BIRDS IN GENERAL. 11 



them for a life in air, and increasing the surface 

 by diminishing the solidity. In the first place, 

 their lungs, which are commonly called the sole, 

 stick fast to the sides of the ribs and back, and 

 can be very little dilated or contracted. But to 

 make up for this, which might impede their 

 breathing, the ends of the branches of the wind- 

 pipe open into them, while these have openings 

 into the cavity of the belly, and convey the air 

 drawn in by breathing into certain receptacles 

 like bladders, running along the length of the 

 whole body. Nor are these openings obscure or 

 difficult to be discerned ; for a probe thrust into 

 the lungs of a fowl will easily find a passage into 

 the belly ; and air blown into the wind-pipe will 

 be seen to distend the animal's body like a blad- 

 der. In quadrupeds this passage is stopped by 

 the midriff; but in fowls the communication is 

 obvious ; and consequently they have a much 

 greater facility of taking a long and large inspira- 

 tion. It is sometimes also seen that the wind- 

 pipe makes many convolutions within the body 

 of the bird, and it is then called the labyrinth; 

 but of what use these convolutions are, or why 

 the wind-pipe should make so many turnings 

 within the body of some birds, is a difficulty for 

 which no naturalist has been able to account. 



This difference of the wind-pipe often obtains 

 in animals that to all appearance are of the same 

 species. Thus in the tame swan, the wind-pipe 

 makes but a straight passage into the lungs ; while 

 in the wild swan, which to all external appearance 

 seems the same animal, the wind-pipe pierces 



