By means of a very simple operation, the ducts of one or both parotid or submaxillary glands can be brought to the skin, and made to pour their secretions on to the surface instead of into the mouth. The flow under various circumstances can be watched. The saliva can be collected and measured.
The nerves of the submaxillary gland are easily isolated. A nerve leaves the seventh (or facial), crosses the drum of the ear, comes out through a minute crevice in the skull, and runs for some little distance as a separate nerve before it applies itself to the lingual branch of the fifth, which runs along the side of the tongue. Owing to its passage across the tympanic cavity (drum of the ear), it is termed “chorda tympani.” As its fibres are very small, they can be recognized wherever they form a part of the lingual nerve. They leave the lingual to go to a ganglion, the submaxillary ganglion, from which the gland is supplied. The gland also receives branches from the sympathetic nerve which ascends the neck. The last-named branches accompany the facial artery. Stimulation of either of these nerves causes the gland to secrete. The flow of saliva which follows stimulation of the chorda tympani is much more copious than that which follows stimulation of the sympathetic, and as a rule it contains far less organic matter, although about the same amount of mineral salts. Under normal conditions the activity of the chorda tympani is brought into play in a reflex manner by impulses which travel up the nerves of taste (the lingual and glosso-pharyngeal) to the cerebro-spinal axis; but almost any other nerve will serve as an afferent path. The gland may also, as we shall presently explain, be called into activity by the cortex of the brain.
It is certain that in the case of the submaxillary gland secretion is not the direct result of increased blood-pressure. It is not a case of filtration from the blood through certain membranes and cells into the salivary duct. Atropin (belladonna) dilates the bloodvessels, increasing blood-pressure, but it stops secretion. After belladonna-poisoning, the mouth, like the skin, is hot and dry. Other drugs there are which provoke a certain amount of secretion, even after the bloodvessels going to the gland have been tied. It is possible, by stimulating the chorda tympani, to obtain a pressure in the fluid in the duct very much greater than that in the bloodvessels which supply the gland. Here we have clear proof that secretion is not filtration. Filtration is the passage of fluid through a filter-bed from a higher to a lower pressure. In filtration, moreover, soluble diffusible salts accompany the water. The saliva contains only half as much of these diffusible salts as the blood. Therefore the gland tissue stops half the salts. Secretion is an active process carried out by the gland-cells, under the influence of nerves, in opposition to the laws of filtration. The gland-cells determine how much water shall pass through them and what percentage of salts shall accompany the water.