304 HANDBOOK OF PHYSIOLOGY. 



all truly secreting structures. They agree in presenting a large extent 

 of secreting surface within a comparatively small space; in the circum- 

 stance that while one end of the gland-duct opens on a free surface, the 

 opposite end is always closed, having no direct communication with 

 blood-vessels, or any other canal; and in a uniform arrangement of 

 capillary blood-vessels, ramifying and forming a network around the 

 walls and in the interstices of the ducts and acini. 



Process of Secretion. It is generally conceded that the process of se- 

 cretion is dependent upon the vital activity of the secreting cells. It is 

 possible, however, in the case of the water and salts, that the physical 

 processes of filtration and dialysis 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 chemical process of secretion various materials which do not exist as 

 such in the blood are manufactured by the agency of the gland-cells from 

 the blood, or to speak more accurately, from the plasma which exudes 

 from the blood-vessels into the interstices of the gland-textures. 



The best evidence in favor of this view is : 1st. That cells and nuclei 

 are constituents of all glands, however diverse their outer forms and other 

 characters, and that they are in all glands placed on the surface or in 

 the cavity whence the secretion is poured. 2d. That certain materials 

 of secretions are visible with the microscope in the gland cells before 

 they are discharged. Thus, granules probably representing the fer- 

 ments of the pancreas may be discerned in the cells of that gland; 

 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. 



Secreting cells, like the cells of other organs, appear to develop, grow, 

 and attain their individual perfection by appropriating nutriment 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 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 ; there- 

 fore not in the extremities of the ducts or in the acini alone, but in great 

 part of their length. 



^Ve will describe elsewhere the changes which have been noticed from 

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

 glands. 



Discharge of secretions from glands may either take place as soon as 

 they are formed; or the secretion may be long retained within the 



