172 BIOLOGICAL LECTURES. 



as they control the food supply, including here oxygen, control 

 also the rapidity of formation and secretion of certain sub- 

 stances of small molecular size. They become, hence, true 

 controllers of secretion. They are secretory nerves in one 

 sense. The secretion of all simple osmosable substances is 

 controlled, then, by their production, which is in its turn 

 modified by the food and oxygen supply. 



How is the secretion of the more complex substances con- 

 trolled } One of the most clearly defined ways in which such 

 secretions from the cells themselves are rendered intermittent 

 is by the action of muscle. The cells, after accumulating in 

 themselves the hylogens to be secreted, are periodically com- 

 pressed by the action of contractile matter, either within the 

 cell itself, or immediately about it. A fine example of this 

 sort is the nematocyst of the Hydrozoa. The hylogen elabo- 

 rated lies coiled in the distended cell. When the projecting 

 irritable point of the cell is properly disturbed, the contractile 

 substance, lying about the cyst in the same cell, suddenly con- 

 tracts and violently ejects the dart. It is not improbable that 

 the cell then forms another dart, which in its turn when fully 

 ready is ejected. If a secretory nerve controlled this secretion, 

 which is probably not the case, it will be seen that it must 

 effect the expulsion of the dart by acting on contractile tissue. 

 It would not produce its action by altering the elaboration of 

 the dart. A similar muscular mechanism is to be found in 

 the unicellular glands of Aplysia, in which the enormously 

 distended cells are surrounded by a well-marked contractile 

 sheath. That these cells are emptied by the contraction of 

 this sheath with a possible coincident dilation of a restraining 

 muscular sphincter can hardly be doubted. If this secretion 

 is under nerve control, and it probably is, as nerves have been 

 traced to the muscular sheath, the nerves which control it must 

 act on the contractile substance about the cell, not upon the cell 

 itself. The large unicellular glands of the carp-louse, Argulus, 

 are also emptied intermittently either by the contraction of the 

 cells themselves or the surrounding musculature. 



A second means by which such cellular secretions are ren- 

 dered intermittent is by the periodic explosions of the cells. 



