50 NATURE AND OBJECTS OF PHYSIOLOGICAL SCIENCE. 



be appropriated to some distinct office which it is specially adapted to per- 

 form, so do we find that the cells which become the instruments of some 

 one particular kind of operation seem to lose their other endowments ; 

 as if the expenditure of the vital force of each cell upon any one pur- 

 pose, unfitted it for any other agency. Of this we shall meet with 

 numerous examples hereafter ; it will be sufficient here to refer to two 

 of the most characteristic. It is necessary for every act of Secretion, 

 that a set of cells should be formed within the ultimate follicles of the 

 Gland which is the instrument of the function ( 238); and these ulti- 

 mate follicles are really to be regarded as parent-cells, which produce 

 the true secreting cells in proportion as the materials of their growth 

 are supplied by the blood. Now these parent-cells themselves possess 

 no secreting power, their vital forces being entirely expended in the 

 production of the true secreting cells. On the other hand, the true 

 secreting cells possess no reproductive power, but die and are cast off 

 when they have reached their maturity ; as if their whole vital force 

 were expended in the secreting process, which is nothing else on their 

 parts than an act of growth. So, again, the cells which constitute the 

 fibrillse of Muscular fibre, and of whose change of form the contraction 

 of the muscle is the result ( 336), exercise no power of chemical transfor- 

 mation, undergo no histological change, and appear to be entirely desti- 

 tute of the power of self-multiplication ; the expenditure of their vital 

 force in the act of muscular contraction involves their death and disinte- 

 gration ; and their renewal appears to be accomplished by a production of 

 new cells from the nucleus of the Myolemma ( 338), which, itself pos- 

 sessing no contractile power, retains its reproductive capacity. 



60. Hence, then, we have reason to believe, that all the truly Vital 

 phenomena, however diversified, are but results of the operation of one 

 and the same Force, whose particular manifestations are determined by 

 the nature of the material substratum through which it acts : the same 

 fundamental agency producing simple growth in one case, transformation 

 in another, multiplication in a third, mechanical movement in a fourth, 

 whilst in a fifth it developes nervous power, which may itself operate 

 in a variety of different modes. Such a view seems fully justified by 

 the consideration, (1) that all these forces are exerted, even in the 

 most highly-organized living being, through a common instrumentality, 

 the simple cell ; (2) that the entire assemblage of cells making up the 

 totality of any organism, have all a common parentage, being lineally 

 descended from the single primordial cell in which it originated ; and 

 (3) that they are manifested in connexion with each other, as parts of 

 the life of each individual cell, in those simple organisms which are the 

 lowest members of the two kingdoms respectively, and in which there 

 is no separation or specialization of function. 



61. The question next arises, what is the source of the Vital Force, 

 of which the phenomena of Life are the manifestations ; and under the 

 guidance of the ideas derived from Physical Science, we shall have no 

 difficulty in referring it to the operation of those external agencies, the 

 influence of which has long been known to be essential to Vital action, 

 and which have been usually designated by the term Vital Stimuli. 

 Thus, the growing Vegetable cell cannot decompose carbonic acid, 



