148 THE VOYAGE OF H.M.S. CHALLENGES. 



twigs to both sides of all the toes, with the exception of the hallux and the outer margin 

 of the minimus (PI. VI. fig. 8, 2). 



The anterior tibial (PI. V. fig. 3, 2) in the Cuscus is a purely motor nerve, and 

 differs from that in the Thylacine by ending on the dorsum of the foot and supplying the 

 extensor brevis of the index and medius, which in this animal has wandered downwards 

 so as to lie upon the dorsum of the foot. 1 In the front of the leg the nerve lies between 

 the extensor longus hallucis and the tibialis anticus. 



The comparative anatomy of the external popliteal nerve has recently received special 

 attention from Dr. Georg Ruge in his elaborate memoir upon the extensor muscles of the 

 leg and foot in Mammalia. He examined three marsupials, viz., the Didelypjhys 

 virginiana, the Didelyphys cancrivora, and the Dasyurus hallucinatus. In all these 

 the general arrangement of the nerve was very similar to that which I have described as 

 existing in the Thylacine and Cuscus. There seems to be, however, a great variation in 

 the relative distribution of the musculo-cutaneous and anterior tibial to the skin on the 

 dorsum of the foot. In Dasyurus as in Cuscus the anterior tibial is purely motor, and 

 although it reaches the dorsum of the foot it gives no branches to the integument. In 

 Didelyphys, as in the TJiylacine, the anterior tibial combines a large proportion of 

 sensory fibres with its motor fibres. Thus in Didelyphys virginiana it supplies the 

 adjacent sides of the index and medius, and also the fibular side of the hallux, whilst 

 in Didelyphys cancrivora it sends twigs to both sides of the index and the tibial side of 

 the medius. This combination of fibres in the anterior tibial, as Ruge shows, seems to 

 be the more usual disposition throughout Mammalia generally. In some animals, indeed 

 (as for instance the A teles), the anterior tibial appears almost to supplant the musculo- 

 cutaneous as the nerve of supply to the dorsal aspect of the digits. 



Ruge is a firm believer in the invariable and immutable relationship between " nerve- 

 supply " and " muscle-homology." 



Perhaps, however, the strongest evidence of any against the immutability of nerve- 

 supply is provided by Dr. Ruge himself. 2 Thus in the Ornithorhynchus he finds the 

 tibialis anticus and the inner portion of the extensor longus hallucis supplied by a branch 

 from the anterior crural, which is prolonged downwards to its destination over the 

 external condyle of the femur. To account for this, he assumes that " the internal part 

 of the extensor longus hallucis and the tibialis anticus are not homologous to the similarly 

 named muscles " in other animals, but " belong rather to the extensor group of the 

 thigh." He believes that the fibres which are supplied by the anterior crural nerve 

 are gradually abolished, and that their place is taken by the external muscles of the leg. 



i In the Vulpine phalanger the anterior tibial on the dorsum of the foot is not expended entirely upon the extensor 

 lirevis, but is continued onwards to supply the skin upon the inner side of the index. It likewise sends many twigs to 

 the integument in the interval between this digit and the hallux, and it communicates with the musculo-cutaneous. 



2 Loc. cit. 



