REPORT ON THE MARSUPIALIA. 29 



longus, both nerves pass under cover of it to reach their destination, and the lower nerve 

 supplies it. As a sensory nerve to the outer aspect of the fore limb, the musculo- 

 cutaneous finds a substitute in the cutaneous division of the circumflex. It is by no 

 means uncommon for the musculo-cutaneous to have a purely motor function. According 

 to Swan, 1 this is the case in the Fox, and its place is here supplied by branches which 

 spring from one of the long roots of the median, a little way above the elbow. In the 

 Dog and the Cat it has the usual double distribution. 2 



Musculo-spiral (PL I. fig. 6, m.s. ; PI. II. fig. 4, m.s.n., and fig. 6, m.s.). — This nerve 

 does not end in front of the elbow by dividing into a radial and a posterior interosseous 

 division. It is carried downwards to the extensor aspect of the forearm as one undivided 

 trunk. 



In its course it closely corresponds with the same nerve in other Mammals. Passing, 

 in the first instance, downwards, it soon pierces the internal head of the triceps in the 

 Thylacine, but in the Cuscus runs behind the humerus between the outer and inner heads 

 of the muscle. It then comes forward through the external intermuscular septum, and 

 continues its course to the elbow-joint upon the supra-condyloid ridge, and lies deeply in 

 the interval between the supinator longus and the brachialis anticus. In the Thylacine, 

 in which the supinator longus is poorly developed, it lies between the extensor carpi 

 radialis longior and the brachialis anticus. At a lower level it lies directly upon the 

 anterior ligament of the elbow-joint, and then diverging outwards and backwards it winds 

 round the neck of the radius under cover of the supinator brevis in Thylacinus, but through 

 its midst in the Cuscus, and thus gains the extensor aspect of the forearm. Here it is 

 placed between the superficial and deep layers of extensor muscles, and ends by giving off 

 numerous branches to the neighbouring muscles and one large cutaneous nerve (PI. II. 

 fig. 5, m.s.n.). 



Branches. — On the inner aspect of the arm, before it pierces the inner head of the 

 triceps, it supplies twigs to the middle and inner heads of the triceps, and also to the 

 dorsi-epitrochlear muscle (PI. II. fig. 4, d.e.n.), thus showing that this latter muscle must 

 be associated with the triceps, and not with the latissimus dorsi from which it springs. 

 Behind the humerus branches are given to the external head of the triceps ; and on the 

 outer aspect of the arm, before it passes under cover of the supinator brevis, it yields 

 twigs to the supinator longus, extensor carpi radialis longior and brevior, and to the 

 superficial aspect of the supinator brevis. On the dorsal aspect of the forearm (PI. II. 

 fig. 5, m.s.n.) it supplies all the extensor muscles, and the anconeus externus in the case 

 of the Cuscus. The cutaneous branch is the direct continuation downwards of the nerve 

 itself. It becomes superficial about the middle of the forearm by coming out between 

 the extensor communis digitorum and the extensor secundus digitorum (i.e., extensor 



1 Swan's Comparative Anatomy of the Nervous System. 



2 Chauveau's Comparative Anatomy. 



