THE CELL DOCTRINE. 67 



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-contin- 

 ued action that even the strongest caustic substances 

 are able to destroy it, whilst it completely resists the 

 caustic alkalies and acids in the degree of concentra- 

 tion usually employed in microscopical investigation. 

 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 retrac- 

 tion in those parts in which they are found in con- 

 siderable 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 pre- 

 sent themselves in tortuous forms. As regards their 

 origin, however, there seems to be no difference be- 

 tween the two kinds; both are derived from the 

 connective tissue cells, and their subsequent arrange- 

 ment is only a reproduction of the original plan. 

 In the place of a tissue, consisting of a basis sub- 

 stance 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 



