44 GENERAL ANATOMY OF THE TISSUES. 



molecular forces of cells, especially of those electrical phe- 

 nomena which will certainly be found in them. 



• 



[Very recently Donders (Nederlandsch. Lancet.) has justly 

 brought forward a character which until now had received no 

 attention, viz. the elasticity of the cell-membranes and the 

 pressure consequently exercised upon the cell-contents. It is an 

 ascertained fact that the cell-membranes are elastic; and it thence 

 naturally follows that, according to the greater or less amount of 

 the contents of the cells, so will these suffer a greater or a less 

 pressure. This, however, reacts again upon the absorptive and 

 excretive processes, so that under a more considerable pressure 

 the latter, under a less, the former prevails, and in certain cir- 

 cumstances it may conduce to the maintaining of a regular 

 interchange of substances. Donders believes, that the greater 

 density of the cell-contents may be derived from their always 

 being under greater pressure than the cytoblastema.] 



§16. 



Excretive processes. — The vegetative functions of animal 

 cells are not limited to mere absorption and metamorphosis, 

 but substances are excreted as a result of their operation. This 

 may take place in two ways. 



1. The cells give out unaltered, the substances which they have 

 received from without. — This occurs in the epithelium cells of 

 those glands which, like the kidneys, lachrymal glands, lungs, 

 &c, simply permit of the discharge of substances from the 

 blood, also in those cells which line the serous membranes, and 

 probably many others. 



2. The cells excrete substances ivhich they have prepared 

 within themselves. — Thus the blood-cells give up their hsematin 

 in dilute blcod-plasma ; the fat cells their fat in emaciated 

 persons ; the hepatic cells, bile ; those of the gastric glands, 

 gastric juice ; those of the mucous glands and membranes, 

 mucus. 



The occurrence of these excretions, of which, in fact, there 

 are assuredly very many with which we are still unacquainted, 

 may in some cases be explained by exosmose; in others how- 

 ever, as in the secretions of the glands, this cannot take place. 

 Here the exit of the contents is a consequence of the pressure 



