286 THE HUMAN BODY. 



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 lobtiles. In the con- 

 nective tissue between the lobes and lobules blood-vessels 

 penetrate, to end in fine capillary vessels around the terminal 

 recesses. They never penetrate the basement membrane. 

 Lymphatics and nerves take a similar course; there is reason 

 to believe that the nerve-fibres penetrate the basement mem- 

 brane and become directly united with the secreting cells of 

 some glands. 



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 circum- 

 stances 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, passed through unchanged ; but solutions con- 

 taining 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 containing less of the 

 imperfectly dissolved body than the unfiltered liquid. The 

 higher the pressure the greater the proportion of such sub- 

 stances which gets through ; and if the pressure is slight the 

 water or other solvent may alone pass, leaving all the rest 

 behind on the filter. Under moderate pressure the blood 

 may thus lose by filtration only such bodies as water and 

 salines ; while an increase 'of arterial pressure may lead to 



