OF CELLS AND CELL-LIFE. 123 



not merely for the generation of their cell-wall, but also for the filling of their 

 cavity with its characteristic contents ; and we shall find that in some instances 

 this pabulum appears itself to contain these peculiar substances already formed, 

 whilst in others the cell seems to exercise a certain converting power, by which 

 it produces them from some other compounds. Not unfrequently, the contents 

 of the cells include a number of minute molecules; and these exhibit an active 

 movement within the cell, especially when water is added, so as to dilute the 

 fluid in which they are suspended. This movement, which is well seen in the 

 interior of the colorless corpuscles of the blood, of the nerve-vesicles, of pus 

 and mucus-corpuscles, of pigment-cells, and occasionally in cells of other kinds, 

 is not to be regarded as having any dependence on vital forces ; for it is nothing 

 else than the " molecular movement/ 7 which (as long since shown by Mr. Rob- 

 ert Brown) is exhibited by almost any very finely-divided particles that are 

 freely suspended in liquids. 1 



102. The nuclei of cells present numerous varieties of structure and aspect. 

 In their simplest condition, they seem to be nothing else than assemblages of 

 minute particles, sometimes of molecular minuteness, in other instances large, 

 well-defined granules; and among these may usually be distinguished some that 

 are of an oleaginous character. In other cases, again, these assemblages of 

 granules appear to have a distinct investment of their own, which separates them 

 from the general cavity of the cell; this is most distinctly seen in pigment- 

 cells and epidermic cells. At or near the centre of the nucleus, one or more 

 corpuscles are frequently seen, very distinct from its general mass, which are 

 termed nucleoli (Fig. 24, c); and these appear in many instances to have the 

 character of minute vesicles. It is probable, however, that the term " nucle- 

 olus" has been attached to bodies which are really very different from each 

 other, both structurally and functionally; and there is yet much to be learned 

 on the subject. The nucleus, as already stated, is usually found attached to the 

 inner wall of the cell, and sometimes even appears to be imbedded in its sub- 

 stance ; but it is occasionally observed to lie freely in the cavity. The form of 

 the nucleus is for the most part nearly circular, and it may be usually observed 

 to continue so, even when the form of the cell has undergone its most remark- 

 able alterations, e. g. becoming fusiform or stellate ; 3 but a very peculiar excep- 

 tion is presented by the nuclei of the " smooth" muscular fibre-cells, which are 

 staff-shaped (Fig. 100, c); this character serving to distinguish these cells from 

 others which closely resemble them in form, but which do not possess their 

 peculiar vital endowments. The size of the nuclei is more constant than that 

 of the cells in which they are found ; their usual diameter being from l-4000th 

 to l-6000th of an inch. Hence the proportion of the cell which the nucleus 

 occupies is extremely variable ; for the whole cell-cavity is sometimes nearly 

 filled by it, especially in young cells, so that it is difficult to make out the pre- 

 sence of a distinct cell- wall, unless by adding water or acetic acid which raises up 

 the latter ; whilst in other instances, especially when the cell is fully formed, 

 the nucleus is comparatively small, being only from one-fourth to one-tenth of 

 the diameter of the cell. The former condition is well seen in the chyle and 

 lymph-corpuscles; the latter in the pavement epithelium-cells (Fig. 24). That 

 the composition of the nucleus is in some respects different from that of the cell- 

 wall, is shown by the fact that the contact of acetic acid neither dissolves it nor 



1 One of the most convenient methods of exhibiting this movement is to rub up a little 

 Gamboge in water; for the resinous particles of this substance, being suspended by the 

 gummy, will continue in motion for any length of time, even when completely secluded 

 from the air, so that evaporation of the liquid can have nothing to do with it. 



2 A stellate nucleus is normally found in the cartilage-corpuscles of Cephalopoda, and 

 abnormally in some cartilaginous tumors in Man. See Mr. Paget's "Lectures on Tumors," 

 "Medical Gazette," Aug. 8, 1851. 



