92 HUMAN PHYSIOLOGY 



distant arcs are less readily brought into play. But in various 

 more complex reflex actions certain special arcs are associated, 

 apparently as the result of their having been educated to 

 act together, and whether this association is an inherited one, 

 or whether it has been acquired, there seems to be a ten- 

 dency for nerve action, which has once travelled by a definite 

 route, to take the same route again. Definite channels of 

 communication connecting ingoing impulses with definite 

 outgoing reaction are thus established. 



The spinal reflexes are modified 1st, By the condition of 

 the neurons (p. 81). 2nd, By stimuli from adjacent areas. 

 The reflex act of sneezing, set up by stimulation of the 

 nasal mucous membrane, may be checked by firmly pressing 

 over the bridge of the nose. 3rd, By the upper arcs. 

 Spinal reflexes are increased when the cord is separated from 

 the brain. 



Cerebral and Cerebellar Reflex Actions. The upper arcs 

 may also act reflexly i.e. inevitably. Thus stimulating 

 certain areas of skin in the dog leads to a reflex scratching 

 in which the body is bent and balanced, while the hind 

 leg performs complex movements. Here the balancing 

 action of the cerebellar arc, as well as the directive action of 

 the cerebral arc, are both involved. But since the action is 

 obviously inevitable, it is classed as a reflex. 



B. More Complex Reactions. 



It has been seen that the spinal reflexes are noc absolutely 

 inevitable since they are modified by the condition of the 

 nervous structures involved. 



When the more complex reactions, in which the cerebral 

 arc is involved, are studied, the resulting action appears 

 to be less inevitable, and to be influenced by the sensations 

 and other changes in consciousness which accompany it. 

 It has, in fact, been assumed that the state of consciousness 

 is the determining factor in the result, and hence such 

 actions have been called voluntary. But since in such 

 conditions as sleep-walking and hypnosis the most complex 

 and selective actions are performed without the intervention 



