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 

 vasculai- action, by which nourishing fluids are circulated through them, but on 

 cellnkir 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 pliysiology by the use of the microscope ; and seems to me to be one of 

 tlie 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, 

 ('. g. by Mr Paget, that " the purpose to which the capillaries iire 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 capUlaries (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, — i. e., in the organs of secretion, the secreted fluids, and in the 

 different solid textures, that additional matter which is always found, whether 

 liguin, 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 Forbes's Medical Keview, July 1843. 



