Expectorants are remedies which facilitate the removal of secretions from the air-passages. The secretion may be rendered more easy of removal, either by an alteration in its character rendering it less adhesive and more easily detached from the air-passages, or by increased activity of the expulsive mechanism.

Our knowledge of the use of expectorants is founded chiefly on empiricism. We are almost entirely indebted to the recent experiments of Rossbach for any precise information as to their mode of action.1

The secretion from the air-passages, like other secretions, depends partly upon the condition of the circulation, and partly on the secreting cells themselves.

In healthy conditions the increased secretion and increased circulation of blood in the mucous membrane go together, but just as in the case of the sweat-glands, these two factors may occur independently of each other, and secretion may take place rapidly when the circulation is diminished and the mucous membrane is anaemic, and, on the other hand, it may stop altogether when the vessels are dilated and the mucous membrane is congested. The latter happens both in cases of disease and in animals poisoned by atropine.

The secretion from the normal respiratory mucous membrane consists of a thin solution of mucin which dries very slowly, and is only secreted in sufficient quantity to keep the mucous membrane moist. It is slightly adhesive, and any particles of dust, etc., which may have found their way into the trachea, will stick to the walls of the air-passages, and will be gradually moved up towards the mouth by the cilia with which the cells of the mucous membrane are furnished. Any excess of mucus secreted in consequence of irritation will also be moved upwards by the cilia in a similar manner. In the ciliated cells of the mucous membrane we recognise a structure which is frequently met with in animals lower down the scale of existence, and the mucous membrane of the respiratory passages appears to resemble the parts of lower organisms, in being very slightly controlled by the central nervous system. When not irritated it secretes slowly and regularly; when irritated locally the secretion is increased, but irritation of the nerves passing to it, such as the vagus, the superior or inferior laryngeal, or the sympathetic, does not cause any increase as it does in the case of the submaxillary gland. These nerves, however, can influence it indirectly through the circulation, for when they are divided an increased dilatation of the vessels occurs in the mucous membrane of the trachea, a freer circulation of blood occurs, and increased secretion is thus indirectly produced. When they are irritated, however, and anaemia of the trachea produced, the secretion is not arrested, but continues.

The circulation in the mucous membrane is readily affected reflexly by irritation of other parts of the body. When, for example, a warm poultice is laid for five or ten minutes on the belly of an animal, and then afterwards replaced by ice, the mucous membrane of the trachea and larynx becomes in half a minute deadly pale from the contraction of its vessels. Though the ice is still allowed to remain on the belly, the tracheal mucous membrane quickly changes colour, and to the paleness succeeds first slight redness, then deep red congestion, and in five or ten minutes lividity. This lividity shows that the congestion is not arterial but venous, and that the circulation, instead of being quicker is really slower. Along with the increase of congestion in the mucous membrane, the amount of mucus secreted increases. When the ice is removed for half an hour, and again replaced by the warm poultice, the bluish-red colour of the mucous membrane almost immediately disappears and gives place to a rosy colour which is, however, redder than normal. Ice again applied will cause a second contraction of the vessels and paleness, though much less than before. These experiments show how sensitive is the mucous membrane of the trachea to reflex stimulation of other parts of the body by heat or cold, and enable us to understand more readily how a draught of cold air on some part of the body should cause inflammation of the respiratory organs.

1 Festschrift der Julius-Maximilian- Universitat zu Wurzburg, Leipzig.