XXX 
INTRODUCTIOX. 
in opposition to Brisson, in his description of the Golden-crested Wren, 
he so positively averred, that this feather was not a quill, on account of its 
shape, insertion and use ; for its form is exactly similar to the next quill, 
and its mode of insertion in no respect differs from it. All the quill 
feathers are placed with the greatest degree of regularity, and with the 
most admirable order, the largest and strongest bearing the heaviest share 
of duty ; and all of them are covered, at their insertion, by layers of thick 
feathers above and below, called the coverts, to preclude the air from 
permeating the wing, and of course lessening the force of its impulse. 
Two powerful muscles^® give vigour to the wing of birds, which are 
infinitely stronger in comparison than those in man, or quadrupedes ; and 
these powerful supporters of flight are so contrived, as to unite enormous 
strength with peculiar elasticity. The pectoral muscle of an Eagle is 
nearly as thick as that in Man, so that, if we take into the estimation 
their relative bulk, the great disparity in the size and force of this 
muscle excites our surprize. 
The bills of birds are uniformly composed of two parts, or mandibles ; 
one above, and the other beneath ; yet they vary almost as much in 
form, as the birds differ in habits. Nature has accommodated all the 
immense multitudes that wing the air with such instruments as are 
The vast power of this muscle has induced many philosophers to presume, that, by 
man, the art of moving in the air was unattainable. They drew this consequence from the 
impossibility of providing springs, analogous to these muscles, of sufficient force to sustain his 
weight in the atmosphere ; but in our days the spirit of curiosity has removed the seeming 
impossibility in a different mode. Machines Irave been fabricated lighter than air, and men, in 
this proud specimen of human ingenuity, have rode triumphantly in an element, which nature 
appeared to have raised against them, as an impassable barrier. 
Pectorales musculi hominis flectentes humeros parvi et parum carnosi sunt ; e contra in 
avibus, vastissimi sunt, imo excedunt et magis pendent, quam reliqui omnes musculi ejusdem avis 
simul sumptj. Borelli, de motu avium. • 
