The mixture under this name is probably variable in its physical and chemical properties, according to circumstances which have not hitherto been examined. When first discharged from the mouth, it always holds suspended a mucus, which is not dissolved, but imparts to it a frothy quality, by enabling it to retain the air which it absorbs from the atmosphere. It is readily separated by merely diluting the saliva as it flows from the mouth with distilled water, when it will gradually subside, and may be collected on a filter. It is a curious circumstance that, although no traces of phosphate of lime can be detected in this mucus by the application of re-agents, yet, after incineration, a considerable proportion appears in the ash 1. It has been doubted whether this mucus be secreted by the salivary glands, or is the common mucus of the mouth; the latter appears more probable. The saliva, deprived of this mucus, consists, according to the analysis of Berzelius, of:

1 It is this mucus that produces the tartar of the teeth, which, at first, is only mucus precipitated on the surface of the enamel; but it soon begins to decompose; its colour changes, by the influence of the air, from -white to yellow; the warmth and moisture of the mouth contribute to complete the decomposition, and the same earthy phosphates, which are produced by oxidation and combustion in open fire, are here formed, and deposited on the surface of the tooth by a slower but a similar process. The tartar is, therefore, as it were, the ash of mucus crystallized on the tooth.

Water...........992.9

A peculiar animal matter...... 2.9

Alkaline muriates ....... 1. 7

Lactate of soda and animal matter . . 0.9 Pure soda.......... 9

The peculiar matter of the saliva is soluble in water, but not in alcohol; and the solution is not precipitated either by alkalies or acids, or sub-acetate of lead, or muriate of mercury, or tannin; neither is it rendered turbid by boiling. The saliva derives its name from the saline qualities which it possesses; and although, under ordinary circumstances, we are not conscious of them, yet, when the stomach has been long empty, and the nervous system acquires increased sensibility, the saline taste is frequently perceptible. The same effect is produced by disease; and the disagreeable taste of which invalids complain, often depends upon chemical changes having been produced in this secreted fluid. The common furred tongue would appear to arise from an increased quantity of vitiated mucus. The peculiar milky tongue1, which appears in certain states of the system, occasionally derives its appearance from an innumerable number of microscopic bubbles of air, as I have ascertained by observation: in such cases, the quantity, rather than the quality of the mucus, appears to be affected.

The black tongue of typhus, on the other hand, is indebted for its character to a decomposed state of the mucus; while a deficiency in the true salivary secretion will explain its dryness.

1 I am induced to consider the milky tongue as rather indicating a sympathetic, than primary derangement of the stomach. It is thus constantly produced by mental anxiety.

29. When we consider the size of the salivary glands, which altogether cannot weigh more than four ounces, the quantity of fluid which they continually secrete is truly astonishing. It would be a difficult matter to ascertain, with accuracy, the proportion of saliva which these glands can separate in a given time; but we know that a person unaccustomed to smoking, will, by the influence of such a stimulus, discharge half a pint in a very short period. In the case of a felon, who cut his throat in prison, and so completely divided both the larynx, a little above the cricoid cartilage, and the oesophagus at the same point, that whatever was introduced into the mouth escaped by the external wound; it was found that, during each meal, there was a discharge of saliva from the mouth, amounting to from five or six, to eight ounces, or even more. This is sufficient to disprove the opinion of Dr. Fordyce, who says, "As far as I can judge, the secretion during a meal, can hardly exceed an ounce or two; and I should think that it serves only to lubricate the passages through which the food is to pass." With respect to this latter assumption, I shall have to offer some observations in a future part of the work.

The influence of the passions over this secretion is extremely curious; the sight of a delicious repast to a hungry man is not more effectual in exciting it, than is the operation of fear and anxiety in repressing and suspending it. If the reader be a medical practitioner, we refer him for an illustration to the feelings he experienced during his examination before the medical colleges; if a barrister, he may remember with what a parched lip he gave utterance to his first address to a jury. There is a trial of ordeal in the Institutes of the Hindus, in which we fancy that we can discern through the dim mist of credulity, a ray of policy that may have been derived from the dawning of a, rude philosophy. In the ordeal alluded to, persons suspected of theft were each made to chew a quantity of dried rice, and to throw it upon some leaves or the bark of a tree; they, from whose mouth it comes dry, or stained with blood, are deemed guilty, while those who are capable of returning it in a pulpy form, are at once pronounced innocent.