590 REFLEX ACTIONS. [BOOK in. 



object such as an arm or a stick, complex reflex movements 

 are excited, the obvious purpose as well as effect of which is to 

 twine the body round the object. A decapitated snake will how- 

 ever with equal and fatal readiness twine itself round a red-hot bar 

 of iron. 



It may be added that the movements evoked by even a 

 segment of the cord may be purposeful in character; hence we 

 must conclude that every segment of the protoplasmic network is 

 mapped out into mechanisms. 



Lastly, the characters of a reflex movement are, as we need 

 hardly say, dependent on the condition of the cord. The action of 

 strychnia just alluded to is an instance of an apparent augmen- 

 tation of reflex action best explained by supposing that the 

 resistances in the cord are lessened. There are probably however 

 cases in which the explosive energy of the nerve-cells is positively 

 increased above the normal. Conversely, by various influences of 

 a depressing character, as by various anaesthetics or other poisons, 

 reflex action may be lessened or prevented ; and this again may 

 arise either from an increase of resistance, or from a diminished 

 action of the nerve-cells themselves. 



In actual life reflex movements, in by far the greater number 

 of instances, are occasioned by stimulation of the skin or of the 

 mucous membrane. They may however occur as the result of 

 stimulation of the organs of special sense. A sound or a flash 

 of light readily produces a start, a bright light causes many 

 persons to sneeze, and reflex movements may even result from a 

 taste or smell. 



Inhibition of Reflex Action. The reflex actions of the spinal 

 cord, like other nervous actions, may be totally or partially inhibited, 

 that is may be arrested or hindered in their developement by 

 impulses reaching the centre while it is already in action. Thus 

 if a decapitated snake be suspended, slow rhythmic pendulous 

 movements, which appear to be reflex in nature, soon make their 

 appearance, and these may be for a while arrested by slight 

 stimulation, as by gently stroking the tail. We have already seen 

 that the action of such nervous centres as the respiratory and vaso- 

 motor centres, which frequently at all events is of a reflex nature, 

 may be either inhibited or augmented by afferent impulses. The 

 micturition centre in the mammal, which is also largely a reflex centre, 

 may be easily inhibited by impulses passing downward to the lumbar 

 cord from the brain, or upward along the sciatic nerves. In the case 

 of dogs, whose spinal cord has been divided in the dorsal region, 

 micturition set up as a reflex act by simple pressure on the 

 abdomen, or by sponging the anus, is at once stopped by sharply 

 pinching the skin of the leg. And it is a matter of common 

 experience that micturition may be suddenly checked by an 

 emotion or other cerebral event. The erection centre in the 



