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



Extensors on the Leg and Foot of Mammalia, will show how variable the long saphenous 

 nerve is in respect to its terminal distribution. In the Mustela foina, Meles vulgaris, 

 Felis domestica, Cercopithecus, it ends before it reaches the foot; in the Omithorhynchus 

 ■paradoxus, Didelphys virginiana, Nasua socialis, Felis leo, Inuus cynomolgus, Cehus, 

 and Orang it reaches the dorsum of the foot, and supplies filaments to the skin of the 

 hallux. In none of these, however, excepting the Omithorhynchus, is its area of supply 

 on the dorsum of the foot so wide as in the discus. 



The obturator nerve 1 (PL V. figs. 5 and 6, o, and fig. 4, 1 ; also PI. IV. fig. 1, o.n) is 

 small in comparison with the corresponding nerve in man. It arises in Thylacinus in 

 common with the anterior crural, and in Cuscus by two roots from the 4th and 5th 

 lumbar nerves. Its course and distribution is the same in both animals. Quitting the 

 pelvis through the upper part of the thyroid foramen it supplies branches to the obturator 

 externus and the adductors longus and brevis, and ends by sinking into the substance 

 of the gracilis. It gives no twigs to the pectineus or adductor magnus, neither does it 

 divide into a superficial and deep division. 



Sacral plexus. — In Thylacinus (PL V. fig. 5) the sacral plexus is apparently formed 

 entirely by the lumbo-sacral cord, which is prolonged into the gluteal region as the great 

 sciatic nerve (g's). No communication could be traced between this great nervous trunk 

 and the first sacral nerve. Before quitting the pelvis it gives origin to (1) the superior 

 gluteal (s.g); (2) a special branch to the ecto-gluteus (g.m); (3) a special nerve to the 

 hamstring muscles (h) ; (4) branches to the pyriformis. 



From the 1st sacral nerve the pudic (p) and small sciatic nerves (s.s) take origin. 



In the Cuscus the arrangement is quite different (PL V. fig. 6). The lumbo-sacral 

 cord is joined by the whole of the 1st sacral nerve, and a small portion of the 2nd sacral 

 nerve, and a large nervous band is thus formed. This divides into two unequal parts, of 

 which the larger is the great sciatic (g'.s) whilst the smaller gives origin to (1) a special 

 branch to the ecto-gluteus (g.m); (2) the small sciatic (s.s); (3) the special nerve to the 

 hamstrings (h). 



The superior gluteal nerve (s.g) arises from the lumbo-sacral cord before it is joined 

 by the sacral nerves, and the pudic ( p) comes from the 2nd sacral nerve. 



Nerves to ecto-gluteus. — The several parts of this muscle (the representatives of the 

 gluteus maximus in man) are supplied differently in the two animals. In Thylacinus one 

 large nerve enters the gluteal region through the anterior part of the great sacro-sciatic 

 foramen, and breaks up into branches for the supply of the four constituent parts of the 

 ecto-gluteus (PL IV. fig. 5, g.n). In Cuscus this branch is much smaller in size, because 

 the ecto-gluteus also draws twigs from the small sciatic. 



Superior gluteal nerve (PL III. fig. 1, s.g.n). — After emerging from the pelvis 

 through the great sacro-sciatic foramen, this nerve ramifies between the meso-gluteus and 



1 In the Fox the obturator springs from the 5th and 6th nerves, and in the Ass from the 4th, 5th, and 6th (Swan). 



