128 SIMPLE HISTOLOGICAL ELEMENTS. 



absorption, secretion, and contraction. These vital actions depend 

 much on physical and chemical conditions, and may, to a great 

 extent, be subjected to microscopic investigation. 



1. Absorption must be distinguished from mere endosmosis, since 

 the nutrition of the cell (a vital and not a mere physical process), 

 depends on the former, and some of the constituents of the sur- 

 rounding fluid are introduced through the cell-membrane, while 

 the rest are rejected. Thus the contents of all cells are chemically 

 different from the surrounding cytoblastema. For instance, the 

 blood-corpuscles contain more potassa than the liquor sanguinis. 

 Doubtless the chemical composition of the cell-contents and the 

 surrounding fluid, and the thickness of the cell-membrane, also 

 exert an influence on this process; nor must endosmosis be entirely 

 overlooked in this connection, since cells are known to dilate in 

 diluted, and to contract in concentrated solutions. 



The vital processes in cells produce changes both in their walls 

 and in their contents. The membranes generally become denser, 

 and of a different chemical constitution, with age ; though whether 

 the membrane itself changes chemically, or an incrustation of salts 

 occurs within, or a deposit on the exterior, are points not con- 

 clusively settled. 



The changes in the coil-contents are various. The primordial cells 

 of the embryo, at first distended with the elements of the yolk^ 

 especially with oil, gradually acquire more fluid and homogeneous 

 contents, the granules becoming dissolved. Then, as development 

 proceeds, various new formations appear in the cells, as hsematine, 

 melanine, fat, &c. But changes in cell-contents occur in adult ani- 

 mals also. Fat-cells, in great deficiency of their nutritive elements, 

 may lose their proper contents, and contain mere serum; or, in case 

 of a superfluity of nutriment, they may even burst from fulness. 1 



The lymph-corpuscles also develop the coloring matter of the 

 blood and the colored corpuscles, within them ; and the cells which 

 secrete the bile undergo marked changes in their contents. (Kblliker^ 



Changes in the/orm of the cells accompany the preceding altera- 



1 Bonders has ascertained that the cell-membranes are elastic, and the contents 

 will suffer a greater or a less pressure, according to their amount. This elasticity 

 may conduce to the maintenance of a regular interchange of substances in the ex- 

 cretive and absorptive processes. And the greater density of the cell-contents than 

 of the surrounding cytoblastema may be due to the fact that they are always 

 under greater pressure. 



