20 PROCEEDINGS OF THE NATIONAL MUSEUM vol.80 



tendinous, the common tendon of this and the short head of the 

 biceps being separable into two tendons a short distance from the 

 coracoid. Occasionally fission takes place somewhat farther down 

 the arm (as in Saiviiri), or the tendon does not divide at all, the 

 fleshy part of both biceps and coracobrachialis originating from the 

 common tendon in a bipennated manner (as in Perodicticus and 

 Galago), the two being separable only over the distal half of the 

 brachium (as in Galago). Or, again, in many rodents that have 

 this muscle in depauperate form the fibers of the coracobrachialis 

 originate directly from the single tendon of the caput breve of the 

 biceps. At times in man we have found the more proximal fibers 

 of the coracobrachialis medius inserting not upon the bone, but 

 directly upon a flat tendon (internal brachial ligament of Struth- 

 ers) that arises from the upper part of the humeral shaft and 

 eventually disappears at the point where the medius begins its 

 osseous insertion. There was a suggestion of such an arrangement 

 in our specimen of Lasiopyga. Testut (1884) has suggested that 

 this long tendinous band may be a possible vestige of the cora- 

 cobrachialis longus (of Wood). It appears to us, however, as ex- 

 tremely unlikely that this could represent the coracobrachialis 

 longus (superficialis) as recognized by us. The band is applied 

 to the deep surface of the coracobrachialis proprius, whereas the 

 pars longa, when present, lies superficial to all the muscles of the 

 brachium except the dorsoepitrochlearis. This feature of the pars 

 longa will be discussed subsequently. 



Another feature that is extremely variable is the position of the 

 musculocutaneous nerve in relation to the muscle. This nerve may 

 cross the brachium above and beneath the whole muscle, or pierce it, 

 or pass entirely below it in a manner that appears indiscriminate, 

 not only in closely related species but also in individuals of the same 

 species. In illustration of such variation we may cite the conditions 

 in 32 adult white and negro cadavers that we have examined in the 

 dissecting rooms of the Johns Hopkins University Department of 

 Anatomy. In all but five cases, both sides were dissected. Out of 

 59 arms, the musculocutaneous nerve pierced the coracobrachialis in 

 55 instances and passed below (superficial to) the muscle in 4 (on 

 the left side in three cadavers and on the right in the other). In 

 those in which piercing occurs there may be as little as approxi- 

 mately one-sixth of the muscle deep to or above the nerve, or prac- 

 tically all of it, but the average is about one-half of the entire muscle 

 located above the nerve. This probably represents with a fair degree 

 of accuracy the condition in man (subject to an unknown degree of 

 racial variation). Apparently it is only very rarely that the nerve 

 passes deep to the entire muscle mass in man, but in mammals below 

 the anthropoids this is the usual condition. In fact Parsons, than 



