334 



PRINCIPLES OF GENERAL PHYSIOLOGY 



FNJ. 88. ALVEOLI OF SEROUS GLAND OF RABBIT. Fresh, without any 

 addition. All figures from the same gland. The boundaries 

 between the cells are made too obvious in all. 



A, At rest. 



B, 1-4S hours later, after 3'65 c.c. of saliva had been secreted under the influence of 



piloearpine in small doses. 



C, Five hours later than A, after stimulation of the sympathetic nerve for alxnit two 



hours, with intervals of rest. It! saliva secreted. The nuclei should not be 

 shown so clearly, although they are unobscured by granules. 



(Langley, Journ. of Ph ysio/. , 2, 

 PI. 7, Figs. 1,2, and 4.) 



where the liquid secreted is blood plasma minus its colloids, with perhaps certain 

 crystalloids adsorbed thereon. 



Although the secretion of water in general is not a simple process, there are 



grounds for hold- 

 ing that osmotic 

 phenomena play 

 an important part 

 in it. 



We have seen 

 (page 163) how a 

 tube containing a 

 solution of some 

 substance, cl< ><<! 

 at one end by a 

 membrane impel - 

 meable to the 

 solute, and at the 

 other end by a 

 membrane per- 

 meable to it, and 

 i mmersed in 

 water, gives a con- 

 tinuous current 

 of water, or rather 



solution, issuing from the permeable end, as long as any osmotically active sub- 

 stance is left in the tube. Such a mechanism has been described by Lepeschkin 

 (1906) in the fungus Pilobolus, and in the hydathodes of higher plants. 



If, therefore, we are 

 justified in assuming 

 that the secreting cells 

 of such organs as the 

 salivary glands or the 

 pancreas are possessed 

 of a membrane on the 

 ends next the blood 

 vessels of such a kind 

 as to be impermeable 

 to some substances pro- 

 duced in the cells, while 

 on the ends next the 

 duct the membrane is 

 permeable to these sub- 

 stances, we can account 

 for a flow of water as 

 long as these osmoti- 

 cally active substances 

 are being formed. 

 They are, of course, 

 carried out with the 

 secretion through the 

 membrane permeable 

 to them. 



, 



a. 



.'X : - ; ' p 'v:' 



FIG. 89. SEROUS GLAND OF THE HUMAN TONOFE. Fixed prepara- 

 tion. Diagrams of the series of functional states (a to (j) 

 from the charged resting state through activity to the state 

 of rest again (h). According to the work of Zimmermann. 



When the secretion 

 has an osmotic pressure 

 mechanism as simple as 



higher than that of the blood, it is clear that a 

 that described is insufficient, and additional compli- 



cations, so-called " protoplasmic activities," must intervene, in order to afford the 

 energy necessary to raise the osmotic pressure. Moreover, in any case, except 

 simple filtration, the mechanism in question requires the continuous production 

 in the cells of osmotically active substances. The osmotic pressure of milk, bile, 



