The sensory mechanism for recognizing the amount of food in the oral cavity and the ability to determine the optimal amount of food to be swallowed are of prime importance to effectively control the functions of food intake and swallowing． Unfortunately， no studies have been conducted in this field．Fujishima et al．first examined these functions in those with dysphagia and normal volunteers to make a comparative study． ln the present study we modified their examination method to perform a more detailed investigation and to understand the sensory mechanism involved in food intake and the ability to determine the amount of food to be swallowed． The number of subjects was increased to allow a reliable statistical analysis． Although Fujishima et al． used jelly as the test food， we used solid food to examine the difference in the physical properties of test food． The subjects studied included 36 healthy female students． At first we measured 10.0 ml of water and gave it to the subjects so that they could memorize the exact amount of water． Then the subjects were asked to measure the standard amount of water with their mouths five times at intervals of one to two minutes． We increased the amount of water up to 20.0 ml and repeated the same experiment． The average amounts of water intake were 10.6 ± 0.86 ml in the first experiment and 20.2 ± 1.45 ml in the second experiment (mean ± SD)．In order to determine the optimal volume to be swallowed， we selected rice crackers as experimental solid food and gave them to the subjects． These rice crackers were uniform in size and weight． The number of rice crackers was gradually increased so that the subjects could recognize how many rice crackers they could chew in their oral cavity without difficulty． The subjects made a subjective evaluation of the changes of their swallowing functions． As results of these experiments which were repeated three times， we recognized that the optimal weight of rice crackers was 1.1 g on the average and that the optimal amount to be swallowed was stable because no subjects reported any change in their swallowing functions．Although the optimal amount to be swallowed was slightly correlated with the height（r ＝0.402， p＜0.05）, correlation with body weight and body mass index was negligible．The present study demonstrated that normal volunteers had excellent ability to recognize the exactamount of food in their oral cavities． We discussed the factors to determine the optimal amount of solid food to be swallowed．

Lips play a very important role in the oral stage， and their functions should be carefully observed． At present， however， few researchers have conducted studies of lip movement from the standpoint of feeding and swallowing functions． Although direct observation has conventionally been regarded as difficult， in this study we tried to observe the dynamic relationship between the tongue， which is a crucial element in the oral stage of swallowing， and lips during swallowing． The subjects studied included 29 healthy adult volunteers． A high-resolution CCD camera was used to study the lip movement and ultrasound sonography was used to study the tongue movement． The data obtained on the same time axis were evaluated quantitatively and qualitatively by two-dimensional dynamic analysis． The results were summarized as follows．

1． The outward traction of the angles of the mouth definitely preceded the elevation of the dorsum of tongue toward the palate． Therefore， the start time of lip movement （LS）was regarded as the common reference point for lip and tongue movement， and the movement changes were carefully observed over the course of time．

2． According to the measurement results of the width of lips during the swallowing period， the average difference between the start of lip movement and LP was 5.6 mm． The existence of a significant difference between the width of lips measured at the beginning of lip movement and that measured at the time of LP and the fact that the average difference accounted for 23.5％ of the horizontal range of lip movement suggested that showed the traction of the angles of the mouth was generally due to swallowing， even in healthy adults．

3． The time required for swallowing using the lip and tongue movement was 2.56 seconds on the average．

4． Because the start of lip movement was used as a reference point in the present study， there was variance in the measurements of other various items． ln spite of these significant individual differences， the mean individual coefficients of variation were small．The lip and tongue movement suggests that healthy adults probably have their own individual mechanism of swallowing．

5． The results of the present study suggested that swallowing movement consisted of the following processes： the initiation of vigorous outward traction of the angles of the mouth after mastication， the acceleration of the elevation of the tongue toward the palate， and the transportation of bolus to the pharynx．

The careful observation of the lip movement enabled us not only to define the initiation of swallowing， but also to dynamically study the tongue movement．

We used five subjects whose hard palates were covered with resin plates to examine their modes of masticatory movement while they were eating soft and solid test food (agar jelly and cookies). We conducted videofluorography in order to investigate the roles of the palate in recognizing the ingested food texture． While the subjects were chewing agar jelly with the resin plates attached to their hard palates， we confirmed that they were chewing more frequently for a longer period of time before swallowing． Some subjects changed their soft food crushing technique from compressing with the tongue and the hard palate to chewing with teeth． This change in food crushing technique corresponded to the time when the subjects recognized the change in the hardness of food． ln these cases the mechanism to recognize the texture of ingested food was impaired by the application of the resin plates and the threshold of recognizing hard food was lowered．While the subjects were chewing cookies with the resin plates attached to their hard palates， we also confirmed that they were chewing more frequently and for a longer period of time before swallowing． ln this experiment we recognized the effects of the motility of tongue and the salivary secretion on this phenomenon which were not observed in the former experiment using soft food． Further work is necessary before the findings obtained in the present study are confirmed．

The repetitive saliva swallowing test (RSST) was developed by Saito in 1996 to detect the difficulty in food ingestion and swallowing．ln this screening test those who swallow their saliva more than three times within 30 seconds are regarded as those with normal food ingestion and swallowing functions． ln order to evaluate the usefulness of RSST as a screening test for the detection of difficulty in food ingestion and swallowing， we studied the relationship between dysphagia and the frequency of saliva swallowing． The subjects were 1098 elderly patients who were under care in 17 nursing homes in Sapporo and the surrounding area． The nursing staff carefully observed their eating and swallowing behavior and judged the presence of dysphagia during or after meals． The ratio of men to women was 1 to 2.5 and their average age was 82.2 years． Of 1098 patients， 1048 were available for RSST．The incidence of dysphagia was 13.1％ (137/1048). According to the results of RSST， 392 patients could swallow their saliva more than three times within 30 seconds． Those who swallowed their saliva more frequently within the specified period showed a lower incidence of dysphagia． We divided the subjects into two groups：a group of elderly patients who swallowed their saliva less than twice （n＝656） and a group of elderly patients who swallowed their saliva more than three times (n＝392). The number of those with dysphagia was 110 patients in the former group and 27 in the latter group． There was a statistically significant difference between these two groups (p＜0.001). The sensitivity and specificity of RSST were 80.3％ and 40.1％ respectively． On the basis of these test results， we concluded that RSST was a useful screening method to detect the difficulty of food ingestion and swallowing in the elderly patients treated in nursing homes．

We prepared a new experimental food to be used in the test to examine the swallowing function． This new food contains agar as its basic material and iopamidol， which is known to have greater safety than barium sulfate and amidotrizic acid． The texture of this new food including its hardness and adhesiveness has been improved so that the food may have sufficient radiopacity for videofluorography． We prepared six types of experimental food with different hardness levels： 2.20×10²N/m²， 6.53×10²N/m²， 2.90×10³N/m²， 7.49×10³N/m²，1.09×10⁴N/m² and 1.86×10⁴N/m²．They were given to an 85-year-old patient with oral cancer． This elderly patient showed difficulty in swallowing depending on the degrees of hardness of the experimental food． Videofluorography revealed the effects of the hardness of food on the patient’s swallowing function． This newly developed food is regarded as a useful test material to be used in the study to examine the relationship between the texture of food and the swallowing function．