402 JAMES CRAWFORD WATT 



radial nerve, and is quite large. Its border blends with that of 

 the long head throughout its extent. 



The medial head lies on the back of the middle third of the 

 humerus, below the groove for the radial nerve. It is over- 

 lapped largely by the long head and blends with the deep surface 

 and medial border of the latter. 



The lower two-thirds of the muscle exhibit a tendon running 

 lengthwise, at the line of junction of the long and lateral heads. 

 Towards this tendon fibers converge in the upper part 

 muscle, and in the lower part they diverge again to their insertion 

 on the bone. 



Insertion. Owing to the absence of the ulna no normal in- 

 sertion is possible, and the whole lower attachment of this muscle 

 is transferred to the humerus. The insertion is into the whole 

 of the lower third of the posterior surface of the shaft of the 

 humerus and to the back of both epicondyles. The radius re- 

 ceives no attachment whatever from this muscle, so extension 

 of the forearm is an impossibility. This explains the early 

 fixation of the forearm in extreme flexion, allowing thus of the 

 development of the skin web and shortening of the brachialis 

 muscle to make this deformity a fixed one. Migration of the 

 attachment of the brachialis down the shaft of the radius is 

 thus permitted by the permanent flexion of the forearm. In 

 this position the further the muscle passes down the radius the 

 shorter it becomes, as its insertion approaches the level of its 

 origin. 



It might be asked why, in absence of the ulna the brachialis 

 muscle becomes attached extensively to the radius but the tri- 

 ceps all ends on the humerus. Why does not the triceps also 

 reach the radius? The difference seems reasonable in view of 

 the following circumstances, comparative anatomy furnishing 

 the answer to the problem. The brachialis is attached to the 

 radius occasionally in man, and as before mentioned, normally 

 in certain lower animals in addition to its ulnar insertion, 

 while in a few species the radial insertion is the only one. In the 

 case of the triceps, insertion on the radius is not normal in the 



