I NVESTIGATION of the process of salivary secretion in the human being has been complicated by an apparent large variability in the rate of secretion not only as applied to large numbers of individuals but even for a single individual during a relatively short period of collection under seemingly constant conditions."' 2 Recent workers have attempted to control this variability by collecting for study the so-called "resting saliva," that is, saliva secreted by a fasting subject under standardized conditions of rest.2-5 Collection of saliva under such conditions of reduced external stimulation has succeeded only partially in reducing this variability in the rate of salivary flow.The crucial question, on which the cause and possibility of control of variability in secretion rate would seem to hinge, is whether salivary secretion in the human being occurs spontaneously or only in response to nervous or humoral stimulation. If secretion occurs spontaneously, then measurement of the rate of secretion under conditions of rigid control of external stimulation may give a true rate of flow of resting saliva. Individual variations and variability within a group would then have to be accepted as a real characteristic of the secretion process.However, if secretion does not occur spontaneously, then there is no true resting saliva, and the secretion collected under so-called resting conditions results, nonetheless, from stimulation. Variability in secretion rate would then reflect variations in intensity of stimulation, either internal or external, and would be extremely difficult to control. The basic question of whether salivary secretion in the human being occurs spontaneously or only in response to nervous or humoral stimulation has not up to the present time received a full or satisfactory answer based on experimental data. The early observations of Mitscherlich6 and of Zebrowskij made on isolated human subjects with fistular openings of a parotid duct, indicate that no measurable secretion by the parotid gland occurs in the absence of stimulation. The experiments of Lashley,8 which involved collection of the secretion of the parotid glands from normal subjects by the use of parotid cups, reveal that, under all ordinary conditions of rest while in the

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THE purpose of the present investigation was to develop a method for measuring the rate of in vitro tooth decay. Since surface softening has been one of the initial signs of in vitro tooth decay, changes in hardness of the enamel surface might be a useful index of the rate of the caries attack. The value of a rapid quantitative method for evaluating anticaries agents is readily apparent.The "artificial mouth"' has been used in this laboratory to produce in vitro tooth decay, the various lesions produced being indistinguishable from those occurring in vivo. This has been shown by the general similarities' and by histologic,2 ultraviolet and polarized light3 studies of the lesions produced.The microhardness of intact surface enamel has also been investigated in this laboratory.4 It was shown that intact surface enamel varies in hardness on a single tooth surface as well as from tooth to tooth. The variation in hardness is generally within the range of 250 to 450 Knoop numbers. The inherent variation in hardness of a single surface poses a real problem when only slight changes in hardness are being measured. However, if a tooth is softened so much that its average hardness no longer falls within the normal range, then a definite softening effect will be demonstrable. EXPERIMENTAL Fig. 1 is a diagram of the apparatus used in the semiquantitive study of in vitro caries. The crowns of caries free, human, anterior teeth were cut into mesial and distal halves. The two halves were mounted, labial surface up, in an acrylic resin box (A) and held securely in place with inlay wax. Ten microhardness measurements were made on each half and the average hardness calculated for each tooth half.The tooth halves were then subjected to a various attack in the "artificial mouth" for 6 to 14 hours and their hardness redetermined. Bacteriologic medium, the same as used in previous work,5 containing 0.25 per cent glucose was maintained at a constant drip rate of 60 drops per minute. The medium was dripped onto a cotton muslin cloth (B) through which it slowly drained.Since some bacterial plaques made in the laboratory tend to develop faster than others, all plaques were pregrown for 24 hours in the "artificial mouth"

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