DIGESTION IN THE MOUTH 755 



there is no doubt that the characteristic effects of stimulating the 

 chorda tympani, namely, the profuse secretion of watery saliva, 

 is partly due to the simultaneous large supply of blood to the gland. 

 A profuse secretion is impossible unless there is a facility for the 

 gland to make up its losses in fluid at the expense of the surrounding 

 blood. If the blood- supply to the gland be diminished during stimu- 

 lation of the chorda tympani, as, e.g. by clamping the carotid artery 

 or by bleeding the animal, there is an actual diminution in the total 

 amount of secretion and a relative increase in the -proportion of 

 solids it contains. It is not easy, however, to imitate exactly in 

 this way the effects of excitation of the sympathetic. On the whole, 

 we may probably assume with Langley that the evidence in favour 

 of the existence of two kinds of secretory fibres is insufficient and that 

 the different results obtained on exciting the two sets of nerve fibres 

 is, largely at any rate, conditioned by the simultaneous changes pro- 

 duced by these nerves on the circulation through the gland. The 

 varying effects of atropine on the two classes certainly indicate a 

 difference in the mode of nerve-ending of the two sets of fibres, but 

 this conclusion does not necessarily involve the conclusion that the 

 influence of the sympathetic and of the chorda tympani respectively 

 on the gland-cells is also different. 



THE ENERGY INVOLVED IN THE ACT OF SECRETION 

 The source of the energy must be sought in the processes of oxida- 

 tion occurring in the cells of the gland, and Barcroft has attempted 

 to determine the total amount of energy put out by the gland in the 

 act of secretion by measuring its respiratory exchanges under the 

 conditions of rest and activity. He found that the resting sub- 

 maxillary gland in a small dog took up 0-25 c.c. of oxygen per minute 

 and put out 0-17 c.c. C0 2 , while during active secretion it absorbed 

 0-86 c.c. 2 and gave off 0-39 c.c. CO.,. Assuming that the total oxygen 

 taken up is employed in the oxidation of a food substance, such as 

 glucose, and that the whole of the energy of the chemical changes is 

 set free in the form of heat, we find that a resting gland weighing 

 about six grammes produces about 1-1 calories per minute. We know, 

 however, that a certain amount of external work is performed in the 

 secretion of a saliva containing less salts than the original blood, 

 and also, when there is any resistance to the flow of saliva through 

 the duct, in raising the hydrostatic pressure of the saliva in the duct 

 to a height greater than that in the blood capillaries. 



Can we from all these data form a conception of the total changes 

 occurring in the gland and involved in the formation of the secretion ? 

 Even during rest, changes are going on in the gland-cells, changes 

 which involve the taking up of food material and its assimilation 



