SALIVATION. 221 



tion the glandular culs-de-sac decrease somewhat in size, and 

 that the mucous (salivary) cells empty their contents grad~ 

 ually without being destroyed. " In short," he says, " the 

 product secreted by the glands nrises from their cells, in order 

 to form which the glandular cells simply yield up the sub- 

 stance which has been elaborated within them, and are not 

 entirely destroyed, as Heidenhain has affirmed. Their active 

 part (nucleus and protoplasm) still remains, and it is this, 

 probably, which makes up for the waste occasioned by secre- 

 tion. 1 



Some agents may cause secretion of the saliva by exciting 

 metamorphoses of the epithelium of the gland, as they excite 

 those of the epithelium of the mouth in general : it is in thia 

 way that mercurial salivation is produced. 



The excretory tubes of the salivary glands appear to be 

 deficient in muscular elements : the saliva flows, not by a 

 movement similar to the peristaltic movement, but by a sort 

 of vis a tergo of the fluid, which, first filling the lower part 

 of the salivary tubes, rises gradually, and at length over- 

 flows. 



The nerve centre of the salivary secretion is found, as we 

 have said, in the spinal cord ; under certain circumstances 

 the intervention of other nerve centres must be admitted : 

 the encephalon, as the organ of the imagination, has great 

 effect on the secretion, and the sight or mere remembrance 

 of food will suffice to increase the effect. Still, properly 

 speaking, the will has no power to produce this secretion : 

 the imagination must call up the memory of a gustatory 

 impression, or produce movements in the mouth which are 

 capable of producing secretion by means of reflex mechan- 

 ism. Under different circumstances, the encephalon appears, 

 on the contrary, to act against secretion by paralyzing the 

 excitatory nerves. Thus, certain emotions of the mind will 

 hinder the secretion of the saliva, while others increase it. 

 Strong emotions produce this effect ; which is shown by ex- 

 cessive dryness of the mouth, and, sometimes, almost entire 

 inability to speak. 



The secretion of the saliva is also more or less under the 

 mechanical influence of the neighboring organs: thus the 

 movements of the jaw, and contraction of the muscles of the 



1 Ranvier, " Notes a la Traduction Franraise de 1'Histologie de 

 Frey," p. 439. 



