206 DR. PETTIGREW ON THE MECHANISM OF FLIGHT. 



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extremities as instruments of locomotion. From this it follows that the water and air 



are acted upon by curves or wave-pressure emanating in the one instance from the body 



of the fish, and in the other from the wing of the bird, the reciprocating and opposite 



curves into which the trunk and wing are thrown in swimming and flying constituting 



in reality a mobile helix or screw, which, during its action, produces the precise kind and 



degree of pressure adapted to fluid media, and to which they respond with the greatest 



readiness. The whole body of the fish is thrown into action in swimming ; but as the tail 



and lower half of the trunk are more free to move than the head and upper half, which 



are more rigid, and because the tendons of many of the trunk-muscles are inserted into 



the tail, the oscillation is greatest in the direction of the latter. The muscular movement 









spiral wave from before (Plate XII. fig. 11) backwards : and 



eacting 



into 



upon the water, causes the fish to glide forwards in a series of curves. Since the head 

 and tail, as has been stated, always travel in opposite directions, and the fish is constantly 

 alternating or changing sides, it in reality describes a spiral course, as represented in 

 Plate XV. fig. 76. These remarks may be readily verified by a reference to the swimming 

 of the Sturgeon (Diagram 2, p. 204), whose movements are unusually deliberate and slow. 

 The number of curves into which the body of the fish is thrown in swimming is 

 increased m the long-bodied fishes, as the Eels, and decreased in those whose bodies 



are short or are comparatively devoid of flexibility. In proportion as the curves 



winch the body is thrown in swimming are diminished, the degree of rotation at the tail 

 or in the fins is augmented, some fishes, as the Mackerel, using the tail very much after 

 he manner of a screw in a steam-ship. The fish may thus be said to drill the water in 

 two directions, viz. from behind forwards by a twisting or screwing of the body on its 

 long axis, and from side to side by causing its anterior and posterior portions to assume 

 opposite curves. The pectoral and other fins are also thrown into curves in action, the 

 movement, as m the body itself, travelling in spiral waves ; and it is worthy of remark 

 that the wmg of the insect, bat, and bird obeys similar impulses, the pinion, as I .hall 

 show presently, being essentially a spiral organ. 



. J h< ; • ntt 8 ° f ^ : PeCt ° ral finS iS WeU Seen in the common Per <* (Percajluviatilis), 



and til better m the 15-spined Stickleback (Gasterosteus spinosus), vlnch latter fre- 

 quently progresses by their aid alone*. In the Stickleback the pectoral fins are so delicate 



motZ P S t I V ! S ° m that ^ 6ye 1S apt t0 0Teri00k them ' Particularly when in 

 unusual t, ! f 1 ^ "" ^ ^^ "* pleaSUre > so that * is 5 *> «neans an 

 and 2 t T g SCe , r^^ Pr ° CeedillS t8il flrst Tht fiDS are rotated or twisted, 



whti 1 1 ma T nS * 6d ab ° Ut by Spiral mOT ™* which elosclv resemble those by 



winch the wings ol msects are propelled t . The rotating of the fish upon its long axis is 



# 



t 



The S„naa » or Pipefishes, s»im chiefly by the undulating movement of the dorsal fin 

 are), il t^Z^X ^ 7 2 h ° m ° ,0gUeS " * — "^ <^" ** -•— % 



mpeds, and so fully d evel .„ d 1 ^ 7,! '* """"^ " hich »'< ,rilC ™ bk »■ th * «*«*«• <* quad- 



pectoral fins," rltt^Z te^h Lit "t "^ *? * *"* "<"" d ' " *» — of the 



vertebrates, they are very smalUew. and 'J££^Z ZEfjS ^ "' ^ h0m0,08 ° US ""^ '" "*" 



mg, advancing, and .,.!„ U yi„ g them pr J £ J™" "* the ^"'f • cements of the fin,_clevating, depress. 



prone and fl„t, by ,„ obbque stroke, upon the sides of the body. The rays or 



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