324 SECBETION. 



fibres ; they contract when irritated, and sometimes manifest 

 peristaltic movements. Bernard and Brown-Sequard, indeed, 

 have observed rhythmic 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 in- 

 stance, are sometimes ejected with much force ; doubtless by 

 the energetic and simultaneous contraction of many of the 

 ducts of their respective glands. The contraction 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 exter- 

 nal conditions on the functions of glands, is manifested chiefly 

 in alterations of the quantity of secretion ; and among the prin- 

 cipal 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 when, on 

 the introduction of food, its glands begin to secrete ; the mam- 

 mary gland becomes much more vascular during lactation ; 

 and it appears that all circumstances which give rise to an in- 

 crease in the quantity of material secreted by an organ, pro- 

 duce, coincidently, an increased supply of blood. In most 

 cases, the increased supply of blood rather follows than pre- 

 cedes the increase of secretion ; as, in the nutritive processes, 

 the increased nutrition of a part just precedes and determines 

 the increased supply of blood ; but, as also in the nutritive 

 process, an increased supply of blood may have, for a conse- 

 quence, an increased secretion from the glands to which it is 

 sent. 



Glands also secrete with increased activity when the blood 

 contains more than usual of the materials they are designed to 

 separate. Thus, when an excess of urea is in the blood, whether 

 from excessive exercise, or from destruction of one kidney, a 

 healthy kidney will excrete more than it did before. It will, 

 at the same time, grow larger : an interesting fact, as proving 

 both that secretion and nutrition in glands are identical, and 

 that the presence of certain materials in the blood may lead to 

 the formation of structures in which they may be incorporated. 



The process of secretion is, also, largely influenced by the 

 condition of the nervous system. 



The exact mode in which the nervous system influences 



