92 



ZOOLOGY. 



a small portion of the gunpowder to be burnt when the 

 train was fired, so that the same train could be fired again 

 and again. 



If we try to trace the origin of the stimulus farther back, 

 we must ask what makes a given nerve-cell start an impulse 

 in its axis-cylinder? The starting of the impulse, like a 

 muscle-fibre's contraction, involves katabolism, and it seems 

 that some nerve- cells (like unstriped muscle-fibres) tend 

 to rhythmical katabolism as a result of mere healthy nutri- 

 tion. But in a vast number of cases, at any rate, the 

 katabolism is the result of a stimulus conveyed by another 

 axis-cylinder. The axis-cylinder of one nerve-cell may end 

 in a ramification which is entangled, as it were, with the 

 dendrons of another nerve-cell, and in this way a stimulus 

 be sent from the one cell to make the other send a stimulus 

 to a muscle -fibre. In the long run the ultimate source of 

 such stimuli is to be found in the sensory nerve-fibres. 

 Light falls on the eye, the air or other external bodies 

 press on the skin or alter its temperature, sound-waves 

 beat on the ear, and so forth, and by these various external 

 agents impulses are started along sensory nerves, whose 

 fibres end within the central nervous system by ramifying 



among the 

 dendrons of 

 nerve- cells. 

 And thus 

 motor im- 

 pulses may 

 be started 

 as a direct 

 or indirect 

 result of a 

 sensory im- 

 pulse. 



11. Re- 

 flex Action. 



A diagram of the simplest case imaginable is shown in 

 fig. 52. Here external impulses directly affect the dendrons 

 of a nerve-cell, A. As a consequence, this cell discharges 



Fig. 52. DIAGRAM OF REFLEX ACTIOM. 



