26 STRUCTURAL COMPOSITION OF HUMAN BODY. 



appearance of structure : it appears sometimes to be an 

 albuminous substance; sometimes a horny matter, as in 

 thick and dried cuticle. In almost all cases (the dry cells of 

 horny tissue, perhaps, alone excepted) the cell- wall is made 

 transparent by acetic acid, which also penetrates into the 

 interior and distends it, so that it can hardly be discerned. 

 But in such cases the cell-wall is usually not dissolved ; it 

 may be brought into view again by merely neutralizing 

 the acid with soda or potash. 



The simplest shape of cells, and that which is probably 

 the normal shape of the primary cell, is oval or spheroidal, 

 as in cartilage-cells and lymph- corpuscles ; but in many in- 

 stances they are flattened and discoid, as in the red blood- 

 corpuscles (fig. 26) or scale-like, as in the epidermis and 

 tesselated epithelium (fig. 2). By mutual pressure they 

 may become many-sided, as are most of the pigment-cells 

 of the choroidal pigmentum nigrum (fig. 12), and those 

 in close-textured adipose tissue ; they may assume a conical 

 or cylindriform or prismatic shape, as in the varieties of 

 cylinder-epithelium (fig. 4) ; or be caudate, as in certain 

 bodies in the spleen ; they may send out exceedingly fine 

 processes in the form of vibratile cilia (fig. 6), or larger 

 processes, with which they become stellate, or variously 

 caudate, as in some of the ramified pigment-cells of the 

 choroid coat of the eye (fig. 13). 



The contents of all living cells, including the nucleus, are 

 formed in a greater or less degree of protoplasm, less as 

 the cell grows older. But, besides, cells contain matters 

 almost infinitely various, according to the position, office, 

 and age of the cell. In adipose tissue they are the oily 

 matter of the fat; in gland-cells, the contents are the 

 proper substance of the secretion, bile, semen, &c., as the 

 case may be ; in pigment-cells they are the pigment-gra- 

 nules that give the colour ; and in the numerous instances 

 in which the cell-contents can be neither seen because they 

 are pellucid, nor tested because of their minute quantity, 



