VI FORM AND FUNCTION 79 
situated where the trachea divides to form the bronchi. 
I shall describe it later on. 
Though a bird has sucha 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 
inacommon domestic pigeon: length 13 inch, depth 
# inch, breadth 4 inch. This gives for cubic content 
} inch, for the two lungs together 4 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 
