OF NEW SOUTH WALES. 411 



toralis, destined for the deltoideus minor. The internal cuta- 

 neous nerve also arises in part from this cord, but its mode of 

 origin presents considerable varieties in different birds. It arises 

 almost always by two roots, which may be both derived from the 

 anterior brachial nerve, or one from the anterior brachial nerve, 

 and the other directly from the spinal nerve following the last of 

 those entering into the formation of the plexus. When the former 

 is the arrangement observed, the anterior brachial nerve is usually 

 joined near its origin by a branch of very small size from the spinal 

 nerve immediately following those which go to make up the great 

 bulk of its fibres, and the spinal nerve from which this slender 

 branch is derived is evidently the equivalent of that from which 

 arises the posterior root of the internal cutaneous, when the last 

 described arrangement holds good ; since in this case the anterior 

 brachial nerve is not joined by any such small accessory root. 

 The spinal nerve which thus so generally assists either 

 directly or indirectly in the formation of the internal cutaneous, 

 seems to be the analogue of the third dorsal nerve of mammals ; 

 and this view is strengthened by the fact that in cases in which 

 it gives off no branch to contribute to the formation of the inter- 

 nal cutaneous, it gives off an intercosto-humeral branch for the 

 nerve-supply of the skin of the upper arm. The two roots of 

 the internal cutaneous nerve join one another about the middle of 

 the upper arm, and the single nerve thus formed courses to the 

 ventral surface of the fore-arm, where it divides into two main 

 branches, which are traceable, giving off numerous branchlets, 

 in the subcutaneous tissue as far as the wrist. 



The above may be regarded as the simplest arrangement of 

 the plexus. In many cases, however, — e.g., Columba aenas, 

 Leucosarcia picata, Phalacrocorax Novaa-Hollandiae — the con- 

 nexions of the nerves are more complex. 



As regards the distribution of the main trunks, the posterior 

 brachial nerve passes from the axilla to the posterior surface of 

 the arm, where it gives off near the head of the humerus a large 

 circumflex branch, which divides for the supply of the deltoideus 

 major, the tensor major and the humero-scapular joint. The main 

 part of the nerve, after giving off branches to the triceps, and, 



