THE ARM. 187 



a superficial fossa is extended for lodging the sub-clavius mus- 

 cle. The sternal two-thirds of the anterior margin are marked 

 by the origin of the pectoralis major; it is there thick: the other 

 part of this margin is thinner, and gives origin to the deltoid 

 muscle. The posterior margin presents, near its middle, one or 

 more foramina for the nutritious vessels. The triangular inter- 

 nal end of the clavicle is unequal where it joins the sternum, 

 and is elongated considerably at its posterior inferior corner. 

 The external flat end presents at its extremity a small oval face, 

 corresponding with that on the acromion scapulae. 



This bone is very strong from the abundance of its condensed 

 lamellated structure; but, like other round bones, the cellular 

 matter predominates at its extremities. 



SECT. n. OF THE ARM, (Os Humeri, ISHumerus.) 



The arm extends from the shoulder to the elbow, and has but 

 one bone in it, the os humeri. The latter, in its general ap- 

 pearance, is cylindrical, with an enlargement of both extremi- 

 ties; the superior end presents a general swell, while the infe- 

 rior is flattened out. 



The superior extremity of the os humeri, which is also called 

 its head, is very regularly hemispherical, and has its axis di- 

 rected obliquely upwards and backwards, to apply itself with 

 more facility to the glenoid cavity of the scapula. The base on 

 which the head reposes is termed neck, it is riot more than four 

 or five lines long, and is marked off by a superficial furrow, 

 surrounding the bone. This furrow is more conspicuous above, 

 where it separates the head from two knobs called the tubero- 

 sities. 



One of these tuberosities, the external, being placed beneath 

 the acromion scapulae, is much larger than the other, and bears 

 on its upper face the marks of the tendinous insertion of three 

 muscles. The most internal mark is for the supra-spinatus 

 scapulae, the middle for the infra-spinatus, and the external, or 

 posterior, for the teres minor. The smaller tuberosity is inter- 

 nal, and placed on a line with the coracoid process; it has but 

 one mark, and that is on its upper face, for the tendinous inser- 

 tion of the sub-scapularis muscle. The two tuberosities are 



