MYOLOGY OF THE ORNITHORHYNCHUS. 149 



broad from side to side, insertion with the remarkably wide posterior 

 border of the olecranon. 



(c'. — From the humerus ; " short.") 



Triceps, its internal head. Vastus interims humeri \ — This muscle 

 is of large size, and wholly distinct from the foregoing; it is not 

 divisible into fasciculi. It is fleshy throughout, and of a somewhat 

 pyramidal shape, being broadest below. It lies almost directly pos- 

 terior upon the humerus, and fills up what would otherwise be a great 

 fossa between the head of the humerus and the olecranon. Its rela- 

 tions are — behind, to the scapular head of the triceps, which is 

 applied flat to its whole surface; in front, mostly to the humerus 

 itself, but also in greater or less part to the brachialis anticus, ori- 

 gins of both inner and outer bundles of forearm muscles, and to the 

 muscle below described as antanconseus ; to the outer side, to exter- 

 nal head of triceps ; to the inner side, to lower part of brachialis 

 anticus, and tendon of latissimus. It takes fleshy origin from the 

 whole of the upper half of the back of the humerus, and is inserted 

 fleshy into the whole width of the edge of the olecranon. At the 

 innermost point, some fibres are collected into a slightly separable 

 bundle, which has more especial and partly isolated insertion into the 

 inner corner of the olecranon. 



Triceps, its external head. Vastus externus humeri! — Like both 

 the foregoing, entirely distinct, and remarkable for its comparatively 

 small size, its isolation from the humerus in its continuity, its defi- 

 nite tendinous origin and insertion, and peculiar, independent action. 

 The muscle is fusiform in general shape, but somewhat prismatic in 

 a transverse section; it lies entirely away from the humerus, upon 

 the outer aspect of the arm, in a bed formed between the brachialis 

 anticus and inner head of triceps. It arises by two tendons ; one is a 

 narrow, flat, strong band from a recess behind the "greater tuberos- 

 ity" of the humerus; the other a broader, shorter, more diffuse and 

 aponeurotic-like fascia from what would be "neck" of an ordinary 

 humerus. It is inserted by a short, definite tendon into the extreme 

 outer corner of the transversely expanded olecranon. 



This last division of the triceps, while extending the forearm like 

 the other two, more especially pulls upon the outer corner of the 

 olecranon, and tips it up sideways, thus producing (in connection 

 with the anconeus) the remarkable rotation of the forearm that 

 answers instead of pronation. The inner head has the reverse action 

 less plainly marked, while the scapular head is the direct extensor ; but 

 both these two last have so broad a fleshy insertion into the olecra- 

 non, that if they contract unequally in their different parts, they may 

 have corresponding effect in tipping the olecranon sideways. The 

 several actions of the triceps, as a whole, are furthered by the follow- 



