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CHAPTER 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 
there is 

other cases, however—as in nerve or retina 
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 anesthetics, 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 
