OF CELLS AND CELL-LIFE. 121 



although by no means identical. 1 The fully-formed cell (Fig. 4) is a membran- 

 ous bag or vesicle, enclosing a cavity, which is occupied by some kind of liquid 

 or solid substance. Its typical form may be con- 

 sidered as globular or spheroidal; but this is com- Fig. 4. 

 paratively seldom exhibited, except in newly-gene- 

 rated cells ; for it is usually more or less altered 

 subsequently, by forces operating either within 

 the cell or externally to it. Thus the cells of 

 adipose tissue, which are usually spheroidal when 

 lying separately in the midst of areolar tissue 

 (Fig. 48), become polyhedral by mutual pressure 

 when compacted into a mass (Fig. 49). Many 



kinds of cells have the form of flattened disks ; Cells from chorda dorgalia of Lam . 

 these being sometimes regularly circular or ellip- pre y : a, a, their nuclei, 

 tical, as the red corpuscles of the blood (Figs. 11 



and 12), sometimes polygonal, as the pigment-cells of the choroid coat (Fig. 39), 

 and some varieties of the pavement-epithelium (Fig. 13) ; but frequently of 

 irregular outline, as is more commonly the case with the pavement-epithelium 

 (Fig. 24) and with the cells composing the parenchyma of the liver (Fig. 157, B). 

 This flattening proceeds so far in the cells of the epidermis, as to render them 

 mere scales (Fig. 36, a). On the other hand, the originally-spheroidal cells may 

 become elongated, instead of depressed ; and may then assume either a very 

 regular prismatic form with flattened polygonal extremities, as in some varieties 

 of cylinder-epithelium, but especially in the enamel of teeth (Fig. 77); or these 

 elongated cells may be more or less cylindrical with pointed extremities, thus 

 becoming fusiform or spindle-shaped, as is well shown in the cells forming the 

 shaft of the hair, but still better in those of which the " smooth" or " non- 

 striated^ muscular fibre is composed (Fig. 99). One of the most curious 

 examples of change of form, however, is presented by those cells, which, while 

 not departing from the spheroidal type, send out caudate processes ; and these, 

 when they issue from the whole circumference of the cell, give to it a stellate 

 character. Both caudate and stellate cells are found in the vesicular substance 

 of the nervous tissue (Fig. 107), and in the pigment-cells of the lower animals 

 (Fig. 87, 3, 3); and it is probable that the " lacunae" and "canaliculi" of bone 

 (Fig. 62) are stellate cells, and that it is also by the inosculation of the peri- 

 pheral extensions of such cells, that 'the ultimate ramifications of capillary blood- 

 vessels, absorbents, and nerves, are at first generated (Fig. 89). The size of 

 cells is not less variable than their forms. Thus even in the Human body, we 

 find them ranging from l-300th of an inch, which is the diameter of many fat- 

 cells and nerve-vesicles, to about l-3200th of an inch, which is the average 

 diameter of the red blood-disks, and thence to as little as l-10,000th or even 

 l-15,000th of an inch, which is the ordinary diameter of the fibrillse of the 

 " striated" muscle, each of which, as will be shown hereafter, is a linear series 

 of very minute cells (Fig. 95). Either lying freely within the cavity of the 

 cell, or, as more commonly happens, attached to some part of its walls, we 

 usually find a body of a somewhat granular appearance, which is called a nucleus 

 (Fig. 4, a). And thus, in examining into the structure and endowments of 

 cells, we have to consider (1) the cell-wall, (2) the cell-contents, and (3) the 

 nucleus. 



100. The Cell-wall, in its primitive state, is an apparently structureless or 

 homogeneous membrane of extreme tenuity; resembling, in fact, the base- 

 ment-membrane to be hereafter described. In composition it agrees uniformly 

 (so far as is yet known) with the protein-compounds; and it is only when ad- 



1 See "Princ. of Gen. and Comp. Phys.," CHAP, iv., Am. Ed. 



