CELLS. 67 



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 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 circumstances 

 it may conduce to the maintaining of a regular interchange of sub- 

 stances. 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 re- 

 ceived from tvithout. 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 which they have prepared ivithin them- 

 selves. Thus the blood-cells give up their hsematin in dilute blood- 

 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 assu- 

 redly very many with which we are still unacquainted, may in some 

 cases be explained by exosmose ; in others, however, as in the secretions 

 of the glands, this cannot take place. Here the exit of the contents is 

 a consequence of the pressure to which they are exposed, which pres- 

 sure is to be referred upon the one hand to the force of the blood, on 

 the other to an attractive force exercised by the cells themselves in ab- 

 sorbing the substances, and to the elasticity of the cell-membranes. 



The excreted matters in general no longer continue in the organism, 

 but are completely removed as in the glands ; in a few localities they 

 remain, taking a solid form, as extra-cellular substance, outside the cells, 

 and form the genuine membranes proprice of the glands (e. g. of the renal 

 canals), the proper envelop of the chorda dorsalis, and probably also the 

 so-called vitreous membranes (capsule of the lens, membrana Demoursii). 

 An intercellular substance is rare in animals, for the matrix of the 

 cartilages and bones, which for the most part is not excreted by the 



