176 DR ALISON’S OBSERVATIONS ON 
tached to previously existing cells; and it seems to be only by the successive 
formation, distension, rupture, and disappearance of cells, that secretions make 
their way into the excreting ducts of glands, or on the surface of membranes. 
The dependence of all living structures, and of all secretions, not simply on 
vascular action, by which nourishing fluids are circulated through them, but on 
cellular action, by which this nourishing fluid is changed, appropriated, and re- 
tained, or restored to the circulation, is the great step which has been recently 
gained in physiology by the use of the microscope; and seems to me to be one of 
the clearest proofs of the dependence of all vital phenomena, on peculiar attrac- 
tions and repulsions, actuating both solids and fluids, and causing motions in the 
latter,—not on any vital powers residing exclusively in solids. When it is stated, 
é. g. by Mr Pacer, that “the purpose to which the capillaries are habitually sub- 
servient, is only the passive one of conveying blood close to those parts of the 
body which either grow or secrete, and that it is proved that if a part be only 
able to imbibe the fluid portion of the blood from an adjacent vessel, it nourishes 
itself as completely, and after the same method, as one whose substance is 
traversed by numerous capillaries,”*-—it becomes obvious that the movements of 
the fluid portion of the blood, whereby they are applied to growth and secretion, 
must be determined by causes quite distinct from the contractions of vessels. 
2. Living and growing cells, therefore, whether acting on the nourishing 
fluid just taken into the system (as in the case of the intestinal villi, or the tufts. 
of the placenta), or on the blood brought to them by the capillaries (as in the 
nutrition of the different textures), appear always to have two functions to per- 
form,—to extract from the nourishing fluid the matter of which they are them- 
selves composed, and to extract from it, likewise, the matter which is contained 
within them,—.e¢., in the organs of secretion, the secreted fluids, and in the 
different solid textures, that additional matter which is always found, whether 
lignin, oil or fat, fibrinous, cartilaginous, or bony substance, in a granular or less 
definite form, incrusting the walls of the cells. It does not appear possible to 
explain what is distinctly seen in all these cases, without supposing that the pre- 
existing cells exert a peculiar attraction or affinity, both for the matter by which 
they are themselves to be nourished, and their successors to be reproduced,—and 
likewise for another matter, different in the different parts of the structure, by 
which they are to be filled or distended. And in the case of vegetables, there 
seems to be this general distinction between the two,-—that the former is a 
matter destitute of azote, and the latter one containing that element. 
3. The cell; growing always by attracting to itself a compound matter, 
existing in the fluid state, and giving it a simple increase of aggregation, the 
nature of the change which takes place as this matter becomes solid, is simply 
* Report in Forses’s Medical Review, July 1843. 

