calcium phosphate and carbonate, admixed with mucus and leptothrix.' The so-called 'tooth-stones' have the same composition.

Quantitative analysis. The quantity of saliva secreted daily by a man varies considerably; estimates varying between 13 oz. and 3 lb. have been given; 500 to 800 grammes is another estimate-oxen and horses may secrete 40,000 to 60,000 grammes daily. Its alkalinity averages in man 08 per cent. expressed as sodium carbonate (Chittenden).

Its specific gravity is 1002 to 1006 in man; 1007 in dogs. It contains in man five parts of solid matter per 1000, of which two are inorganic.

Submaxillary saliva.—A cannula is inserted into Wharton's duct, and the saliva obtained by the stimulation either of the chorda tympani or sympathetic can be readily collected and examined.

The saliva thus obtained is colourless, clear, transparent, and sticky, especially if obtained by stimulation of the sympathetic. It is markedly alkaline, and soon becomes cloudy in the air from deposition of calcium carbonate.

Its composition is in the main the same as that of mixed saliva ; the mucin is more abundant; the proteid coagulable by heat is not always present. Ptyalin is present in human submaxillary saliva, except in infants under the age of two months (Zweifel). It is present in most animals, but not in dogs. Potassium sulphocyanide is present in human, but not in dogs' submaxillary saliva. The inorganic salts are calcium carbonate, calcium and magnesium phosphate, potassium and sodium chloride."

Quantitative analysis (in parts per 1000 dog's submaxillary saliva) :

1 Vergne, 'Du tartre dentaire et de ses concrétions,' Thèse, Paris, 1869.

2 Frerichs, Wagner's Handwörterbuch d. Physiol. iii. 758.

3 C. Schmidt and Jacubowitsch, Ann. Chem. Pharm. lxxix. 156.

4 Herter, Hoppe-Seyler's Physiol. Chem. p. 188.

5 Zweifel, Untersuchungen ü. d. Verdauungsapparat. d. Neugeb. Strasburg, 1874.

6 Longet, Compt. rend. xlii. 480; Oehl, La saliva umana, Pavia, 1864.

The main facts concerning the difference in the secretion produced by excitation of the two nerves of the gland have been already mentioned (p. 617). The following are actual analyses (given in percentages) of the saliva thus obtained :

I. By stimulation of the chorda tympani.

A strong stimulus thus produces an increase in the total solids, especially of the organic solids, and particularly of the mucin. That the percentage of salts in the saliva also increases with the rate of secretion was also noted by Werther, who obtained as high a percentage in some cases as 0.77. Langley and Fletcher have more recently obtained the same results, both by means of stimulating the chorda, or injecting small doses of pilocarpine, a drug which increases the rate of salivary secretion. Such a fact goes to prove that the secretion of even water and salts is an act of the secreting cell, and not simply due to increased transudation from the blood.

1 Bidder and Schmidt, Ann. Chem. Pharm. vol. lxxix. Hoppe-Seyler's Physiol. Chem. p. 191.

3 In rabbits' submaxillary saliva, mucin is absent.

4 Herter, Loc. cit.

5 Pflüger, in Heidenhain's Studien des Physiol. Inst. Breslau, Leipzig, Heft iv. p. 25. 6 Heidenhain, Ibid.

8 Phil. Trans. 1889, vol. clxxx. B, p. 109.

Pflüger's Archiv, xxxviii. 293.

9 A very complete account of the antagonistic action of atropine and pilocarpine on salivary secretion will be found in Journ. Physiol. i. 339 (Langley). Another important contribution on the influence of nicotine on salivary secretion, by the same author, will be found in Journ. Physiol. xi. 123.

II. By stimulation of the sympathetic (dog).

Here we get a small quantity of saliva, which is richer in solids than chorda saliva, especially in mucin and formed elements. Heidenhain, moreover, found that the percentage of solids falls after prolonged stimulation; thus:—

Sublingual saliva. The secretion of the sublingual gland does not materially differ from that of the submaxillary. It is, however, the richest of the salivas in solids (2.75 per cent. Heidenhain), formed elements, mucin, and inorganic salts; it is thus the most viscid and the most alkaline (Heidenhain, Werther, Langley).

In certain birds this gland is much enlarged, and secretes the viscid material out of which they build their nests (see Neossin, edible bird's-nest, p. 486).

Parotid saliva. The parotid gland yields a watery secretion, free from mucin, and rich in ptyalin, even in the new-born child. The gland is more highly developed in vegetable feeders than in carnivora. The saliva can be easily collected, especially in dogs, by means of a cannula in Stenson's duct. Its characters, with the exception of sliminess, and its constituents, with the exception of mucin, are the same as in submaxillary saliva. It always contains a small quantity of a globulin.

Quantitative analysis.—On the next page are some analyses in parts per 1000. The table is compiled by Hoppe-Seyler.1

The Secretion of the Mucous Membrane of the Mouth

When the ducts of all the salivary glands have been ligatured a small quantity of very viscid secretion is poured into the mouth by the mucous glands of its lining membrane. Jacubowitsch gives the following analysis of this secretion obtained from a dog :

It has no diastatic action. The mucus secreted by the tongue of the frog (an animal with no salivary glands) is, however, diastatic.

The poison-glands of snakes are modified salivary glands. The secretion is

1 Physiol. Chem. p. 199.

SS

rich in proteids, and the poison is a proteid one (see Proteids as Poisons, p. 137). The specific gravity of snake poison is over 1040. Its reaction is in some cases alkaline, in others weakly acid; it is usually described as yellowish and viscid.

4. THE ACTION OF SALIVA

The active principle of saliva is ptyalin. This belongs to the class of unorganised ferments, that are called either amylolytic (starchsplitting), or diastatic (resembling diastase, the similar ferment in germinating barley and other grains).

Ptyalin may be prepared from a watery infusion of a minced salivary gland or from the saliva itself. Dilute phosphoric acid is added, and this is neutralised with lime-water; the precipitate of calcium phosphate which is formed carries down the ptyalin with it; this is collected on a filter and water added; the water dissolves out the ptyalin, leaving the phosphate on the filter. The ptyalin is then precipitated from its aqueous solution by adding excess of alcohol. The precipitate may be collected, dried, and preserved for future use. It may be purified by re-dissolving in water, and again precipitating with alcohol. To obtain a glycerine extract, a minced salivary gland is covered with absolute alcohol for twenty-four hours; the gland substance freed from alcohol is dried, powdered, and allowed to macerate in strong glycerine for several days; the ptyalin may then be precipitated from the glycerine solution by alcohol as before.

1 R. Kulz, Zeit. Biol. xxiii. 321.

The only important chemical action of saliva is that due to the presence of ptyalin. It has various physical actions; it dissolves certain substances, enabling us to taste them; in virtue of its mucin, it lubricates the bolus before it is swallowed; in virtue of its viscidity and alkalinity, it has a feeble, emulsifying action on fats.

The diastatic activity of saliva may be readily demonstrated by the following simple experiment :—

A few cubic centimetres of starch solution are placed in a test-tube, and a few drops of saliva added; the tube is placed in a warm bath at 35° C. and by means of a glass rod a drop is removed every halfminute, and mixed with a drop of dilute solution of iodine on a testing slab. At first the drop strikes a deep blue from the presence of starch; after a few minutes, another drop gives a violet colour; this is because the starch is gradually disappearing, but some is still left, and the violet colour is produced by admixture of the blue tint, due to starch, and the reddish tint, due to dextrin into which the starch is being converted; in a few minutes more a fresh drop strikes a reddish brown with iodine, showing that all the starch has disappeared; and in a few minutes more, a fresh drop gives no colour at all with iodine, showing that the dextrin which gave the red colour has also gone. If at this stage a little of the fluid be withdrawn and alcohol added in excess, a white precipitate is produced; this cannot be starch, as all the starch has long ago disappeared; it cannot be sugar, as sugar gives no precipitate with alcohol; it cannot be the dextrin that gave the red colour with iodine, as there is no longer a red colour given with iodine; if analysed, however, it is found to have the same composition as dextrin, and thus it is called achroo-dextrin; while the dextrin which gave the red tint is called erythro-dextrin. If we test the liquid at the various stages by means of Trommer's test or Fehling's solution for sugar (p. 95), we shall find sugar present as soon as dextrin appears; it increases as the dextrin disappears. Achroo-dextrin is, however, only partially, and with great difficulty, converted into sugar. This simple experiment teaches us that starch is transformed into dextrin and sugar, and that ultimately the greater part of the dextrin is also changed into sugar.

2

Nasse was the first to show that this sugar is not dextrose, and called it ptyalose. v. Mering and Musculus conclusively proved that ptyalose and maltose (the sugar formed by diastase in malting) are identical.

We have already in our consideration of the carbohydrates seen

1 Pflüger's Archiv, xiv. 473.

2 See Seegen's paper, Pflüger's Archiv, xl. 38.