646 THE HUMAN BODY. 



true of the protoplasmic cells forming the walls of the capil- 

 laries. When a highly differentiated tissue is replaced in 

 the body of mammals after breaking down or removal, it is 

 usually by the activity of special cells set apart for that pur- 

 pose, 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 licematoblastic cells retained throughout 

 life in the red marrow of bone and perhaps in the spleen. 

 The nervous tissues are highly differentiated and a nerve is 

 often regenerated after division, but this is by outgrowth of 

 the ends of axis cylinders still attached to their cells and by 

 secondary formation of a medullary sheath around these, and 

 not by division or multiplication of already existing fibres. 

 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 fibres may be found in the cicatrix, 

 but their origin is not known; it is probably not from pre- 

 viously developed muscle fibres. On the other hand, the less 

 differentiated unstriated muscle has been observed to be re- 

 paired in some cases after injury by true karyokinetic division 

 of previously formed muscle cells. Although many gland- 

 cells in the performance of their physiological work are par- 

 tially broken down and lost in their secretion, 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 

 consists essentially of cells. An excised portion of a salivary 

 or parotid gland is never regenerated : the wound is repaired 

 by connective tissues. 



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

 ing 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 reproductive 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 progenitive activity of survivors of the same kind as those 

 destroyed. In none of the higher animals, therefore, do we 

 find multiplication by simple division, or by budding : no one 



