146 



ANIMAL LOCOMOTION. 



vertical direction of the loops formed by the wing of the bat 

 and bird will readily be understood by referring to figs. 

 74 and 75 (p. 145), which represent the wing of the bird 

 making the down and up strokes, and in the act of being ex- 

 tended and flexed. (Compare with figs. 64, 65, and 66, p. 

 139 ; and figs. 67, 68, 69, and 70, p. 141.) 



The down and up strokes are compound movements, — the 

 termination of the down stroke embracing the beginning of 

 the up stroke; the termination of the up stroke including the 

 beginning of the down stroke. This is necessary in order 

 that the down and up strokes may glide into each other in 

 such a manner as to prevent jerking and unnecessary retarda- 

 tion. 



The Margins of the Wing throion into opposite Curves during 

 Extension and Flexion. — The anterior or thick margin of the 

 wing, and the posterior or thin one, form different curves, 

 similar in all respects to those made by the body of the 

 fish in swimming (see fig. 32, p. 68). These curves may, 

 for the sake of clearness, be divided into axillary and distal 

 curves, the former occurring towards the root of the wing, 

 the latter towards its extremity. The curves (axillary and 

 distal) found on the anterior margin of the wing are 

 always the converse of those met with on the posterior 

 margin, i.e. if the convexity of the anterior axillary curve 

 be directed downwards, that of the posterior axillary curve 

 is directed upwards, and so of the anterior and posterior 

 distal curves. The two curves (axillary and distal), occurring 

 on the anterior margin of the wing, are likewise antagonistic, 

 the convexity of the axillary curve being always directed 

 downwards, when the convexity of the distal one is directed 

 upwards, and vice versa. The same holds true of the axillary 

 and distal curves occurring on the posterior margin of the 

 wing. The anterior axillary and distal curves completely 

 reverse themselves during the acts of extension and flexion, 

 and so of the posterior axillary and distal curves (figs. 76, 77, 

 and 78). This antagonism in the axillary and clistal curves 

 found on the anterior and posterior margins of the wing is 

 referable in the bat and bird to changes induced in the bones 

 of the wing in the acts of flexion and extension. In the 



