202 DR. PETTIGREW ON THE MECHANISM OF FLIGHT. 



The extremities, when present, are provided with their own muscular cycles of extensor 

 and flexor, abductor and adductor, pronator aud supinator muscles, — these running longitu- 

 dinally and at various degrees of obliquity, and enveloping the hard parts according to their 

 direction — the bones being twisted upon themselves and furnished with articular surfaces 



which reflect the movements of the muscular cycles, whether these 



straight 



lines anteriorly, posteriorly, or laterally, or in oblique lines in intermediate situations. 

 The straight muscles are principally brought into play in the extension and flexion of the 

 extremities of quadrupeds in walking, and the oblique ones in the twisting of the pec- 

 toral fins of fishes and the flippers of the Whales and Seals in balancing and swimming, 

 and the wings of insects, bats, and birds in flying. The straight and oblique muscles 

 arc usually found in combination, and cooperate in producing the movements in ques- 

 tion, the amount of rotation in a part always increasing as the oblique muscles prepon- 

 derate. It is with a view to preparing the reader for the peculiar and complicated spiral 

 and rotatory movements of the wings that this digression on muscular arrangements and 

 actions has been thought desirable. The combination of ball-and-socket and hinge-joints, 

 with their concomitant oblique and non-oblique muscular cycles (the former occurring in 

 their most perfect forms where the extremities are united to the trunk, the latter in the 

 extremities themselves), enables the animal to present, when necessary, an extensive 

 resisting surface and a greatly diminished and comparatively non-resisting one, and 

 secures that subtlety and nicety of motion demanded by the several media at different 

 stages of progression. 



In those land-animals which take to the water occasionally, or the reverse, the feet, as a 

 rule, are furnished with membranous expansions extending between the toes. Of such 

 the Otter (Plate XII. fig. 2), Ornithorhynchus (Plate XII. fig. 3), Seal (Plate XII. fig. 4), 

 Crocodile, Sea-Bear (Plate XII. fig. 6), Walrus, Erog, and Triton (Plate XII. fig. 8) 

 may be cited. The Crocodile and Triton, in addition to the membranous expansion 

 occurring between the toes, are supplied with a powerful swimming-tail, which adds 

 very materially to the extent of surface engaged in natation. Those animals, one and all, 

 walk awkwardly, it always happening that when the extremities are modified to operate 

 upon two essentially different media (as, for instance, the land and water), the maximum 

 of speed is attained in neither. For this reason those animals which swim the best, walk, 

 as a rule, with the greatest difficulty, and vice versd, as the movements of the Auk and 

 beal in and out of the water amply testify. 



In addition to those land-animals which run and swim, there are some which preci- 

 pitate themselves, parachute-fashion, from immense heights, and others which even fly. 

 in these the membranous expansions are greatly increased, the ribs affording the necessary 

 '".pport m the Dragon or Mying Lizard (Plate XIII. fig. 13), the anterior and posterior 



powers possessed by Serpents. 



), "that the serpent has 



;t« nrn„Mn„« « : i v • % ' r c j<-iuua, auu, suaaemy ioosiug tneciose cons ui 



1 „ I, ' ; T SPnnS m '°, * e ^ and "* * e hM **» *• ™*" « * » "The serpent has neither 

 Ids " Z ? f 7 ° UtWreSt ! e ' he atMete ' and C ™ h the «W« in the emhrace of its ponderous overlapping 



tnenrseHes in an undeveloped or latent form in the trnnk of the reptile. 



