402 BIOLOGY: GENERAL AND MEDICAL 



this phase of regeneration, probably because of the 

 greater difficulty of operating upon the internal organs 

 of small animals. 



Among the vertebrates there is very little true regen- 

 eration of the internal organs. If a kidney be removed, 

 the animal lives on and the other kidney continues to 

 functionate for both, increasing in size for the purpose, 

 not by the formation of new glomerules, but by hyper- 

 trophy, or increase in the size of those already present. 

 In cases in which a kidney is damaged, by operation or 

 disease, new tubules have been found to bud out from 

 the pre-existing tubules and to extend for a considerable 

 distance either among the older tubules or in the scar 

 tissue, but there is no new formation of glomerules, and 

 hence no true regeneration. When large portions of the 

 liver are removed by operation or destroyed by disease, 

 the remaining portions hypertrophy to carry on its func- 

 tion, and not infrequently offshoots from the bile ducts 

 are found extending some distance into the cicatrices, 

 as though new liver cell columns might form, but the 

 attempt seems to be abortive and to include only the 

 cells of the ducts and not those of the parenchyma. 

 The removal of the spleen is compensated for by enlarge- 

 ment of other lymphatic organs without any new for- 

 mation corresponding to the splenic structure. 



When a lung is removed or destroyed by disease, no 

 new tissue forms, though the entrance of an unusual 

 quantity of air may cause inflation of the undisturbed 

 tissue an injurious rather than a beneficial effect. 



The loss of the heart or the brain is certainly, though 

 not immediately, fatal. Life, however, is maintained 

 under these circumstances for a very short time only in 

 most cases. Destruction of the spinal cord results in 

 hopeless palsy. 



As the phylogenetic series is descended, the tenure of 

 life, after such mutilations, increases, and the ability 

 to repair damage becomes greater. Thus, in man, well 

 authenticated cases of regenerative changes in the eye 



