PROCESSES CONCERNED IN SECRETION. 2G3 



deepest parts. Surrounding each subdivision and bind- 

 ing it to its neighbors is the gland stroma formed of con- 

 nective tissue, a layer of which also commonly envelops, 

 the whole gland, as its capsule. Usually on looking at 

 the surface of a large gland it is seen to be separated by 

 partitions of its stroma, coarser than the rest, into lobes, each 

 of which answers to a main division of the primary duct; 

 and the lobes are often similarly divided into smaller parts 

 or lobules. In the connective tissue between the lobes and 

 lobules blood-vessels penetrate, to end in fine capillary 

 vessels around the terminal recesses. They never pene- 

 trate the basement membrane. Lymphatics and nerves 

 take a similar course; there is reason to believe that the 

 nerve-fibres penetrate the basement membrane and be- 

 come directly united with the secreting cells. 



The Physical Processes in Secretion. From the struc- 

 ture of a gland it is clear that all matters, derived from the 

 blood and poured into its cavity, must pass not only through 

 the walls of the capillary blood-vessels, but also, by filtra- 

 tion or dialysis, through the basement membrane and the 

 lining epithelium. By filtration is meant the passage of a 

 fluid under pressure through the coarser mechanical pores 

 of a membrane, as in the ordinary filtering processes of a 

 chemical laboratory; and the higher the pressure on the 

 liquid to be filtered the greater the amount which, other 

 things being equal, will pass through in a given time. 

 Since in the living Body the liquid pressure in the blood 

 capillaries is nearly always higher than that outside them, 

 filtration is apt to take place everywhere to a greater or less 

 extent, and will be increased in amount in any region by 

 circumstances raising blood-pressure there, and diminished 

 by those lowering it. To a certain extent also the nature 

 of the liquid filtered has an influence. True solutions, as 

 those of salt in water, pass through unchanged; but solu- 

 tions containing substances such as boiled starch or raw 

 egg albumen, which swell up greatly in water rather than 

 truly dissolve, are altered by filtration; the filtrate contain- 

 ing less of the imperfectly dissolved body than the unfil- 

 tered liquid. The higher the pressure the greater the pro- 



