THE RECEPTORS AND EFFECTORS 



85 



cold only. The free nerve-endings found here he assumes to be pain recep- 

 tors and the end-bulbs of Krause (Fig. 27) to be cold receptors. By an 

 analogous argument he assumes that the "genital corpuscles" of Dogiel and 

 some similar endings widely distributed in the skin are warmth receptors. 

 By some other physiologists these types of corpuscles are regarded as belong- 

 ing to the tactile system. Stimulation of the somatic nerves of deep sensi- 

 biEty causes no temperature sensations. (For temperature sensations in the 

 viscera see p. 242.) 



4. End-organs for Pain. Some physiolo- 

 gists believe that there are separate nerve- 

 endings for pain; others regard pain as a 

 quality which may be present in any sense, 

 and not as itself a true sensation (pp. 249 

 ff.). The free nerve-endings among the cells 

 of the epidermis are regarded by von Frey 

 as the pain receptors, because these endings 

 alone are present in some parts of the body 

 where susceptibility to pain is the only sense 

 quality present, such as the dentin and pulp 

 of the teeth (Fig. 28), the cornea, and the 

 tympanic membrane of the ear (J. G. Wilson). 



Similar endings are found throughout the 

 epidermis (Fig. 29) and in many deep struc- 

 tures. The nerves of deep sensibility of the 

 somatic sensory type may also carry painful 

 impressions. (For visceral pain see pp. 243, 

 250.) According to Head, cutaneous pain is 

 wholly of protopathic type, and in case of in- 

 jury to the peripheral nerves it disappears 

 and reappears in regeneration simultaneously 

 with the protopathic type of tactile and tem- 

 perature sensation. This cutaneous pain is 

 not accurately localizable unless epicritic cu- 

 taneous sensibility is also present. 



5. End-organs of General Chemical Sen- 

 sibility. In man this type of sensibility is 

 found only on moist epithelial surfaces, such 

 as the mouth cavity; but in fishes it may be 

 present over the entire surface of the body. 

 The sense organ is probably the free nerve 

 terminals among the cells of the epithelium, 

 never special sense organs like taste-buds, 

 for these when present in the skin belong 

 to a quite different system. Coghill has 



recently shown that the supposed sensitivity of the amphibian skin to acids 

 is really due to a destructive action of the reagents upon the epithelium, and 

 the entire question of diffuse chemical sensibility requires further study. 



6. Organs of Hearing. The stimulus is material vibrations whose 

 frequency ranges from 30 to 30,000 per second (see p. 70). The receptor is 

 the spiral organ (organ of Corti) in the cochlea of the ear (see p. 197), and 

 perhaps also the sensory spots in the saccule and utricle. There are two 

 forms of auditory sensations: (1) noise, stimulated by sound concussions or 

 irregular mixtures of aerial vibrations; (2) tone, stimulated by sound waves 

 or periodic aerial vibrations. 



Fig. 28. Longitudinal 

 section of a tooth of a fish, 

 Gobius, showing nerve ter- 

 minals: d, Dentin; n, 

 nerve-fibers entering the 

 cavity of the dentin and 

 ending free. (After Ret- 

 zius, from Barker's Ner- 

 vous System.) 



