740 SPECIAL SOMATIC AND VISCERAL RECEPTORS 



The Sense of Pain. — Uncomfortable and painful sensations may be 

 mediated by any sense-organ and even if the intensity of the stimulus 

 is slight. This is true particularly of obnoxious odors, noises, very loud 

 sounds and high intensities of light. As commonly interpreted, how- 

 ever, the word pain refers to a very definite sense quality which allows 

 of a rather precise localization, while the painful sensations just alluded 

 to, are indefinite and general in their character. Pain is widely dis- 

 tributed throughout the body, and is a common phenomenon even in 

 the deeper tissues and organs. The most sensitive part, however, is 

 the skin, as may be gathered from the fact that an incision in the 

 integument always elicits a more intense pain than the handling and 

 cutting of the deeper structures. According to Von Frey,^ more than 

 one hundred pain points are allotted to each 1 sq. mm. of skin. It 

 also appears that the visceral receptors for pain cannot be activated 

 by ordinary mechanical means. Thus, it is possible to operate upon 

 the stomach and intestine without causing an acute sensation of pain, 

 while inflammatory reactions in these organs or their distention by 

 gases and subsequent spasmodic contraction give rise to intense 

 gastralgia and colic. Quite similarly, it is commonly noted that the 

 passage of biliary calculi through the common duct or of renal cal- 

 culi through the ureters, evokes an intense pain in otherwise practically 

 insensitive structures. In all these cases, it appears that the adequate 

 stimulus is distention, pain resulting only if the degree of the distention 

 exceeds that ordinarily required to obtain the sensation of physiological 

 fulness. 



As far as the cutaneous sensation of pain is concerned, it may be 

 held that it is caused either by an overstimulation of the receptors 

 for pressure and touch or by the excitation of specific sense-organs for 

 pain. The second view is more commonly accepted to-day, ^ because 

 the sense of pain possesses a punctiform distribution and is mediated 

 by end-organs which yield solely this particular sensation. Thus, 

 while the hyperexcitation of the touch points may give rise to an 

 unpleasant sensation, the quality of the latter is distinctly different 

 from that of true pain.^ In addition, it might be mentioned that the 

 tactile and temperature senses may be absent in certain regions of the 

 body, but not the pain sense. Thus, while the stimulation of the 

 cornea readily elicits a painful sensation, it does not give a distinct 

 sensation of touch. Furthermore, it is a common observation that 

 pathological processes may give rise to an analgesia or loss of the pain 

 sense, but not to an anesthesia or loss of the sense of touch. 



Assuming, therefore, the separate existence of pain points, it seems 

 most plausible to refer this sensation to the free endings of the nerve 



1 Arbeiten aus dem physiol. Inst, zu Leipzig, 1896. 



2 Brown-Sequard, Jour, de physiology, vi, 1864, and Funcke, Hermann's 

 Handb. der Physiol., ill, 1883. 



3 Blix, Zeitschr. fur Biol., xx and xxi. 1884-85. 



