98 THE PHILOSOPHY OF BIOLOGY 



into the cavity of the gland. If this were the case, 

 then the liquid in the gland would be the same in com- 

 position and concentration as the liquid part of the 

 blood — the plasma. But it is really different in com- 

 position and it is not so concentrated. Now osmotic 

 pressure — on the action of which so much is based — 

 cannot help us, for the liquid in the gland is less con- 

 centrated than that in the blood vessels, so that water 

 ought to pass from gland to blood instead of from blood 

 into gland. Again, if we tie the duct, so that the saliva 

 cannot escape, secretion still goes on, though the hydro- 

 static pressure of saliva in the cavity of the gland may 

 be considerably greater than that of the liquid in the 

 blood vessels. Yet again, if we stop the blood flow by 

 tying the artery, secretion of saliva may still go on 



for a time. 



Therefore the only physical agencies we can think 

 of do not explain the secretion. The latter is actually 

 the work of the individual cells, stimulated by the 

 nerves. If the volume of the gland be measured just 

 while it is being stimulated to secrete, it will be found 

 that the organ becomes smaller, yet while it is being 

 stimulated the blood-vessels are being dilated so that 

 the volume of the whole structure ought to become 

 greater. Obviously part of the substance of the gland 

 is being emptied out through its duct as the secretion. 



If we examine the cells of the gland in various 

 states we see clearly that granules of some material, 

 different in nature from the substance of the proto- 

 plasm itself, are being formed within them. Evidently 

 these granules swell up during secretion and discharge 

 their contents into the ducts. Further changes in the 

 characters of the cell-substance, and in the nucleus, 

 can be observed, and all these indicate that the proto- 

 plasm of the cells, as the result of stimulation, elaborates 



