BIRDS. 309 



the body, and were attached to the skin; and a beautiful fan- 

 shaped muscle was also spread over the anterior surface of the 

 large air-cell just mentioned. " The use of these muscles 

 appeared to he to produce instantaneous expulsion of the air 

 from these external cells, and by thus increasing the specific 

 gravity of the bird, to enable it to descend with the rapidity 

 necessary to the capture of a living prey, while swimming near 

 the surface of the water." 



This is one of those beautiful adaptations of means to an 

 end which Natural History records in every department. 

 " The descent of the Gannet on its prey has been, not inaptly, 

 compared to that of an arrow, the beak of the bird forming 

 the arrow-head, and the body and wings the feathered shaft 

 of the weapon we here have the secret of its heavy fall; the 

 same machinery restores the buoyancy at the proper moment, 

 and the bird rises with its fish aloft." 



Moulting. The plumage of birds is periodically renewed, 

 and the process of this change of feathers is termed ' * moulting. ' ' 

 The aspect of the bird, in many instances, changes, not only 

 with age, but also with the season; the summer dress, as we 

 shall have occasion to mention, is often very unlike that of the 

 winter. The changes in the plumage of birds have been 

 investigated, with great care, by Mr. Yarrell; and, in the 

 opinion of that able zoologist, the different appearance which 

 it presents may be explained, 



1st. By the feather itself becoming altered in colour; 

 2d, By the birds obtaining a certain addition of new fea- 

 thers, without shedding any of the old ones ; 

 3d, By an entire or partial moulting, at which old feathers 

 are thrown off, and new ones produc ed in their 

 places; and, 



4th, By the wearing off of the lengthened lighter-coloured 

 tips of the barbs of the feathers on the body, by 

 which the brighter tints of the plumage underneath 

 are exposed. 



In spring, the change which takes place prior to the pairing 

 season is to be attributed to the first two modes; and at that 

 time, also, there is a partial moulting of old feathers a laying- 

 aside, as it were, of a portion of the warm garments of winter. 

 The entire moulting is that absolute change of feathers which 

 takes place in autumn. 



Digestive Organs. If, quitting for a moment the con- 



