210 DIGESTION. 



Mixed Saliva. Although the study of the distinct secretions discharged into the 

 mouth possesses considerable physiological interest and importance, it is only the fluid 

 resulting from a union of them all, which can properly be considered in connection with 

 the general process of insalivation. In man it is necessary that the cavity of the mouth 

 should be continually moistened, if for nothing else, to keep the parts in a proper condi- 

 tion for phonation. A little reflection will make it apparent that the flow, from some of 

 the glands at least, is constant, and that, from time to time, a certain quantity of saliva 

 is swallowed. This is even more marked in some of the inferior animals, as the rumi- 

 nants. The discharge of fluid into the mouth, though diminished, is not arrested during 

 sleep. In the review of the different kinds of saliva, it has been seen that the flow from 

 none of the glands is absolutely intermittent ; unless it be so occasionally from the pa- 

 rotid, the secreting function of which is most powerfully influenced by the act of masti- 

 cation and the impression of sapid substances. 



Upon the introduction of food, the quantity of saliva is enormously increased ; and 

 we have already noted the influence of the sight, odor, and occasionally even the thought 

 of agreeable articles. Many persons present a marked increase in the flow of saliva at 

 the sight of a lemon ; and we are all familiar, in a general way, with the impressions 

 which bring " water into the mouth." The experiments of Frerichs on dogs with gas- 

 tric fistulae, and the observations of Gardner on a patient with a wound in the oesopha- 

 gus, have demonstrated that the flow of saliva may be excited by the stimulus of food 

 introduced directly into the stomach without passing through the mouth. 



Quantity of Saliva. It is not easy to estimate, in the human subject, the entire 

 quantity of saliva secreted in the twenty-four hours ; and great variations in this regard 

 undoubtedly exist in different persons, and even in the same individual at different times. 

 An approximate estimate may be arrived at by noting, as nearly as possible, the average 

 quantity secreted during the intervals of digestion and adding to it the quantity ab- 

 sorbed by the various articles of food. Some of the earlier physiologists investigated 

 this subject with much patience. Be"rard quotes the experiments of Siebold, who col- 

 lected the saliva by holding the mouth open with the head inclined, receiving the 

 fluid in a vessel as fast as it was secreted. An estimate of this kind can only be ap- 

 proximative, and those made by Dalton are apparently the most satisfactory. This ob- 

 server found that he was able to collect from the mouth, without any artificial stimulus, 

 about five hundred and fifty-six grains of saliva per hour ; and he also found that wheaten 

 bread gained in mastication fifty-five per cent., and lean meat, forty-eight per cent, in 

 weight. Assuming the daily allowance of bread to be nineteen ounces and the allow- 

 ance of meat to be sixteen ounces, and estimating the quantity of saliva secreted during 

 twenty-two hours of interval, the entire quantity in twenty-four hours would amount to 

 20,164 grains, or a little less than three pounds avoirdupois, of which rather more than 

 one-half is secreted during the intervals of eating. 



Eemembering that the quantity of saliva must necessarily be subject to great varia- 

 tions, this estimate may be taken as giving a sufficiently close approximation of the quan- 

 tity of saliva ordinarily secreted. It must be borne in mind, however, with reference 

 to this and the other digestive secretions, that this immense quantity of fluid is at no 

 one time removed from the blood, but is reabsorbed nearly as fast as secreted, and that, 

 normally, none of it is discharged from the organism. 



General Properties and Composition of Saliva. The mixed fluid taken from the 

 mouth is colorless, somewhat opaline, frothy, and slightly viscid. It generally has a faint 

 and somewhat disagreeable odor very soon after it is discharged. If it be allowed to 

 stand, it deposits a whitish sediment, composed mainly of desquamated epithelial scales, 

 with a few leucocytes, leaving the supernatant fluid tolerably clear. Its specific gravity 

 is variable, ranging from 1004 to 1006 or 1008. Its reaction is almost constantly alka- 



