Under ordinary circumstances very little saliva is secreted, only sufficient being poured into the mouth to keep the surface moist. When, however, food is introduced into the mouth, and the process of mastication commences, the secretion goes on more or less rapidly, according to the stimulating or non-stimulating character of the food.
The activity of a salivary gland is at once brought about by means of special nervous agencies when a stimulus is applied to the mouth. We know that the nervous mechanism which regulates this secretion is called a reflex act. The stimulus traveling from the surface of the mouth to the nerve centres is reflected thence to the glands. We speak, then, of afferent nerves, which carry the impulses to the nerve centre, and effere?it nerves, which carry them from the centre.
If we review the ordinary circumstances giving rise to a flow of saliva, there will be no difficulty in determining the nerves which act as the afferent channels in the simple reflex act.
Stimulation of the mucous membrane of the tongue and mouth, whether chemically, as with irritating condiments, or mechanically, as by the motions of mastication, is generally transmitted to the centre by the sensory branches of the fifth cranial nerve, which supply the mouth, and by the branches of the glosso-pharyngeal.
The stimulus of the sense of taste is sent by the nerves of that sense, mainly the glosso-pharyngeal, to the taste centre in the cortex cerebri, and from thence to the secreting centre by means of inter-central fibres.
Fig. 64. Diagram of Nerves of the Submaxillary Gland. The dark lines show the course of the nerves going to the gland.
The stimulating of the olfactory region with certain odors induces salivation through a channel of a similar kind passing along the olfactory nerve to the brain, and thence to the special salivary centre. Even in the absence of taste or smell, mental emotion may be excited by seeing or thinking of food, and may cause activity of the salivary glands; here the inter-central channel is the only one occupied in bearing the impulse to the special secreting centre. 12
Irritation of the gastric mucous membrane stimulates the salivary glands, as may be seen with a gastric fistula, or by the sudden flow of saliva which commonly precedes vomiting. In this case the impulses are carried by the gastric branches of the vagus. The stimulation of the central end of the cut sciatic is said to cause an increase in the flow of saliva, so that it would appear that even an ordinary sensory nerve can excite the centre to action. Lastly, many drugs, when introduced into the blood, cause a flow of saliva; among these are pilocarpin, physostigmin and digitalin, while atropin and daturin, on the other hand, check the action of the glands.
From this we learn that the nerve centre controlling the activity of the salivary glands receives impulses from many distant and diverse sources, or may be influenced directly by the quality of the blood flowing through the nerve centre itself.
The channels traversed by the efferent impulses going to the salivary glands have been demonstrated by experiment.- In the case of the submaxillary, the route is especially distinct and instructive, so that from this gland we obtain most of our knowledge concerning the direct influence of nerve impulses on the gland cells. This question, therefore, will be treated somewhat in detail.
Fig. 65. Diagram of Nerves supplying the Parotid Gland. The dark lines indicate the course of the nerves of the gland.
(v) Inferior division of the fifth nerve and its (a t) auriculo-temporal branch. (vii) Portio dura, (S. C. G.) Superior cervical ganglion sending a branch to the carotid plexus around the artery.
There are two sets of nerves going to the salivary glands, one belonging to the sympathetic and the other to the cerebrospinal systems, both of which have been proved to exert a certain amount of influence on the action of the glands, the share taken by each apparently differing in different animals.
The sympathetic branches for the submaxillary and sublingual gland come from the plexus which embraces the facial artery, those for the parotid come from the plexus surrounding the internal maxillary as that artery traverses the gland. Both of these nervous plexuses are derived from the superior cervical part of the sympathetic nerve.
The cerebro-spinal fibres for the submaxillary and sublingual glands lie in the complex nerve known as the chorda tympani, which comes from the portio dura of the seventh, and joins the lingual branch of the fifth. They pass thence through the submaxillary ganglion to the glands.
The cerebro-spinal parotid branches pass through the lesser superficial petrosal nerve from the tympanic plexus to the otic ganglion, and thence to the auriculo-temporal nerve which sends twigs to the gland. (Fig. 65).
I. The effects of experimental stimulation of the cerebrospinal glandular branches are, so far as we know, alike for all the glands. But owing to the greater facility with which the submaxillary gland can be reached, and its nerve isolated, research has been chiefly devoted to it, by operating on the chorda tympani and the other nerves supplying the gland.
It has been found that section of this nerve, or of the portio dura near its origin, removes the possibility of exciting the glands to action by stimulating the mouth, so that the cerebrospinal and not the sympathetic are the channels traversed by the reflected impulse on its way to the gland from its centre.