178 A MANUAL OF ANATOMY 



inous collections fuse, and thereafter the primary centres coalesce Ossifi- 

 cation from these two centres proceeds at first in the precartilaginous tissue, 

 but subsequently in the cartilage to which this tissue gives place. The 

 primary centres may fail to join, w^ith the result that the clavicle may persist 

 in two halves. 



The secondary centre appears in the cartilage of the sternal end about the 

 zoth year, and this epiphysis joins the shaft about the 25/A year (Mall and 

 Fawcett) . 



The law of ossification applicable to bones having a shaft and 

 one epiphysis is as follows: the medullary foramen and the canal 

 to which it leads are directed towards that extremity which has no epi- 

 physis. This law is illustrated in the clavicle and the metacarpal, 

 metatarsal, and phalangeal bones. 



The Scapula. 



The scapula or shoulder-blade is situated on the posterior aspect 

 of the thorax, where it extends from the second to the seventh 

 rib, being separated by muscles from the thoracic wall. It consists 

 of a body and three processes, namely, a spine, an acromion and 

 a coracoid process. 



The body is a thin triangular plate, and it presents two surfaces, 

 three borders, and three angles. The anterior surface, venter, or 

 subscapular fossa is concave, and the bone forming it is for the 

 most part thin, except near the external or axillary border, where 

 there is a thick, round, elongated ridge. It gives origin to the 

 subscapularis, except (i) along the anterior aspect of the base from 

 the superior to the inferior angle, where the serratus magnus is 

 inserted, and (2) over the front of the neck. The venter is crossed 

 by three or four oblique ridges, which extend upwards and outwards 

 from the base, and give attachment to tendinous septa intersect- 

 ing the subscapularis muscle. 



The posterior surface or dorsum is irregularly convex, and is divided 

 into two unequal parts by the spine. The upper division, along with 

 the superior surface of the spine, forms the supraspinous fossa. It 

 represents about one-fourth of the dorsum, and gives origin over its 

 inner two-thirds to the supraspinatus. In the region of the neck 

 it presents a nutrient foramen for a branch of the suprascapular 

 artery. The lower division, along with the inferior surface of the 

 spine, forms the infraspinous fossa, and it represents about three- 

 fourths of the dorsum. Towards the external or axillary border 

 it presents an elongated concavity, external to which is an oblique 

 line extending from its upper end downwards and inwards to the 

 base near the inferior angle. The infraspinous fossa, as far out 

 as this oblique line, gives origin to the infraspinatus, except at 

 the upper and outer part, and it presents a nutrient foramen 

 superiorly, close to the spine near the centre, for a branch of the 

 dorsalis scapulae artery. The oblique line marks off impressions for 

 the teres muscles and dorsalis scapulae artery, as follows: the teres 

 minor arises from about the upper two-thirds, near the centre of 

 which there is a groove for the dorsalis scapulae artery, and the 



