SKELETON AND TEETH 



79 



is the rounded, convex head, which fits into 

 the glenoid cavity of the shoulder-blade, form- 

 ing the joint of the shoulder; in front of 

 the head are two prominent and sometimes 

 very large projections for muscular attach- 

 ment, the external and internal tuberosities, sep- 

 arated by a groove, in which play the two ten- 

 dons of the biceps muscle and is therefore called 

 the bicipital groove. In a few mammals, such as 

 the Horse, Camel and Giraffe, 

 the groove is divided into two 

 by a median tubercle or ridge. 

 From the external tuberosity 

 there generally passes down 

 the front face of the shaft a 

 rough and sometimes very 

 prominent ridge, the deltoid 

 crest, to which is attached the 

 powerful deltoid muscle. At 

 the lower end of the humerus is the trochlea, 

 an irregular half -cylinder, for articulation with 

 the two bones of the fore-arm and vary- 

 ing in form according to the relative sizes 

 of those bones. On each side of the troch- 

 lea is frequently a rough prominence, the epi- 

 condyle, and above the inner one is, in many 

 mammals, a perforation, the epicondylar fora- 

 men, for the passage of a nerve. Extending 

 up the shaft from the outer epicondyle is a 

 rough crest, the supinator ridge, to which is 

 attached one of the muscles that rotate the 

 hand and is conspicuously developed in those 

 Fig. 27.— Left hu- mammals which have the power of more or 

 merus of Man, iggg fj-ge rotatiou and especially in burrow- 

 tera as in Kg. 25. ers. On the posterior face of the humerus, 



Fig. 26. — Left hu- 

 merus of Horse, 

 front side. i.<., in- 

 ternal tuberosity. 

 ex.t., external tu- 

 berosity, be, outer 

 part of bicipital 

 groove, dt., del- 

 toid ridge. »., su- 

 pinator ridge, tr., 

 trochlea. 



