THE DIGESTIVE SYSTEM. 



299 



final stages of the secretory process, for rnucin itself is precipitated by 

 acetic acid. The material contained in the granules must be a precursor 

 of mucin, and it has therefore been called mucinogen. 



The cells of the serous type, treated in the same way, are more 

 cubical in shape, with a central nucleus, and in the resting state are 

 diffusely filled with much finer granules, which, as they are supposed to 

 consist of a precursor of ptyalin, have been called zymogenic. As in 

 the case of the mucous gland, the serous cell becomes smaller during 

 activity and the granules are diminished in number, especially towards 

 the base of the cell (fig. 118). In sections of the hardened gland which 

 have been stained, 'the cells even in the resting stage always appear to 

 contain a relatively greater amount of protoplasm than the mucous cells. 



In both mucous and serous cells the formation of granules is pre- 



FIG. 118. Alveoli of a serous gland. (Langley.) 

 Essentials of Histology. 



From Schiifer's 



A, at rest. B, after a short period of activity. C, after a prolonged period of activity. 

 In A and B the nuclei are obscured by the granules of zymogen. 



ceded by the appearance of filaments, basophile in character, which have 

 received the general name of ergastoplasm. 



(3) In the resting condition of a salivary gland an electrical current 

 can be detected by the capillary galvanometer, the direction of which in 

 the gland is from the acini towards the duct. This current, in the case 

 of the submaxillary gland, undergoes a diphasic variation on stimulation 

 of the chorda tympani, becoming first increased in intensity and then 

 reversed in direction. Stimulation of the sympathetic nerve is followed 

 by a negative variation only. 



DEGLUTITION. 



The bolus of food formed in the mouth is conveyed through the 

 pharynx and oesophagus into the stomach by the act of deglutition, 

 which originates as a voluntary process and is continued by nervous 

 reflexes. In the first or voluntary stage the jaws are closed and the 

 tongue is raised so as to press against the palate, the latter movement 

 being due to contraction of the mylohyoid aided by the intrinsic muscles 

 of the tongue itself. At the same time the base of trie tongue is drawn 



