REFERRED PAIN 195 



This relationship between the nerve-supply of the skin of the 

 trunk and the nerve-supply of the viscera suggests a very 

 intimate connexion between the viscera and the covering 

 parietes. 



It may be pointed out that viscera which contain muscle 

 fibres give rise to pain of much greater severity and with much 

 more frequency than viscera which contain few or no muscle 

 fibres. A striking contrast exists between the intense pain 

 which may be caused by a minute calculus in the pelvis of 

 the ureter and the entire absence of pain in advanced disease 

 of the liver, lungs or kidneys. 



Enough has been said to make it clear that any viscus may 

 give rise to the viscero-sensory reflex, and it is equally clear 

 that our knowledge of this important subject is as yet very 

 deficient. 



"A Focus of Irritation." When a viscero-sensory reflex 

 is established in connexion with a pathological process in 

 a viscus, the stream of abnormal afferent impulses from that 

 viscus and the constant " overflow" may cause a temporary 

 increase in the excitability of the nerve-cells secondarily 

 affected. This condition has been termed by Mackenzie 

 a "focus of irritation." As a result of the increased excit- 

 ability, an exaggerated interpretation is given to ordinary 

 stimuli passing through the affected cells. For example, if a 

 " focus of irritation " is set up in the eighth thoracic segment 

 by a gastric ulcer, the result may be an increase in the 

 excitability of the nerve-cells which are accustomed to receive 

 impulses from the terminal branches of the eighth intercostal 

 nerve. When this is the case, it is found that an area of 

 cutaneous hyperalgesia is present in the epigastric region, 

 i.e. gentle stroking of the skin over a certain area gives rise 

 to a feeling of discomfort or pain, because the afferent impulse 

 from the skin becomes exaggerated in its passage through 

 the cells which are in a state of increased excitability. The 

 discovery of such an area is of value, because, when the 

 spinal nerve which supplies it is known, the site of the 



