412 SECRETION. 



only occasional, such as the testicle, are usually retained 

 in the ducts during the periods of the gland's inaction. 

 And there are glands which are like both these classes, 

 such as the lachrymal and salivary, which constantly 

 secrete small portions of fluid, and on occasions of greater 

 excitement discharge it more abundantly. 



When discharged into the ducts, the further course of 

 secretions is effected partly by the pressure from behind ; 

 the fresh quantities of secretion propelling those that were 

 formed before. In the larger ducts, its propulsion is 

 assisted by the contraction of their walls. All the larger 

 ducts, such as the ureter and common bile-duct, possess in 

 their coats organic muscular fibres; they contract when 

 irritated, and sometimes manifest peristaltic movements. 

 Bernard and Brown- S^quard, indeed, have observed rhyth- 

 mic contractions in the pancreatic and bile ducts, and 

 also in the ureters and vasa deferentia. It is probable 

 that the contractile power extends along the ducts to a 

 considerable distance within the substance of the glands 

 whose secretions can be rapidly expelled. Saliva and 

 milk, for instance, are sometimes ejected with much force ; 

 doubtless by the energetic and simultaneous contraction of 

 many of the ducts of their respective glands. The contrac- 

 tion of the ducts can only expel the fluid they contain 

 through their main trunk ; for at their opposite ends all 

 the ducts are closed. 



Circumstances influencing Secretion. The influence of 

 external conditions on the functions of glands, is mani- 

 fested chiefly in alterations of the quantity of secretion; 

 and among the principal of these conditions are variations 

 in the quantity of blood, in the quantity of the peculiar 

 materials for any secretion that it may contain, and in the 

 conditions of the nerves of the glands. 



In general, an increase in the quantity of blood traversing 

 a gland, coincides with an augmentation of its secretion. 

 Thus, the mucous membrane of the stomach becomes florid 



