276 



ANIMAL MECHANISM. 



nervure towards its base. The membranous part commences 

 only at the point where rapidity of movement begins to be 

 of some use, and the membrane goes on increasing in breadth 

 till near the extremity of the wing. Such is (fig. 116) the 

 type of the wing essentially active — that is, intended only to 

 strike the air. 



FiQ. 116. —Wing of an insect. 



In the bird, on the contrary, one of the phases of the 

 movement of the wing is, to a certain extent, passive ; that 

 is to say, it receives the pressure of the air on its lower sur- 

 face, when the bird is projected rapidly forward by its 

 acquired velocity. Under these conditions, the whole bird 

 being carried forward into space, all the parts of the wing 

 are moved with the same rapidity ; the regions near to the 

 body are as useful as the others to take advantage of the 

 notion of the air which presses on them as on a kite. 



Fig. 117.— Active and passive parts of the bird's wing. 



Thus, the base of the wing in the bird, far from being re- 

 duced, as in the insect, to a rigid but bare rib, is very wide, 

 and furnished with feathers and wing coverts which constitute 

 a large surface, under which the air presses with force, and 

 in a manner very efficacious to sustain the bird. Fig. 117 

 gives an idea of the arrangement of the wing of the bird, at 

 the same time active and passive. 



The inner part, deprived of sufficient velocity, may be 



