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sympathetic saliva; but in all these animals the sympathetic saliva issmaller in quantity than the chorda saliva, and in all of them the blood-vessels are constricted.

To explain this difference between the action of the two nerves of the gland, Heidenhain has advanced the theory that the cells of a secreting gland are supplied by two kinds of nerves; the one, trophic, exciting chemical processes in their protoplasm; the other, secretory, having to do with the separation of the secreted products. In all cells, gland-cells among the number, two processes are continually occurring : one the building up of their substance and contents (anabolism), the other the breaking down of the same (katabolism). That each of these processes is governed by a special nerve-filament was an ingenious speculation, which it turns out, on further investigation, is supported in several ways. The existence of the two kinds of fibres, and their admixture in various proportions with one another, and with vaso-motor fibres, will explain very largely the result of stimulation of the nerves we have mentioned; to take the case of the dog's submaxillary again, the chorda contains many secretory fibres and few trophic fibres; hence the secretion which follows its stimulation is copious and watery. The sympathetic, on the other hand, contains few secretory and many trophic fibres; hence the secretion which follows its stimulation is scanty and viscid.

Bayliss and Bradford 2 have confirmed the probable existence of Heidenhain's two sets of fibres by demonstrating that the electrical changes in the glands are of the opposite kind on stimulation of the two nerves; and that atropine destroys the chorda variation (hilus positive to surface of gland), but only slightly lessens the sympathetic variation (hilus negative to surface). .

Langley, however, considers that the existence of more than one kind of secretory fibre is very doubtful; and he shows, too, that this assertion is not irreconcilable with the conclusions of Bayliss and Bradford. The reasons for Langley's conclusions are entered into fully in the papers quoted below, and briefly they are these:-

3

(1) The phenomena of atropine-poisoning give no indication of the existence of more than one kind of secretory nerve-fibre. By the use of very small doses of atropine, administered successively, all varieties of secretory nerve-fibre are equally and simultaneously paralysed.

(2) Experiments on the submaxillary, in which the two nerves supplying the gland are alternately stimulated, also tend to throw The symdoubt on the existence of two varieties of nerve-fibre.

1 Hermann's Handbuch, 1880, vol. v.

2 Proc. Roy. Soc. xl. 203.

3 Journ. Physiology, ix. 55; x. 291.

pathetic saliva is largely increased in amount by previous stimulation of the chorda, that is, after the increased supply of blood produced by dilatation of the blood-vessels. Unless the gland has been thus previously supplied richly with oxygen, the secretory fibres of the sympathetic (which are comparatively few in number and masked by admixture with vaso-constrictor nerves) are non-effective or nearly so. Langley thus considers that the action of the nerve-fibres on the size of the vessels has more importance than Heidenhain was inclined to give to it; and that the secretory fibres being in the two nerves mixed with vaso-motor fibres of opposite kinds, explains the difference in the actions of the nerves quite as well as or better than the hypothesis that the secretory fibres are themselves of opposite kinds.

Whichever explanation is ultimately shown to be correct--and there is much to be said on both sides--there is little doubt that the parotid and the sublingual are governed by nervous influences in the same way as is the submaxillary gland. Stimulation of the sympathetic in the dog produces no secretion of saliva from the parotid gland, or only when the gland has been previously thrown into a state of increased irritability by the previous stimulation of a nerve which corresponds to the chorda tympani in relation to the submaxillary this nerve is a branch of the glosso-pharyngeal nerve called Jacobson's nerve, which may be reached within the tympanum, in the tympanic plexus.

Paralytic secretion. This is a thin, watery secretion that occurs about twenty-four hours after section of the secretory nerve. The gland of the opposite side is also affected (antilytic secretion; Langley). It begins to diminish about the eighth day. It has been explained as a degeneration effect comparable to the fibrillar contraction of muscle, and also as caused by the action of venous blood, increasing the excitability of local centres in the gland.

2. THE STRUCTURE OF THE CELLS THAT SECRETE SALIVA

As much as is known concerning the chemical constituents of the salivary glands en masse has been already referred to (p. 558). Microscopical examination of the cells of the glands in relation to the time and amount of secretion teaches us certain important facts concerning the elaboration of the important substances ptyalin and mucin.

The first fact of importance noted in sections from hardened specimens is that certain cells are filled with a highly refracting substance called mucinogen, which is subsequently extruded as mucin when these cells. degenerate; they press the more protoplasmic and more easily stainable cells to the basement membrane, where they form the so-called crescents

of Gianuzzi. Cells of this nature are called mucous cells; the alveoli in which they occur, mucous alveoli or acini; the glands which contain mucous alveoli are called mucous glands; these are in man the submaxillary and the sublingual; the secretion of the mucous glands is called mucous saliva. There are other cells arranged also in acini, which according to their state of secretory activity are more or less filled with fine granules; the granules, however, will be invisible in a section of the hardened organ; there are no crescents; these are termed serous cells, serous acini, serous glands respectively. Such a gland is the parotid. Some acini of the submaxillary and sublingual are also serous. The saliva secreted by these is called serous saliva. The saliva is in other words limpid and not viscid. The use of the word serous in this connection is established by long usage, but is singularly unfortunate; the parotid saliva and the serum are alike in that mucin is absent from both, but here the resemblance ends. Foster suggests the word albuminous instead.

The changes in the cells and their structure can be made out better by teasing fresh preparations of the gland in serum, or aqueous humour, or after exposure to weak osmic acid, or better still osmic acid vapour (Langley); this reagent preserves the granules, and the preparations can be subsequently stained with carmine, hæmatoxylin, &c., and kept.

When we study an albuminous gland in the fresh, living condition, the changes during activity are like those already described in more general terms in connection with secreting epithelium. In the stage represented in A (fig. 82) the cells are large, their outlines very indistinct, and the cell-substance studded with minute granules.

This stage is generally called the stage of rest; rest is here a comparative term. The building up of these granules within the protoplasm is really a stage of activity, but a different kind of activity to that which follows. These granules are composed of or indicate the presence of the precursor of the ferment. Ferment precursors are also

1 Foster's Physiology, 5th edition, ii. 406.

called zymogens; and this particular zymogen may be called ptyalinogen, the precursor of ptyalin. The stages which follow (fig. B and C) are usually spoken of as stages of activity; they are more correctly the stages seen while secretion is taking place, or after it has occurred. The cells become smaller (B) as they shed out the secretion, their outlines and nuclei more distinct, and the granules disappear, especially from the outer parts of each cell. After prolonged activity (C) such as is produced by the injection of pilocarpine, or by stimulation of the secretory nerve, these changes are all more marked; only a few granules are left at the free border of the cells, which now abut on a conspicuous lumen.

These granules are dissolved or rendered indistinct by alcohol, chromic acid, and other hardening reagents.

In a mucous gland the changes that take place are of a like kind. If a piece of resting, or, to speak more correctly, loaded submaxillary gland be teased out, spherules are seen which are larger than the granules of the parotid and less dense

and solid than those of the pancreas. These crowd the cell-protoplasm (fig. 83, a). In a discharged gland, that is, one which has been secreting for some time, the granules are less numerous and largely confined to the part of the cell near the lumen (fig. 83, b). The distinction between an inner 'granular zone' and an outer clear zone' next to the basement membrane is less distinct than in the serous or albuminous acini, partly because the granules do not disappear in so regular a manner as in the parotid and pancreas, and partly because the outer zone of the mucous cell is less homogeneous.

b

b'

FIG. 83.-Mucous Cells from a fresh Sub maxillary Gland of Dog (from Foster, after Langley).

In fig. 83, a' and b' represent cells in a loaded and discharged condition respectively, which have been irrigated with water or dilute acid. The mucous granules (mucinogen) are swollen into a transparent mass of mucin traversed by a network of protoplasmic cell-substance ; the appearance of mucous cells in sections hardened by alcohol and stained in the usual way is very similar.

1 Langley, Phil. Trans. 1889.

3. THE COMPOSITION OF SALIVA

Mixed saliva. The saliva as found in the mouth is a mixture of that from all the different glands. On microscopic examination, a few epithelial scales from the mouth and salivary corpuscles from the salivary glands are seen. The liquid is transparent, slightly opalescent, of slimy consistency, and may contain lumps of nearly pure mucin. On standing, it becomes more cloudy, owing to the precipitation of calcium carbonate, the carbonic acid which held it in solution as bicarbonate escaping.

Its constituents are:

Organic. (a) Mucin. Acetic acid precipitates this in a stringy

form.

(b) Ptyalin: an amylolytic ferment discovered by Leuchs in 1831. It is constantly present in human saliva, even in new-born children; 2 it is usually absent in dog's saliva; 3 it was not found by Roux in the saliva of horses; Schiff found it in that of rabbits and guinea-pigs.5 It has since been found in nearly all animals.

4

(c) Proteid. A trace of a proteid, coagulable by heat, of the nature of a globulin is constantly present.

(d) Sulphocyanide of potassium (KSCN) is usually, but not always, present in human saliva. It is absent in dog's saliva.7

Inorganic.-Small quantities of chlorine and phosphoric acid in combination with potassium, sodium, calcium, and magnesium; also small quantities of sodium carbonate. Sodium chloride is the most abundant salt. Schönbein, observed that saliva contains a substance which, like nitrous acid, colours blue a mixture of starch and potassium iodide. The nature of this substance is doubtful.

Impurities. If putrefactive processes be taking place in the mouth, bacteria will be found; indeed, they are never altogether absent. The saliva may under these circumstances be acid. It is also acid in some cases of diabetes mellitus; sugar, is however, always absent; and so is bile in cases of jaundice. Certain drugs, especially iodine, appear quickly in the saliva after they have been administered. In fevers the amount of saliva secreted is much lessened. Tartar consists chiefly of 1 Kastner's Arch. 1831. See also Schwann, Pogg. Ann. xxx. 358.

2 Schiffer, Arch. f. Anat. und Physiol. 1872, p. 469; Korowin, Centralbl. med. Wiss. 1873, No. 17.

3 Hoppe-Seyler, Physiol. Chem. p. 186. 4 Gazz. med. veterin. de Milano, 1871. 5 Schiff, Leçons sur la physiol. de la digestion, 1868.

6 Treviranus, Biologie, vol. iv. 1814, p. 330; Tiedemann and Gmelin, Die Verdauung nach Versuchen, vol. i. 1826, p. 9.

7 Hoppe-Seyler, Physiol. Chem. p. 186.

8 Journ. prakt. Chem. lxxxvi. 151. See also Schaer, Zeit. Biol. vi. 467.