Digestion. 107 



separates the manufactured products. Building up, anabo- 

 lism, and breaking down, katabolism, are occurring constantly 

 in all the cells of the body. The cranial nerves are chiefly 

 secretory, whilst the sympathetic are trophic, hence stimula- 

 tion of the chorda gives a watery saliva poor in solids, whilst 

 stimulation of the sympathetic gives a scanty saliva rich in 

 solids. 



Langley considers that this view is not tenable, and that 

 it is more reasonable to believe that there is only one kind 

 of fibre engaged in secretion, which, being mixed with 

 nerves having opposite actions on the bloodvessels, pro- 

 duces the difference in the results observed. 



During secretion the temperature of the gland rises 27 F., 

 and the blood in the veins is warmer than the blood in the 

 arteries. 



The method by which secretion in the parotid gland is 

 carried out differs in no essential respect from that of the 

 submaxillary. The nerves supplying the parotid are the 

 glosso-pharyngeal (the action of which corresponds to the 

 chorda of the submaxillary) and the sympathetic ; both of 

 these contain secretory fibres and dilator and constrictor 

 nerves for the bloodvessels. 



The changes occurring in the cells of the salivary glands 

 during secretion depend upon the nature of the gland. We 

 will therefore describe separately, from Langley's observa- 

 tions, the changes in the cells of a serous gland, such as the 

 parotid, and the changes in a mucous gland, of which the 

 submaxillary is a type. In both we distinguish certain 

 differences depending upon whether the gland is at rest 

 or whether it is active, viz., whether the cells are charged 

 or whether they are rapidly getting rid of the material 

 which has been formed. In connection with this point, 

 we must remember that differences in the cells may be 

 observed which are more accidental than real, and depend 

 upon the methods which have been employed in demon- 

 strating them. Discordant results are not, however, obtained 

 when by suitable means the gland-cells in the living animal 

 are examined during rest and activity. 



