EEPORT ON THE SPHENISCID^. 161 



(B) The Ulnar Nerve (10) extends along the inner surface of the humerus, and 

 crossing the elbow joint in company with the humeral vein, passes along the ulnar margin 

 of the wing as far as the middle in length of the metacarpal bones, where it divides into 

 two branches. 



(i.) The first of these passes round the anterior margin of the wing to reach the dorsal 

 surface of that organ, where it is distributed to the skin. 



(ii.) The other is distributed to the cutaneous structures on the concave surface of tlie 

 wing, as far down as the first phalanx. 



The Dorsal Bra^ich or Musculo- Spiral Nerve. 



The second branch, derived from the single cord of the brachial plexus, passes back- 

 wards, to be distributed on the dorsal surface of the wing. From its course, with refer- 

 ence to the humerus, it may be named the musculo-spiral nerve. 



Leaving the cord of the brachial plexus, the musculo-spiral nerve (11) passes back- 

 wards over the posterior margin of the humerus under cover of the scapular heads of 

 the triceps. Having reached the dorsal surface of the wing, it crosses the elbow joint 

 and passes downwards, lying in the interval between the radius and ulna, under cover of 

 the extensor carpi radialis brevis, as far as the wrist joint, beyond which it can be traced 

 as far as the distal end of the metacarpal bone. 



The musculo-spiral nerve gives off the following branches. 



(a) A branch to the tensor patagii longus muscle (12) winds round the inner side of 

 the neck of the humerus, and enters the posterior border of that muscle. This branch 

 closely resembles the circumflex nerve of the mammal. 



(b) A branch to the latissimus dorsi (13) passes upwards and backwards round the 

 axUlary margin of the scapula to reach the lower border of the latissimus dorsi, where it 

 divides into two twigs, one of which is distributed to each of the two portions of that 

 muscle. 



(c) A branch (14) which supplies the dorsal scapular muscles winds round the 

 axillary border of the scapula, and divides into two twigs, one of which supplies the supra- 

 and the other the infra-spinatus muscle. 



(d) Another branch (15) passes to the triceps, and supplies both the humeral heads of 

 that muscle. 



(e) A cutaneoxis branch (16) is given off by the musculo-spiral nerve opposite the 

 middle in length of the humerus. It reaches the outer side of the elbow joint, and 

 supplies the skin of that region. 



(/) A second cutaneous branch (17) leaves the trunk of the nerve opposite the elbow 

 joint. It passes along the anterior margin of the radius as far as the wrist joint, beyond 

 which it could not be traced. It was apparently distributed to the skin. 



(g) Small branches (19) are given off to supply the radial extensors of the wrist. 



(ZOOL. CHALL. EXP.— PAET XVIII. — 1883.) S 21 



