326 HAND-BOOK OF PHYSIOLOGY. 



the testicles; granules of uric acid in those of the kidneys (of fish); fatty 

 particles, like those of milk, in the cells of the mammary gland. 



Secreting cells, like the cells or other elements of any other organ, 

 appear to develop, grow, and attain their individual perfection by appro- 

 priating nutriment from the fluid exuded by adjacent blood-vessels and 

 elaborating it, so that it shall form part of their own substance. In this 

 perfected state, the cells subsist for some brief time, and when that 

 period is over they appear to dissolve, wholly or in part, and yield their 

 contents to the peculiar material of the secretion. And this appears to be 

 the case in every part of the gland that contains the appropriate gland- 

 cells; therefore not in the extremities of the ducts or in the acini alone, 

 but in great part of their length. 



We have described elsewhere the changes which have been noticed 

 from actual experiment in the cells of the salivary glands, pancreas, and 

 peptic gland (pp. 235, 259, 265). 



Discharge of Secretions from glands may either take place as soon 

 as they are 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 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 partly by the pressure from behind; the fresh quantities of secre- 

 tion 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 

 plain muscular fibres; they contract when irritated, and sometimes mani- 

 fest 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 con- 

 siderable 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. 



Circumstances Influencing Secretion. Amongst the principal 

 conditions which influence secretion are (1) variations in the quantity of 

 blood, (2) in the quantity of the peculiar materials for any secretion that 

 it may contain, and (3) in conditions of the nerves of the glands. 



(1.) An increase in the quantity of Hood 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 



