146 MANUAL OF PHYSIOLOGY. 



made out. Possibly the removal of some trophic nerve influences 

 induces abnormal nutritive changes which cause stimulation of 

 the cells, and ultimately lead to their degeneration. 



The histological investigation of the elements of these glands 

 in the various stages of secretion throws considerable light on the 

 behavior of the cells during their periods of activity and rest. 



It is now certain that the different stages are accompanied by 

 constant structural changes in the cells, which, doubtless, are inti- 

 mately connected with secretory activity. During the period of 

 rest, that is, the time when the gland is not discharging its secre- 

 tion, the cells slowly undergo a change in their appearance, which 

 is obvious in proportion to the ease with which the material they 

 secrete is recognized in the protoplasm of a cell. Thus, in mucous 

 glands, or in mucus-yielding salivary glands, the changes are 

 conspicuous ; whilst in those which give a watery secretion they 

 are less easily seen. 



As an example, we may take a mucous gland, such as the orbital 

 gland of the dog, and follow the changes which occur in one of 

 its cells, beginning at the end of its cycle of activity. (Fig. 66.) 



Immediately after the prolonged and active discharge of the 

 secretion of the gland, the cells have all the characters of ordinary 

 protoplasmic units, and the distinction between the polygonal cells 

 and those next the wall of the acinus (demi-lune cells) is made 

 out with great difficulty, because all the cells stain evenly with 

 carmine, and have no special characters except those belonging to 

 active protoplasm. 



During rest certain changes gradually appear in those gland 

 cells which are next the lumen of the saccule. They appear to 

 swell toward the lumen, and at the same time become clear and 

 resist staining with carmine, their protoplasm becoming impreg- 

 nated with mucous-like material (mucigen), while the demi-lune 

 cells remain protoplasmic and stain easily, and are thereby readily 

 distinguished from the cell in the cavity of the saccule. 



Now, if the discharge of secretion be called for, either by 

 normal reflex excitation, or by direct stimulus of the chorda 

 tympani nerve, the cells discharge the contained specific material, 

 some of them most probably being destroyed by the act. If the 



