CHAPTER VIIL 
THE RESPIRATORY SYSTEM. 
I. ORGANS OF RESPIRATION. 
HESE are commonly known as the organs of breath- 
ing. They comprise the nostrils, nasal chambers, 
larynx, trachea, bronchial tubes and the lungs. 
1. The Nostrils. — The nostrils are two openings, one on 
each side of the nose. They are held open by the aid of 
cartilage or gristle, and muscles. About one and one-half 
inches up the nostril on the under side is a small opening 
about the size of a grain of shot. Through this opening the 
tube or duct which carries the tears down from the eyes 
empties into the nose. The nostril is lined with a thin deli- 
cate skin which changes into mucous membrane as it passes 
up into the chambers of the head. 
2. Nasal Chambers — Chambers of the Head. — These 
give passage to the air from the nostril into the larynx or 
Adam's apple. There are two of these chambers, divided in 
the center by a thin partition or cartilage called the septum 
nasi. These communicate with the sinuses of the head. The 
horse cannot breathe through the mouth on account of the 
formation of the throat, and this compels him to always 
breathe through the nostrils. This is a point which should 
be remembered. 
3. Larynx or Adam*s Apple. — This is a box made of 
cartilage, or gristle. It gives passage to the air and also the 
organ of voice. It is situated in the floor of the gullet. This 
box has an opening on its upper side, guarded by a valve, 
which is always open except when the animal is swallowing 
food or water. When the food is being swallowed it passes 
over the valve which closes the opening while the food passes 
over it. This is important, for if the valve does not close 
properly, thus allowing either food or water to drop into 
the windpipe, the animal will have a fit of coughing. This 
is sometimes referred to as "the food going the wrong way." 
On the outside of this box of cartilage are found several small 
muscles which help to hold it in its place. It is lined inside 
by a continuation of the same membrane as tha-t of the cham- 
