212 THE HUMAN BODY 



or taste sensations. Pain, however, is always recognized as a 

 distinct sensation having its own modality. Its function seems 

 to be wholly one of warning; only when something is amiss do 

 we feel it. Since danger results from strong stimulation but not 

 from feeble stimulation pain receptors are less irritable than other 

 sorts: it is estimated that the sense of touch is one thousand times 

 as delicate as the sense of pain. Harm may result from excessive 

 stimulation of any sort. Pain receptors, therefore, are irritable 

 to all forms of energy except that of light. 



Because pain results from any sort of stimulation, but only 

 when excessive, it was formerly thought to be not a distinct sense 

 but the result of overstimulation of the other senses. On this 

 theory it would be hard to account for the fact that skin pain is 

 so very different in modality from a touch or temperature feeling, 

 and to understand why it gives rise in consciousness to concep- 

 tions concerning a condition of the Body and not of some external 

 object: it is not extrinsically referred by the mind to a quality of 

 anything but the painful part itself, as a dazzling light sensation 

 or a fetid odor is. There is also experimental and pathological 

 evidence that the paths taken in the spinal cord by nerve impulses 

 causing pain are different from those leading to a consciousness 

 of touch. If certain parts of the cord are cut in the thoracic region 

 of a rabbit, gentle touches on the hind limb appear to be felt; the 

 animal erects its ears or moves its head : but powerful stimulation 

 of the sciatic nerve causes no signs of pain, while if the dorsal white 

 columns be cut the animal still can feel stimuli applied to the hind 

 limb and sufficient to cause pain under normal conditions, but it 

 appears insensible to gentle pressure on the skin. In human beings 

 very similar phenomena have been observed in cases of spinal 

 cord disease: and in a certain stage of chloroform or ether narcosis 

 the patient feels the surgeon's hand or his knife where it touches 

 the skin, but he experiences no pain when deeper parts are cut. 

 /Such considerations seem to lead to the conclusion that the 

 nerve-fibers and receptors concerned with painful sensations are 

 quite distinct from those of the other senses. If that be so we 

 must assume that there are "pain" fibers very widely distributed 

 over the skin and through most other parts of the Body. In 

 accident or disease these are stimulated powerfully enough to 

 arouse perception and imperiously call attention to danger. 



