Nostrils.—The nostrils are two openings, one on each side of the nose. They are held open by the aid of cartilage 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 duct or tube which carries the tears down from the eyes empties into the nose. The nostril is lined with a thin, delicate skin which changes into mucous membrane as it passes up into the chambers of the head.
Nasal Chambers.—These give passage to the air from the nostril into the larynx. 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.
Larynx.—This is a cavity made of cartilage. It gives passage to the air and also the organ of voice. It is situated in the floor of the gullet. This cavity has an opening on its upper side, guarded by a valve, which is always opened 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 down the wrong way.” On the outside of this cavity 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 chambers of the head. The vocal cords which come into play when the animal is whinnying are found along the inside of this cavity. These cords are not nearly so well marked as in the human being, and if they or the cartilage of the larynx become affected it generally gives rise to the disease called wind-broken or roaring.
Trachea (Windpipe).—This is a tube which conveys 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 the larynx 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.
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 small tubes which pass all through the lungs and terminates 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 membrane 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.
Lungs.—Lungs are the most important organs of respiration. 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 separated by a partition known as the mediastinum, by the heart which is in the folds of this partition, and also 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 of the chest. It is made up of two folds, one being attached around the outer surface of the lungs, while the other is attached to the ends of the ribs at the side and at the back to the large curtain which separates the lungs from the bowels. The little glands situated in this membrane secrete an oily fluid which serves to lubricate these parts while the lungs are working in the chest so as not to cause friction. When this membrane becomes inflamed from a chill or injury it sets up the disease called pleurisy.
Respiration.—The number of respirations per minute varies with the different classes of animals; as a rule, the larger the animal the slower the respiration.
| The horse | 8 to 10 |
| Cattle | 12 to 15 |
| Sheep and goats | 12 to 20 |
| The dog | 15 to 20 |
| Swine | 10 to 15 |
The rate of breathing is increased from the process of digestion immediately after eating, or may increase from exercise.