712 PHYSIOLOGY 



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 submaxillary 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-dilalation. It was shown by Claude Bernard that on exciting 

 the chorda tympani the flow from the vein of the submaxillary gland 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 submaxillary 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 submaxillary saliva obtained on 

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

 the saliva obtained from the submaxillary 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 

 carries the vaso-constrictor fibres to most or all of the vessels of the head and 



