CHAPTEE XX 



GENERAL SURVEY AND CONCLUSION 



WE have seen that stimulus produces a certain ex- 

 citatory change in living substances, and that the 

 excitation produced sometimes expresses itself in a 

 visible change of form, as seen in muscle ; that in many 

 other case.s, however as in nerve or retina there is 

 no visible alteration, but the disturbance produced by 

 the stimulus exhibits itself in certain electrical changes, 

 and that whereas the mechanical mode of response 

 is limited in its application, this electrical form is 

 universal. 



This irritability of the tissue, as shown in its 

 capacity for response, electrical or mechanical, was 

 found to depend on its physiological activity. Under 

 certain conditions it could be converted from the 

 responsive to an irresponsive state, either temporarily 

 as by anaesthetics, or permanently as by poisons. 

 When thus made permanently irresponsive by any 

 means, the tissue was said to have been killed. We 

 have seen further that from this observed fact that 

 a tissue when killed passes out of the state of responsive- 

 ness into that of irresponsiveness ; and from a confusion 

 of ' dead ' things with inanimate matter, it has been 

 tacitly assumed that inorganic substances, like dead 



