504 THE HUMAN BODY 



breaking down or removal, it is usually by the activity of special 

 cells set apart for that purpose, or by repair or outgrowth of the 

 cells affected and not by their division. The red blood-corpuscles 

 are constantly being broken down and replaced, but the new ones 

 are not formed by the division of already fully formed corpuscles 

 but by certain special hematoblastic cells retained throughout 

 life in the red marrow of bone. The nervous tissues are highly 

 differentiated and a nerve is often regenerated after division, but 

 this is by outgrowth of the ends of axons still attached to their 

 cells and by secondary formation of a myelin sheath around these, 

 and not by division or multiplication of already existing fibers. 

 A striped muscle when cut across is healed by the formation of a 

 band of connective tissue; after a very long time it is said that 

 true muscular fibers may be found in the cicatrix, but their origin 

 is not known ; it is probably not from previously developed muscle- 

 fibers. On the other hand, the less differentiated unstriated 

 muscle has been observed to be repaired in some cases after 

 injury by true karyokinetic division of previously formed muscle- 

 cells. Although many gland-cells in the performance of their phys- 

 iological work are partially broken down and lost in their secre- 

 tion, and then repaired by the residue of the cell, multiplication by 

 division of fully differentiated gland-cells does not appear to occur, 

 if we except such organs as the testes, the secretion of which con- 

 sists essentially of cells. An excised portion of a salivary or pa- 

 rotid gland is never regenerated: the wound is repaired by con- 

 nective tissues. 



We find, then, as we ascend in the animal scale a diminishing 

 reproductive power in the tissues generally: with the increasing 

 division of physiological labor, with the changes that fit pre- 

 eminently for one work, there is a loss of other faculties, and this 

 one among them. The more specialized a tissue the less the re- 

 productive power of its elements, and the most differentiated 

 tissues are either not reproduced at all after injury, or only 

 by the specialization of amoeboid cells, and not by a progeni- 

 tive activity of survivors of the same kind as those destroyed. 

 In none of the higher animals, therefore, do we find multi- 

 plication by simple division, or by budding: no one cell, and 

 no group of cells used for the physiological maintenance of 

 the individual, can build up a new complete living being; but 



