CIRCUMSTANCES INFLUENCING SECRETION 295 



yield their contents to the peculiar material of the secretion. The changes 

 which have been noticed from actual experiment in the cells of the salivary 

 glands, pancreas, and peptic glands will be described more fully in the chapter 

 on Digestion. 



Discharge of secretions from the glands may either take place as soon as 

 formed, or the secretion may be long retained within the gland or its ducts. 

 The former is the case with the sweat glands. But the secretions of those 

 glands whose activity of function is periodical are usually retained in the cells 

 in an undeveloped form during the period of the gland's inaction. 



When discharged into the ducts, the further course of secretions is affected: 



(1) partly by the pressure from behind; the fresh quantities of secretion pro- 

 pelling 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 fibers; 

 they contract when irritated, and sometimes manifest peristaltic movements. 

 Rhythmic 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 variations in the quantity of blood, and varia- 

 tions in nerve impulses passing to the gland cells over secretory nerve fibers. 



An increase in the quantity of blood traversing a gland, as in nearly all 

 the instances before quoted, coincides generally with an augmentation 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 lactation. All circumstances which give 

 rise to an increase in the quantity of material secreted by an organ produce, 

 coincidently, an increased supply of blood. But we shall see 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 to the immediate formation of secretion, 

 but that it favors the prolonged activity of glands. 



Influence of the Nervous System on Secretion. The process of 

 secretion is largely regulated through the nervous system. The exact mode 

 in which the influence is exhibited must still be regarded as somewhat obscure. 

 In part, it exerts its influence by increasing or diminishing the quantity of 

 blood supplied to the secreting gland, in virtue of the power which it exercises 

 over the contractility of the smaller blood-vessels. It also has a more direct 

 influence, as is described at length in the case of the ^ubmaxillary gland, upon 

 the secreting cells themselves. This may be called trophic influence. Its 

 influence over secretion, as \veil as over other functions of the body, may be 



