i 1 4 KE GENERA 7 'ION 



be fulfilled, and the animal with a broken leg or wing will most prob- 

 ably be killed. Nevertheless, since the bones have this power at 

 whatever level they may be broken (but only if they are kept together 

 artificially), the process can scarcely have been acquired through the 

 liability of the parts to injury. We find here another instance of a 

 useful process existing in animals, but one that could not have been 

 acquired by exposure of the part to injury. It is probable that this 

 same property is found in all the bones of the body, in those that 

 may occasionally be injured, and in those that are not. 



The muscles have also the power of regenerating, although few 

 experiments have been made except in those forms in which the 

 whole leg can regenerate, yet there are a few observations that show 

 that even in mammals, in which the leg or the arm cannot regenerate 

 as a whole, a certain amount of regeneration of the muscles them- 

 selves may take place. 



It has been known for a long time that if a nerve is cut a new 

 nerve grows out from the cut-end, and may extend to the organs sup- 

 plied by that nerve. The process takes place more successfully if 

 the peripheral part is left near the cut-end from which the new nerve 

 grows. Whether this old part only serves to guide the new part to 

 its proper destination, or whether it may also contribute something to 

 the new nerve, as, for instance, cells for the new sheath, is not finally 

 settled. The general opinion in regard to the origin of the new nerve 

 fibres is that the central axis or fibril grows from the cut-end. That 

 this power could have been acquired for each nerve as a result of its 

 liability to injury is too improbable to discuss seriously. 



The central nervous system of the higher vertebrates s,eems to 

 have very little power of regeneration, and although in some cases 

 a wounded surface may be covered over and a small amount of con- 

 nective tissue be formed, the development of new ganglion cells does 

 not seem to occur. In other animals, as the earthworm, planarian, 

 and even in the ascidian, as shown by Loeb, a new entire brain may 

 develop after the removal of the old brain, or of that part of the 

 body in which it is contained. 



This examination of the power of regeneration of internal organs 

 in the vertebrates has shown that it is highly improbable that there 

 can be any connection between their power of regeneration and their 

 liability to injury. That the internal organs may be occasionally 

 injured by bacteria, or by poisons made in the body, may be admitted, 

 but that injuries from this source have been of sufficient frequency 

 to establish a connection, if such were indeed possible, between their 

 power of regeneration and their liability to injury from these causes 

 is too improbable a view to give rise to much doubt. These results 

 taken in connection with those discussed in the preceding chapter go 



