153 



first of these classes, including the supinators and the pronators ; the 

 muscles attached to the metacarpal bones constitute the second class, 

 and the muscles set apart for the motions of the phalanges constitute 

 the third. 



Of these three groups, the second presents us with the principal 

 varieties, both in the way of anomalies and in individual variations, 

 throughout the orders of the vertebrate sub-kingdom ; it constitutes a 

 most remarkable class ; it seems as if typically there had been five pair 

 of muscles developed — a flexor and extensor for each metacarpal bone. 

 Thus we find the first bone extended in the foot by the tibialis anticus, 

 flexed by the tibialis posticus ; in the hand an aberrant muscle, extensor 

 carpi radialis accessorius, is developed occasionally in place of the 

 former, and sometimes a few tendinous fibres of the flexor carpi radialis 

 occur in the room of the latter. The anomalous muscle mentioned 

 above was described by Mr. "Wood, and my friend and colleague, Mr. 

 Richardson has communicated to me a description of an instance of it 

 which occurred in his dissections. For the second metacarpal bone we 

 have a flexor in the ordinary flexor carpi radialis, and an extensor in the 

 extensor carpi radialis longior ; the foot has the first of these represented 

 by the tibialis secundi of the hare (named so by Mr. Huxley), and the 

 second probably by the second tibialis anticus described by Mr. Mivart 

 in the echidna ("Tr. Linn. Soc." vol. xxv., p. 392), or the tibialis 

 anticus of the agouti (" Proceedings of the Zoological Society,'' 1866, 

 p. 411), although in the former animal the tendon is inserted into the 

 hallux. The third metacarpal has an extensor — the extensor carpi 

 radialis brevior ; and as a flexor it has the flexor tertii metacarpi of 

 Wood, or flexor carpi radialis profundus ; these have no ordinary re- 

 presentatives in the foot. The fourth metacarpal and its correspond- 

 ing metatarsal have no separate muscles attached to them, as in the con- 

 solidated state of the foot there could be no use for them as specialized 

 muscles. Of the flexor quarti metatarsi we have the trace in the slip 

 of the peroneus longus, so frequently connected to the base of the fourth 

 metatarsal bone. In the hand a slip of the flexor carpi ulnaris is some- 

 times attached to the base of the fourth metatarsal, or a fibrous band 

 from the pisiform is attached to that bone ; the muscles of the fifth 

 metatarsal bone are easily recognized ; the peroneus longus is evidently, 

 as Meckel has stated, of the same type as the flexor carpi ulnaris. Its 

 course and its- sesamoid bone (representing the pisiform), and the 

 transverse palmar course of the tendinous slips of the latter, in the 

 TIrsus arctos and sloth ; the peroneus brevis is the obvious representa- 

 tive of the extensor carpi ulnaris, even though in hyrax, Messrs, 

 Murie and Mivart found them going, the longus in front of the 

 malleolus, and the brevis behind it. This is but an accidental change 

 in position. 



Having thus homologated the metacarpal flexors and extensors, it 

 may be interesting to reduce our results to a tabular form at this 

 stage : — 



