THE HUMERUS 83 



The shaft is thick and cylindrical above, expanded transversely 

 and three-sided below. It is divided into external, internal, and 

 posterior surfaces by anterior and lateral borders. Superiorly 

 is the bicipital groove lodging the long tendon of the biceps 

 and a branch of the anterior circumflex artery. This groove, 

 descending, is bounded by rough margins, the external or pec- 

 toral ridge for the pectoralis major muscle, and the internal 

 for the latissimus dorsi and teres major muscles. These muscular 

 attachments end at the junction of the upper fourth with the 

 lower three-fourths. 



The anterior border is the pectoral ridge continued to the 

 coronoid depression below. It becomes rounded and smooth 

 below, and gives attachment to the brachialis anticus muscle. 



The inner border is the inner bicipital ridge continued to 

 the inner condyle, called below the internal supracondylar 

 ridge. About the centre of this border is a rough linear mark 

 for the coracobrachialis muscle, and just below it the orifice 

 of the medullary canal directed downward. 



The external border runs from the back part of the great 

 tuberosity to the external condyle. Its centre is traversed 

 by a broad spiral groove, which is limited above by the deltoid 

 eminence and below by the external supracondylar ridge. This 

 ridge gives origin by its upper two-thirds to the supinator 

 longus muscle; hence it is called the supinator ridge, which 

 is very large in burrowing animals; its lower third attaches 

 the extensor carpi radialis longior. The posterior lip of either 

 supracondylar ridge is for the triceps, and a middle portion 

 for intermuscular septa. The external surface presents near 

 its middle the deltoid eminence; above this it is smooth and 

 rounded; below it, it is smooth and looks outward and forward, 

 giving attachment to a part of the brachialis anticus. 



The internal surface is narrow above, and forms the bicipital 

 groove; near its centre is the insertion of the coracobrachialis. 

 Below this level it is smooth, looks inward and forward, and 

 gives attachment to parts of the brachialis anticus. 



The posterior surface is twisted, so that its upper part is 

 directed a little inward, its lower part backward and outward. 

 It is nearly all covered by the external and internal heads of 

 the triceps, which are separated by the spiral groove running 

 down and out. At the upper part of this groove is generally 

 a second medullary foramen for a branch of the superior pro- 

 funda artery. 



