12 THE VOYAGE OF H.M.S. CHALLENGER. 



series of short fleshy fasciculi which arise from the scapular border above this, and which 

 curve downwards and forwards in the form of a thin fleshy layer to blend with the fibres 

 of the scapular head in its upper third. The external head takes origin from the posterior 

 aspect of the humerus in its upper half, and is here blended with the brachialis anticus 

 through the intervention of the external intermuscular septum. The inner head arises 

 from the inner and posterior aspect of the humerus in all its length. It is pierced by the 

 musculo-spiral nerve, which splits it into two distinct portions. The three heads of the 

 triceps do not fuse until they reach the olecranon, into which the muscle has the usual 

 insertion. 



The Cuscus and Phascogale differ from the Thylacine in the following points: — (l) The 

 long head does not spring from more than one-third of the scapular border, and all the 

 fibres are derived from the one tendon of origin; (2) the inner head is weakly developed, 

 and does not extend upwards upon the humerus beyond the insertion of the teres major ; 

 moreover, it is not pierced by the musculo-spiral nerve ; (3) the three heads blend higher 

 up the limb than in the case of the Thylacine. The triceps in Cuscus and Phascogale 

 resemble very closely the triceps of man. 



Macalister 1 states that in Marsupials "the lateral heads of the triceps are united into 

 one large humeral muscle, inseparable from each other." I did not find this to be the 

 case in the three animals in question. 



Anconeus externus. — In the Thylacine the anconeus externus can hardly be said to 

 exist. It is merely represented by a few of the lower fibres of the inner head of the 

 triceps which are inserted upon the outer surface of the upper part of the olecranon. In 

 the Cuscus (PI. II. fig. 5, a.e.) it is largely developed, but inseparably united with the 

 triceps. Springing from the back of the external condyle of the humerus, it is inserted 

 into the outer surface of the olecranon and into the shaft of the ulna in its upper half. 

 In the Phascogale the anconeus, externus is almost identical with the same muscle in the 

 Cuscus, but it is not so well marked, and does not extend so far down the shaft of the 

 ulna. This muscle varies very greatly in its development in the different members of 

 Marsupial group. 



Anconeus interims {Epitrochleo-anconeus). — This little muscle is present in each of 

 the three animals. It presents the usual attachments, viz., on the one hand, to the back of 

 the internal condyle of the humerus, and on the other, to the inner side of the tip of the 

 olecranon process. Its lower border is in opposition with a fibrous cord which bridges 

 across the hollow between the condyle and the olecranon, and which gives origin to the 

 fibres of flexor carpi ulnaris. In the Cuscus (PL II. fig. 4, a.i.) the anconeus internus is 

 firmly united to the upper border of this cord, so that the two muscles are as it were 

 merely separated by a fibrous raphe. In all cases the ulnar nerve passes under cover of 

 the anconeus internus, and gives to it its nerve-supply. 



1 Annals and Magazine of Natural History, vol. v. 



