THE CELL DOCTRINE. 73 



homogeneous, and to a certain extent sclerotic, and 

 acquires an incredible power, of resisting the influ- 

 ence of reagents, so that it is only after long-con- 

 tinued action that even the strongest caustic sub- 

 stances are able to destroy it, whilst it completely 

 resists the caustic alkalies and acids in the degree of 

 concentration usually employed in microscopical in- 

 vestigation. The farther this change advances, the 

 more does the elasticity of the parts increase, and in 

 sections we usually find these fibres, not straight or 

 elongated, but tortuous, curled up, spirally coiled, or 

 forming little zigzags (.Fig. 17, c, e). These are the 

 elements which by virtue of their great elasticity, 

 cause retraction in those parts in which they are 

 found in considerable quantity, as, for example, in the 

 arteries. The fine elastic fibres, which are those 

 which possess the greatest extensibility, are usually 

 distinguished from the broader ones, which certainly 

 do not present themselves in tortuous forms. As re- 

 gards their origin, however, there seems to be no 

 difference between the two kinds : both are derived 

 from the connective tissue cells, and their subsequent 

 arrangement is only a reproduction of the original 

 plan. In the place of a tissue consisting of a basis 

 substance and anastomosing reticulated cells, there 

 afterward arises a tissue with its basis substance 

 mapped out by long elastic networks with extremely 

 compact and tough fibres." This may be looked 

 upon as the least well determined of the important 

 points of Virchow's doctrine, though most German 

 histologists also favor it. Among these may be 



