XVlll 
IXTUODUCTION. 
for emission, cannot naturally repass through the cesophagus, because little 
fleshy protuberances encircle it at its insertion in the gizzard, and hinder 
the food from returning without the most violent exertion. To prevent 
the bile from regurgitating into the stomach, the outlet of the duct, called 
the ductus choledochus, that conveys it into the intestines, is distinct from 
the stomach, else the bird, in the rapidity of its flight, would be subject to 
trequent regurgitations ; and in order that it may be properly blended and 
mixed with the chyle, this duct enters downward, opposite to the course of 
the food. The lungs of birds are not divided into lobes, nor are they loose 
in the cavity of the thorax, but adhere to the back-bone and sides through- 
out their extent, in order to render their flight more even and steady ; 
though in some of the smaller tribes, they are but slightly adhesive. 
They are soft, reddish, spongy, flat substances, covered with a thin, 
pervious membrane, supplying the place of the diaphragm, and commu- 
nicating with the vesicles, or air bags, that spread over the thorax and 
abdomen. The two extremities of the branches of the windpipe terminate 
within the lungs ; and the air, thus received into them, is emitted through 
little orifices, or perforations, into the thin, transparent, membranous 
bladders ; many of which are divided by slight, filmy, transverse septa, or 
little boundaries ; and the air is conveyed over the body by these bladder- 
like tubes ; which seem to serve another purpose, that of confining the 
viscera, and preventing them from moving violently in the cavity of the 
body, by the motion of the bird in its flight. This property of being 
enabled to imbibe a considerable quantity of air, renders the bird speci- 
fically lighter when the lungs are 'distended, and some of the feathered tribes 
have air cells so generally distributed over them, that they are found even 
in their quills, pinions, and thigh-bones. Indeed, a large portion of the 
skeleton of birds is formed into receptacles, which contain air. In the 
'■* In the dissection of several of the smaller birds I have discovered, that the membrane, 
which connects the lungs with their sides and back, is so thin and lax, that the lungs are but 
slightly adhesive. 
