335 



saliva, and sweat is, in fact, lower than that of the blood, but urine, as it leaves the 



kidney, has an osmotic pressure considerably higher than that of the blood, owing 



to increase of concentration 



as the filtrate from the glom- 



eruli passes along the tubules. 



Osmotic work must therefore 



be done by the kidney cells. 



The reader will probably note 

 that a mechanism the reverse of 

 that sketched above, namely, cells 

 with their permeable and semi- 

 permeable membranes interchanged 

 in position, would account for ab- 

 sorption of water from such cavities 

 as that of the intestine. 



FIG. 90. ALVEOLI OF THE RABBIT'S PANCREAS, LIVING, 

 IN SITU. 



a, Inner granular zone. 



&, Outer transparent zone, with faint striation in B. 

 e, Lumen of alveolus, obvious in B, indistinct in A. 

 d, Indentation at junction of two cells, resulting from decrease 

 in volume. 



(Kiihne and Lea. ) 



A fact which points to the 

 intervention of osmotic pro- 

 cesses in the secretion of water 

 is the discovery of Ludwig 

 (1851) that the pressure in the 

 duct under which saliva still 

 continues to be secreted is con- 

 siderably higher than that in 



the arteries. Hill and Flack (1912) found, with an arterial pressure of 130 mm. 

 of mercury, a pressure of saliva as high as 240 mm. of mercury. This pressure 



might be brought about either by the produc 

 tion within the cells of osmqtically active 

 substances, in conjunction with a membrane 

 impermeable to them, and situated next the 

 blood vessels, or by some process of imbibi- 

 tion, by which swelling of some constituents 

 of the cells takes place. It is more difficult, 

 on the latter view, to see how a continuous 

 secretion could be provided for. 



Changes in the Microscopic Appearance of 

 gland cells might perhaps be expected to 

 throw some light on the question before us, 

 but the interpretation of those phenomena 

 which have been observed is not an easy 

 matter. For details, the reader is referred to 

 the article by Metzner (1907, 1). It must 

 suffice to mention here that it appears to be 

 almost universal that granules (" zymogen " 

 granules) make their appearance in the more 

 or less homogeneous protoplasm of the resting 

 cell, and slowly increase in size. When the 

 gland is excited to secretion, these granules 

 usually undergo a process of solution, and are 

 regarded as the source of the special con- 

 stituents of the secreted fluid. Sometimes 

 the granules pass directly into the secretion 

 without preliminary solution. It is con- 

 ceivable that, in the breaking up of these 

 granules, substances of a much smaller mole- 

 cular weight and higher osmotic pressure 

 than themselves might be formed. During the process of continuous secretion, 

 it is clear that the granules must be renewed, although it is not difficult to 

 obtain, by artificial stimulation of the gland, a nearly complete disappearance 

 of the granules, those that are left being situated next the lumen of the duct. 

 Figs. 88 to 90 show this result in different cases. See also Fig. 92 on page 347 below. 



FIG. 91. AN ACINUS OF THE STTB- 

 MAXILLARY GLAND OF MAN (EXE- 

 CUTED). Fixed and stained by 

 Flemming's method. Obj. 2 mm., 

 Oc. 8. 



e, Ergastoplasm. 



6, Secretory capillaries. 



(Garnier, " Cellules glandulaires 

 sereuses,"- Nancy, 1899, Fig. 16.) 



