LOCOMOTION OF FISHES. 243 



is inserted into the fore part of the lower three-fourtlis of the 

 tibia. The tibialis posticus, with the usual origin, is inserted into 

 the astragalus, fig. 44, a. A peroneus arises from the outer 

 femoral condyle, and from the outer side of the leg-bone ; its 

 tendon bifurcates, one part being attached to the outer malleolus, 

 the other to the base of the calcanemn, ib. d. An extensor hnu/us 

 di(/itorum arises from the outer malleolus, passes between the two 

 portions of the tibialis anticus, and, after sending an insertion to 

 the astragalus, is continued to the three middle toes. The 

 extensor brevis arises from the whole length of the calcancum, and 

 divides into six parts, an external to the metatarsal of the hallux, 

 an internal to that of the minimus, and the intermediate four to 

 the plialanges of the four outer toes ; these unite with the tendons 

 of interossei extend, to which mirdit be referred the extensor of the 

 hallux. Both this toe and the outermost have their abductors 

 for S2ireading the web. There is also an abductor of the ento 

 cunciform, fig. 44, ci, wliich resembles a small accessory digit. 

 The plantar aponeurosis, which receives a fleshy fascicle from the 

 tibio-tarsal capsule, gives oi-igin to a muscle inserted into the 

 w]H)le length of the astragalus, divides into six fascicles, whicli 

 form sheaths for the flexor tendons, two of which lielono- to the 

 foiu'th toe ; and, finally, is resolved into three tendons, of which 

 two go to the fifth toe, and one to the fourth. Thej?c'xo?- hmfjus 

 diijitorum arises from the tibio-tarsal capsule, and is ex})ended 

 u])on the three outer toes. The several insertions of the fore- 

 going digital flexors give one tendon for the ungual phalanx, a)Kl 

 two for the other phalanges. 



§ 48. Locomotion of Fishes. — Hitherto the osteology and myo- 

 logy of the cold-blooded Vertebrates have been considered chiefly 

 from a homological point of view. I have aimed at relie'S'ing the 

 dryness of descriptive detail, and at connecting the multifarious 

 particulars of this difficult part of Comj)arative Anatomy in 

 natural order, so as to lie easily retained in the memory, by 

 referring to the relations which the bones and muscles of Fishes 

 and Reptiles bear to the general plan of vertebrate organisation, 

 and by indicating their analogies to transitory states of structure 

 in the embryo of higher animals, and to those answerable con- 

 ditions of the mature skeleton wliich, in longer lapse of time, 

 have successively prevailed and ])assed away in the generations 

 of species that have left recognisable remains in the superimposed 

 strata of the earth's crust. 



To determine the parts of the vertebrate structure which are 

 most constant — to trace their general, serial, and special lionio- 



R 2 



