252 Journal of Comparative Neurology. 



The nervous elements and the neuroglia have a most strik- 

 ing interrelation. In other organs Ave meet only the parenchy- 

 ma and mesoblastic tissue, and one can recognize the general 

 law, that a deterioration of the parenchyma finally leads to hy- 

 perplasia of the mesoblastic tissue ; if a sufficient amount of 

 tissue has become disintegrated without being regenerated a 

 'scar' forms, consisting largely of mesoblastic tissue which 

 contracts and becomes more or less fibrous. In the nervous 

 system, the (epiblastic) neuroglia takes a very prominent part 

 in the repletory function, and the mesoblastic vascular tissue 

 steps in more prominently only when both the nervous ele- 

 ments and a great part of the neuroglia have perished. Regen- 

 eration of nervous elements being a great exception, the neu- 

 roglia proliferation is the constant result of their decay. At- 

 tention has already been drawn to the importance of the gen- 

 eral firmness of the tissue. In the new-born, the tissues are so 

 plastic, that a defect is almost completely resorbed and the out- 

 line of the organ adapts itself to the defect ; if, however, the 

 form has become stable and more rigid, the filling-in is partly 

 explained by the natural equilibrium of tension which must be 

 estabHshed again if possible (compare the difference of neuro- 

 glia reaction in the spinal cord of (infantile) porencephaly and a 

 secondary degeneration of the pyramid in the adult in Fig. 7. 

 If the equilibrium of tension is disturbed too strongly so that 

 collapse occurs before the gap can be filled up with neuroglia 

 alone, a scar of vascular tissue plus neuroglia forms. This is 

 the case where a disorder of nutrition destroys at once all the 

 elements, so that an ischaemic necrosis of the entire district of 

 a blood-vessel is established. It is not improbable that the oc- 

 currence of porencephaly and of lobar sclerosis is due to a very 

 acute ischaemia in the case of porencephaly and a subacute or 

 less complete ischaemia in the case of lobar sclerosis. 



This compensatory relation between neuroglia and nervous 

 elements does not, however, cover the only possible variations 

 in the constitution of tissue-portions ; indeed Nissl and Alz- 

 heimer insist very strongly on the observation that an increase 

 of glia is possible without being a consequence of decay of 



