298 ANATOMY AND PHYSIOLOGY OF THE FOWL 
lungs, just as in the human system. (Refer to text-books on physi- 
ology for a description of the human circulation.) 
Respiratory System.—Breathing is carried on in the lungs. The 
air enters these through the trachea or windpipe, whichis a long tube, 
dividing just before it reaches the lungs into two bronchial tubes, 
one of which enters each lung. It is in the lungs that the blood 
gives up the poisonous material-which it has gathered, and takes 
up its supply of oxygen to carry through the body. The lungs are 
surrounded by large air sacs 
in the breast and abdomen, 
there being nine in all. These 
increase the size of the bird in 
proportion to its weight and 
enable it to fly better. 
Reproductive Systems.— 
The female’s reproductive 
system consists of two main 
organs, the ovaries and ovi- 
duct. The hen has two ova- 
ries, only one of which is 
active. In general this re- 
sembles a bunch of grapes, 
the ova (or eggs) varying in 
size from small specks to full- 
size yolks, when the bird is 
in condition for laying. Each 
ovum (or egg) as it comes to 
maturity enters the oviduct, 
where it may be fertilized by 
the sperm of the male; as it 
- passes down the oviduct suc- 
oaca . . 
eae cessive layers of albumin are 
Fia. 144.—Male reproductive system. added, and finally just before 
being laid the shell is put on. 
A description of the formation of the egg is given in Chapter XVIII. 
The reproductive glands of the male bird, called testicles, two 
in number, are near the backbone just in front of the kidneys and 
about opposite the last ribs. These secrete the male sperm, which 
is transported through two tubes, called vas deferens, to the 
cloaca (Fig. 144). 
The Development of the Senses.—The fowl has a well-devel- 
vas deferens 
