THE RECEPTORS AND EFFECTORS 77 



The organs belonging to each of these two groups do much of 

 their work independently of the other group, i. e., visceral stimuli 

 call forth visceral responses and external or somatic stimuli 

 call forth somatic responses. Nevertheless, the two groups of 

 organs are by no means entirely independent, for external excita- 

 tions may produce strong visceral reactions, and conversely. 

 Thus, the sight of luscious fruit (exteroceptive stimulus) natu- 

 rally calls forth movements of the body (somatic responses) to go 

 to the desired object and seize it. But if one is hungry, the 

 mouth may water in anticipation, a purely visceral response. 

 On the other hand, the strictly visceral (interoceptive) sensation 

 of hunger is apt to set in motion the exteroceptive reactions 

 necessary to find a dinner. 



Sherrington, whose analysis with some modifications is here 

 adopted, recognizes three types of sense organs or receptors: 

 (1) the interoceptors, or visceral receptive organs, which respond 

 only to stimulation arising within the body, chiefly in connection 

 with the processes of nutrition, excretion, etc.; (2) the extero- 

 ceptors, or somatic sense organs, which respond to stimulation 

 arising from objects outside the body; (3) the proprioceptors, a 

 system of sense organs found in the muscles, tendons, joints, 

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

 of the exteroceptors. This third group is really subsidiary 

 to the somatic group, or exteroceptors, and will be considered 

 more in detail below. 



The proprioceptive sense organs are deeply embedded in the 

 tissues and are typically excited by those activities of the body 

 itself which arise in response to external stimulation. The 

 proprioceptors then excite to reaction the same organs of re- 

 sponse as the exteroceptors and regulate their action by reinforce- 

 ment or by compensation or by the maintenance of muscular 

 tone. All reactions concerned with motor coordination, with 

 maintenance of posture or attitude of the body, and with 

 equilibrium involve the proprioceptive system. 



The important point to bear in mind here is that stimulation 

 of the visceral sense organs typically calls forth visceral responses, 

 i. e., adjustments wholly within the body, while stimulation of 

 the somatic (exteroceptive) sense organs typically calls forth 

 somatic responses, i. e., a readjustment of the body as a whole 



