T 



CHAPTER VIII. 

 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 that of the cham- 



M 



