REPORT ON THE MARSTJPIALIA. 135 



"nerve-supply" and "muscle-homology " is invariable and immutable. It is well at this 

 stage to inquire bow far the nerve arrangements in the Mammalian foot bear out this 

 doctrine. 



In the great majority of cases the pedal nerves have precisely the same course and 

 distribution. The two plantar nerves enter the sole by the hollow in the os calcis. The 

 internal plantar nerve supplies the abductor hallucis and flexor brevis hallucis (when such 

 muscles are present). The external plantar nerve turns outwards to reach the fibular 

 margin of the foot, where it breaks up into a superficial and deep part. The former 

 supplies the flexor brevis and abductors of the little toe, whilst the deep part turns 

 inwards across the sole under cover of the plantar adducting muscles, and is distributed 

 to all the adductors and to the other intrinsic muscles of the index, medius, and annularis. 

 The rule therefore is, that with the exception of the flexor brevis and the abductor of 

 the hallux, the external plantar nerve supplies all the intrinsic muscles of the foot. But 

 this is not the invariable rule. Certain well-marked exceptions are met with. 



In the Elephant the internal plantar nerve supplies the flexor brevis indicis; in the 

 Hyrax the internal plantar nerve supplies the flexor brevis indicis, the adductor indicis, 

 and the second dorsal interosseous muscle; in the Beaver, a still more remarkable devia- 

 tion is found. From the internal plantar nerve proceed the twigs of supply for the 

 abductor hallucis, flexor brevis indicis, flexor brevis medii, and the first and third dorsal 

 interossei. Lastly, in the Fox-bat there is an example of the external plantar nerve 

 encroaching upon the domain of the internal plantar by supplying a twig to the outer 

 head of the flexor brevis hallucis. 



The arrangement of the digital nerves is also not without interest. It follows a more 

 constant plan than that of the muscular twigs. In a pentadactylous foot the internal 

 plantar nerve gives off four digital branches, which are distributed to the hallux, the 

 index, the medius, and the tibial side of the annularis. The external plantar nerve on the 

 other hand provides two digital branches — one for the fibular side of the minimus, and 

 the second for the adjacent sides of the minimus and annularis. In only one instance 

 have I observed a marked deviation from this arrangement, viz., in the Echidna, in which 

 the digital nerve for the contiguous margins of the annularis and medius comes from the 

 deep division of the external plantar, and the twig, for the outer side of the minimus 

 proceeds from the internal plantar. 



The digital nerve for the adjacent margins of the minimus and annularis is somewhat 

 variable in its origin. In the lower members of the Mammalian class (Ornithorhynchus, 

 Echidna, Cuscus, Vulpine phalanger, Paca, &c.) this branch arises from the deep 

 division of the external plantar, and reaches its distillation by coming out from under 

 cover of the adductor minimi digiti. In the Fox-bat, Quadrumana, and Man it springs 

 from the superficial division of the external plantar nerve, and in its course downwards 

 it is not covered by any muscular structure. In Thylacine an intermediate condition is 



