THE ANATOMY OF THE HORSE 
66 
bers of the head. The vocal chords which come into play 
when the animal is whinnying are found along the inside of 
this box. These chords are not nearly so well marked as in 
the human being, and if they or the cartilage of the larynx 
become aflfected it generally gives rise to the disease called 
roaring. 
4. Trachea or Windpipe. — This is a tube which carries 
the air down from the larynx to the bronchial tubes in the 
lungs. It is made up of forty or fifty rings of cartilage which 
are united to each other by strong elastic ligaments. They 
give to the windpipe its flexibility, that is, the power to bend 
in any direction almost like a piece of elastic. From Adam's 
apple the windpipe enters the chest where it terminates into 
two small tubes, one going to the right lung and one to the 
left. These are called the bronchial tubes. 
5. Bronchial Tubes and Air Cells. — These are made up 
of the same material as that of the windpipe, but are only 
about half the size. After passing into the substance of the 
lungs they break up into other small tubes which pass all 
through the lungs and terminate into what is known as the 
air cells. These small tubes and air cells are lined inside by 
a very thin mucous membrane, a continuation of the mem- 
brane lining the other organs already mentioned. Just inside 
this thin mucous membrane is found the capillary network 
of the lungs, and while the blood is slowly passing through 
this network of vessels it gives off to the air in the air cells 
carbonic acid gas and takes in the oxygen from the pure air 
while it is in the lungs. 
6. The Lungs are the most important organs of respira- 
tion or breathing. They are spongy, yellowish organs, two 
in number, one situated on the right side and the other on 
the left. The right lung is the largest because of the left one 
having a hollow in its side for the heart. The lungs are sep- 
arated by a partition known as the mediastinum, by the heart 
which is in the folds of this. partition, and also by the large 
blood vessels and oesophagus. They are made up of light 
elastic tissue and are full of air cells and tubes. While the 
animal is alive they are very large and fill up nearly the 
whole chest cavity, but after death they collapse and are not 
nearly so large. Between the lungs and the ribs is found a 
serous membrane called the pleura or the lining membrane 
