$3 THE PHILOSOPHY 



From this general view of the external figure and ftruc- 

 ture of birds, it is apparent, that Nature has defigned them 

 for two diftin(St kinds of motion. They can, at pleafure, 

 either walk on the furface of the earth, or mount aloft, and 

 penetrate the airy regions with prodigious fwiftnefs. 



Some peculiarities in the internal flrudlure of birds deferve 

 our notice. 



Like quadrupeds, the feathered tribes are divided into 

 granivorous and carnivorous •, and their manners and difpo- 

 tions correfpond with their internal and external conformation. 



In the granivorous clafs, the oefophagus or gullet runs 

 down the neck, and terminates in a pretty large membranous 

 fac, called the ingluvles^ or craw, where the food is macerat- 

 ed, and partly diiTolved by a Hquor fecreted from glands 

 fpread over the furface of this fac. Some birds, as the 

 rooks and the pigeon kind, have the power of bringing up 

 the food from this fac into their mouths, and feeding their 

 young with it in a half digefted form. After macerating for 

 fome time, the food pafTes through the remainder of the gul- 

 let into another fpecies of flomach denominated 'uentnculus 

 fiiccentiiriatusy which is a continuation of the gullet. Here 

 the food receives a farther dilution. From this fecond 

 flomach, the food is tranfmitted to the gizzard, or true 

 flomach, which confifts of two very ftrong mufcles, covered 

 externally v/ith a tendinous fubftance, and lined with a thick 

 firm membrane. The remarkable flrength of the gizzard 

 was formerly fuppofed to affift the digeftion of granivorous 

 birds by attrition. But this notion has of late been entirely 

 exploded ; for Do<Stor Stevens, and, after him, Spalanzani, 

 have demonftrated, by unequivocal experiments, that digef- 

 tion is performed f olely by the diiTolving powers of the gaftric 

 juices.* 'The other inteftines are proportionally larger, and 

 much longer than thofe of the carnivorous birds. 



See Stevens Diflbrt, Med. Inauj. De Aliment orum Cojicodione, Edin, i777v 

 ^nd Spalanzani, 



