166 GENERAL PHYSIOLOGY 



from practical considerations. The substances given off from the 

 cell, among which occur gaseous, liquid, and solid substances in all 

 grades of consistency, are distinguished as secretions when they 

 play a still further useful role in the life of the organism, and as 

 excretions when they are removed to the outside as useless residue. 

 Accordingly, secretions are contrasted with excretions. We will 

 look for a moment somewhat in detail at the two groups of sub- 

 stances and at the mode of their output. 



1. The Mode of Output of Substances ly the Cell 



Like the taking-in of food, so also the manner of output of 

 substances varies, according as the latter are gaseous, dissolved or 

 solid. 



The output of gaseous or dissolved substances evidently takes 

 place under the same conditions and in the same manner as such 

 substances are taken in, for here there is the same process reversed. 

 In many cells, e.g., in many unicellular organisms, it is very probable 

 that the so-called contractile vacuole (Fig. 57), a drop of liquid 

 within the cell which is alternately emptied and filled by rhyth- 

 mical contractions of the protoplasm of its wall, attends to the 

 expulsion of dissolved substances. It is supposed that the latter, 

 together with the water that during the diastole of the vacuole 

 streams in from all sides out of the protoplasm, accumulate in the 

 vacuole and at its systole are given off to the outside. 



It is clear that every cell excretes primarily substances that are 

 derived from its own metabolism. But in the compound cell- 

 community, especially of the animal organism, there exist also cells 

 which in addition have undertaken for the whole body the excre- 

 tion of certain other materials. Thus, the cells in the convoluted 

 uriniferous tubules of the kidney excrete the urea that is prepared 

 by the liver-cells and passed into the blood, by receiving it from 

 the blood and giving it off to the outside. Other cells of the 

 kidney, those of the so-called glomeruli, the microscopic capsules 

 in which the blood-capillaries are twisted into knots, greedily suck 

 up the water from the blood to excrete it as the water of urine 

 into the pelvis of the kidney. 



In the mode of output of solid substances two types again are dis- 

 tinguished. They are essentially different according as the excreted 

 substances either occur in the cell itself in a dissolved condition, and 

 become solid only at the moment of excretion, or lie within the 

 living substance as solid masses, which are to be given off as such 

 to the outside. 



In the former case, which is realised in the excretion of 

 most skeletal substances, such as chondrin, chitin, and lime, the 

 same conditions are present as in the excretion of dissolved sub- 





