The respiratory apparatus may be influenced in two principal directions by the administration of drugs, which may be divided into stimulants and depressants. The reader who would properly understand their modus operandi is referred to the chapter dealing with the physiology of the breathing; organs.

It will be there seen that the minute blood-vessels of the lungs are spread over the walls of the air-vessels which constitute the parenchyma or lung substance in the form of a net-work, where it is brought into contact with the air inspired, or with gases accidentally or intentionally drawn into the lungs in the act of inspiration.

The diameter of the minute air-tubes is capable of being increased or diminished by their involuntary muscular fibres, while the larger tubes derive greater firmness from the presence of rings of cartilage which prevent them from being altogether closed under any circumstances.

The lining membrane of the air-passages contains mucous glands which secrete a bland fluid for lubricating and moistening the surface, and is further clothed with fine hair-like processes (cilia), which, waving gently, like a field of corn in the wind, carry any superfluous secretion to be expectorated along the bronchial tubes towards the larynx, whence it passes out by the nose.

The structure of the lungs facilitates the exchange of gases in the process of respiration, and this is largely regulated by the movements of the heart; upon its force and frequency the amount of work thrown upon the lungs will depend. A proper understanding of the subject is essential to the treatment of pulmonary diseases upon any rational basis. The confusion which exists in the minds of horse-owners and others as to congestion and inflammation of these organs arises from want of knowledge of the distribution of blood from the different sides of the heart, and its destination. Circulation of blood in the lungs is quickened by any agent which stimulates the heart. Remedies having this effect are referred to under the head of stimulants, as alcohol, ammonia, strophanthus, digitalis, essential oils, etc.

A stimulating effect upon the air-passages and the membranes which line them is produced by warm food and copious draughts of fluid. Some alkaline preparations, as acetate of ammonia, and other drugs, derived both from the vegetable and mineral kingdoms, have the effect of increasing the amount of secretion poured out from the respiratory surfaces when congestion has induced undue dryness.

Among the remedies that thus increase the amount of material in the air - tubes may be mentioned iodide of potassium, ipecacuanha, squills, camphor, turpentine, benzoin, balsams of Tolu and Peru, stramonium, etc.

Notwithstanding the disadvantage our equine patients possess of being unable to expectorate in the ordinary sense of the term, they yet derive great benefit from the class of remedies known in human practice as expectorants. The dryness of the membranes in cases of bronchitis may by their judicious administration be relieved, and the superfluous mucus got rid off by way of the nostrils.

By reducing the force and frequency of the heart's action, the circulation of blood in the lungs may be reduced in speed as well as volume, and for this purpose aconite is often prescribed. Counter-irritants, as mustard plasters, turpentine, and ammonia liniments, by withdrawing blood from the pulmonary vessels to contiguous structures, relieve the overloaded lungs, and aid in restoring the circulation to its normal condition.

In some chronic forms of disease, as in the bronchial catarrh of old horses, it may be desirable to reduce the activity of the glands and diminish the output of mucus. Warm applications to the skin, and such drugs as opium, ether, chloroform, belladonna, and hyoscyamus, have this effect.

While remedies employed in the treatment of diseases of the air-passages and lungs are broadly divided into stimulants and sedatives or depressants, there are some which act in both ways.