THE AUTONOMIC NERVOUS SYSTEM 477 



conducted without any anaesthetic and without causing any pain to the 

 patient. On the other hand, impulses may arise in the afferent nerve- 

 endings of the viscera, as a result of disease or certain forms of stimulation, 

 e.g. stretching or compression, which may reach consciousness and give rise 

 to the sensation of pain. This pain is not localised in the viscera, but 

 is referred to certain parts of the surface of the body. When the afferent 

 autonomic fibres of a nerve are the seat of pain, the primary referred pain is in 

 the area of the cutaneous somatic fibres of the nerve. It has been shown by 

 Mackenzie and by Head that visceral disease may cause hyper-sensitivity of 

 the corresponding areas of the skin, and a method has been elaborated by 

 these observers for utilising this referred pain or skin tenderness as a means 

 of localising the site of the disease. 



