122 DIGESTION 



pressure, which in turn causes water to pass into tiie cell 

 from the blood. It is ditticult to understand how continued 

 secretion can thus be explained. Physical factors may play 

 a part in secretion, but they cannot cause it. The best 

 proof of this is that secretion is always accompanied by an 

 increase in the consumption of oxygen and in the production 

 of CO2. In the act of secretion, therefore, work is being 

 done by the cells of the gland. 



The sahvary glands are innervated by branches from the 

 cranial nerves and by the sympathetic. In the case of 

 the submaxillary gland stimulation of the chorda tympaui 

 causes a secretion accompanied by vaso-dilatation, stimu- 

 lation of the sympathetic, secretion accompanied by vaso- 

 constriction. In some animals the chorda secretion is 

 thin and copious, while the sympathetic secretion is thick 

 and scanty. The question therefore arises whether this 

 difference in the character and amount of the secretion is 

 due to differences in the nerve fibres or to the accompanying 

 differences in the state of the blood-vessels. It was believed 

 by Heidenhain that each nerve contains two kinds of 

 fibres, " trophic " fibres which cause secretion of water 

 and salts, and " secretory " fibres which cause secretion 

 of organic substances; in the chorda trophic fibres, and 

 in the sympathetic secretory fibres preponderate. The 

 following facts seem to support' this view. The presence 

 of meat in the mouth causes a secretion much richer in 

 organic constituents than dOes the presence of acid. The 

 difference is just as marked after removal of the superior 

 cervical gangha, indicating that different nerve-fibres in the 

 chorda tympani are called into play. 



Changes occurring in the Mouth 



We habitually speak of the sight and smell and even of 

 the idea of food making the mouth water. To what extent 

 is this idea justified? For the full answer to this question 

 we are indebted to the researches of the Kussian physio- 

 logist, Pavlov. Pavlov diverted the duct of the dog's sub- 



