44

PROPERTIES OF MIXED SALIVA.

secretions of the various salivary glands. It may be doubted whether the method of obtaining it sometimes recommended, by makProperties of mixed salivas. ing pressure under the chin and tickling the fauces with a feather, yields it of normal constitution. It is described as an alkaline juice, of a bluish color or colorless, in consistency glairy, readily frothing, and therefore well adapted for entrapping atmospheric air. It contains, of solid matter, from 0.348 to 0.841 per cent. Its alkali appears, for the most part, to be combined with an organic substance, ptyaline, from which it may be separated by the weakest acids, such as carbonic. In the ash of saliva the alkali occurs chiefly as phosphate: this arises from rearrangement of the constituents during incineration. The saliva contains but a trace of alkaline sulphates, the chlorides of sodium and potassium preponderating over all the other mineral ingredients.

On standing, saliva separates into two layers: a transparent one, which is supernatant, and a grayish turbid one below, which consists of a deposit of particles of pavement epithelium and mucus corpuscles, derived from the lining membrane of the mouth and the salivary ducts. Its chemical reaction varies to some extent with the state of the system; thus, after long-continued fasting, from being alkaline, it may approach the neutral state. By some it is asserted that under these conditions it may even become acid. There is no proof that this is owing to the appearance of lactic acid: it may be due to butyric acid, or even the acid phosphate of soda. In morbid conditions this reaction is by no means infrequent it has been commonly observed in intestinal inflammation, acute rheumatism, intermittent fever. Donné and Frerichs assert that acidity of the saliva depends on an irritation of the buccal mucous membrane.

The specific gravity of mixed saliva varies from 1.004 to 1.009. These variations depend on many different causes, there being a diminution after the taking of drink, and a greater increase after taking food, than even is observed in the fasting state. An animal diet especially increases it. Under ordinary circumstances, the saliva is secreted to an amount of Quantity of from 15 to 20 ounces daily. The exudation is more copious saliva. during mastication, speaking, reading, more being produced by the use of hard than soft food. Mental emotions exert a control over its flow, sometimes diminishing it, as in moments of anxiety, sometimes increasing it, as by the anticipation of food. After eating, the flow continues to a considerable extent; it is also provoked by the use of aromatics. On irritation of the interior of the stomach through a gastric fistula, the flow is simultaneous with that of the gastric juice.

The movements of the jaw and the pressure of the food give rise to variations in the quantity of saliva. It is perhaps for these reasons that the parotid gland on that side of the mouth which is most used in mastication secretes more than the other. Of the proportion of the different kinds of

CONSTITUTION OF SALIVA.

45

saliva in the mixed secretion, nothing is known with certainty in the case of man, but it is said that in horses the parotids furnish two thirds, the submaxillaries one twentieth, and the sublinguals and mucous follicles the rest. The secretion of the saliva goes on during sleep.

To the active organic substance of the saliva the designation of ptyaline has been given. It is regarded as a ferment, possessing in several respects the properties of diastase, and hence has been called by Mialhe diastase salivaire.

Ptyaline.

For the purpose of analysis, saliva should be obtained in a perfectly fresh state, a condition not easily fulfilled, for it decomposes or changes with rapidity.

During these changes, alkaline carbonates, for example, are formed in abundance, though they may have existed but to a small extent at first. We have already seen that in this way parotid saliva, ex- Constitution of posed to the air, yields crystals of carbonate of lime. The saliva. following table is presented as offering an example of the average constitution of mixed saliva.

Of the fixed salts the chief are, the phosphates of soda, lime, and magnesia, and the chlorides of sodium and potassium. The sulphocyanide of potassium varies in amount considerably: it increases after meals, and especially after the use of condiments, salt, pepper, spices. Those articles which contain sulphur, as mustard, garlic, radishes, increase its amount in a very marked manner.

Not only does the saliva, as derived from the different glands, present differences of constitution; it likewise differs in various ani- Modifications mals, and in the same animal according to its age. This is of saliva. observed even in the case of man. The saliva of an infant at the breast possesses very little power of saccharizing starch, a transmutation which that of the adult accomplishes with energy.

The action of this secretion appears to be limited to starch, and certain kinds of sugar, which first yield lactic and then butyric acid. It does not exert any influence in transforming albuminous matter.

The saliva discharges many functions. It is a necessary intermedium in the sense of taste, for substances to be sapid must be more Functions of or less soluble in this juice. If insoluble, they are tasteless. saliva. It also moistens the interior of the mouth, and prevents the sensation of

46

SALIVARY DIGESTION IN THE STOMACH.

dryness. But its chief duty seems to be that of promoting the digestive operation; for, though the food remains in the mouth but a short time, the action of the saliva is prolonged after the masticated mass has been deposited in the stomach. Though the direct admixture of saliva with gastric juice injures the power of the latter, this effect does not ensue in the stomach, since they act for the most part separately. The action of the gastric juice is superficial, and two distinct operations are therefore conducted at the same moment, the surface of the food changing under Action of the the influence of the gastric juice, and the inner portion under saliva is con- that of the saliva. I believe that in this manner the salivary stomach. juice lends itself to stomach digestion, for it is well known that by its aid starch changes into grape sugar, and the transmutation does not stop at that point, but goes on to the production of lactic acid. An acid juice is essential to stomach digestion.

tinued in the

sugar from

starch in the

saliva.

After the administration of balls of starch to animals in which gastric Production of fistulæ have been established, sugar may be detected in the stomach in the course of ten or fifteen minutes. It does stomach by the not appear that there is any relation between the quantity of saliva incorporated by mastication and the quantity of starch in the food. Animals which swallow their food without mastication have either no parotids, or those organs exist in only a rudimentary state; commonly, however, their submaxillary glands are large. Under the most favorable circumstances, the digestion of starchy food is 'scarcely ever complete, a considerable portion being found in the excrement. The true function of the saliva has been well illustrated by inserting amylaceous food into the stomach of dogs with gastric fistulæ, after tying the salivary ducts, in which case no sugar can be detected.

It has been suggested that the eventual arrest of the action of saliva on reaching the stomach may be due to the digestion of its ptyaline by the gastric juice. In artificial experiments, however, such a digestion or destruction can not be accomplished.

The double digestion, partly salivary and partly gastric, occurring in the stomach, is doubtless one of the causes of those differences which have been noticed between the natural action of that organ and the artificial imitations of it. The influence of the saliva, even under these, which may seem at first sight to be unfavorable circumstances, is far from being trivial, an effect which is well illustrated by the instantaneous manner in which a solution of starch in water, mixed with an equal quantity of saliva and agitated, is transmuted into a solution of sugar. In a few moments its viscidity is lost, it fails to give the blue reaction with iodine, becomes sweet to the taste, and readily answers to Trommer's test.

Besides the duties which have been mentioned, the saliva incidentally

RELATION OF THE SALIVARY GLANDS AND KIDNEYS.

47

accomplishes a secondary object by its power of retaining gases in its froth or foam. Atmospheric oxygen by this means is incor- Saliva carries porated with the food during mastication, and is thus enabled air into the to exert an important influence in promoting the action of stomach. the gastric juice. For to the inception of the change which that juice impresses on the food, oxygen is necessary. It is brought into the cavity of the stomach entangled or dissolved in the saliva.

ciencies of hy

It has just been mentioned that the action of saliva on starch is not restricted to the production of sugar, but that it may end in the Lactic acid formation of lactic acid. If, therefore, any thing intervenes to supplies deficheck the supply of hydrochloric acid, which usually gives drochloric. acidity to the gastric juice, the system possesses within itself the means of compensating for the difficulty. In the interior of the digesting mass lactic acid is being set free. This acid, as has long been known, can replace hydrochloric acid in its physiological duty.

Though so large a quantity of saliva as 20 ounces may be secreted in a day, this being about one half of the urinary discharge, it is to be remembered that the water is not lost to the system, as in the latter case. When the impure habit of profuse spitting is indulged in, it Disgusting efis interesting to remark the reflected effect which takes place fect of profuse in the reduced quantity of the urine, and an instinctive desire expectoration. for water, a kind of perpetual thirst. It is probable that, under these disgusting circumstances, the percentage amount of saline substances in the saliva is increased, and that, so far as that class of bodies is concerned, the salivary glands act vicariously for the kidneys, and the mouth is thus partially converted into a urinary aqueduct.

The relation between the salivary glands and the kidneys is very well shown after the administration of such substances as the Relation of the

and the kid

iodide of potassium. If five grains of this salt be taken in salivary glands pills, and the mouth be then thoroughly washed, in the course neys. of a quarter of an hour the saliva will readily strike a blue tint when tested with nitric acid and starch, but the urine will not show that reaction until after a considerable interval, perhaps even an hour or more. It would therefore appear that such a salt must pass again and again through the salivary glands before it is finally disposed of by the kidneys, which offer the only outlet for its total removal.

Among the functions of the saliva we ought not to overlook the influence which its rapid secretion must exert on the state of tension of the blood-vessels, an influence which probably favors the absorption going on in the stomach and intestines.

Thus prepared by mastication and insalivation, the food descends into the stomach, passing along the pharynx, which dilates to receive it. The rima glottidis spontaneously closes, and additional security is given to the

48

THE DIGESTIVE TRACT.

respiratory passage by the valve-like shutting of the epiglottis. Through the oesophagus the morsel advances by the contraction of the muscular coat, with a wave-like or undulating motion onward. The food is now delivered at the cardiac orifice of the stomach, and, entering that organ, is submitted to the gastric juice, which is exuding from the mucous membrane.

The digestive tract may be considered as presenting six prominent reIllustration of gions-the mouth, the pharynx, the oesophagus, the stomach, the digestive the small intestine, the large intestine. Their relative position and subdivisions are illustrated in Figure 3.-1, the

tract.

tongue; 2, 2, the pharynx; 3, 3, the œsophagus; 4, the velum pendulum palati; 5, section of the larynx; 6, the palate; 7, the epiglottis; 8, the thyroid cartilage; 9, the medulla spinalis; 10, 10, bodies of vertebræ; 11, 12, spinous processes of ditto; 13, cardiac orifice of stomach; 14, splenic extremity; 15, pyloric extremity; 16, 16, greater curvature; 17, the less curvature; 18, pylorus; 19, superior transverse portion of duodenum; 20, middle or perpendicular portion; 21, inferior transverse portion; 22, gallbladder; 23, cystic duct; 24, hepatic duct; 25, ductus communis choledochus; 26, its aperture in the duodenum; 27, duct of the pancreas, emptying into the duodenum near to the place of entry of the ductus communis choledochus; 28, commencement of jejunum; 29, 29, 29, jejunum ; 30, 30, 30, ileum; 31, ileum opening into great intestine; 32, ileo-colic valve; 33, ileo-cocal valve; 34, coecum; 35, appendix vermiformis; 36, 36, the ascending colon; 37, transverse arch of colon; 38, descending colon; 39, sigmoid flexure; 40, rectum; 41, anus.

From the interior or mucous coat of the stomach the gastric juice exudes. This fluid may be best obtained for examination by gastric fistulæ artificially established in animals. As respects the