346 THE MECHANICAL FUNCTIONS. 



or furcular bones of birds, are always curved in- 

 wards. This part of the trunk requires to be 

 alternately dilated and contracted during flight, 

 and hence the several pieces of which its dorsal 

 portion is composed are loosely connected to- 

 gether by ligaments.* 



The shape of the wings is more or less trian- 

 gular. They are moved by numerous muscles, 

 which occupy a large space in the interior of 

 the trunk, and consist of various kinds of 

 flexors, extensors, retractors, levators, and de- 

 pressors ; the w hole forming a very complicated 

 assemblage of moving powers. The largest, 

 and consequently most powerful of these muscles, 

 are those which depress, or bring down the 

 wings. They form a large mass, marked a in 

 Fig. 144. All these muscles exert great force in 

 their contractions, which are capable of being 

 renewed in very rapid succession : for, indeed, 

 unless they had this power, even so light a 

 body as that of an insect could not have been 

 sustained for a moment in so rare a medium as 

 the atmosphere, far less raised to any height by 

 its resistance. 



The simple ascent and descent of the wings 

 would be sufficient, without any other movement 

 being imparted to them, to carry forwards the 



* See Chabrier's " Essai sur le Vol des Insectes," Memoire.s 

 du Museum d'Histoire Natuielle; vi. 410, vii. 297, and viii. 47 

 and 349. See also Zoological Journal ; i. 391. 



