REPORT ON THE MARSUPIALIA. 27 



this, fewer of the nerves take part in the formation of the plexus. This is very well 

 exemplified in the case of the Dolphin and Ass respectively. In the former, where the 

 cervical vertebrae are all compressed together so as to reduce the neck to a minimum, we 

 find that the plexus is formed by the posterior five cervical nerves and the first two 

 dorsal nerves. 1 In the Ass, 2 on the other hand, the last two cervical nerves and the first 

 dorsal nerve are the only nerves which take part in the formation of the plexus. In the 

 Horse and in Euminants 3 the brachial nerves are represented by the last three cervical 

 and the first and second dorsal nerves, and in man and animals with a neck of medium 

 length the plexus is formed by the last four cervical nerves and the first and second 

 dorsal nerves, as, for example, the Dog, 4 the Cat, 5 &c. 



But this rule does not hold good in every case. In the Pig, 6 whose neck cannot be 

 called long, the plexus is formed by the same nerves as in the Ass, and the Jaguar and 

 the Fox 6 in this respect resemble the Horse. 



In both the Tlujlacine (PL I. fig. 6) and Cuscus (PI. II. fig. 6) the brachial plexus is 

 formed, as in man, by the anterior primary divisions of the posterior four cervical nerves 

 and of the first dorsal nerve. In the Thijlacine, however, only a portion of the 5th 

 cervical nerve enters the plexus. The 5th cervical nerve in this animal divides into two 

 equal parts close to the intervertebral foramen, and diverging widely from each other, the 

 posterior of these joins the plexus, whilst the anterior proceeds forwards under cover of 

 the omo-hyoid muscle (a), and joins the 4th cervical nerve. It also gives cutaneous 

 twigs to the neck and outer and dorsal aspects of the shoulder (b and c). 



In the Thylacine the branches entering into the constitution of the brachial plexus 

 join so as to present a more or less looped appearance, but the general plan of the plexus 

 is very indefinite, irregular, and complicated (PI. I. fig. G). 



The branches which proceed from it arise in the following manner: — (l) From the 

 5th and 6th cervical nerves the suprascapular (s.), the phrenic (p.), and the nerve to the 

 subclavius (s.c.) ; (2) from the 6th and 7th the subscapular nerves (s.s.) and the external 

 respiratory (e.r.) ; (3) from the 7th muscular branches to the panniculus carnosus (p.c.) 

 and pectoral muscles (p.m.), the circumflex (c.f.), and the nerves corresponding to the 

 musculo-cutaneous in man (m.c.) ; (4) from the 7th, 8th, and 1st dorsal the median (m.) 

 and musculo-spiral (m.s.) ; (5) from the 8th and 1st dorsal the ulnar («.) and internal 

 cutaneous (i.e.). 



In the Cuscus the brachial nerves have a more definite arrangement (PL II. fig. 6). 

 They first unite so as to form four nervous loops, and from these two large trunks 

 proceed — the one deriving fibres from the 5th, 6th, and 7th cervical nerves, and the other 



1 Spinal Nervous System of the Porpoise and Dolphin, Jour. Anat. and Phys., p. 217, 1876. 



2 Swan's Comparative Anatomy of the Nervous System. 3 Chauveau's Comparative Anatomy. 



4 Graduation Thesis on the Anatomy of the Dog, by J. H. Scott, M.D., unpublished, but to be consulted in the 

 Library of the University of Edinburgh 



5 Chauveau's Comparative Anatomy. ° Swan's Comparative Anatomy of the Nervous System. 



