122 OP THE STRUCTURAL ELEMENTS OF THE HUMAN BODY. 



ventitious deposits have been made upon its interior, from the peculiar contents 

 of the cell as in the case of the deposit of horny matter within the cells of 

 the epidermic tissues that its character seems altered. This uniformity in 

 the composition of the cell-wall is indicated, independently of other consider- 

 ations, by the uniform action of acetic acid upon it; for except in the cases in 

 which the original cell-walls are thickened by secondary deposit, this reagent 

 renders them so transparent, that they become for the time almost invisible, 

 though brought into view again on the addition of potash. It is one of the 

 most remarkable peculiarities of this membrane, that whilst it keeps together 

 the liquid contents of the cell, and can afford resistance to any ordinary mechani- 

 cal force tending to their expulsion, it may still permit the most ready transu- 

 dation of fluids, although not the slightest trace of pores for their passage 

 through it can be seen with the highest magnifying powers. And thus, in the 

 ordinary current of nutrition, fluids may pass from cell to cell, apparently by 

 endosmotic action, with considerable rapidity, notwithstanding the presence of 

 the intervening septa. There is no evidence that this membrane possesses any 

 distinctly vital property; and its function appears essentially to consist in the 

 limitation of the cell-contents, which are drawn together by other agency. For 

 when we examine into the history of cytogenesis or cell-formation, we shall find 

 that when cells originate de novo, the cell-membrane is generated at a compara- 

 tively late stage, not making its appearance until the other components are 

 already in existence ( 106). 



101. The Cell-contents are as varied in their composition, as the cell-walls are 

 uniform. In the first place, they may be either solid or liquid. Of the solid, 

 we have examples in the prismatic cells of the enamel of teeth (Fig. 77), which 

 are completely filled with mineral matter in a state of crystalline aggregation, 

 just as are the prismatic cells which form the external layer of many bivalve 

 shells. 1 So, again, the contents of the cells which constitute the horny layer of 

 the epidermis and the substance of the nails (Fig. 40), become solid by the 

 desiccation of the fluid in which the horny matter seems to have been originally 

 dissolved. It is obvious, however, that no vital action can go on, in cells whose 

 contents are of such a character ; and we accordingly find that the tissues thus 

 consolidated are completely thrown out of the current of change, and that their 

 functions are purely physical that of the Epidermis and its horny appendages, 

 as well as of Shell, being simply protective, and that of the Enamel of teeth 

 being to resist pressure. The liquid cell-contents are extremely various; and it 

 is, in fact, in their diversity, that the peculiarity of many tissues consists. Thus, 

 as we shall hereafter see, all the glands are formed upon a plan essentially the 

 same; and their different endowments are entirely due to the diverse properties 

 of the cells of which they are essentially composed, one set filling themselves 

 with the components of bile (Fig. 30), another with those of milk, another with 

 sebaceous matter, whilst within another set are generated the moving particles 

 characteristic of the seminal secretion (Plate I. Fig. 3). Again, we shall find 

 one set of cells drawing fatty matter into their interior from the contents of 

 the alimentary canal (Fig. 135), whilst another set, eliminating a similar sub- 

 stance from the blood, stores it up as a part of the bodily fabric (Fig. 48). 

 And the color which is characteristic of particular parts, as, for instance, the 

 lining of the choroid coat of the eye, the mammary areola, the hair, and the 

 entire epidermis of the dark races of mankind, the red corpuscles of the blood, 

 and the vesicles of nervous matter, is due to the presence of pigmentary matter, 

 either of a deep black, or of some lighter hue, which forms either a part or the 

 whole of the contents of particular cells. These several sets of cells cannot be 

 formed, therefore, unless that pabulum be supplied to them which they require, 



1 See "Prin. of Gen. and Comp. Phys.," 197, Am. Ed. 



