Respiration can be modified by such very various influences that it is difficult to decide upon the exact mode of action of any drug which affects it. For example, alterations in the blood and in the air will modify it; the respiratory centre itself may be influenced, either directly or reflexly, from almost any organ in the body; or, again, the movements of the respiratory muscles themselves may be interfered with; and, lastly, respiration is much under the influence of the circulatory apparatus. Furthermore, the chief object in medicine is to remove the cause of the respiratory difficulty rather than to act upon respiration itself.

We have already spoken of those drugs which produce changes in the blood and the circulation, and the consideration of those modifications of the temperature, moisture, and pressure of the air which are of value in medicine, belongs to a book on general therapeutics. We will, therefore, now consider the respiratory drugs under the following heads:

A. Drugs altering the Composition of the Air inhaled. - This division is devoted to drugs which, inhaled, have some direct effect on the respiratory mucous membrane, on bronchial and pulmonary contents, and have certain remote effects. It is found convenient to administer some drugs, although they are not given for their influence on respiration, by making the patient inhale them; such are anaesthetics and amyl nitrite.

Some drugs when inhaled are particularly irritating to the bronchial mucous membrane, causing dilatation of the vessels, increased secretion, and reflexly, cough from the stimulation of the sensory nerves of the bronchial mucous membrane.

Such are cold dry air, iodine, bromine, chlorine, senega, ipecacuanha, sulphurous anhydride, nitric acid fumes, ammonia, and tobacco. These are rarely used therapeutically as inhalations, and their inhalation is to be particularly avoided in irritable conditions of the bronchi.

The drugs which, when inhaled, are soothing to the bronchial mucous membrane, but are rarely employed, are -

Hydrocyanic acid.

Conium.

Inhalations which are used to stimulate the bronchi, that is to say, to increase their vascularity, secretion, and muscular power, are -

(1) Carbolic acid (2) Oil of cajuput

Division VIII Drugs acting on Respiration 156

xx.; 1.20 c.c.

(3) Oleum pini sylvestris,

Division VIII Drugs acting on Respiration 157

xxx.;

2.00 c.c.

(4) Tinctura ben-zoini composita,

(5) Creosote,

(6) Oil of cubeb.

Division VIII Drugs acting on Respiration 158

ss.; 15 c.c.

The amounts given after each are the quantities that should be added to a pint 500 c.c. of water at 1400 F.; 6o° C.

Inhalations which are used to disinfect foul secretions from the bronchial mucous membrane are those of -

(1) Creosote.

(2) Iodoform.

(3) Mild solutions of benzoin.

(4) Carbolic acid.

(5) Sulphurous anhydride.

(6) Oil of juniper.

(7) Oil of cubeb.

(8) Oil of eucalyptus.

Inhalations for relieving spasm of the bronchial tubes are those of -

(1) Conium.

(2) Stramonium.

(3) Chloroform.

(4) Ether.

(5) Amyl nitrite.

B. Drugs acting on the Respiratory Centre. - If the drug, when injected into the carotid artery, very quickly produces its effect on respiration, it is concluded that it acts on the respiratory centre. Another experiment often used to determine whether the drug acts on the centre, or on the vagal terminations in the lung, is to cut the vagi and to observe whether it acts similarly before and after the section.

Drugs which directly stimulate the respiratory centre are -

(1) Strychnine.

(2) Ammonia (very powerful).

(3) Apomorphine.

(4) Belladonna.

(5) Stramonium.

(6) Hyoscyamus.

Drugs which depress the respiratory centre are -

(1) Physostigmine (very powerful).

(2) Chloral hydrate.

(3) Chloroform.

(4) Ether.

(5) Alcohol.

(6) Opium.

(7) Hydrocyanic acid.

(8) Codeine.

(9) Aconite.

(10) Veratrine.

(11) Conium.

(12) Caffeine.

(13) Quinine.

(14) Ipecacuanha.

(15) Antimony salts (very weak).

Alcohol, ether, chloroform, caffeine, and quinine slightly excite, before they depress the respiratory centre.

Therapeutics

The drugs which excite the respiratory centre may be used when there is any difficulty in respiration, especially with the view of increasing the force of the respiratory act whilst other means are employed to get rid of the cause of the difficulty. They are, of course, most frequently required in diseases of the lungs, especially bronchitis. Ammonia and apomorphine are often employed, as they are also powerful expectorants; and belladonna is useful when there is too much secretion from the bronchial tubes.

Substances which depress the respiratory centre are very little needed for this action; but the centre for the reflex act of coughing is in the close neighborhood of the respiratory centre, and opium, hydrocyanic acid, codeine, conium, and ipecacuanha are often very valuable in allaying the continual hacking cough which so frequently accompanies disease of the heart and lungs.

The drugs which relieve cough are very numerous, for it may be reflexly set up by irritation of so many peripheral parts, viz., nose, throat, pharynx, ear, teeth, larynx, trachea, bronchi, lungs, pleura, stomach, and liver; and consequently its removal may depend upon the removal of peripheral irritation in any of those organs.

C. Drugs affecting the Bronchial Secretion.

(a) Those increasing it:

(1) Apomorphine.

(2) All alkalies, especially ammonium carbonate and other salts.

(3) Cocillana.

(4) Ipecacuanha.

(5) Senega