34 



ANIMAL LOCOMOTION. 



ties of quadrupeds, bipeds, etc. in walking ; in the move- 

 ments of the tails and fins of fishes, whales, etc. in swimming ; 

 and in the movements of the wings of insects, bats, and 

 birds in flying. The straight and oblique muscles are 

 usually found together, and co-operate in producing the 

 movements in question ; the amount of rotation in a part 

 always increasing as the oblique muscles preponderate. The 

 combination of ball-and-socket and hinge-joints, with their con- 

 comitant oblique and longitudinal muscular cycles (the former 

 occurring in their most perfect forms where the extremities 

 are united to the trunk, the latter in the extremities them- 

 selves), enable the animal to present, when necessary, an exten- 

 sive resisting surface the one instant, and a greatly diminished 

 and a comparatively non-resisting one the next. This arrange- 

 ment secures the subtlety and nicety of motion demanded by 

 the several media at different stages of progression. 



The travelling surfaces of Animals modifiecl and adai^ted 

 to the medium on or in tvhich they move. — In those land 

 animals which take to the water occasionally^ the feet, as a 



Fig. 10. Fig. 11. Fig. 12. Fig. 13. Fig. 14. 



Fig, 10. — Extreme form of com]>ressed foot, as seen in the deer, ox, etc., 

 adapted specially for land ivRTiiiii.— Original. 



Fig, 11. — Extreme form of expanded foot, as seen in the Ornitlwrhynclviis, 

 etc., adapted more particularly for i^wmimm^. — Original. 



Figs. 12 and 13. — Intermediate form of foot, as seen in the otter (lig. 12), 

 frog (fig. 13), etc. Here the foot is equally serviceable in and out of the 

 water, — Original. ■ . 



Fig. 14. — Foot of the seal, which opens and closes in the act of natation, 

 the organ being folded upon itself during the non-effective or return stroke, 

 and expanded during the effective or forward stroke. Due advantage is 

 taken of this arrangement by the seal when swimming, the animal rotating 

 on its long axis,' so as to present the lower portion of the body and the 

 feet obliiiuely to the water during the return stroke, and the Hat, or the 

 greatest available surface of both, during the effective or forward stroke. — 

 Original- 



rule, are furnished with membranous expansions extend- 

 ing between the toes. Of such the Otter (fig. 1 2), Ornitho- 

 rhynchus (fig. H), Seal (fig. 14), Crocodile, Sea-Bear (fig. 37, 

 p. 76), Walrus, Frog (fig. 13), and Triton, may be cited. 

 The crocodile and triton, in addition to the membranous 



