THE PHYSIOLOGY OF SECRETION. 173 



In such cells, for example, as those lining the gut of Ptycoptera 

 larvae, a sort of mucin is formed. This absorbs water, distends 

 the inner end of the cell until finally the turgor becomes so 

 great that the cell actually explodes. As the nucleus remains 

 behind, the cell is reformed, again breaks down into mucin, 

 again explodes, and so on. The cause in this case is perfectly 

 constant, the result only is intermittent. The secretion of 

 the goblet cells of the intestine is probably rendered inter- 

 mittent in this manner, although in this case the variation in 

 blood supply hastens or delays the final explosion. 



Finally the secretion of the cells of the vertebrate glands is 

 controlled beyond doubt, in part at least, by the blood supply. 

 In the vertebrate we have a highly elaborated, carefully coor- 

 dinated, closed vascular system which enables an organism to 

 turn food, oxygen and liquid, on or off from a given area at 

 will. The cells may be thrown at once into an anaemic state or 

 overwhelmed with oxygen. In all glands of vertebrates which 

 are strikingly intermittent, the glands during rest are almost 

 white ; during secretion, pink, from the dilated blood vessels. 

 That these secretions are controlled by the blood system the 

 following facts will show : in the kidney and liver the rapidity 

 of secretion rises and falls with blood pressure and the rapidity 

 of flow through the glands ; it is very dependent on oxygen ; 

 in the pancreas every agency which induces vaso-constriction 

 stops secretion; every agency which induces dilation in the 

 normal gland causes secretion ; in the intestine and stomach 

 similar relations prevail ; in all vertebrate glands the principal 

 secretory nerves are dilator nerves, the inhibitory, constrictors. 

 Evidently, in the vertebrate, one of the chief ways of controlling 

 secretion is through the vascular system. 



Let us now consider the manner in which the discharge of 

 the secretion from the gland is brought about. This is the 

 process which is incorrectly called secretion by most authors. 

 We will call it glandular secretion in distinction from cellular 

 secretion. Let us consider the invertebrate glands first. Prac- 

 tically the only manner in which the glandular secretions of 

 the invertebrates are rendered intermittent is through muscle. 

 This muscle acts in either of two ways ; first, as a sphincter 



