STRTJOTURB OP BIRDS, 



series of long stiff feathers, variously pjioportioned, according 

 to the kind of flight required by the species. This adaptation 

 of the form and structure of birds to flyiug, or progression in 

 the air, is obvious and intelligible. Their body is oval, with 

 the larger end forwards, and the mojre powerful muscles placed 

 on the breast, so that when the bdrizontal direction is assumed, 

 the centre of graivity comes between the wings, and is kept 

 near the lower part by the weight of the pectoral muscles. 

 The length and fiesibility of the neek enable the bird to make 

 the necessary changes in the centre of gravity, while the solidity 

 of the dorsal spine gives advantage to the action of the muscles ; 

 the head is terminated by a pointed bill, which aids in cleaving 

 the air; the feet, when short, are drawn up and concealeid 

 imder the feathers; when long, sbsetAfid out beneaUi or 

 beyond the tail, which is more or less expanded, and helps 

 to support the body in the air, as well as, by acting in the 

 manner of a rudder, to change its direction, or, by being 

 stretched out, to break its descent. Jn proportion to their 

 bulk, birds are also much lighter than other vertebrate 

 animals, and theif lightness is produced by iha introduction 

 of air into their tissue, and even into the bones, as well as by 

 the great bulk of the feathers, which, in these having a very 

 buoyant flight, as owls and guHs, is much greatrar than that of 

 the body. 



" Wien a bird intends to fly, it loosens its wings from their 

 ordinary position, throws its body forward, and gives it a 

 sudden impulse by means of the legs, which would merely 

 produce a leap, but the wings, being in the meantime spread 

 out and devated, they are again brought down with force, so 

 that their points generally strike against the ground. 'Wbether 

 or not, they act as levers, and, by repeated strokes, carry the 

 bird upwards. Were its ascent vertical, the rapid action of 

 the wings in the same plane would suffice to raise it, provided 

 the downward stroke were much more powerful than the 

 upward, the wing, moreover, being drawn in during the latter, 

 aoxd stretched out during the former. But, for progression in a 

 horizontal direction, it is necessary that the downward stroke 

 shoidd be modified by Use elevation, in a qertain degree, of the 

 free extremiides of the quills, and that the pinion should be 

 ptJled backwards. The best subject in which to Btaay: the 

 motions of the wings during flight is one of the larger gulls, 

 in whidh the wings being very k)ng and the flight remarkably 

 buoyant, and performed by slow beats, one may trace their 



