320 SHOULDER-GIRDLE IN MONOTREMES, 



Insertion. The origin is by a well marked tendon, and 

 the muscle runs upwards and outwards. As it approaches its 

 insertion the fibres, which spring from the upper part of the 

 glenoid ridge, become superticial, while those from the lower part 

 of the glenoid and the coracoid become deep, and thus two 

 distinct layers are formed, the upper one being inserted on the 

 inner two-thirds of the posterior border of the olecranon, while 

 the deeper one is inserted on the whole posterior border of the 

 olecranon and also on the summit (i.e., dorsal surface) of the 

 olecranon. (Fig. 10, Trc. S.). 



Relations. At its origin the muscle is related to the infra- 

 spinatus and teres minor anteriorly, and subscapularis posteriorly. 



Origin, (b) External humeral head. — This arises by a thin 

 tendon from a spot situated between the articular head of the 

 humerus proximally, and the origin of the supinator longus and 

 brachialis internus distally. 



Insertion. The muscle, small and narrow, runs to be 

 inserted on the external extremity of the dorsal surface of the 

 olecranon in conjunction with the insertion of the proximal one of 

 the internal heads (c). 



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

 from an area on the proximal third of the internal portion of the 

 posterior surface of the humerus. This area is bounded externally 

 by the ventral third of the supinator ridge with the supinator 

 longus and brachialis internus arising from it distally by the origin 

 of the fourth head of the triceps ; internally by the sharp internal 

 border of the humerus and the insertion of the teres major, and 

 ventrally by the insertion of the teres minor. 



Insertion. The muscle is inserted on the whole length of 

 the summit (dorsal surface) of the olecranon. 



Origin. {d) Distal internal humeral head. — This arises 

 from an area on the distal part of the internal portion of the 

 posterior surface of the humerus. This area is bounded ventrally 

 (proximally) by the origin of the third (c) head of the triceps ; 

 externally by a line parallel and close to the supinator ridge ; 



