These are remedies which increase the secretion of saliva.

Anything which is chewed, or even turned about in the mouth, such as a pebble, will increase the secretion of saliva; but the chief sialagogues have a stimulating action of their own.

Action. - In the secretion of saliva there are two factors - first, the activity of the secreting cells; secondly, the supply of new material to them, from which they may manufacture the secretion. This depends on the circulation.

Secreting cells do not derive the new material from which they form the secretion directly from the blood. They obtain it from the lymph which fills the adjacent lymph-spaces. Hence they may continue to secrete for a short while after the circulation has ceased, as in the sweat-glands of an amputated limb, or in the salivary glands after the head of the animal has been separated from the body. But the supply of lymph soon becomes exhausted unless a supply of fresh lymph in the spaces is kept up by exudation from the blood-vessels. We therefore find that abundant secretion is usually, though not invariably, associated with an abundant blood-supply. If the flow of blood is not rapid the secretion must soon diminish or come to a stop, for, although it may occur rapidly at first, the lymph which has accumulated in the lymph-spaces supplying the cells soon becomes exhausted.

Fig. 124.   Diagram representing the general relation of nerves to the secreting cells and vessels of a gland. For the sake of simplicity only one afferent nerve and one nerve centre and one set of secreting and vascular nerves are here represented.

Fig. 124. - Diagram representing the general relation of nerves to the secreting cells and vessels of a gland. For the sake of simplicity only one afferent nerve and one nerve-centre and one set of secreting and vascular nerves are here represented.

In the salivary gland, when the secretion is going on, the arteries usually dilate, and the blood flows rapidly through them. The submaxillary gland, in which secretion has been best studied, appears to receive four kinds of nerves - two sets being contained in the chorda tympani and two in the sympathetic.

The chorda contains some fibres which act on the bloodvessels, causing them to dilate and allow the blood to flow freely through the gland, and others which stimulate the cells of the gland to secrete a thin, watery saliva. These two kinds are spoken of as vaso-dilating and secreting, or secretory, fibres (Fig. 124).

At present the usually accepted theory is that the secretory nerves have a direct influence upon the tissue-change in the cells of the gland. During secretion the granules in the cell decrease in number and generally in size, the hyaline substance increases, and the network within the cell grows.1 It is not at all improbable, however, that in addition to their action upon secreting nerves some drugs influence the amount of fluid poured out from the vessels. For if we inject a solution of quinine into the duct of the gland and thus destroy its secreting power, and afterwards irritate the chorda tympani, the lymph poured out from the blood-vessels will accumulate in the gland and render it cedema1 Langley, Proc. Camb. Phil. Soc, Nov. 12, 1883.

tous; but if an animal be poisoned with atropine the gland does not become cedematous when the chorda tympani is stimulated - although the blood-vessels going to it are dilated and its power of secretion is completely destroyed. We might suppose that the gland did not become cedematous because the lymph, although not used up by the gland, had been carried away by the cervical lymphatics. But this is not the case, for Heidenhain has found that the flow of cervical lymph is not increased under these circumstances.

It appears to me that the circumstance can hardly be explained otherwise than by supposing that atropine not only paralyses the secreting fibres of the chorda, but acts upon the vessels in such a manner as to greatly diminish or prevent the exudation which would usually take place from them into the lymph-spaces on irritation of the chorda.

The sympathetic contains some fibres which cause the vessels of the gland to contract and the blood to flow slowly through it, and others which stimulate the cells to secrete a thick and viscid saliva.

Besides the ordinary secretion of saliva regulated by the action of the nerves, there is a secretion which is usually termed paralytic, because it occurs, not after irritation, but after paralysis of the nerves going to the salivary gland. It occurs in the subexcited.

Fig. 125.   Diagram to show the afferent nerves by which the secretion of saliva may be reflexly maxillary gland, when its nerves have either been paralysed by the injection of small doses of curare into the artery going to the gland, or by a section of the combined lingual nerve and chorda tympani, or extirpation of the submaxillary ganglion.

Fig. 125. - Diagram to show the afferent nerves by which the secretion of saliva may be reflexly maxillary gland, when its nerves have either been paralysed by the injection of small doses of curare into the artery going to the gland, or by a section of the combined lingual nerve and chorda tympani, or extirpation of the submaxillary ganglion. It is not improbable that morphine also, like curare, produces it, because in moderate doses it causes dryness of the mouth, but in enormous doses causes excessive salivation.

The secretion of saliva may be stimulated by the direct action of drugs upon secreting nerves in the gland itself, or reflexly through the sensory nerves of the mouth, stomach, eye, or nose (Fig. 125). The mere smell, or sight, of appetising food, causes secretion of saliva, which is probably due to the nerves of smell and taste acting through the brain upon the medulla. The brain, when excited by mere recollection, may also stimulate the secretion of saliva.

Increased salivation is a common accompaniment of sickness or nausea. The afferent nerve here appears to be the gastric branches of the vagus.

The nerve-centre which regulates the secretion of the thin chorda-saliva is probably the nucleus of the seventh nerve situated in the medulla oblongata.

Efferent fibres pass out along the chorda tympani and reach the gland, some directly, and some after passing through the submaxillary ganglion.

The afferent fibres, which convey stimuli from the mouth to the medulla are contained in the lingual branch of the fifth, and the glossopharyngeal nerves. Those which convey stimuli from the stomach, and excite the salivation which accompanies nausea, are contained in the vagus. The salivary centre may also be stimulated by impulses sent down from the brain, and the nerves of sight and smell may act as afferent nerves to the salivary centre indirectly through the brain (Fig. 125).1

Besides the nerve-centre in the medulla oblongata there are subsidiary nerve-centres. These are the submaxillary ganglion and small ganglionic masses in the submaxillary gland itself.

Sialagogues have been divided into two classes : 1st, topical, or direct; and 2nd, specific, remote, or indirect. The names direct and indirect are complete misnomers, and ought not to be used; inasmuch as the so-called direct sialagogues are those which act directly on the mouth, but do not act directly on the substance of the gland, or on the nervous structures contained within it or immediately connected with it.

Sialagogues are better divided according to their mode of action into reflex sialagogues, specific sialagogues, and those which act both reflexly and specifically, and may be called mixed sialagogues.

1 The nasal branches of the fifth nerve probably also act as afferent nerves for the salivary secretion, for I have noticed that on dipping the tip of the nose into hot water containing a little compound tincture of benzoin, salivation occurred, ceased when the nose was withdrawn, and again occurred regularly whenever the nose was again introduced into the mixture. The mere inhalation of the vapour had no effect.