INTRODUCTION. 
ix 
cells difperfed through its body ; that the Woodcock, and fome 
other flying birds, are not fo liberally fupplied with thefe cells ; 
yet, he elfe where obferves, that it may be laid down as a general 
rule, that in birds of the higheft and longeft flights, as Eagles, 
this extenfion or diffuflon of air is carried further than in o- 
thers ; and, with regard to the OArich, though it is deprived of 
the power of flying, it runs with amazing rapidity, and confe- 
quently requires flmilar refources of air. It feems therefore to 
be proved, evidently, that this general diffuflon of air through 
the bodies of birds is of infinite ufe to them, not only in their 
long and laborious flights, but likewife in preventing their refpi- 
ration from being flopped or interrupted by the rapidity of their 
motion through a refilling medium. Were it pofiible for man 
to move with the fwiftnefs of a Swallow, the adlual refiftance of 
the air, as he is not provided with internal refervoirs flmilar to 
thofe of birds, would foon fuffocate him.* 
Birds may be diflinguilhed, like quadrupeds, into two kinds 
or claffes, granivorous and carnivorous ; like quadrupeds too, 
there are fome that hold a middle nature, and partake of both. 
Granivorous birds are furnilhed with larger inteftines, and pro- 
portionally longer than thofe of the carnivorous kinds. Their 
food, which confifts of grain of various kinds, is conveyed whole 
and entire into the firft ffomach or craw, where it undergoes a 
partial dilution by a liquor fecreted from glands fpread over its 
furface ; it is then received into another fpecies of ftomach, 
where it is further diluted ; after which it is tranfmitted into 
the gizzard, or true ftomach, confifting of two very ftrong muf- 
cles covered externally with a tendinous fubftance, and lined 
with a thick membrane of prodigious power and firength ; in 
this place the food is completely triturated, and rendered fit for 
the operation of the gaftric juices. The extraordinary powers 
* May not this univerfal diffufxon of air through the bodies of birds ac- 
count for the fuperior heat of this clafs of animals ? The reparation of oxy- 
gen from refpirable air, and its mixture with the blood, by means of the 
lungs, being fuppofed by the ingenious Dr Crawford to be the efficient 
caufc of animal heat. 
