THE HIGHER ORGANISMS 153 



body itself which arise in response to external stimulation. 

 The proprioceptors then excite to reaction the same organs of 

 response as the exteroceptors and regulate their activity by 

 reinforcement or by compensation or by the maintenance of 

 muscular tone. All actions concerned with motor coordination 

 and with the maintenance of posture or attitude of the body 

 and with equilibrium involve the proprioceptive system." 



The controller is a nerve center. It consists of a group 

 of nerve cells communicating with one another by den- 

 drites. 



The effector is the nerve ending in muscle, gland, or 

 other tissue to be activated. 



In every reflex arc or circuit the parts must be so con- 

 nected that a stimulation is followed by a useful and 

 adaptive response. The response is called a reflex or 

 reflex act, and can be exemplified by the sudden jerking 

 away of the hand when pricked by a needle. 



Such an effect is only possible when a sensory nerve 

 ending in the hand (receptor) is irritated by the needle, 

 the impulse transmitted to a nerve cell along the neuraxis, 

 then transmitted to other nerve cells by the dendrites, 

 and then new impulses sent out along efferent neuraxes 

 from these cells to the muscles moving the hand and arm. 



How, one may ask, does such a withdrawal differ from 

 the withdrawal of its pseudopodia by an irritated ameba? 

 Only in the more complicated and more systematic 

 manner in which it is accomplished. Verworn sees no 

 essential difference. He says: 



" The most primitive form of a reflex arc exists in unicellular or- 

 ganisms, the cell body of which possesses both the sensory and motor 

 elements and even functions also as the control bond for the two. 

 . . . What is in the unicellular organism accomplished within a 

 single cell is, in animals with a nervous system, distributed among 

 several cells. In the most simple nervous systems, such as are seen 

 among the invertebrates, one cell, the sensory, receives the stimulus; 

 from this a centripetal nerve path conducts to a central cell, the 

 ganglion cell, and from here a centrifugal nerve path conducts to a 

 cell that performs the reaction, the motor end cell." 



The only practical difference, therefore, between a 

 tropin and a reflex reaction is to be found in the greater 



