910 



THE EXTERNAL SECRETIONS 



ogen. They remain inactive until discharged into the ducts, when 

 they are immediately converted into the active enzyme ptyalin. 



Histological Changes During Activity. — A most interesting feature 

 of the activity of glands is that they undergo certain very characteris- 

 tic changes in their structure. While this is true of the lacrimal, 

 gastric and pancreatic glands, none exhibit them in a more striking 

 manner than the salivary glands. They were first studied by Heiden- 

 hain^ in fixed and stained preparations of the parotid and submaxillary 

 glands of the rabbit, but have also been observed by Langley ^ and others 

 in fresh preparations. When resting these cells are large and faintly 

 outlined against one another by delicate cell walls. Their cytoplasm 

 is evenly packed with granules which stain deeply with the ordinary 

 dyes. Near the basement membrane are found their somewhat 

 irrgular and dark nuclei. Contrary to this picture, a cell which has 

 been made to secrete for a considerable length of time, is smaller, more 



Fig. 490. — Acini of the Submaxillary Gland During Rest (R) and Activity (A). 

 The Dark Outer Cells Represent the Demilune Cells. 



translucent, and contains a rounded nucleus which occupies a position 

 near its center. Many of these cells, in fact, appear merely as a nar- 

 row frame of cytoplasm, investing a very prominent rounded nucleus. 

 The granular material has disappeared from the entire inner part of the 

 cell and is now arranged in the form of a narrow zone along its margin 

 next to the duct. 



These changes clearly prove that these cells lose a certain part of 

 their substance in the course of their activity; in fact, Heidenhain 

 states that many of them disappear altogether, but reform their con- 

 tents during the subsequent period of rest. Of special interest is 

 the fact that the active cell gradually discharges its zymogen granules 

 which, as has been mentioned above, give rise to the enzyme ptyaUn. 

 Upon the cessation of this stimulation, the stage of dissimilation is 

 followed immediately by a stage of assimilation during which the 

 material lost is again replenished. A clear non-granular material 

 is then seen to invade the basal segment of the cell which is gradually 



1 Noll, Ergebn. der Physiol., iv, 1905. 



2 Jour, of Physiol., x, 1889, 433. 



