THE TISSUES 87 



joining together, regenerate the nerve, so that frequently when 

 the detached end is united to the central part, the nerve is 

 very rapidly capable of performing its functions again. Other 

 investigators maintain that the regeneration is always due to 

 outgrowths from the axis cylinder, either of the central end 

 of the cut nerve or from adjacent nerves, and this is sup- 

 ported by the fact that a cut nerve will grow down into any 

 other nerve with which it is connected (see p. 81). 



The cell of the neuron appears to have the power of accu- 

 mulating a reserve of material as Nissl's granules, for it 

 has been found that after continued action these granules 

 diminish in amount. The nucleus, too, would seem to have 

 the power of giving off material for the nourishment of 

 the neuron, since in conditions of excessive activity it has 

 been found shrunken and distorted. Whether the cells play 

 any other part in the physiology of the neuron is not known. 



But the cell is also dependent for its proper nutrition upon 

 the condition of the rest of the neuron. When the axon is 

 cut, the chromatin of the cell nucleus decreases, and the 

 nucleus becomes displaced to one side, and ultimately the 

 whole cell degenerates. This is sometimes called Nissl's 

 Degeneration (see Fig. 35). 



The passage of excitation from one neuron to others 



occupies a very appreciable time. This may be readily 

 demonstrated in what is called reflex action, which may be 

 denned as the response through outgoing neurons which 

 follows the stimulation of ingoing neurons. As examples of 

 this the drawing up of the leg when the sole is tickled, or 

 the winking of the eye when the eyeball is touched, may be 

 taken. In each of these the end of an ingoing neuron is 

 excited ; the change passes in and sets up changes in out- 

 going neurons, which act upon muscles. In the case of the 

 eye, about '06 second elapses between the touching of the 

 eye and the resulting " wink." 



Knowing the rate at which nerve changes pass along 

 nerves, and knowing the length of the ingoing and of the 

 outgoing neurons, the time taken in the passage of the 

 change along these is readily calculated. In a reflex wink 

 it is about '01 second. 



