140 THE CELL DOCTRINE. 



contents, and is therefore a part of the formed or 

 non germinal matter, and is in no way essential to 

 the constitution of the cell. 



In structure the cell-wall when present is usually 

 homogeneous, transparent, in a word structureless. 

 Recent observations have, however, shown this to be 

 not invariable. Thus, it is not difficult to demon- 

 strate with the higher powers certain lines or strise 

 in the outer thickened edge of the columnar cells 

 capping the villi of the small intestine, which are 

 perhaps correctly interpreted by Funke, KoUiker, 

 and others, as porous canals in the cell-wall, through 

 which the tine particles of the emulsified fats are 

 absorbed. These may at least be discovered occupy- 

 ing corresponding situations with great distinctness. 

 Schroen has also described similar markings in the 

 thickened cell-wall of the rete Malpighii of the 

 human skin. 



Intercellular Substance. — Closely allied to the cell- 

 M^all is the so-called intercellular substance of tissues. 

 Although it is true that all tissues originate from 

 cells, yet there are comparatively few which in their 

 perfect state are composed purely of cells, but the 

 latter are more or less separated by a substance be- 

 tween them. Thus cartilage or " gristle " is com- 

 posed of cells and an intercellular substance, which 

 is either hyaline or fibrous, and all the connective 

 tissues, under which are included bone, cartilage, 

 already alluded to, white fibrous tissue, yellow elastic 

 tissue, and even teeth, are similarly composed of 

 cellular elements, and a something between them, 

 which is either homogeneous or structured. 



