SURVEYS OF NATURE. 5 



I fufpefl alfo, that as fome birds have a vacuity between the 

 flefh and fkin, fo that air blown between fwells them exter- 

 nally (/. e* their ikin), while their flefli remains little affeded ; 

 fo birds, which rife to great heights, may fwell and dilate them- 

 felves when high, more than when low ; to counteradt the effed: 

 of the rarer medium at thofe heights, againil which ftrength of 

 wing is a feeble refource. 



Birds have, properly fpeaking, but one flom.ach ; but different 

 in different kinds. In the rapacious kinds that live on animal 

 food, and in fome of the iiih-feeding tribes, the gullet is replete 

 with glandulous bodies, which dilate and moiften the food as it 

 paflds ; the ftomach is always large, and generally wrapped round 

 with fat, toincreafe its warmth. 



In granivorous birds, the gullet dilates juft above the breafl:- 

 bcne, and forms a pouch or bag, called the crop; this contains 

 falivary glands to moiften its contents. They are very numerous, 

 with longitudinal openings, which emit a whicifli andvifcous lub- 

 ftance. After a convenient time the food paiTes into the belly, 

 where (inftead of a foft, fat, moift ftomach, as in the rapacious 

 kinds) it is ground between two pair of mulcles, commonly called 

 the gizzard, covered on the infide with a ridgy coat, almoft car- 

 tilaginous. Thefe ridgy coats, rubbing againft each other, are 

 capable of bruifing and attenuating the hardefl fubftances, even 

 iron or ftcnes. 



Birds are in general very much longer-lived in proportion than 

 quadrupeds : the age of quadrupeds is calculated at about feveii 

 times the interval between their birth and maturity ; that of birds 

 is probably ten or twelve times that interval. Their difeafes 

 are few: the moft remarkable is their moulting. We have {ttw 

 lomcwhat refernbling this among quadrupeds ; but in birds it is 

 mofi: notorious. We have feen the ilag caft his horns, the camel 

 his coat, the rabbit his fur ; we have ieen winter alliruilatie to its 

 fnows the clothing of polar animals, and their varying colours 

 return in fpring; but in birds, this change is fo powerful, that cney 



Part IV, No. 24. B become 



