548 MAMMALIAN ANATOMY 



and posterior and external in the lower third. It arises above in the 

 posterior middle line, below the overhanging posterior hook of the 

 articular part of the head. It is smooth, rounded, and not prominent; 

 it passes gradually outward to reach the external aspect of the bone 

 below the middle; here it becomes sharper, and, as the external supra- 

 condyloid ridge, is continued on the lower extremity of the bone, where 

 it forms the anterior margin of its external surface. The upper part 

 of the external border is included in the area of insertion of the 

 posterior humeral head of the triceps muscle on the adjoining posterior 

 and external surfaces. Below this area is attached a strong aponeu- 

 rosis, which gives common origin above to the brachialis anticus, the 

 supinator longus, and the anconeus, and at the lower part to the 

 extensor carpi radialis longus, extensor radialis brevior, anconeus, and 

 extensor communis digitorum muscles. 



The internal border (Fig. 429) is not well defined. It begins 

 above at the lower end of the lesser tuberosity as a prominent, 

 rounded ridge ; it then arches forward and downward for from five to 

 fifteen millimetres parallel with the external border and not far from 

 it, but is soon lost, and is hardly manifest again until it appears below 

 as the ridge limiting the anterior edge of the supracondyloid foramen. 

 On its course, considerably below the middle, is the well-marked 

 nutrient foramen piercing the bone obliquely downward and inward ; 

 it transmits the principal artery of the shaft. The coraco-brachialis 

 muscle is inserted into the prominent upper part of this border. 

 Lower down the anterior edge of the shield-shaped area of origin 

 of the inner humeral head of the triceps reaches the internal border. 

 For the rest of its course it is free from muscular attachment. 



Of the three surfaces into which the shaft is divided, the external 

 surface (Fig. 428) is the largest, equalling in extent both the posterior 

 and internal surfaces together. It lies between the anterior and the 

 external border. Owing to the position of the external border, it is 

 wider above than below. Above, it faces forward and backward as 

 well as outward, and below, in front of the supracondyloid ridge, it 

 faces forward as well as outward. Its upper part, nearly a half of the 

 surface, is divided into two parts by the oblique deltoid ridge, which 

 begins above at the back of the greater tuberosity, is directed down- 

 ward and forward, and joins the anterior border of the shaft at or 

 below its middle. The triangular area above and in front of this line 



