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



minimi digiti), and giving a few twigs to the skin in this region, it is continued down- 

 wards in the Cuscus to the dorsum of the hand, where it ends in three branches which 

 go one to the skin on each side of the middle finger, and the third to the radial side of 

 the ring finger. In the Thylacine the nerve does not give any dorsal digital branches. 

 It ends on the back of the wrist. 



This cutaneous branch of the musculo-spiral is not the representative of the radial 

 nerve in man. It is clearly the posterior interosseous neiwe. A branch somewhat 

 analogous to the radial is given off by the median. 



Median (PI. I. fig. 6, m. ; PI. II. fig. 4, m.n., and fig. 6, m.). — This is the largest branch 

 of the brachial plexus. In the upper arm it lies in close company with the humeral artery, 

 and with it passes through the supra-condyloid foramen. In front of the elbow-joint it 

 sinks deeply under cover of the pronator radii teres, and ends by dividing into (l) a 

 superficial or radial branch, (2) the median proper. In the upper arm the main trunk 

 gives off no branches ; as it lies under cover of the pronator radii teres, however, it 

 supplies twigs to each of the divisions of the flexor muscle, and also to the flexor carpi 

 radialis. The twig to the ulnar portion of the flexor muscle — that part which represents 

 the flexor profundus digitorum is very small. 



Superficial or medio-radial nerve.- — This nerve seems to take the place of the 

 radial branch of the musculo-spiral which we have seen to be absent. It becomes 

 superficial about the middle of the flexor aspect of the forearm, by appearing in the 

 interval between the pronator teres and the flexor carpi radialis. It ends a short way 

 above the wrist by dividing into a dorsal and a palmar branch (PI. II. fig. 3, m.r.n.). 

 In the forearm, before it comes to the surface, it gives off a few twigs to the pronator 

 teres, and a long slender branch to the superficial aspect of the pronator quadratus. 

 The latter accompanies the anterior interosseous artery, and is the representative of the 

 interosseous branch of the median in man. 



The palmar branch (PL II. fig. 3, m.r.n.) of the medio-radial nerve enters the palm 

 by passing over the annular ligament in company with the superficial artery of the 

 forearm, and after giving numerous twigs to the palmar pads, and a branch to the 

 abductor pollicis and the radial and ulnar parts of the flexor brevis pollicis, it ends as 

 the palmar digital nerve for the radial side of the thumb. 



In the Thylacine the dorsal branch is much the larger of the two, and winding round 

 the radial border of the forearm to the dorsal aspect of the wrist, it gives off a few twigs 

 to the skin, and ends in dorsal digital branches for both sides of the thumb, index, and 

 middle fingers, and the radial side of the ring finger. In the Cuscus it merely supplies 

 digital branches to the pollex and index. The middle finger and the radial side of the 

 ring finger in this case derive their nerve supply from the superficial branch of the 

 musculo-spira]. 



Median proper. — This nerve, accompanied by the main artery of the forearm, proceeds 



