320 SECRETION IN GENERAL 



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 the 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 contrac- 

 tile power extends along the ducts to a considerable distance within the sub- 

 stance 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 in- 

 crease of blood supply, and so it may be inferred that this condition of in- 

 creased 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 submaxillary gland, upon 

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

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

 excited by causes acting directly upon the nervous centers, upon the nerves 



