THE RECEPTORS 859 



Very commonly pain arising in the internal organs is referred to some 

 remote region on the surface of the body. This phenomena is known as 

 referred pain, Such pains are sharp or aching in character, whereas pain 

 which seems to come from the internal organs is dull or heavy (Head 13 ). 



The afferent nerves from the viscera apparently terminate in the cord 

 in close association with afferent nerves from certain skin areas. Just 

 how one group of afferents affects the other is not known, but the fact is 

 that the sensation from the viscus is referred to the peripheral distribu- 

 tion of those fibers which terminate in the same segment of the spinal 

 cord. The nerves of cutaneous sensation have a definite segmental dis- 

 tribution (Fig. 214) and consequently the reference of internal pain 

 will be to one or another of these segmental regions. Not only is the 

 visceral pain referred to these segmental areas, but these parts of the 

 skin become hypersensitive, so that the sensation of pain and sometimes 

 of heat and cold arising from them is greatly exaggerated. Thus the 

 location of the referred pain and of hypersensitivity may be taken as an 

 accurate indication of the internal situation of the source of irritation. 

 The table on page 858 indicates the segmental distribution of referred 

 pain from the major viscera. 



The sensitivity of the viscera is of a low order. When the sensitivity 

 of a region of the skin is reduced to a similar condition by disease, sen- 

 sations arising therefrom may be referred to other parts, just as the vis- 

 ceral pains ordinarily are. This phenomenon is known as allocheiria, 

 If the hyposensitive area is limited to one side of the body, the sensation 

 arising from it is referred to the corresponding part of the other side 

 of the body. If both sides are hyposensitive, the reference is to the 

 next segment above or below. Like the reference of visceral pain, this 

 condition must be attributed to the central relationship between the 

 tracts carrying afferent impulses from symmetrical points on the skin 

 and from neighboring segments to the sensory centers. When the mecha- 

 nism of sensation for one part is depressed, the sensation is referred to 

 the most closely associated normal region. 



Cutaneous and Deep Sensibility 



The physiology of receptors may be illustrated by a consideration 

 of sense organs which have a general distribution throughout the body. 

 These comprise the receptors of chief interest in practical neurology, 

 and will serve to illustrate principles which apply as well to the spe- 

 cial sense organs of the head (of the eye, ear, gustatory and olfactory 

 epithelia), the consideration of which space will not allow. They may 

 be divided into organs of cutaneous and of deep sensibility, depending 

 on whether the receptors lie in the skin or in the internal parts of the 

 body. We can recognize four primary qualities in the sensations to 



