PAIN 



463 



threshold is a constant from man to man or from one 

 time to another in a given man." He attributes the 

 variability to the 'psychic reaction component' rather 

 than the 'original sensation.' Precise determinations of 

 pain threshold find perhaps their greatest practical 

 utility in the pharmacologic appraisal of analgesic 

 agents — a subject beyond our scope. To me it appears 

 that firmer conclusions are tenable when one works 

 with stimuli in animals and man which, in normal 

 man, consistenth' evoke unequivocal and unalloyed 

 pain. The subsequent discussion will seek to emphasize 

 such work. 



Other methods for the stud\' of pain either deliber- 

 ately evoked or arising in pathological states will now 

 be considered. 



Distention of Viscera 



Incision into and passing a needle through skeletal 

 mu.scle are almost painless, and the abdominal 

 viscera exposed under local anesthesia may be cut, 

 torn or burned as long as the parietal peritoneum and 

 roots of the mesentery are not stimulated (164). These 

 viscera are capable of initiating impulses for pain upon 

 the appropriate stimulus, however, and it was Len- 

 nander (165) who demonstrated that it was distension 

 of the human kidney pelvis which was painful. Hurst 

 (131) extended this principle, that distension is the 

 pain-evoking stimulus for hollow viscera, to studies of 

 the alimentary canal. Davis et al. (54, 56) have applied 

 the same tactic to studies of the gall bladder. Rapid 

 expansion of the capsule of solid organs like the li\er 

 and kidney also hurts. 



Arterial Constriction with Ischemia and Arterial Dilatation 



Both arterial constriction or occlusion to the point 

 of ischemia and severe arterial dilatation at times 

 associated with excessive pulsation in a part are pro- 

 ductive of pain. Sutton & Lueth (257) thought that 

 myocardial anoxia was the physiological stimulus for 

 cardiac pain when they found that lightly anesthetized 

 dogs gave responses suggestive of pain when a coronary 

 artery was occluded by a ligature. Gorham (107) con- 

 firmed these results and added the ob.servation that if 

 three ligatures were placed in the wall of the coronary 

 artery so that divergent traction on them would tend 

 to distend the vessel, responses of 'pain' also occurred. 

 The headache of migraine is one of the better studied 

 examples of a pain probably brought on by arterial 



dilation. Wolff, a prominent exponent of this view, 

 summarized the e\idence for it in 1948 C303, pp. 265 

 to 288). Histamine produces, among other effects, 

 painful distension of arteries and presents a means of 

 evoking headache experimentally, although this 

 differs from that seen in migraine in at least eight 

 respects according to W'olff (303, pp. 289 to 290). The 

 reverse situation, ischemia as a cause of pain, is seen 

 both in intermittent claudication affecting especially 

 the lower limbs and in cardiac angina. Lewis, Picker- 

 ing and Rothschild have developed a testing pro- 

 cedure (171, p. 97) involving voluntary manual grip- 

 ping movements at the rate of i per sec. developing a 

 tension of 20 to 28 lb. Such movements, normally 

 painless for many minutes, soon cause pain if the 

 circulation to the arm is stopped by inflation of a 

 proximally placed cuff. Lewis (170) initiated and 

 Kellgren (140) followed with another type of test, the 

 injection of hypertonic saline into muscles, tendons, 

 ligaments and joints to provide and permit the analysis 

 of pain from these deeper structures. 



Inflammation 



Inflammation, arising from disease or produced 

 experimentally, is apparently another process whereby 

 previously painless stimuli appear to become painful; 

 this is true for skin (i 72), for deeper somatic structures 

 (150) and for the viscera (142). Thus the inflamed 

 appendix hurts when pinched, but not the normal 

 appendix. 



Excellent summaries of various experimental 

 methods appear in Hardy et al. (118, Chapter III) 

 and in Beecher (16, Section V); Lewis (171, Chapter 

 I) gives a useful catalogue of the effective stimuli for 

 each of the pain-sensitive tissues of the body. 



Qjiantitation 0/ Seventy of Pain 



The purely subjective character of pain has given 

 rise to great difficulty in efforts at quantitation, but 

 Hardy et al. (118, p. 156) have thought that trained 

 observers can distinguish as many as 2 1 different 

 degrees of pain, from zero to maximum, arising from 

 radiant heat. That is to say, there were 21 steps or 

 'just noticeable differences (jnd's)' as the amount of 

 radiant heat was increased. They suggested a unit for 

 pain sensation, a ' dol' equivalent to the sum of 2 

 jnd's; pain of ceiling intensity has a value of lo''^ 

 'dols.' Armstrong et al. (9) have found their trained 



