THE RECEPTORS AND EFFECTORS 89 



12. Organs of static and equilibratory sensation arising from 

 stimulation of the semicircular canals of the internal ear (Fig. 32). This is 

 the most highly specialized member of the proprioceptive group and acts 

 in conjunction with all of the other somatic senses to maintain equilibrium, 

 posture, and the tone of the muscular system (see p. 189). The eyes and 

 most of the other exteroceptive sense organs, so far as they act in the way 

 just suggested, also serve as proprioceptors. 



II. VISCERAL RECEPTORS 



The visceral or interoceptive senses fall into two well-defined groups: 

 First, the general visceral systems are without highly specialized end-organs 

 and are innervated through the sympathetic nervous system. Their reac- 

 tions are chiefly unconsciously performed. Second, the special visceral 

 senses are provided with highly developed end-organs which are in- 

 nervated directly from the brain without any connection with the sympa- 

 thetic nervous system. The special visceral sense organs may in some 

 cases serve as exteroceptors as well as interoceptors. Their reactions 

 may be conscious and voluntary. 



A. GENERAL VISCERAL GROUP 



Many of the sensations of this group are obscure and a number of excito- 

 motor and excito-glandular reactions may be included here which never 

 come into clear consciousness, particularly those concerned with nutrition, 

 excretion, and vasomotpr adjustments. The number of these reactions 

 might be considerably increased; for further discussion of these reflexes 

 see p. 234. 



13. Organs of Hunger. The stimulus is strong periodic contractions 

 of the muscles of the stomach. Hunger is apparently a variety of muscle 

 sense, but other factors are also present (see p. 240) . 



14. Organs of Thirst. The specific stimulus here is probably a dry- 

 ing of the pharyngeal mucous membrane, together with more general 

 conditions. 



15. Organs of Nausea. The stimulus is probably an antiperistaltic 

 reflex in the digestive tract (see p. 243). 



16. Organs giving rise to respiratory sensations, suffocation, etc. (see 

 p. 235). 



17. Organs giving rise to circulatory sensations, flushing, heart panics, 

 etc. (see p. 234). 



18. Organs giving rise to sexual sensations. 



19. Organs of sensations of distention of cavi.ties, stomach, rectum, 

 bladder, etc. This is a variety of muscle sense. 



20. Organs of visceral pain (see pp. 243, 250). 



21. Organs of obscure abdominal sensations associated with strong 

 emotions of fright, anger, affection, etc., characterized (probably correctly) 

 by the ancients by such expressions as "yearning of the bowels," etc. The 

 stimulus is probably a tonic contraction, of the unstriped visceral muscula- 

 ture. 



The nerve-endings of the general visceral receptors are generally either 

 simple terminals in the visceral muscles or free arborizations in or under the 

 various mucous surfaces, without the development of specialized accessory 



