Vomiting is a frequent symptom of cerebral disturbance. The fluctuations of pressure which the brain experiences as it rocks about on its “water-bed” within the skull is the cause of sea-sickness. Yet the motion of a ship may produce violent headache without nausea, the brain only, not the stomach, appearing to be troubled by the motion. Not that headache is a pain “inside the head.” Nor is it properly described as a pain in the scalp, although the messages which are felt in consciousness as headache originate in the endings of the nerves of the skin which covers the skull. The excessive sensitiveness of these nerves is due to vaso-motor conditions, usually the dilation, occasionally the constriction, of the bloodvessels of the scalp. But the vaso-motor condition is sympathetic with the disturbance of the brain; and the special urgency or efficiency of the messages from the skin results from their being delivered into excited brain-tissue. Nausea and headache are equally symptoms of the irritability of the brain caused by the motion of the ship. In one case messages from the stomach, in the other case messages from the scalp, acquire undue importance, owing to the agitated condition of the brain-tissue through which they pass. Not uncommonly the voyager, who wakes in the morning reconciled to the changes of pressure which he has experienced while recumbent, finds, when he stands upright, that the base of his brain is as sensitive as ever. Visual sensations also contribute to the brain-disturbance. So, too, do the movements of endolymph in the semicircular canals ([cf. p. 335]). It is, indeed, possible that this last factor is more important than the variations in pressure on the surface of the brain. Probably it accounts for the after-image of rolling which almost everyone experiences for at least a day after leaving the ship. Its cause being cerebral, the tendency to sea-sickness can be controlled by drugs which, like the bromides, chloral, alcohol, etc., deaden the brain.
Salivary Glands.—The secretion which accumulates in the mouth is the combined product of the sublingual, submaxillary, and parotid glands. It is a very thin, watery solution containing not more than 0·5 per cent. of solid substance. If red litmus-paper is moistened with saliva, it becomes blue, showing that the secretion is alkaline. It contains a ferment, ptyalin, which digests starch. The action of this ferment can be demonstrated by holding in the mouth for half a minute some warm starch mucilage—boiled arrowroot, for example. It quickly loses its viscidity owing to the conversion of starch into sugar. Chemically this change may be demonstrated by adding iodine-water to a specimen of the starch before and after action. Before the starch is taken into the mouth the iodine turns it blue (a characteristic reaction for starch). After it has been exposed to the digestive action of the saliva, iodine fails to colour the mixture, which now contains no starch. All the starch has been converted into dextrin and sugar. If unboiled arrowroot is placed in the mouth, some sugar is produced, but the process of conversion is very slow. It is almost impossible to digest raw starch in the mouth sufficiently to render it insusceptible to the colouring action of iodine. The sugar produced by the action of ptyalin is of the same nature as that which appears during the malting of barley. It is therefore termed “maltose.” It closely resembles grape-sugar, but is not identical with it.
The Secretion of Saliva.—The accessibility of the salivary glands, and especially of the submaxillary, has led to their being used for a very large number of experiments. They have been studied with the aim of coming to an understanding of the mechanism of secretion in general. The glands consist of tubes of gland-cells, each tube suspended in a basket of connective tissue, in a bath of lymph ([cf. Fig. 3]). Innumerable capillary bloodvessels traverse the lymph-bath. The arteries which carry blood to the gland are supplied with nerves, which regulate their calibre, and therefore determine the amount of blood which passes through the capillaries into which they break up. The glands also are supplied with nerves which influence their functional activity. Nutrient substances and oxygen pass out of the blood into the lymph. Carbonic acid passes into the blood from the lymph. Waste products are either carried away in the lymph-stream, or make their way through the walls of the capillaries into the blood. Many problems present themselves for solution. How does the amount of work done by the gland affect its supply of blood? Does the quantity of saliva secreted vary directly with the pressure of lymph in the spaces by which the gland is surrounded? Is this pressure wholly dependent upon the pressure of the blood? Are the substances secreted by the gland supplied as such by the blood, or does the gland make the ptyalin and mucus which it secretes? If it makes its secernable products, what materials does it abstract from the blood for the purpose of their manufacture? Does it use the whole of these materials, whatever they may be, or does it use part only and return the residue to the lymph? Does it make its products only when it is actively secreting, or is it always making them, and storing them in its cells in order that it may have a supply to discharge when called upon by the stimulation which results from the presence of food in the mouth? Is their discharge merely a washing out due to the rush of fluid which occurs when the bloodvessels are dilated, or can the gland-cells expel their products in response to nervous action? In what way do the nerves of the gland influence secretion? Do they call for increased production, or increased output, or both? These are some of the problems which the exposed situation of the submaxillary gland allows physiologists to tackle.
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.
How does a gland-cell make the substance which it secretes? There is no reason for supposing that the ptyalin or the mucus which the salivary glands secrete is present in the blood, either ready formed, or, as it were, half formed, in combinations which can be easily broken up. All the evidence obtainable points to the conclusion that the gland-cells take out of the lymph proteid materials from which they manufacture the peculiar substances which they secrete. During rest, granules accumulate in the cells. During activity they disappear. It has been shown in the case of the gastric glands that these granules consist of the special ferment which the gland secretes, in an inactive form. It may be that it is combined with a substance which prevents it from exerting its digestive action on the cells within which it is made; damped, as gunpowder is damped during transit. Or it may be that it is not a finished ferment; it may need a further addition to its molecule. During activity, while the granules disappear, proteins accumulate at the bases of the cells, giving to a tube of gland-cells the appearance of a peripheral non-granular zone. This proteid substance must have come from the lymph, and the inference seems inevitable that the cells have taken into their protoplasm a supply of material which will serve for the manufacture of additional granules. Each gland-cell is therefore an independent unit. By its own activity it takes up materials from the lymph, out of which it manufactures its own special products. It stores its products until they are wanted. Then by its own activity it extrudes them into the lumen of the gland-tube. It has, indeed, been shown that, when the nerve going to a salivary gland is stimulated, the gland shrinks, notwithstanding the great dilation of its bloodvessels. Under the influence of the stimulation the granules in the gland-cells imbibe water, swell up, and escape from the cells. The cells discharge their accumulated stores, in the first instance, more rapidly than they take up materials (even fluid) from the blood. For its knowledge (if the term may pass) of what is wanted the gland-cell is dependent upon messages which reach it through the nervous system. These messages take origin in the endings of the sensory nerves of the mouth, pass up to the brain, and are reflected down the nerves to the gland. So accurate is the information conveyed to the glands, that when a horse transfers the work of mastication from one side of its mouth to the other, as it is in the habit of doing about every quarter of an hour, the flow of saliva from the parotid gland on the masticating side is increased; on the other side it is diminished. Two or three times as much saliva is poured out on the one side as on the other.
Not only is the amount of saliva poured out in response to stimulation proportional to the needs of mastication, but the kind of saliva is adapted to the nature of the food. In a dog—and this is an observation which can be made only on an animal which lives on a mixed diet—it is possible to determine the amount of the two kinds of saliva secreted and the relation of flow to food. When meat is given to the animal, the submaxillary gland yields its secretion; when it is fed on biscuit, abundance of the watery parotid saliva is poured forth. A mouthful of sand also causes the parotid saliva to flow, in order that the sand may be washed out of the mouth.
More remarkable than the response to direct stimulation is the effect produced by the sight and smell of food. When meat is shown to a dog, submaxillary saliva begins to flow; when it is offered bread, parotid saliva is secreted. And the activity of the glands is not merely a nervous reflex independent of the animal’s mind. The moment the dog realizes that it is being played with—that there is no intention of giving it the coveted food—the flow of saliva ceases. An emotion may check secretion when every physiological condition is demanding it. This is the explanation of the Rice Ordeal. Dry rice provokes a flow of saliva in the mouth of all save the guilty man. Response to mental impressions is a matter of the greatest consequence in the physiology of digestion. It holds good in the case of the secretion of gastric juice equally with that of saliva. The sight and smell of food sets the juice flowing into the stomach, and the more desirable the food, the more attractive its appearance, the more stimulating its smell, the more rapidly does the secretion flow. Here we touch upon a theme which hardly needs exhaustive treatment. It is not the stoutest people who eat the most, although an impartial survey of one’s well nourished friends will show them to be persons who “take kindly to their victuals.” A small quantity of food perfectly digested is more nourishing than much food which the digestive organs do not efficiently prepare for assimilation. Good digestion waits on appetite; and appetite, in civilized man, is something more than a mere physical need of food. The hunger which leads to the bolting of food without pleasurable anticipation, without mastication, without any consideration of the quality of the viands, is a harmful craving which ends in imperfect assimilation. It is more profitable to toy with a hors d’œuvre than to engulf, unthinking, a plateful of beef. But we have said enough to suggest reflections to those who take no thought as to what they shall eat or what they shall drink; and few who take thought need to be convinced.
The Stomach.—The sight and smell of food, its presence in the mouth, and the performance of mastication, which induces a secretion of saliva, gives rise at the same time to a flow of gastric juice. It is psychic stimulation and the act of eating which cause gastric juice to ooze from the gland-tubes of the stomach at the commencement of digestion, not the stimulation of nerve-endings by food which has passed down the œsophagus. As a consequence of gunshot wounds, or as the result of operations performed for the purpose of relieving patients whose œsophagus has become blocked, numerous cases have been recorded in which a fistulous opening into the stomach has made it possible to study the interior of this organ. Such cases present an opportunity of watching the digestion of various foods introduced through the opening, and of collecting gastric juice for purposes of analysis. A similar condition has been established in animals by operative means. The œsophagus having been cut, and the cut end sutured to the margins of an aperture in the skin, food taken by the mouth escaped by this opening instead of passing into the stomach. A similar opening was made into the stomach for the insertion of food, and for the purpose of studying the effects of reflex stimulation of the gastric glands. As soon as food was introduced into the mouth, gastric juice began to flow. The advantage of this experimental method lies in the fact that the juice secreted was a pure juice—not mixed with food, as in all the earlier experiments in which, the stomach being opened without diversion of the œsophagus, the presence of food within it was the stimulus which led to secretion. No juice flowed in the absence of stimulation; nor was the secretion normal in composition when provoked by a mechanical stimulus, such as the tickling of the gastric mucous membrane by a feather.