THE PHYSIOLOGY OF SECRETION. 169 



A fourth method largely employed for removing these sub- 

 stances from the cell is by bursting the cell through internal cell 

 tension. We have seen that the hydrolytic splittings taking 

 place in protoplasm, particularly during vaso-constriction in the 

 absence of oxygen, render the cell contents more soluble and 

 of smaller molecular size. The absorption of water by the cell 

 and its turgor is thereby greatly increased. A typical picture 

 of such a cell is afforded by any mucous cell, particularly 

 those of the submaxillary gland. The whole cell is enormously 

 distended, and the nucleus, reduced to a granular mass, is driven 

 against the bottom of the cell. Now, if the cell membrane be 

 resistant enough to hold in these substances and prevent their 

 escape by filtration, obviously there must come a time when 

 the cell will burst. When this happens, the cell contents rush 

 forth often with such violence that they drag the nucleus with 

 them. More frequently, however, the nucleus remains behind 

 and builds up a new cell. This is the manner in which the 

 mucin appears to escape from the goblet cells of the intestine ; 

 from' the intestinal epithelial cells of Ptycoptera larvae; from 

 the cells of the submaxillary and sublingual glands. Probably 

 the secretion from the pancreas cells is of a similar kind, though 

 this is by no means so certain. 



Finally there is the method employed by the cells of the 

 mammary glands in which the inner ends of the cells are dis- 

 organized and break off, apparently losing all coherence after 

 their fatty transformation. This process is also seen in the 

 calcareous glands of the earthworm. 



In these different ways, then, do these non-osmosable sub- 

 stances escape from the cell. Each of these methods consti- 

 tutes a mechanism of secretion, and it is readily seen that there 

 are many mechanisms. It is hence absurd to speak of a special 

 secretory activity of cells, as Heidenhain and many other phys- 

 iologists do, as if there was one process common to all. To 

 which of these secretory processes does such a term have refer- 

 ence ? We shall come back to this in treating of the question 

 of the existence of secretory nerves. 



The secreting cell does not, as a rule, secrete equally well in 

 all parts. It discharges always in one given direction. How 



