CHANGES IN TISSUE AFTER FORMATION. 95 



arrangement of nerve fibres, which can be readily 

 explained by supposing movements of the bioplasm, 

 while I believe in no other manner can the facts be 

 satisfactorily accounted for. So also by the careful 

 study of the arrangement of the twisting of nerve 

 fibres in many tissues, we shall become convinced of 

 the never-ceasing movement of the masses of bioplasm, 

 not only during development, but afterwards, during 

 the adult period of life. In this way only can 

 many of the highly intricate structural arrangements, 

 familiar to us in many organs of man and the higher 

 animals, be explained. 



13*J. Changes in Tissue after formation. Changes, 

 however, take place in many kinds of tissue after the 

 formative act has been completed. In some cases the 

 part which was first produced dries up, and gives 

 rise to irregularities or cracks, which appear as 

 peculiar markings, and may be characteristic of the 

 fully formed structure. Sometimes a tissue, which 

 for a long time may appear homogeneous and clear, 

 gradually acquires a fibrous appearance from the 

 tendency of the old tissue to split, or cleave in certain 

 directions, which will, in fact, be found to correspond 

 to the lines in which new tissue material was de- 

 posited at an early period of formation. 



138. Epithelial Tissue. One of the simplest forms 

 of tissue found in man and animals, and perhaps that 

 which is produced most easily and most quickly, is 

 cuticular epithelium, 115. Possessing elasticity, 

 and considerable extensile property, performing the 

 passive office of protecting more important textures 

 beneath it, upon which it rests, and with which it is 

 often connected, this tissue is readily repl-aced, if 

 removed, and when injured is quickly and effectually 

 repaired. Epithelial tissues exhibit, however, re- 

 markable differences in property in different situations. 

 One may be dry and firm, hard and resisting, forming 

 a sharp point or cutting edge, as in certain kinds of 



