

DIGESTION IN THE MOUTH 



667 



to that which would be obtained from the gland under normal conditions if 

 it were provided with a permanent fistula. The concentration of the saliva 

 as well as its rate of secretion depends on the strength of the stimulus. The 

 following Table (Heidenhain) shows the effect on the amount and composition 

 of sub- maxillary saliva obtained by weak and strong stimulation of the 

 chorda tympani nerve : 



With the strong stimulus the amount of saliva was increased over four- 

 fold, while the percentage of organic substances in the saliva was raised from 

 0-84 to 2-06 per cent. There was at the same time an increase in the per- 

 centage of salts. If the excitation be continued for a considerable time, 

 there is a gradual rise in the percentage of inorganic salts and a fall in the 

 percentage of organic matter. 



The cranial nerves going to these glands have another important effect, 

 namely, vaso- dilatation. It was shown by Claude Bernard that if the 

 outflow from the vein of the sub-maxillary gland were measured, on exciting 

 the chorda tympani the flow might increase four to eight times, and indeed 

 to such an extent that the blood passing through the gland did not stay there 

 long enough to lose its oxygen. Moreover the dilatation of the arterioles 

 removes the normal resistance which serves to damp and obliterate the pulse 

 between the arteries and the veins. As the result of exciting the chorda 

 therefore the blood coming from the vein may show distinct pulsation, and 

 may have a brilliant scarlet hue just as if it were derived from an artery. 

 The same dilatation has been observed to attend excitation of the cranial 

 supply to the parotid gland. 



The effects of exciting the sympathetic nerve-supply differ according to 

 the gland and the animal which is the subject of experiment. In the dog 

 excitation of the cervical sympathetic causes the secretion of a few drops 

 of thick viscid saliva from the sub-maxillary gland. In this animal no 

 secretion is obtained at all from the parotid gland on exciting the sympathetic, 

 but the influence of the excitation is shown by the occurrence of histological 

 changes in the gland-cells. In the cat the sub-maxillary saliva obtained on 

 sympathetic excitation may be as copious as and even more watery than 

 the saliva obtained from the sub-maxillary on stimulation of the chorda 

 tympani. We shall have later on to discuss how far these results are to be 

 ascribed to a fundamental difference in the point of attack of impulses 

 carried to the secreting cells by the two sets of nerve fibres, and how far to 

 the varying effects of the cranial and sympathetic nerves respectively on 

 the blood-vessels. It must be remembered that the sympathetic nerve 



