flat, presents an articular surface, 

 and covers the coracoid process. 



Its Structure is compact and cellular. The former is very 

 condensed and strong in the centre of the bone ; the latter 

 abounds mostly at the extremities. 



Function. To support the scapula and prevent its fall- 

 ing forward or inward, and also to protect the vessels and 

 nerves as they pass to the extremity. Its development is 

 from two points of ossification, (one for the body and the 

 other for the sternal end) and is found to be very perfect in 

 the foetus. Ossification begins in the clavicle sooner than 

 in any other bone, as early even as the fifth week of intra- 

 uterine life. The sternal epiphysis is ossified during the 

 fifteenth and twentieth years. It is articulated to the ster- 

 num and scapula. 



SECTION II. 



THE HUMERUS OR ARM-BONE. 



The humerus is the largest bone in the upper extremity, 

 and is situated between the scapula above, and the radius 

 and ulna below. It consists of a body and two extremities. 



The body or shaft is cylindrical ; its upper anterior fourth 

 is divided by the bicipital groove which lodges the long head 

 of the biceps muscle. The anterior edge of this groove has 

 the pectoralis major inserted into it ; its posterior edge 

 receives the insertions of the latissimus dorsi and teres major 

 muscles. About the centre of this bone is seen a rough tri- 



