152 BIOLOGY: GENERAL AND MEDICAL 



short and finely branched, and serve to effect communica- 

 tion between the nerve cells themselves. 



The afferent neuraxes begin in specialized structures, 

 differing according to the particular stimuli they are de- 

 signed to collect, and diverging widely in form in the dif- 

 ferent sense organs. The efferent neuraxes terminate in 

 structures that differ as widely according to the particular 

 kind of tissue they are to excite. The neuraxes constitute 

 the bonds of communication between the cells, the sources 

 of stimulation, and the parts affected. 



The reflex circuit is foreshadowed in what is seen in some 

 of the higher ccelenterates, where certain ectodermal cells 

 appear to transmit such impulses as they receive to spe- 

 cialized nerve cells, which, in turn, excite muscle cells to 

 action through the intermediation of certain fibres ex- 

 tending from one to the other. Here one nerve cell re- 

 ceives the impulse and passes it on, but in higher animals 

 this rarely if ever occurs, because the neuraxis of the nerve 

 cell receiving the impulse ends in that cell which must 

 pass it on through its dendrites to one or more neighboring 

 nerve cells whose neuraxes extend to the parts to be af- 

 fected. 



Three elements are then concerned in reflex action, and 

 constitute the reflex arc or reflex circuit. They are known 

 as the receptor ', the controller, and the effector. 



The receptor is the specialized beginning of an afferent 

 neuraxis, and, as has been said, is diversified in form and 

 position according to the source and nature of the im- 

 pulses it has to collect. Sherrington recognizes three 

 types of receptors: 



Interoceptors or visceral receptive organs, which only respond to 

 stimulations arising within the body, chiefly in connection with 

 the process of nutrition, excretion, etc. 



Exteroceptors or somatic sense organs, which respond to stimula- 

 tions arising from objects outside of the body. 



Proprioceptors or sense organs found in the muscles, tendons, 

 joints, etc., to regulate the movements called forth by the 

 stimulation of the exteroceptors. 



"The proprioceptive sense organs are deeply embedded in 

 the tissues and are typically excited by those activities of the 



