134 OF THE STRUCTURAL ELEMENTS OF THE HUMAN BODY. 



we usually observe that the whole texture retains its primitive cellular charac- 

 ter ; and that the development of higher forms of tissue only occurs in those 

 whose growth is slower, their cells having ceased to multiply themselves thus 

 rapidly, when they underwent histological change. But perhaps one of the 

 most striking examples of this principle is presented by those glandular follicles 

 which act as parent-cells, developing in their interior a successional progeny, 

 which are the true secreting cells : for the former possess no secreting power, 

 their vital force being expended in the production of the latter ; whilst on the 

 other hand, the latter possess no reproductive power, but die and are cast off 

 when they have reached their maturity, even their own cell-walls being usually 

 very imperfectly developed ( 117), as if their whole vital force had been ex- 

 pended in the secreting process. So, again, the cells whose vital force is exerted 

 in mechanical movement, seem exclusively adapted for this purpose, apparently 

 performing no other vital changes than those involved in their own develop- 

 ment. Thus, the ciliated epithelium-cells which line the respiratory passages 

 and the ducts of many glands, appear never to perform that secretory function 

 which is discharged by other non-ciliated cells of the same stratum ; so that, 

 their mode of production and the general conditions of their development being 

 essentially the same, we can scarcely fail to regard the ciliary movement and 

 the secreting action as, however dissimilar in themselves, two modes of opera- 

 tion of one and the same " cell-force/ 7 Again, the elongated cells which con- 

 stitute the non-striated muscular fibre, and the minute cellules of which the 

 fibrillae of the striated muscular fibre are made up, seem to exercise no chemical 

 change, to undergo no further development, and to undergo disintegration 

 without having previously multiplied themselves ; so that all increase and re- 

 generation of muscular tissue appear to take place, either by production de novo, 

 or possibly (in the case of the striated fibre) by the continued development of 

 new cells from the nucleus of the parent-cell, which, like that of the glandular 

 follicle, performs no other function than that of multiplication. The cells com- 

 posing the nervous tissue, again, do not show any indication of reproductive 

 power, and seem to undergo disintegration as the direct consequence of their 

 production of nerve-force ; so that they, too, appear to expend their whole vital 

 energy in one particular mode of action, and to have no power to spare for any 

 other. 



114. We have hitherto spoken only of that part of the Life of the Cell, which 

 intervenes between its origin and its epoch of maturity ; we have now to advert 

 to its decline and death ; and these are to be regarded, no less than the pre- 

 ceding, as part of that regular series of changes by which it is distinguished as 

 an organized structure. For it may be stated as a general rule, that the amount 

 of vital action which can be peformed by each living cell has a definite limit ; 

 an4 that when a certain point has been once reached, a diminution in the vital 

 activity of the cell must ensue, and it must become more and more subject to 

 those influences which are constantly tending to degrade it to the condition of 

 inorganic matter. Hence there is for the most part a limit to the duration of 

 each component cell of the organism, which is quite irrespective of that of the 

 organism at large; the life of the latter being maintained by the continual de- 

 velopment of new tissues, in the place of those which are degenerating and de- 

 caying. But this limit is by no means constant ; since a cell may live faster or 

 slower, that is, it may put forth a greater or less degree of vital energy in a 

 given time, in accordance with the conditions under which it is placed ; and the 

 more rapid the rate of life, the more brief, cseteris paribus, is its duration. Of 

 this general principle, abundant examples are afforded to us in the Vegetable 

 kingdom, in which we can readily trace the operation of varying degrees of light 

 and heat ; but it is not difficult to trace them out also in cold-blooded animals \ 

 and the only reason for the greater constancy in the rate of decay and renova- 



