submaxillary of the rabbit and guinea-pig (fig. 175). In this kind the alveolar lumen is small, and the cells lining the tubule are short granular columnar cells, with nuclei presenting the intranuclear network. During rest. the cells become larger, highly granular, with obscured nuclei, and the lumen becomes smaller. During activity, and after stimulation of the sympathetic, the cells become smaller and their contents more opaque; the gran-ules first of all disappearing from the outer part of the cells, and then being found only at the extreme inner part and contiguous border of the cell. The nuclei reappear, as does also the lumen. (2) In the true mucus-secreting Fig. 175. From a section through a true salivary gland. a, the gland alveoli, lined with albuminous "salivary cells;" b, intralobular duct cut transversely. (Klein and Noble Smith.) glands, as the sublingual of man and other animals, and in the submaxillary of the dog, the tubes are larger, contain a larger lumen and also have larger cells lining them. The cells are of two kinds, (a) mucous or central cells, which are transparent columnar cells with nuclei near the basement membrane. The cell substance is made up of a fine network, which in the resting state contains a transparent substance called mucigen, during which the cell does not stain well with logwood (fig. 176). When the gland is secreting, mucigen is converted into mucin, and the cells swell up, appear more transparent, and stain deeply in logwood (fig. 177). Fig. 176. From a section through a mucous gland in a quiescent state. The alveoli are lined with transparent mucous cells, and outside these are the semilunes of Heidenhain. The cells should have been represented as more or less granular. (Heidenhain.) During rest, the cells become smaller and more granular from having discharged their contents. The nuclei appear more distinct. (b) Semilunes of Heidenhain (fig. 176), which are crescentic masses of granular parietal cells found here and there between the basement membrane and the central cells. The cells composing the mass are small, and have a very dense reticulum, the nuclei are spherical, and increase in size during secretion. In the mucous gland there are some large tubes, lined with large transparent central cells, and having besides a few granular parietal cells; other small tubes are lined with small granular parietal cells alone; and a third variety are lined equally with each kind of cell. Fig. 177.-A part of a section through a mucous gland after prolonged electrical stimulation. The alveoli are lined with small granular cells. (Lavdovski.) (3) In the muco-salivary or mixed glands, as the human submaxillary gland, part of the gland presents the structure of the mucous gland, whilst the remainder has that of the salivary glands proper. Nerves and blood-vessels. Nerves of large size are found in the salivary glands, they are principally contained in the connective tissue of the alveoli, and in certain glands, especially in the dog, are provided with ganglia. Some nerves have special endings in Pacinian corpuscles, some supply the blood-vessels, and others, according to Pflüger, penetrate the basement membrane of the alveoli and enter the salivary cells. The blood-vessels form a dense capillary network around the ducts of the alveoli, being carried in by the fibrous trabeculæ between the alveoli, in which also begin the lymphatics by lacunar spaces. Saliva. Saliva, as it commonly flows from the mouth, is mixed with the secretion of the mucous glands, and often with air bubbles, which, being retained by its viscidity, make it frothy. When obtained from the parotid ducts, and free from mucus, saliva is a transparent watery fluid, the specific gravity of which varies from 1004 to 1008, and in which, when examined with the microscope, are found floating a number of minute particles, derived from the secreting ducts and vesicles of the glands. In the impure or mixed saliva are found, besides these particles, numerous epithelial scales separated from the surface of the mucous membrane of the mouth and tongue, and the so-called salivary corpuscles, discharged probably from the mucous glands of the mouth and the tonsils, which, when the saliva is collected in a deep vessel, and left at rest, subside in the form of a white opaque matter, leaving the supernatant salivary fluid transparent and colourless, or with a pale bluish-grey tint. In reaction, the saliva, when first secreted, appears to be always alkaline. During fasting, the saliva, although secreted alkaline, shortly becomes neutral; especially when it is secreted slowly and is allowed to mix with the acid mucus of the mouth, by which its alkaline reaction is neutralized. The presence of potassium sulphocyanate (or thiocyanate) (CN KS) in saliva, may be shown by the blood-red colouration which the fluid gives with a solution of ferric chloride (Fe., Cl.6), and which is bleached on the addition of a solution of mercuric chloride (Hg C12), but not by hydrochloric acid. Rate of Secretion and Quantity.-The rate at which saliva is secreted is subject to considerable variation. When the tongue and muscles concerned in mastication are at rest, and the nerves of the mouth are subject to no unusual stimulus, the quantity secreted is not more than sufficient, with the mucus, to keep the mouth moist. During actual secretion the flow is much accelerated. The quantity secreted in twenty-four hours varies: its average amount is probably from 1 to 3 pints (1 to 2 litres). Uses of Saliva.-The purposes served by saliva are (1) mechanical and (2) chemical. I. Mechanical.-(1) It keeps the mouth in a due condition of moisture, facilitating the movements of the tongue in speaking, and the mastication of food. (2) It serves also in dissolving sapid substances, and rendering them capable of exciting the nerves of taste. But the principal mechanical purpose of the saliva is, (3) that by mixing with the food during mastication, it makes it a soft pulpy mass, such as may be easily swallowed. To this purpose the saliva is adapted both by quantity and quality. For, speaking generally, the quantity secreted during feeding is in direct proportion to the dryness and hardness of the food. The quality of saliva is equally adapted to this end. It is easy to see how much more readily it mixes with most kinds of food than water alone does; and the saliva from the parotid, labial, and other small glands, being more aqueous than the rest, is that which is chiefly braided and mixed with the food in mastication; while the more viscid mucous secretion of the submaxillary, palatine, and tonsillitic glands is spread over the surface of the softened mass, to enable it to slide more easily through the fauces and œsophagus. II. Chemical. The chemical action which the saliva exerts upon the food in the mouth is to convert the starchy materials which it contains into some kind of sugar. This power the saliva owes to one of its constituents ptyalin, which is a nitrogenous body of uncertain composition. It is classed among the unorganized ferments, which are substances of uncertain composition capable of producing changes in the composition of other bodies with which they come into contact, without themselves undergoing change or suffering diminution. The conversion of the starch under the influence of the ferment into sugar takes place in several stages, and in order to understand it, a knowledge of the structure and composition of starch granules is necessary. A starch granule consists of two parts: an envelope of cellulose, which does not give a blue colour with iodine except on addition of sulphuric acid, and of granulose, which is contained within, and which gives a blue with iodine alone. Brücke states that a third body is contained in the granule, which gives a red with iodine, viz., erythro-granulose. On boiling, the granulose swells up, bursts the envelope, and the whole granule is more or less completely converted into a paste or gruel, which is called gelatinous starch. When ptyalin or other amylolytic ferment is added to boiled starch, sugar almost at once makes its appearance in small quantities, but in addition there is another body, intermediate between starch and sugar, called erythro-dextrin, which gives a reddishbrown colouration with iodine. As the sugar increases in amount, the erythro-dextrin disappears, but its place is taken in part by another dextrin, achroo-dextrin, which gives no colour with iodine. However long the reaction goes on, it is unlikely that all the dextrin becomes sugar. 22 Next with regard to the kind of sugar formed, it is, at first at any rate, not glucose but maltose, the formula for which is C12 H2 O. Maltose is allied to saccharose or cane-sugar more nearly than to glucose; it is crystalline; its solution has the property of polarising light to a greater degree than solutions of glucose; is not so sweet, and reduces copper sulphate less easily. It can be converted into glucose by boiling with dilute acids, and by the further action of the ferment. According to Brown and Heron the reactions may be represented thus :-One molecule of gelatinous starch is converted by the action of an amylolytic ferment into n molecules of soluble starch. 12 20 2 One molecule of soluble starch = 10 (C12 H 010) + 8 (H, O), which is further converted by the ferment into 1. Erythro-dextrin (giving red with iodine) + Maltose. (C12 H22 011) then into 2. Erythro-dextrin (giving yellow with iodine) + Maltose. Test for Sugar.-In such an experiment the presence of sugar is at once discovered by the application of Trommer's test, which consists in the addition of a drop or two of a solution of copper sulphate, followed by a larger quantity of caustic potash. When the liquid is boiled, an orange-red precipitate of copper suboxide indicates the presence of sugar. |