150 MECHANISM OF FLIGHT IN BIRDS. 



curves or saltations. Natatorial birds sometimes need 

 very extended flight ; they take a very oblique position 

 in the air, stretch out their legs behind and their neck in 

 front ; they have the ribs greatly lengthened, the integu- 

 ments of the abdomen are long and flexible, which enables 

 them greatly to enlarge the abdominal portion of their 

 bodies by inflating it with air ; this causes a decrease in 

 the specific gravity of that part, and raises it to a hori- 

 zontal position; the compensating power consists in the 

 posterior half of the body becoming specifically lighter, 

 while the specific gravity of the anterior half remains un- 

 altered. When they alight they drop the legs, throw back 

 the trunk by bending the knee-joint, and bring the head 

 over the trunk by a graceful sigmoid curve of the long 

 neck, as in Fig. 24. The act of swimming is rendered 

 easy by the specific gravity of the body, by the boatlike 

 shape of the trunk, and by the conversion of the hinder 

 extremities into oars, in consequence of the membranes 

 unitinoj the toes too^ether. The effect of these web-feet in 

 water is further assisted by the toes having their mem- 

 branes lying close together when carried forwards ; whilst, 

 on the contrary, they are expanded in striking backwards. 

 The oarlike action of the legs is still further favored by 

 their backward position — an arrangement, however, which 

 is unfavorable for walking. 



Borelli was the first who, by comparison of the ana- 

 tomical pecidiarities of the human frame and the structure 

 of birds, demonstrated, to a certain extent, the impossi- 

 bility of the realization of the cherished project of flying 

 by man. He arrived at this conclusion from a compari- 

 son of the form and strength of the muscles of the wings 

 of birds with the corresponding muscles of the human 

 body, 



