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



from the 8th cervical and 1st dorsal nerves — and embracing the axillary artery, they 

 unite in the form of a flattened nervous band which lies upon the inner aspect of the 

 axillary artery, between it and the vein. The principal branches of the plexus spring 

 from this band. 



The branches of the plexus may be conveniently classified under two heads : — (1) 

 Those arising from the primary looped plexus ; (2) those which take origin from the 

 flattened nervous band or plexus proper. 



The first group includes — (1) the suprascapular (s.), phrenic (p.), and nerve to the 

 subclavius (s.c), all proceeding from the 5th and 6th nerves ; (2) the subscapulars (s.s.) 

 coming — one from the 5th and 6th, and two from the 6th. 7th, and 8th ; (3) the external 

 respiratory (e.r.) from the 6th and 7th ; (4) the circumflex (c.) from the 6th, 7th, and 

 8th ; (5) the musculo-spiral (m.s.) from the 7th, 8th, and 1st dorsal nerves. 



The plexus proper gives orgin to the musculo-cutaneous (m.c), median (to.), ulnar (u.), 

 internal cutaneous (i.e.), and muscular branches to the panniculus carnosus (p.c.) and 

 pectoral muscles (p.m.). 



In the upper arm and forearm the distribution of the brachial nerves is very similar 

 in the two animals, and, consequently, in this part of their course one description will 

 suffice for both. In the manus, however, certain points of dissimilarity must be noted. 



Circumflex (PI. I. fig. 6, c.f. ; PI. II. fig. 4, c.n., and fig. 6, a). — This is a large nerve, 

 and has a more extensive cutaneous distribution than the corresponding nerve in man. 

 It takes the usual course through the quadrilateral space, and round the neck of the 

 humerus to reach the deep surface of the deltoid. Here it divides into two branches of 

 equal size. Of these one immediately splits up into twigs for the supply of the 

 deltoid and the teres minor. The other branch is cutaneous, and has a wide area of 

 distribution. Emerging from under cover of the deltoid (PI. I. fig. 5, c) it becomes 

 superficial upon the outer aspect of the upper arm, and not only supplies the skin in this 

 locality, but is also continued downwards to the outer aspect of the forearm. This 

 portion of the circumflex nerve is to be regarded as the substitute for the cutaneous part 

 of the musculo-cutaneous in man, which is absent in these animals. It is interesting to 

 note that in the Porpoise and Dolphin 1 a great pajt of the circumflex nerve is distributed 

 as in Thylacine and discus to the skin over the radial aspect of the arm and forearm. 



Musculo-cutaneous (PI. I. fig. 6, m.c. ; PL II. fig. 6, m.c, and fig. 4, m.c.n. and m.c.n 1 .). 

 — This is a purely motor nerve, distributed only to muscles, and consequently the term 

 "musculo-cutaneous" is a misnomer. It comes off in two parts. One of these passes 

 outwards to supply the short coraco-brachialis, and the superficial biceps (PI. II. 

 fig. 4, m.c.n 1 .), whilst the other passes obliquely downwards and outwards towards the 

 elbow-joint, to supply the deep portion of the biceps and the brachialis anticus 

 (PL II. fig. 4, m.c.n.). In the discus, in which there is a well developed coraco-brachialis 



1 Cunningham, Spinal Nervous System of the Porpoise and Dolphin, Jour. Anat. and Phys., 1876. 



