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



The Nerves of the Hind-limb of the Thylacine and Cuscus. 



So much stress is now laid upon " nerve supply " in the determination of " muscle 

 homology," that it becomes a matter of importance to add in every way possible to our 

 knowledge of the comparative anatomy of the nervous system. With this end in view 

 I have examined with great care the arrangement of the nerves in the hind-limb of the 

 Thylacine and Cuscus. 



Lumbar plexus. — In both the Thylacine and Cuscus the anterior primary divisions of the 

 lumbar nerves are six in number, but the part which these play in the formation of the 

 lumbar plexus differs in the two animals. In Thylacinus (PI. V. fig. 5), the plexus is 

 formed by the first four nerves, whilst the fifth and sixth lumbar nerves join to constitute 

 the lumbo-sacral cord. In Cuscus (PL V. fig. 6) the first lumbar nerve is quite inde- 

 pendent of the plexus, and is expended like the last dorsal in the abdominal wall, whilst 

 the plexus is formed by the union of the 2nd, 3rd, and 4th nerves, and the greater part 

 of the fifth nerve. The lumbo-sacral cord in this case is composed of the remaining part 

 of the fifth and the whole of the sixth nerve. In both animals the plexus has the usual 

 looped arrangement. 



Branches. — In Thylacinus (PL V. fig. 5) the branches of the plexus come off in the 

 following manner : — (1) from the 1st lumbar nerve a large branch to the abdominal 

 parietes ; (2) from the loop between the 1st and 2nd, two twigs to the abdominal 

 parietes ; (3) from the loop between the 2nd and 3rd, the genito-crural nerve (g.c.) ; (4 

 from the loop between the 3rd and 4th, the external cutaneous nerve (e.c.) ; (5) from the 

 3rd and 4th, the anterior crural (a.c), and obturator nerves (o) ; (6) from the 2nd, 

 3rd, and 4th, twigs to the psoas. 



In the Cuscus (PL V. fig. 6) the branches have the following origin : — (l) From the 

 2nd, a branch to the abdominal parietes ; (2) from the 3rd, a long nerve to the 

 abdominal wall, and a small twig to the psoas ( p) ; (3) from the loop between the 3rd 

 and 4th, the genito-crural (g.c) ; (4) from the 3rd and 4th, the external cutaneous (e.c) ; 

 (5) from the 3rd, 4th, and 5th, the anterior crural (a.c) ; (6) from the 4th and 5th, the 

 obturator (o). 



If we exclude the 1st lumbar nerve and enumerate the others from the second back- 

 wards, there is a striking resemblance between the lumbar plexus in the Cuscus and that 

 in Man, both in its mode of formation and in the manner in which the branches take 

 origin 



The three branches which are given off both in Thylacinus and Cuscus to the 

 abdominal parietes correspond to the ilio-inguiiial and ilio-hypogastric in man, and they 

 are distributed to the muscles and skin upon the same plan. 



The genito-crural (PL V. figs. 5 and 6, g.c and PL IV. figs 1 and 4, g.c.n) proceeds 

 downwards through the psoas parvus to reach the large cremaster, into the deep surface 



