322 SECRETION. 



of the bloodvessels. But, in the production of the other se- 

 cretions, such as those of mucous membranes and all glands, 

 other besides these mechanical forces are in operation. Most 

 of the secretions are indeed liable to be modified by the cir- 

 cumstances which affect the simple exudation from the blood- 

 vessels, and the products of such exudations, when excessive, 

 are apt to be mixed with the more proper products of all the se- 

 creting organs. But the act of secretion in all glands is the 

 result of the vital processes of cells or nuclei, which, as they 

 develop themselves and grow, form in their interior the proper 

 materials of the secretion, and then discharge them. 



The best evidence for this view is : 1st. That cells and 

 nuclei are constituents of all glands, however diverse their 

 outer forms and other characters, and are in all glands placed 

 on the surface or in the cavity whence the secretion is poured. 

 2d. That many secretions which are visible with the micro- 

 scope may be seen in the cells of their glands before they are 

 discharged. Thus, bile may be often discerned by its yellow 

 tinge in the gland-cells of the liver ; spermatozoids in the cells 

 of the tubules of 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. 



The process of secretion might, therefore, be said to be 

 accomplished in, and by the life of, these gland-cells. They 

 appear, like the cells or other elements of any other organ, to 

 develop themselves, grow, and attain their individual perfec- 

 tion by appropriating the nutriment from the adjacent blood- 

 vessels and elaborating it into the materials of their walls and 

 the contents of their cavities. In this perfected state, they 

 subsist for some brief time, and when that period is over they 

 appear to dissolve or burst and yield themselves and their con- 

 tents to the peculiar material of the secretion. And this ap- 

 pears 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. 



In these things there is the closest resemblance between 

 secretion and nutrition ; for if the purpose which the secreting 

 glands are to serve in the economy be disregarded, their for- 

 mation might be considered as only the process of nutrition 

 of organs, whose size and other conditions are maintained in, 

 and by means of, the continual succession of cells developing 

 themselves and passing away. In other words, glands are 

 maintained by the development of the cells, and their con- 

 tinuance in the perfect state ; and the secretions are discharged 

 as the constituent gland-cells degenerate and are set free. 

 The processes of nutrition and secretion are similar, also, in 



