Reasons for Rci^^ciicratioii. 235 



in this sense. According to ]\larchand many cells have a tendency 

 to adhere to their support with as broad a base as possible until 

 forced off by other cells of the same type, as may be seen in the 

 wav young connective tissue cells force their way into the smallest 

 cellulose meshes of elder pith, necessarily passing through its nar- 

 row pores and changing their shape to marked degrees. This tactile 

 stimulus perhaps explains the tendency of connective tissue cor- 

 puscles to apply themselves about foreign bodies, fibrin fibrils and 

 analogous objects. 



In most cases the causes enumerated are combined to give origin 

 to the stimulus for cellular proliferation ; for example, where granu- 

 lation tissue grows up and out of a wound in a luxurious manner 

 instead of ceasing to grow after filling the wound and then 

 cicatrizing, the condition may be ascribed to the combined influence 

 of a number of the conditions above indicated, as the absence of 

 pressure opposition, excess of nutrition and the action of special 

 irritants (saliva, medicaments). 



Regenerative capacity is possessed by the cells and tissues of 

 animals in varying degree. In in,vertebrates and the lower verte- 

 brates the power of replacement or regeneration of important parts 

 after loss is common to all the tissues. Both pieces of a divided 

 earthworm grow into perfect creatures ; salamanders and lizards 

 can reproduce completely the loss of a tail, with bones, muscles, and 

 even the portion of spinal cord belonging thereto ; salamanders can 

 form a new leg in place of one cut oft", or an inferior maxilla 

 (Samuel, Ribbert). This extensive regenerative capacity has been 

 tentatively explained by the supposition that such low forms of 

 animals are more commonly subjected to injuries of the type indi- 

 cated and so their cells, naturally and because of their, less com- 

 plicated structure, retain their power of adaptation better than those 

 of the higher animals. In the latter the cellular construction is 

 more complicated and more fully developed for special functional 

 requirements, and possess a less marked tendency to divide in 

 comparison with the simple cell forms, are less independent, and in 

 some degree require the co-operation of other cells in their nutritive 

 processes. The more a type of tissue or cell departs from its 

 embryonic state and the higher differentiation it attains, the more 

 difficulty it experiences in attempting regeneration. The epithelium 

 of the skin and mucous membranes and connective tissue retain 

 their power of adjustment in response to the numerous faults to 

 which they are liable, and easily and quickly regenerate in the 

 higher as well as the lower forms of life ; but the ganglionic 



