OP CELLS AND CELL-LIFE. 131 



folding of the leaves of the Mimosa, and the flexure of the stamen of the Ber- 

 berry, upon mechanical or chemical stimulation, are specimens of the latter. 

 In all these cases, the aptitude for the movement, whether " spontaneous" or 

 " excited/ 7 is so clearly dependent upon the general vital activity of the cells 

 which perform it, that we can scarcely regard it in any other light than as an 

 expression or manifestation of their peculiar force. And this view derives con- 

 firmation from all that we know of the conditions under which movement is 

 originated in Animals. For here, too, may it be said that all motion is depend- 

 ent upon the change of form of cells ; and that this change may be either 

 " spontaneous" or excited," but in both cases must be regarded as a mani- 

 festation of vital force. Of the spontaneous movement, we have the most re- 

 markable example in the incessant rhythmical action of the cilia (Fig. 26) 

 which line many of the tubes and cavities of the body ; these cilia being, it 

 would appear, nothing else than prolongations of the cell itself, though of ex- 

 treme tenuity. Their movement is obviously dependent upon the life of the 

 cell from which they are put forth, and is retarded or brought to a stand by 

 agencies which tend to depress its vital power. But these are not the only 

 cases in which cells appear to undergo rhythmical changes of form, as a part of 

 their regular vital operations; for the alternating contractions and dilatations of 

 the cavities of the heart appear due to an action of this kind in its muscular 

 walls; and we may not improbably regard the contraction of the uterine struc- 

 ture, when it has reached a certain stage of its development, in the same light. 

 In both these cases, it is true, it is to be observed that the structures in question 

 are peculiarly amenable to the influence of stimuli; thus the rhythmical action 

 of the heart may be re-excited, after it has been suspended, by a slight mechani- 

 cal irritation, and its regular movements may be either accelerated or retarded 

 by nervous influence ; so, again, the parturient efforts of the uterus may be in- 

 duced, when they would not otherwise take place, by mechanical or nervous 

 stimulation, or, when they are already going on, they may be accelerated or re- 

 tarded by the same agency. But still, when every source of excitement is ex- 

 cluded, we cannot but perceive that these actions take place with a spontaneity 

 which can scarcely be accounted for in any other way than by considering them 

 as expressions of the vital activity of the component cells of these forms of 

 muscular tissue, which manifests itself in this mode when the developmental 

 life of the cell has attained its maturity. And this view is strikingly confirmed 

 by what we know of the origin and termination of these movements. For the 

 action of the heart commences, when as yet its contractile parietes consist but 

 of an assemblage of ordinary-looking cells, no proper muscular tissue being 

 evolved, and no nervous system being yet developed from which the stimulus 

 to the movement can proceed ; and it is impossible to assign any other cause for 

 the movement under such circumstances, than the attributes inherent in the 

 tissues which perform it. So, again, as will be shown hereafter (CHAP. XIX. 

 SECT. 3), the parturient action of the uterus cannot be fairly attributed to any 

 of the agencies which have been supposed to excite it ; but must be looked 

 upon as one of those phenomena, whose periodical recurrence is due to the 

 regularity of the operations of growth, whereby the tissue attains its maturity 

 in a certain limited time, and then discharges itself of its vital force in the 

 shape of motor power; just as another cell will consume it in the development 

 of a brood of young ones, or in the performance of chemical transformations, 

 setting free the product, when completely formed, by its own rupture or deli- 

 quescence. And the subsequent result is just what on this view might be an- 

 ticipated ; for the vital power of the tissue being thus exhausted, it speedily 

 undergoes degeneration and death ; and thus it is that the reduction of the 

 uterus to its normal size takes place so much more rapidly after parturition, than 

 does the wasting of ordinary muscles by simple disuse. Hence we may look 



