DR. PETTIGREW ON THE MECHANISM OF FLIGHT. 205 



it obliquely to the water, the tail meanwhile twisting upon itself and being in sobm 

 instances more or less compressed vertically. During extrusion, or when the effective 

 stroke is delivered, these conditions are reversed; the body is retilted so as to apply 



its flat or maximum of surface, and the curves into which it was thrown reduced bv a 

 violent recoil, in which the concavity or biting surface of the tail is vi orously urged from 

 right to left, and the corresponding surface of the head from left to right, or the reverse. 



When the head and tail travel beyond the axis of motion— and )>y tin's I mean an 

 imaginary line (say a b of Diagrams) on either side of which they oscillate— the curves 

 are gradually reversed and the back stroke commenced. The axis of motion, therefore, 

 corresponds with the long axis of the fish when the body is straight. It is to this line 

 that the movements of the fish are to be referred, the upper and lower portions of tin 

 body being curved and carried away from it in flexion (Diagram 2, c<7), and straightened 

 and forced towards it during extension. The double curve or spiral into which the fish 

 throws itself when swimming maybe conveniently divided into an upper or cephalic 

 curve (Diagram 2, d) and a lower or caudal one (c). When the concavity of the caudal 

 curve is biting or laving hold of the water, and when the concave surface of the tail is 

 being forced during extension with great violence in the direction of the axis of mot ion (a b 

 of Diagram 2), where the concave surface is suddenly converted into a convex one, th< 

 concavity of the cephalic curve, i. e. the concave surface of the upper halt of the fish, is 

 being urged with less vigour in the direction of the same line from the opposite Bide of 



it. As the caudal and cephalic curves are obliterated when the line in question is 

 reached, there is consequently a period (momentary it must be) between the effective and 

 non-effective strokes in which the body of the fish is comparatively straight, and conse- 

 quently in a position to advance almost without impediment. When, further, the tail i- 

 being forced from right to left of the axis of motion, the head is reciprocating by travel lin - 



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in a contrary direction, or from left to right, the head and tail and the cephalic and 

 caudal curves complementing each other. I shall perhaps be better understood if I say 

 the concave surfaces of the cephalic and caudal portions of the fish bite or seise the 

 water at the same time, the cephalic concave surface in conjunction with the water 

 affording the resistance or point d'appui necessary for the effective action of the concav. 

 surface of the tail, while it conducts the head in an opposite direction to the tail, the 

 upper part of the body acting as a guide or pilot to the lower portion. The fish has, 

 therefore, every part of its body perfectly under control, both during extension, when 

 the effective stroke is given, and during flexion, when the non-effective, or back stroke, is 

 made. The terms extension and flexion, I may remark, are commonly confined to the 

 lateral oscillations of the lower half of the body. This, however, is a mistake, since the 

 head and upper half are likewise extended and flexed, the only difference between the 

 extension and flexion of the upper and lower portions of the body amounting to this, 

 that they occur on the opposite sides of a straight line— this line corresponding to the 

 axis of motion. It is a curious circumstance, and as suggestive as curious, that this is 

 precisely what happens in the axillary and distal curves of the anterior and posterior 

 margins of the wing of the bird in extension and flexion (vide n,fra). A certain analogy 

 mav therefore be traced between the water and air as media, and between the trunk and 





vol. \xvi. 



2f 



