SECRETION". 305 



gland or its ducts. The former is the case with the sweat glands. But 

 the secretions of those glands whose activity of function is only occa- 

 sional are usually retained in the cells in an undeveloped form during 

 the periods of the gland's inaction. And there are glands which are 

 like both these classes, such as the lachrymal, 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 

 affected (1) partly by the pressure from behind; the fresh quantities of 

 secretion propelling those that were formed before. In the larger ducts, 

 its propulsion is (2) assisted by the contraction of their walls. All the 

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

 coats plain muscular fibres; they contract when irritated, and sometimes 

 manifest peristaltic movements. Ehythmic contractions in the pancreatic 

 and bile-ducts have been observed, 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. 



Circumstances Influencing Secretion. The principal conditions which 

 influence secretion are (1) variations in the quantity of blood, (2) varia- 

 tions in the quantity of the peculiar materials for any secretion that the 

 blood may contain, and (3) variations in the condition of the nerves of 

 the glands. 



(1.) An increase in the quantity of blood traversing a gland, as in 

 nearly all the instances before quoted, coincides generally with an aug- 

 mentation of its secretion. Thus the mucous membrane of the stomach 

 becomes florid when, on the introduction of food, its glands begin to 

 secrete ; the mammary gland becomes much more vascular during lacta- 

 tion ; and all circumstances which give rise to an increase in the quan- 

 tity of material secreted by an organ produce, coincidently, an increased 

 supply of blood ; but we have seen that a discharge of saliva may occur 

 under extraordinary circumstances, without increase of blood-supply, 

 and so it may be inferred that this condition of increased blood-supply 

 is not absolutely essential. 



(2.) An increase in the amount of the materials which the glands are 

 designed to separate or elaborate, contained in the blood supplied to them, 

 increases the amount of any secretion. Thus, when an excess of nitro- 

 genous waste is in the blood, from destruction of one kidney or whatever 

 cause, a healthy kidney will excrete more urea than it did before. 



(3.) Influence of the Nervous System on Secretion. The process of 

 secretion is largely influenced by the condition of the nervous system. 

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