OF NATURAL HISTORY. 73 



mufcles, covered externally with a tendinous fubftance, and lined 

 •with a thick, firm membrane. The remarkable ftrength of the giz- 

 zard was formerly fuppofed to alTift the digeftion of granivorous 

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

 ploded ; for Do£lor Stevens, and, after him, Spalanzani, have de- 

 nionftrated, by unequivocal experiments, that digeftion is perform- 

 ed folely by the diflblving powers of the gaftric juices *. The other 

 inteftines are proportionally larger, and much longer than thofe of 

 the carnivorous birds. 



The ftruiSure of the heart, in granivorous birds, is nearly the 

 fame with that of quadrupeds. 



The lungs hang not loofe in the cavity of the thorax, but are fix- 

 ed to the back-bone : Neither are they divided into lobes, as in man 

 and other animals whofe fpines admit of confiderable motion. They 

 are red, fpongy bodies, covered with a membrane that is pervious, 

 and communicates with the large v^ficles or air-bags which are 

 fpread over the whole abdomen. Thefe veficles, when diftended 

 with air, render the bodies of birds fpecifically light. They like- 

 wife fupply the place of a diaphragm, and ftrong abdominal muf- 

 cles. They produce the fame effeds on the vifcera as thefe mufcles 

 would have done, without the inconveniency of giving an addition- 

 al weight to the body. 



Birds have no bladder of urine : But a blueifh-coloured canal, or 

 ureter, is fent off from each kidney, and terminates in the redtum. 

 Their urine is difcharged along with the faeces. It is a whitifh 

 fubftance, and turns chalky when expofed to the air. 



t K The 



• See Stevens Diflert. Med. Inaug. De Alimentorum Concoftione, Edin. 1777, 

 -and Spalanzani. 



