20 i 



PETTIGREW ON THE MECHAN 



OF FLIGHT. 



muscles in the fish, as has been explained, are for this purpose arranged along the spinal 

 column, and constitute the bulk of the animal, it being a law that when the extremities are 

 wanting, as in the Water-snake, or rudimentary, as in the fish (Plate XII. fig. 11), 



are largely developed, 



o 



Lepidosiren *, Proteus, and Axolotl, the muscles of the trunk are 

 the onus of locomotion, as a consequence, falling chiefly, if not entirely, upon the tail 

 and lower portion of the body. The operation of this law is well seen in the metamorphosis 

 of the Tadpole, the muscles of the trunk and tail becoming modified, and the tail itself 

 disappearing as the limbs of the perfect Prog are developed. The same law likewise 

 prevails in certain instances where the anterior extremities are comparatively perfect, as 

 in the Whale, Porpoise (Plate XII. fig. 10), Dugong, and Manatee (Plate XII. fig. 9), 

 and where both anterior and posterior extremities are perfect, as in the Crocodile, Triton 

 (Plate XII. fig. 8), and Salamander. The Whale, Porpoise, Dugong, and Manatee em- 

 ploy their anterior extremities in balancing and turning, the great organ of locomotion 

 being the tail. The same may be said of the Crocodile, Triton, and Salamander, these 

 using their extremities in quite a subordinate capacity as compared with the tail. The 

 peculiar movements of the trunk and tail in swimming are seen to most advantage in the 

 fish, and may now be briefly described. 



Sw 



the Fish, Whale, Porp 



When the fish gathers itself to 



gether, preparatory to making the stroke which sends it forwards, it bends or flexes 

 its tail laterally, not, as is commonly represented f, by throwing its body 



into a single 



(Diagram 1, c), whose convexity is turned in the direction of the mesial line of 



Diagram 1. 



Diagram 2. 



d 



a 



I 



a 



\ & 



Of 



■cl 



fish (a b\ and which would have the effect of projecting it along the arc 

 d), but by converting it into a double curve (Diagram 2, c d), the convexities of both 



The object of this arrange- 



of which are directed away from the line in question (a b) 



ment is twofold : — first, to 



that the fish shall pursue 



mparatively straight 



d secondly, to 



course, its head and tail oscillating on either side of a given line (a b) 



diminish the amount of resistance experienced by the upper and lower portions of the 



body during flexion, or when the back stroke is being made, the resistance occasioned by 



a concave surface (c of Diagram 1) 

 being as nearly as possible as two to 



compared with 



one 



of Dia 



9 



stroke is further diminished by the body being made to rotate slightly 



The resistance encountered during the back 



so as to present 



with 



rudimentary 



nprfflof *™o u-i xi. n , *v B «.« t „w F o 110 01 ruuimeniary nmos turmshed with tour urn- 



treltv r f ^f""" haS anteriOT «•»■»» »™a with three toes each, and , very feeble posterior ex- 



treraiU terminating m two tae* J F 



an^^L> D b e v 7^ Anim ,t m V. ^^ "• ^ 5 ' £ra ' 4t °' 2 VOlS " W > 1680 -> C ?- A -t. & Phy, 



art. Motion, by Bishop, p. 438 ; Owen, op. cit. pp. 252-253. 



y 



ol. iii. 



