BY W. J. S. McKAY, 321 



distally by a line running from the supinator ridge to the internal 

 condyle and placed just proximal to the snpra-condyloid foramen ; 

 internally by the sharp inner edge of the humerus. 



I n s e r t i o*n. The muscle is inserted in conjunction with the 

 proximal part (c) on the summit of the olecranon. 



Relations. The musculo-spiral nerve passes from the 

 internal border of the humerus between the proximal and distal 

 *' internal humeral heads," and then passes beneath the outer head. 



Ornithorhynchus. 



The triceps arises by four heads. 



Origin, (a) Scapular 2Jortion. — This arises from the ventral 

 three-fourths of the glenoid border of the scapula. The origin is 

 fleshy for the most part, but becomes tendinous near the glenoid 

 cavity. (Fig. 12, Trc. S.). 



Insertion. The muscle runs towards the olecranon, becom- 

 ing twisted on its axis. As it approaches its insertion it becomes 

 split into three layers ; an upper, middle and deep. The upper 

 layer is inserted on the internal one-fourth of the posterior edge 

 of the olecranon, the middle on the external two-fifths of the 

 posterior edge of the olecranon, and the deep on the whole length 

 of the summit (dorsal surface) of the olecranon. 



Relations. At its origin the muscle is placed between the 

 infraspinatus and teres minor anteriorly, and subscapularis 

 posteriorly. 



Origin, (b) External humeral liead. — This arises by two 

 narrow tendons, one from the proximal (ventrjtl) extremity of the 

 supinator ridge close to the articular head of the humerus ; the 

 other more externally close to the insertion of the infraspinatus. 



Insertion. The two heads run separate for a short distance 

 and then coalesce, and the muscular fibres run towards the 

 olecranon, where the muscle is inserted on the external one-fourth 

 of the posterior edge and summit of the olecranon. 



Origin, (c) Proximal internal humeral head. — This muscle 

 arises from the proximal half of the internal portion of the 



