254 STUDIES IN ADVANCED PHYSIOLOGY. 



tion occurs. If, now, between such bunches of grapes we 

 should imagine a very full network of blood-vessels and a 

 supply of nerves, we should have a coarse but possibly a 

 helpful analogy. 



2. The Process of Secretion. Such an arrangement of 

 a gland with a basement membrane and blood-vessels natur- 

 ally suggests the possibility of secretion being a mere case 

 of physical filtration. We know, for instance, that if a 

 liquid containing things in solution be put into a membran- 

 ous bag, such a liquid by the process of osmosis reaches 

 the outside. There are, however, certain facts which make 

 it at once perfectly plain that the process of secretion is not 

 a process of physical filtration. Quite a number of glands 

 secrete substances which are not found in the blood at all. 

 They secrete substances which they themselves have pro- 

 duced. Thus, for instance, the pepsin of the stomach, or 

 trypsin of the pancreas, and the ptyalin of the salivary 

 glands are not contained in blood at all. Such special pro- 

 ducts which the glands have themselves produced are called 

 specific elements. In this sense we speak of the hydro- 

 chloric acid, rennet, and pepsin of the stomach as the spe- 

 cific elements of that organ. If secretion were a physical 

 filtration, specific elements would be impossible. 



But the question then recurs, may not the remaining 

 elements of a secretion be merely filtered through. This 

 question must be answered in the negative for several rea- 

 sons. First, glands secrete at certain times only, while if it 

 were a physical filtration the process ought to go on all the 

 time, for even when a gland is at rest the blood circulates 

 through it freely. Second, a certain poison called atropine 

 (of frequent use in the physiological laboratory) destroys 

 the secreting power of a gland at once, but it does not 

 change the blood pressure in the gland at all. Such a 

 poisoning action would be impossible in the case of simple 

 filtering. Third, glands may be stimulated to such increased 

 action that the pressure of the secretion may actually ex- 

 ceed the pressure of the blood in the gland; a condition of 



