SIMPLE REFLEX ACTION 153 



to it which come directly from the ganglion instead of 

 from a receptor. Thus it is possible for nervous disturb- 

 ances to pass from receptor to effector, but only by way 

 of the center or ganglion, and only by passing through 

 two nerve-cells. It is customary to call the nerve-cell 

 which extends between the receptor and the ganghon a 

 sensory nerve-cell, and the one which extends from the 

 ganglion to the effector a motor nerve-cell. The arrange- 

 ment which is described is easily understood because, as 

 stated above, it is comparable to a telephone installation. 

 Just as in the latter messages coming in over one line 

 can be directed out over another and their course can be 

 directed from the center, so that the outgoing line is now 

 one and now another, the nerve impulses coming into the 

 ganglion from the receptors are passed through it and 

 out into pathways leading to one or another effector. 



Simple Reflex Action. — A fact of the behavior of 

 animals which can be clearly demonstrated by careful 

 study is that the stimulation of a given contact receptor 

 nearly always results in the action of a particular effector, 

 the response of the effector coming with no more delay 

 after the stimulation of the receptor than is necessary 

 for the nervous impulse to pass from the receptor up to 

 the center and from the center out to the effector. So 

 far as the majority of actions of the simple animal go, 

 one might dispense with the ganglia and suppose the re- 

 ceptor and effector to be directly connected, as they are 

 in the very primitive forms described in a former para- 

 graph. Evidently there are within the center or ganglion 

 preferred or usual connections between sensory and motor 

 nerve-cells so that under ordinary circumstances the ner- 

 vous disturbance arriving from any particular sensory 

 cell is passed on at once to a particular motor cell and 

 arouses^ a particular effector to activity. A case of this 

 sort, in which a given action follows the stimulus of a 

 given receptor with regularity, is called reflex action, and 

 the particular sequence of nerve-cells between the re- 



