THE PROCESS OF SECRETION 319 



formed of epithelium of various forms, supported by a basement membrane. 

 The larger tubes may have an outside coating of fibrous areolar or muscular 

 tissue. The salivary glands, pancreas, Brunner's glands, kidney, testes, with 

 the lachrymal and mammary glands, are examples of this type, but present 

 more or less marked variations among themselves. 



3 . The racemose glands, in which a number of vesicles or acini are arranged 

 in groups of lobules, C, figure 247. The Meibomian follicles are examples 

 of this kind of gland. There seem to be glands of mixed character, com- 

 bining some of the characters of the tubular with others of the racemose type; 

 these are called tubulo-racemose or lubulo-acinous glands. The acini are 

 formed by a kind of fusion of the walls of several vesicles, which thus combine 

 to form one large cavity with recesses lined or filled with secreting cells. The 

 smallest branches of the gland-ducts sometimes open into the centers of these 

 cavities; sometimes the acini are clustered round the extremities or by the 

 sides of the ducts; but, whatever secondary arrangement there may be, all 

 have the same essential character of rounded groups of vesicles containing 

 gland cells, and opening by a common central cavity into minute ducts, 

 which in the large glands converge and unite to form larger and larger 

 branches, and at length one common trunk which opens on a free surface. 



The Process of Secretion. The process of secretion is dependent 

 upon the activity of the secreting cells. In the case of the water and salts the 

 physical processes of filtration and diffusion may play a part. 



The chemical processes constitute the process of secretion properly so 

 called, as distinguished from mere transudation spoken of above. In the 

 process of secretion various materials which do not exist as such in the blood 

 are manufactured by the agency of the gland cells, using as a nutrient fluid 

 the blood or, to speak more accurately, the lymph which fills the interstices 

 of the gland textures. 



Evidences in favor of this view are: i. That gland cells are constituents 

 of all glands, however diverse their outer forms and other characters, and 

 they are placed in all glands on the surfaces or in the cavity whence the secre- 

 tion is poured. 2. That certain materials of secretions are visible with the 

 microscope in the gland cells before they are discharged. Thus, granules 

 probably representing the precursors of the ferments of the pancreas may 

 be discerned in the cells of that gland. Granules of uric acid are found in 

 the cells of the kidneys of birds and fish, and fatty particles, like those of milk, 

 in the cells of the mammary gland. 



Certain secreting cells, like the cells of the sebaceous glands, appear to 

 develop, grow, and attain their individual perfection by appropriating nutri- 

 ment from the fluid exuded by adjacent blood vessels and building it up so 

 that it shall form part of their own substance. In this perfected state the cells 

 subsist for some brief time and then appear to dissolve, wholly or in part, and 

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



