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



External plantar nerve (PL V. fig. 2, 1).— This nerve proceeds down the back of the 

 leg in close company with the internal plantar, and near the heel it is joined by the large 

 reinforcing branch from the external saphenous. The further course of the external 

 plantar differs so much in the two animals that a separate description for each becomes 

 necessary. 



In Tlujlacinus (PL VI. fig. 1, 1, and fig. 4, 1) it lies close against the os calcis, and 

 entering the sole diverges to the outer margin of the foot under cover of the flexor brevis 

 digitorum. Here it gives twigs to the abductor ossis metatarsi minimi digiti and the 

 long abductor minimi digiti, and then divides into («) a superficial, and (b) a deep 

 portion. 



The superficial division is a purely cutaneous nerve, and almost immediately divides 

 into two digital branches, viz.: — one to the fibular side of the minimus (Fig. 1, 4), and 

 a second which bifurcates to supply the adjacent margins of the minimus and annu- 

 laris (3). The twig which goes to the tibial side of the minimus is reinforced by a 

 slender filament from the deep division of the external plantar nerve. 



The deep division of the external plantar (Fig. 1, 2) turns inwards, and, disappear- 

 ing from view under cover of the great flexor tendon, insinuates itself between the 

 plantar or adductor, and the intermediate or flexor groups of intrinsic muscles, where it 

 divides into an outer and an inner branch. The external branch proceeds downwards, 

 and after giving twigs to the short abductor minimi digiti, the flexor brevis minimi digiti 

 and the fourth dorsal interosseous muscle (i.e., the abductor annularis), it emerges from 

 under cover of the adductor minimi digiti in the cleft between the minimus and annu- 

 laris, and joins the tibial digital nerve to the minimus. The internal branch is carried 

 transversely across the sole upon the intermediate muscles, and gradually exhausts itself 

 in the supply of the other intrinsic muscles of the foot. It also effects a communication 

 by means of a long slender filament with the digital nerve from the internal plantar to 

 the adjacent margins of the index and medius. 



In Cuscus (PL VI. fig. 5, 2, and fig 7, 3) the external plantar nerve gains the sole by 

 passing under cover of the ligament which binds the plantar cartilage to the tuber of the 

 os calcis, and immediately divides into a superficial and a deep division. 



The superficial branch proceeds outwards and downwards over the abductors of the 

 minimus, and ends as the fibular plantar nerve of this digit. 



The deep branch supplies twigs to the three abductors and the opponens of the 

 minimus, and passing under cover of the last muscle reaches the interval between the 

 plantar and intermediate muscles. Here it divides as in the Tliylacine into an outer and 

 inner branch. 



The external branch emerges from under cover of the adductor minimi digiti, and 

 divides to supply the adjacent sides of the minimus and annularis. The internal branch 

 ends by supplying twigs to all the intrinsic muscles, with the exception of (1) the abductors 



