bZ GENERAL ANATOMY OF THE TISSUES. 



substances occur. Thus, there exist cells with much protein, as the 

 nerve-cells ; and with much fat, as the fat-cells, the cells of the seba- 

 ceous follicles, of the milk-glands, &c. ; then such as contain haematin, 

 pigment, biliary, and urinary constituents, mucus (epithelium-cells), milk, 

 sugar, &c. &c. 



The phenomena manifested by these, so variously constituted cell- 

 contents, during life, may be best enumerated as absorption, assimila- 

 tion, and excretion. These depend principally upon chemical and 

 physical conditions, and are to a great extent capable of microscopic 

 investigation, since very frequently the changes of form in the cell and 

 the changes of its contents go hand in hand. Absorption is manifested 

 in all cells, but to far the greatest extent in those which at first have 

 little or no contents save the nucleus. In these, the primary cause of 

 the absorption is not to be sought in endosmose, but, as Schwann has 

 indicated, in this that while the membranes grow by the attraction of 

 material from the surrounding fluid, by virtue of their porosity they 

 allow substances to penetrate into the interior. This filling, however, 

 does not take place by the cells admitting every kind of matter indis- 

 criminately, but they exhibit peculiar relations to the cytoblastema, 

 varying with the period and with the locality ; so that they take up one 

 constituent and reject another ; and the like occurs with the absorptive 

 powers of those cells which possess contents from their earliest exis- 

 tence. 



That this is actually the case, is demonstrated, for instance, by the 

 fact, that in embryos, notwithstanding the identity of the formative 

 material in all cells, i. e. the plasma of the blood, some take up more of 

 one substance, some more of another ; and it is still more clearly evi- 

 denced by the fact, that the cell-contents of probably all cells are 

 chemically different from the cytoblastema out of which they are formed 

 and by which they are nourished, as has been clearly shown lately, in 

 the ova and blood-corpuscles, which for example contain far more po- 

 tassa than the blood. The reason of this phenomenon may be generally 

 stated to be, that the cell-membranes do not act as mere filters, but 

 allow one substance or another to permeate them, according to their 

 chemical composition, the constitution of the fluid which imbues them, 

 their condition of aggregation, and their thickness. 



Endosmose must also be taken into account as a condition in the ab- 

 sorptive actions of cells, though hitherto it has been too freely appealed 

 to, and cells have been too often considered as vesicles provided with 

 merely indifferent porous membranes. That endosmose operates is not 

 to be denied, when it is observed how the addition of concentrated or 

 diluted solutions, causes cells to dilate or to collapse, yet it is not easy 

 to determine what influence such conditions have during life, nor what 

 results are produced by the combined operation of the cell-membranes 



