v , FORM AND FUNCTION 79 



situated where the trachea divides to form the bronchi. 

 I shall describe it later on. 



Though a bird has such a splendid "wind," his lungs 

 are small. They will be found lying close against the 

 back, and, if the body is laid down breast uppermost, 

 under the heart and liver. They extend from the 

 first rib to where the kidneys begin, and may easily 

 be known by their sponginess and their scarlet colour. 

 It is difficult to measure them exactly, but these are 

 the measurements as nearly as I could take them 

 in a common domestic pigeon: length if inch, depth 

 § inch, breadth | inch. This gives for cubic content 

 | inch, for the two lungs together \ inch. There is 

 no reason to suppose that in a Homer pigeon the 

 dimensions are appreciably larger. These small lungs 

 are a wonderful feature in a bird, to whom, under 

 favourable conditions, a flight of fifty miles in an 

 hour is no great exertion. In all birds we find 

 the same striking contrast between the excellence 

 and the small size of the lungs. Though they are 

 spongy, they have but little elasticity. When a man 

 expands his chest, the lungs are distended and the air 

 rushes in to fill the vacuum caused. A bird's lungs 

 vary little in size. They are prolonged into spacious 

 air-sacks, the most characteristic part of the breath- 

 ing apparatus, which renders elasticity of lungs 

 unnecessary. These air-sacks are extensions of the 

 membrane which forms the walls of the bronchi. 

 The two bronchi we have already described as 

 leading to the lungs. They run through the lungs 

 dividing as they go, and end in these great ex- 

 pansions by the help of which a bird is able to get 



