DIGESTION. 



163 



As soon as secretion becomes active, however, the granules begin to 

 disappear from the outer region of the cell and move toward the inner 

 border and into the lumen of the acinus. From these observations 

 it might be inferred that during rest the protoplasm of the cells gives 

 rise to granular material, and that during and after secretory activity 



FIG. 65. CELLS OF THE ALVEOLI or A SEROUS OR WATERY SALIVARY GLAND. A. 

 After rest. B. After a short period of activity. C. After a prolonged period of 

 activity. (Yeo's " Text-Book of Physiology"} 



there is an absorption of new material from the lymph and a recon- 

 struction of the granular material. 



In the submaxillary gland, a portion of which may be taken 

 as a type of aTmucous gland, similar changes have been observed 

 (Fig. 66). During rest the epithelial cells are large, clear in appear- 

 ance, highly refractive, and loaded with small globules resembling 

 mucin. The nucleus, surrounded by a small quantity of proto- 

 plasm, lies near the margin of 

 the cell. That the granules 

 are not protoplasmic in char- 

 acter is shown by the fact 

 that they do not stain on the 

 addition of carmine. When 

 treated with water or dilute 

 acids, the globules swell up, 

 coalesce, and form a uniform 

 mass. The chemic relations 

 of this substance indicate that 

 it is the precursor of mucin 

 namely, mucigen. During 

 secretory activity the cells' dis- 

 charge these mucigen granules into the lumen of the acinus, where 

 they are transformed into mucin. Though the appearance of the 

 gland-cell appears to indicate it, there is no evidence for the view 

 that the cell itself undergoes disintegration in the process. 



The Physiologic Actions of Saliva. The constant presence of 

 salivary glands in the different classes of animals and the large amount 



FIG. 66. THE APPEARANCE PRESENTED BY 

 THE CELLS or THE SUBMAXILLARY 

 GLAND OF THE DOG AFTER PROLONGED 

 SECRETION. 



