one or both parotid ducts. 3. Rabbits chew unilaterally ... - NCBI

J. Physiol. (1985), 364, pp. 19-29 19With 6 text-figgure8Printed in Great Britain

THE POSSIBLE RELATION BETWEEN MASTICATION AND PAROTIDSECRETION IN THE RABBIT

BY D. J. ANDERSON*, M. P. HECTORt AND R. W. A. LINDENFrom the *Department of Physiology (Oral Biology), University of Bristol,

Bristol BS8 JTD, and the Department of Physiology, King's College London,Strand, London WC2R 2LS

(Received 14 December 1984)

SUMMARY

1. Salivary flow has been recorded from conscious rabbits during 1 min periodswhilst continuously chewing standard laboratory dry pellets or pieces of carrot and,in some animals, a mash of pellets with water. Flow was measured using contact droprecorders or a continuous flow recorder via Polythene tubes permanently inserted intoone or both parotid ducts.

2. Large variations in flow were obtained with unilateral recordings particularlyduring dry pellet chewing. Bilateral recordings showed that the flow was alwaysgreater on one side than on the other and that dominant secretion alternated fromside to side in an apparently random manner.

3. Rabbits chew unilaterally. Videotaped recordings of chewing movementsshowed that the greater secretion was always produced on the chewing side.

4. To test the possibility that drying of the oral mucosa, or the prolonged hardnessof the pellets may result in higher flow rates in animals with cannulated ducts thanwould normally be seen in intact animals, water was injected downstream into themouth through a third cannula. This was inserted in an anterograde direction in theparotid duct on one side. Significant reductions in flow were recorded during dry pelleteating, but not during carrot eating. When animals were fed a soft pellet mash,salivary flow was significantly lower than with dry pellets.

5. Recordings have been made from strain gauges attached to the ascending ramusofthe mandible. Previous findings that dry pellets produce greater strain than carrotshave been confirmed. It has also been shown that less strain is produced with a softpellet mash. The strain gauge data suggested that a relation exists betweenmasticatory force and parotid salivary flow.

6. The results are compatible with the hypothesis that intra-oral mechanoreceptorsmay be involved in a masticatory-salivary reflex.

t Present address: Department of Oral Medicine and Pathology, United Medicine and DentalSchools (Guy's), London SE1 9RT.

20 D. J. ANDERSON, M. P. HECTOR AND R. W. A. LINDEN

INTRODUCTION

Gjorstrup (1980a) reported that in conscious rabbits the output of parotid salivawas approximately three times greater when the animal chewed standard laboratorypellets (range 45-1260 #I/min) than when they chewed carrots (11-425Il/min). Inthese experiments he collected saliva from the right parotid gland into small test tubesattached behind the animal's ear and calculated the rate of flow per minute from 2-3min samples collected for amylase determination. The method did not allow thedetection of short term variations in flow. Colin (1854) observed in both the horseand mule, with bilateral parotid cannulae, that during unilateral chewing the flowof parotid saliva was greater on the chewing side than on the opposite side.

In man, Lashley (1916) also recorded parotid flow from both glands simultaneously,and demonstrated the same relation between the chewing side and the dominantsecretary side. He went on to show that the output of saliva increased with the bitingforce and from these and other observations he concluded that 'each gland seems tobe most intimately associated with the receptors of its own side'. No informationwas provided as to the number of subjects involved in the study. In only one subject,Kerr (1961) confirmed Lashley's (1916) observations on the relation between chewingside and parotid secretion and, by implication, between chewing force and secretion,using tasteless waxes of different consistencies.

In the rabbit, Weijs & de Jongh (1977) used cine-photography to establish thechewing side and strain gauges to measure mandibular strain. They found that thestrain was twice as high with pellets as with carrots, and with both types of food loadswere higher on the chewing side. Salivary flow was not recorded.

Gjorstrup (1980b) suggested that the very large variation in parotid flow foundin his experiments might be accounted for by secretion alternating from side to side,as seen in the horse and mule (Colin, 1854) and man (Lashley, 1916; Kerr, 1961),associated with changing chewing sides.The experiments to be reported here have been designed (1) to record parotid

secretion so that short term variations in flow could be measured, (2) to testGjorstrup's (1980b) suggestion that parotid secretion in the rabbit is related to thechewing side, (3) to record bilaterally in order to detect alternating secretion andobtain videotaped data on chewing movements and strain gauge recordings ofmandibular strain to establish the chewing side and (4) to examine the possibility thathardness and dryness of the food might contribute to the differences in flow betweeneating pellets and carrots.

Preliminary accounts of some of these experiments have been given (Anderson,Rogers & Sirett, 1981; Anderson, Hector & Rogers, 1982).

METHODS

The experiments were conducted on twenty-nine New Zealand White rabbits weighing 2-0-3-4 kg.Before surgical preparation the animals were trained to take food in a daily experimental sessionof 10-20 min duration, seated unrestrained in a box open at the top and one end. They were offereda standard laboratory pellet diet (consisting mostly of compressed grass and hay) or pieces of carrotalternately in quantities which ensured uninterrupted chewing for at least 1 min for each material.Most animals adapted rapidly to this regime; those not responding to training after several weekswere excluded from the investigation.

PAROTID SECRETION IN THE RABBIT

To ensure that the animals maintained their weight, they were given 50-75 g of pellets and alsopieces of carrot daily, in addition to whatever they ate during the feeding session.

All surgery was performed under intravenous sodium pentabarbitone anaesthesia (30 mg/kg)which was supplemented as necessary by further injections through an indwelling cannula in alateral ear vein. Access to the parotid ducts for cannulation was made through a small incisionapproximately 1 cm below the inferior orbital margin. Initially the right parotid duct only andsubsequently both right and left ducts were cannulated in a retrograde direction using fine,transparent Polythene tubes (i.d. 0-58 mm, o.d. 0-97 mm) which were reflected back subcutaneouslyover the masseter muscle to emerge behind the ear. After surgery and while the animal was stillanaesthetized, the patency of the system was tested by an intravenous injection of carbachol(3-5 ,ug/kg). The animals were given intramuscular penicillin post-operatively (300 mg/kg, Tripl-open, Glaxo Laboratories Ltd.).

Ac.-~~~~~~~~~~~Tc

0-2in.i,L F S. S.t

Fig. 1. Scale drawing of Teflon and titanium device. Key: T.c., Teflon cap; Ch., chamber;S.n., spanner notch; S.s.t., stainless-steel tube; R.c., retrograde cannula; A.c., anterogradecannula.

An event marker was operated manually to signal the onset of each chewing sequence and theflow rate was determined over a 1 min period thereafter. We waited until the animal stoppedchewing one type of food (generally signalled by licking of the lips) before presenting it with thenext; this almost always resulted in a longer interval between pellets and carrots than vice vera.

In order to test the effect of replacing the fluid lost by diverting saliva from the mouth, the parotidduct on one side was also cannulated in the anterograde direction and this cannula was also threadedsubcutaneously to emerge behind the ear. In the early experiments the anterograde cannula or theremaining portion of the duct frequently became blocked after a few days and it seemed thatpatency would be more likely to be maintained if saliva flowed into the mouth at all times exceptduring the short recording periods. This was achieved with a titanium and Teflon device which wasused on ten animals and is shown in Fig. 1. For recording flow and injection of water downstreamat I ml/min with pellets, and 05 ml/min with carrots (comparable with the maximum flowsrecorded with each food), the Teflon cap was removed and two fine-bore tubes attached to thestainless-steel tubes protruding into the chamber. An infusion pump with glass syringe (Harvard)was used for injection of water downstream. At the end of a recording session the cap was replaced.Saliva passing up the retrograde cannula filled the chamber and passed down the anterogradecannula into the mouth. The device was sutured into position onto the skin behind the animal'sear, using perforations on a flange around its base. The device was 12-4 mm long, 13-0 mm wideand weighed 2-8 g.

21


Some of the rabbits were trained to accept a mash of pellets made with water in order to testthe effect of the consistency of the food and therefore the possible relation between intra-oralmechanoreceptor input and salivary flow. A comparison was then made between salivary flow ratesrecorded with the pellet mash and with dry pellets.

In earlier experiments salivary flow was determined using electrical contact drop recordersconnected to a pen recorder. Values were expressed as drops/min. Since drop size is highly flowdependent, the possibility that there might be a significant error in the measurement of flow usingdrops/min was overcome by using an instantaneous flow recorder in later experiments. This wasbased on the apparatus described by Burgen (1964) but using a differential micromanometer(± 10 mmH2O, Mercury Electronics Ltd.). The apparatus was calibrated with an infusion pump(Harvard) over a range of flow rates which encompassed the rates encountered. With this apparatus,used in conjunction with strain gauges attached to the bone of the ascending ramus (see below fordetails), it was possible to determine the latency between the onset of chewing and the salivaryresponse and to examine the relation between mandibular strain (and presumably thereforemasticatory force) and salivary flow. This combination of data also complemented the videotapeevidence for the relation between chewing side and the dominant secretary side, since themandibular strain is greater on the chewing side than on the other (Weijs & de Jongh, 1977).

In four animals with bilateral parotid cannulae, indelible ink marks were made on the forehead,snout and chin in the mid line. The animals had been trained as previously described and recordingsof chewing movements were made simultaneously with salivary flow recordings, with the videocamera in front of the head and slightly below the horizontal. Because rabbits chew very rapidly(up to 5 Hz) it was necessary to replay the videotape at slow speeds to establish the movementpatterns. The video recorder was equipped with a timing mechanism with which it was possibleto determine the time relation between the movements seen on the video recording and the salivaryresponse displayed on the pen recorder.The type of strain gauges used were either Delta rosette (Micro-Measurements EA-09-015YD-120

option S.E.) or single (Kyowa, KFD-I-0 11) with an over-all size of 5 x 4 mm and a gauge lengthof 1 mm and 1-5 mm respectively. The method for attaching them to the mandible was essentiallythe same as described by Weijs & de Jongh (1977). In order to avoid damage to the parotid ductand the facial nerve, these structures were located before the strain gauge was attached. The straingauge was attached to the ascending ramus of the mandible anterior to the insertion of the massetermuscle and the leads (Teflon-coated, 025 mm diameter) were brought out behind the ear. The straingauge output was amplified using an a.c. bridge amplifier.The strain gauge data were intended for within-animal comparison to confirm the chewing side,

determine the chewing frequency, relate the onset of chewing to the commencement of salivationand confirm previous reports of differences in strain with foods of different consistency and relatethese differences to salivary flow. No attempt was made to obtain absolute values of strain.

All salivary flow and strain gauge data were recorded simultaneously on a pen recorder and alsoon magnetic tape for further analysis.The statistical significance of differences between data was calculated using Student's t test for

independent means. A P value of less than 005 was considered to be statistically significant.

RESULTS

The mean (±S.D.) output of saliva with pellets (18-86+0-88 drops/min, n = 397)was significantly greater than that with carrots (3-20+0-29 drops/min, n = 191.P<0 001).

In the nine animals in which only one duct was cannulated, there were frequentlyperiods of several minutes during each recording session when little or no saliva wasproduced even with pellets. This is demonstrated by the output of the left parotidgland in Fig. 2, where the output during pellet chewing is substantially higher thanduring carrot chewing in the early part of the record. Later in that recording thereis a period of more than 3 min during which the output of the gland fell substantiallyeven while pellets were consumed, but increased soon afterwards. It seems unlikely


that this was due to fatigue of the gland, because after the pause in secretion the glandsecreted at rates comparable with those before the flow had ceased. There is a remotepossibility that the cannulae became blocked intermittently but this could hardlyhave occurred in all nine animals. However, a satisfactory explanation for thereduction in flow from one gland during chewing follows the observations in the nextseries of experiments.

DropsIaro Pe Ill lllllllllllllllllllllllllllllellllllll 11111 I L1

Carrot Pellets

1 minFig. 2. Continuous record showing the output of parotid saliva (each spike representing

1 drop) during a unilateral recording in response to eating pellets and carrots.

In the ten rabbits with both parotid ducts cannulated, it appeared that secretionalternated in an apparently random manner, with one gland invariably assuming adominant role. This was seen in all the recordings with pellets and in seven of theten recordings with carrots. Fig. 3 shows the results of four recordings from oneanimal with bilateral cannulation, and is typical of all animals tested, clearly showingthe quantitative differences in the response to pellets and carrot and the pattern ofalternating secretary activity.

Replaying the video recordings at a slow speed showed that clockwise rotation ofthe mandible relative to the rest of the head occurs with chewing on the right andanticlockwise with chewing on the left; as shown by Weijs & de Jongh (1977) withcine-photography. We were also able, by matching the strain gauge records with thevideo recordings, to confirm their finding that the mandibular strain was greater onthe chewing than on the non-chewing side. The simultaneous videotape and salivaryflow recordings established that the dominant secretary side was also the chewingside. Therefore, taking videotape, strain gauge and salivary data together, there isa strong suggestion that mandibular strain and salivary flow are directly related.

Fig. 4 presents two histograms of the data from eleven rabbits in this study,demonstrating the wide range of flows associated with alternating chewing. The flowswith pellets (Fig. 4A) were 10-57 drops/min on the chewing side and 0-10 drops/minon the opposite side. With carrots (Fig. 4B) however, the flows on the chewing siderange between 0-35 drops/min and 0-8 drops/min on the opposite side.

23


In the series of experiments with anterograde and retrograde cannulation of theparotid duct, injection of water (1 ml/min) into the mouth during pellet chewing wasaccompanied by a significant reduction in flow (see Table 1). This effect was seen onboth sides. However, the rates of flow with pellets and injection of water were alwaysgreater than those recorded with carrot alone. In six rabbits, injection of water(0 5 ml/min) had no significant effect on the rates of flow with. carrot. Injection ofwater, unaccompanied by chewing, produced no measurable salivary response.

60 R

F]

40~~~~~~~~~~~~~~~~. 77: ..!. 1]40

301 r

20 41

1 0

0 0 L

.> 40

30

20I

1 0~~~~~~~~~~~~1

1 minFig. 3. The output of parotid saliva in drops/min from the left (L) and right (R) glandsfrom four recording sessions in the same animal, during eating ofpellets (stippled columns)and carrot (unstippled columns). Each bar represents a 1 min chewing period.

The findings with pellet mash compared with dry pellets showed that substantiallyless flow occurred with the mash than with the dry pellets, and in the animals in whichstrain gauges were used less strain was recorded with the mash than with dry pellets.During periods of continuous pellet mash or dry pellet chewing, the mean (± S.D.)flows (ml/min using the instantaneous flow recorders) were calculated over 20 s

epochs of mash or dry pellet chewing. The findings with pellet mash compared withdry pellets showed significantly less flow (P < 0-001) with the mash(0-19+0-07 ml/min, n = 16) than with dry pellets (0-31 +0-07 ml/min, n = 17).Fig. 5 is a continuous record showing the output from a strain gauge attached to theright side of the mandible, and the ipsilateral parotid flow recording. In addition tothe fact that with mash the flow was reduced compared with dry pellets, the straingauge output with mash was reduced by up to 50% (mean 30 %) compared with drypellets. There was a significant positive correlation between the mean flows and themean strain gauge outputs during the same 20 s epochs (r = + 0 49, P < 0 01, n = 33)when mash and dry pellets were eaten.


A F req. 7 l Contralateral to the120 L J chewing side

U --- Ipsilateral to the100 - chewing side

o n 397

80-a)

0

4- 60 - _0

E 40-

20

0-4 10-14 20-24 35-39 45-49 30-345-9 15-19 25-29 40-44 50-54 55-59

Drops/m inD Contralateral to the

8 60 - [ chewing side

1 Ipsilateral to the50 - L chewing side

n= 1910

4040

o 30-0

t 20

10

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 31 32Drops/min

Fig. 4. Two histograms of data from eleven rabbits demonstrating the wide range of flowsassociated with alternating chewing during (A) pellet and (B) carrot chewing.

In order to relate the onset of salivation to the commencement of chewing, fiverabbits were trained to accept one pellet at a time. During the recording sessions theanimal was given time to finish the pellet and stop chewing. The flows were allowedto return to resting levels before the animal was offered another pellet. In this waythe onset and termination of discrete episodes of chewing activity could be recorded.The mean latency between the first chewing stroke and an increase in flow was0-85+0-18 s (±S.D., n = 154).

Fig. 6A-C shows one such episode, and clearly illustrates the close relation betweenchewing activity and ipsilateral parotid flow. Fig. 6D demonstrates the remarkablyclose association between the strain gauge record and flow when the flow record wasinverted, shifted (to allow for the latency) and superimposed on the strain gaugerecord.

25


TABLE 1. The effect of injecting water (1 ml/min) into the mouth on flow rates (drops/min) recordedduring pellet eating in four rabbits (*P < 005; **P < 0001)

Ipsilateral Contralateral

Rabbit Pellets Pellets + water P Pellets Pellets + water P

1 494+50 34-7+12-8 * 3-7+3-5 2-3+3-1 *n= 18 n= 18 n= 18 n= 18

2 41-9+9-0 33-0+6-9 * 10-9+139 0+0 **n= 19 n= 19 n= 19 n= 19

3 22-7+5-8 14-3+68 ** 63+31 29+2-1 **n = 23 n =23 n =23 n = 23

4 33-9+6-3 21-3+6-1 ** 49+40 1-9+22 **n= 18 n=18 n= 18 n= 18

Chewing frequency in rabbits varies between 2 and 5 Hz. The mean frequencieswere calculated by counting the number ofpeaks in the strain gauge record over every10 s period of continuous chewing and the results were 3-42 + 049 Hz (n = 220);3-45+ 0 50 Hz (n = 88); 4-25+ 0-27 Hz (n = 88) (mean+ S.D.) for dry pellets, carrotsand pellet mash respectively. The mean chewing frequency with the pellet mash wassignificantly greater (P < 0 001) than with dry pellets or carrots.During the course of this study it became clear that high salivary flows were

maintained for as long as the animal continued to chew and frequently 10 min ormore of continuous chewing would pass with little or no change in the secretion rate.

C to

Right parotid flow

Right mandibular strain

Hard pellets Soft pellets Hard pellets

1._c10 _E 0.5

Hard pellets Soft pellets 15 s

Fig. 5. A continuous record showing the output from a strain gauge attached to the rightside of the mandible and the ipsilateral parotid flow recording during eating of both hardpellet and soft pellet (mash).

PAROTID SECRETION IN THE RABBIT 27

Times) -

A

E 0*75E 0*5

1-- 0-25.2 E 0

B

!.C 050

C

R ightstrain gauge

D

Fig. 6. Record showing: A, right parotid flow; B, left parotid flow; C, the output froma strain gauge attached to the right side of the mandible, during the chewing of a singlehard pellet on the right side. D, demonstrates the remarkably close association betweenthe strain gauge record and ipsilateral flow when the flow record is inverted, shifted (toallow for the latency) and superimposed onto the strain gauge record.

DISCUSSION

These experiments have shown that parotid secretion in the rabbit alternates fromside to side and that the alternating pattern coincides with the changing of chewingsides. This is in accord with the observations by Colin (1854) in the horse and mule;Patterson, Brightfing & Titchen (1982) in the sheep; and Lashley (1916) & Kerr (1961)in man. The output of saliva was substantially greater with dry pellets than withcarrots, as shown by Gj6rstrup (1980a). However, although the distributions of floware similar in both this study and that of Gj6rstrup (see Fig. 1, Gjdrstrup 1980a) thewider ranges in Gj6rstrup's study can be accounted for by the fact that in hisunilateral collections of saliva, the effect of alternating chewing on secretion wasmasked. Our method ofcontinuous recording, designed to detect short-term variationsin the flow, often revealed zero flow on the non-chewing side. Presumably chewingcontralateral to the cannulated gland accompanied by zero flow in that gland never


occurred throughout the entire 2-3 min periods of unilateral collections of saliva inGjorstrup's experiments. Separating ipsi- and contralateral flows in our data, showsa narrower range for both pellets and carrots than for the pooled data. It will be ofinterest to discover whether bilateral collection of parotid saliva reveals a linkbetween amylase content and the chewing side.

Gjorstrup (1980a) injected sapid substances into the mouth and concluded thattaste is not an important stimulus to secretion of fluid by the parotid in the rabbit,and in addition that unilateral section of the lingual and glossopharyngeal nerves didnot reduce the salivary response during chewing. We did not examine gustatorystimuli. However, it is likely that the gustatory stimulus of pellets is increased duringdownstream injection of water and while the animal is chewing a pellet mash, andin both circumstances salivary flow was reduced.The major contribution of the strain gauge data is to establish a clear but indirect

relation between stimulus amplitude and duration and the parotid salivary response.The use of strain gauges made possible the first recording in conscious animals of thelatency between the commencement of chewing and the onset of salivary flow butdid not allow us to determine the stimulus threshold for the salivary response.However, given the narrow range of chewing frequencies observed with pellets, pelletmash and carrots, these data provide the most direct evidence for a relation betweenchewing force and parotid flow. A previous attempt using the rectified and integratedmasseter electromyogram as an indirect measure of chewing force failed to establishsuch a relation (Anderson & Robinson 1981).

It is possible that the greater flow with pellets and carrots on the chewing side thanon the non-chewing side could be due to mechanical squeezing of the duct and glandbecause of the close association of the gland with masseter muscle and ascendingramus of the mandible, and the fact that the duct passes through the buccinatormuscle before opening into the mouth. Evidence obtained in this study does notsupport this suggestion, although it is possible to detect very small oscillations, justwithin the resolving power of the flow meter, synchronous with chewing movements.However, it is not possible to account for the sustained increase in flow (up to1 ml/min), which can last for 10 min or more during continuous chewing, bymechanical expulsion of pre-formed saliva. The fact that the duct is cannulated withPolythene tubing which lies over the jaw muscles insulates it against externalmechanical effects. Although the jaw muscles on the non-chewing side are notexerting as much force as on the chewing side, they are active nevertheless instabilizing the jaw, yet zero flow often occurs on the non-chewing side. In man, ithas been shown that during repeated clenching of the jaws with maximum effort, butwith nothing between the teeth, little or no parotid saliva is produced (Anderson &Hector, 1982).

Pellets are obviously drier and harder than carrots and it seemed likely that thehigher flows in response to pellets would be the result of these differences. Drynessof the mouth caused by diversion of saliva from the mouth for recording purposeswould be expected to augment this effect. In the event, the data with water injectionand with pellet mash supported these expectations, but did not allow us to decideon the relative importance of the hardness of the food and the dryness of the mouth.

In conclusion, the results obtained with the different techniques of this study


clearly point to a direct relation between chewing force (and presumably intra-oralmechanoreceptor input) and parotid flow, and therefore the existence of a mastic-atory-salivary reflex, since: (1) flow and mandibular strain were greater with pelletsthan with carrot, and were greater on the chewing than on the non-chewing side withboth types of food, (2) flow and mandibular strain were both greater with dry pelletsthan with pellet mash, (3) injections of water along the anterograde parotid cannulainto the mouth reduced the salivary response to pellets but not to carrots, and(4) instantaneous flow recordings superimposed on recordings of mandibular strainshowed a remarkable similarity of pattern.

This work was supported by the Medical Research Council. One of us (D.J. A.) wishes toacknowledge the assistance of Miss Nancy Sirett and the facilities provided in the Department ofPhysiology, University of Otago, Dunedin, New Zealand, during the early stages of this study.

REFERENCES

ANDERSON, D. J. & HECTOR, M. P. (1982). The effect of anaesthesia of various intra-oral afferentson reflex parotid secretion in man. Journal of Physiology 336, 15-16P.

ANDERSON, D. J., HECTOR, M. P. & ROGERS, I. (1982). The possible effect ofdiverting parotid salivafrom the mouth on the recorded flow rates in rabbits chewing laboratory pellets. Journal ofPhysiology 327, 10-1 1P.

ANDERSON, D. J. & ROBINSON, P. P. (1981). A masticatory-salivary reflex? Journal of DentalResearch 60 (B), 1111.

ANDERSON, D. J., ROGERS, I. & SIRETT, N. (1981). Alternating parotid secretion in feeding rabbits.Journal of Physiology 320, 61-62P.

BURGEN, A. S. V. (1964). Techniques for stimulating the auriculo-temporal nerve and recordingthe flow of saliva. In Salivary Glands and their Secretions, ed. SREEBNY, L. M. & MEYER, J.,pp. 303-307. Oxford: Pergamon.

COLIN, G. (1854). Traite de Physiologie Comparge des Animaux Domestique, vol. 1. Paris: Bailliere.GJORSTRUP, P. (1980a). Parotid secretion of fluid and amylase in rabbits during feeding. Journal

of Physiology 309, 101-1 16.GJ6RSTRUP, P. (1980b). The secretion of amylase caused by sympathetic nerves in the parotid gland

of the rabbit and its dependence on a parasympathetic activity. Ph.D. Thesis, Lund, Sweden.KERR, A. C. (1961). The Physiological Regulation of Salivary Secretions in Man. Oxford:Pergamon.

LASHLEY, K. S. (1916). Reflex secretion of the human parotid gland. Journal of ExperimentalPsychology 1, 461-493.

PATTERSON, J., BRIGHTLING, P. & TITCHEN, D. A. (1982). Beta-adrenergic effects on compositionof parotid salivary secretion of sheep on feeding. Quarterly Journal of Experimental Physiology67, 57-67.

WEIJS, W. A. & DE JONGH, H. J. (1977). Strain in mandibular alveolar bone during masticationin the rabbit. Archives of Oral Biology 22, 667-675.

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