THE HUMERUS. 93 



The shaft or body, thick and cylindrical above, becomes expanded transversely 

 and somewhat three-sided below. It is divided into anterior and posterior faces by 

 lateral lines, slightly marked in the upper part, but more prominent in the lower, 

 where they pass into the supracondylar ridges. Superiorly, on its anterior aspect 

 is the bicipital groove, so named from lodging the long tendon of the biceps muscle ; 

 this groove, commencing between the tuberosities, descends with an inclination 

 inwards, and is bounded by two rough margins, the external and more prominent 

 of which, pectoral ridge, gives attachment to the pectoralis major muscle, the internal 



to the latissimus dorsi and teres major. To- 

 wards the middle of the shaft, on the inner 

 lateral line, is a rough linear mark where the 

 coraco-brachialis muscle is inserted, and a little 

 lower down is the foramen by which the chief 

 medullary artery enters the bone, directed down- 

 wards. On the external part of the shaft, near 

 its middle, in a line anteriorly with the pectoral 

 ridge, is a large, elevated, and rough surface, of 

 a triangular shape, the deltoid eminence, for the 

 insertion of the muscle of the same name. Below 

 this, the ridge is continued into a smooth 

 elevation which, descending on the front of 

 the shaft to the inferior extremity, separates an 



IF ULNAR NERVE 



Fig. 92. LOWER TWO-FIFTHS OF THE RIGHT 



HUMERUS, FROM THE OUTER SIDE. (Drawn 



by T. W. P. Lawrence. ) 



Fig. 93. LOWER EXTREMITY OF THE RIGHT HUMERUS, 



FROM BELOW. (Drawn by T. W. P. Lawrence.) 



external from an internal surface ; while at the sides these are separated from the 

 flat posterior surface by the supracondylar ridges, which descend, the external more 

 prominent than the internal, to the condyle on each side. About the middle of the 

 shaft externally, a broad depression, the spiral groove, winds downwards and 

 forwards, limited above by the deltoid eminence, and below by the external 

 supracondylar ridge: the hollow is for the most part occupied by a process of the 

 brachialis anticus muscle, but posteriorly, where there is often to be recognized a 

 second smaller groove, the musculo-spiral nerve and superior profunda vessels rest 

 against the bone. At the upper part of the groove there is generally a second 

 foramen for a branch of the superior profunda artery. 



The external and internal surfaces below the deltoid eminence are occupied by the origin of 

 the brachialis anticus muscle. Posteriorly, the outer head of the triceps arises from the surface 

 above the spiral groove, the inner head from the surface internal to and below the groove. 

 The external supracondylar ridge gives origin in its upper two-thirds to the supinator longus, 

 in its lower third to the extensor carpi radialis longior. The external and internal inter- 

 muscular septa of the arm are also attached to the corresponding supracondylar ridges. 



The inferior extremity is much enlarged laterally, flattened from before 

 backwards, and curved slightly forwards. Projecting on either side are the external 



