i 9 2 THE NERVOUS SYSTEM 



nerve, the interpretation will be a painful sensation in the area 

 supplied by that nerve. It is at present impossible to say 

 whether this theory is correct or not, but at least it offers a 

 reasonable working hypothesis. 



A precisely similar sequence of events may occur in con- 

 nexion with any of the sensory cerebral nerves. Thus, 

 stimulation of one of the terminal branches of the inferior 

 alveolar (dental) nerve by an abscess at the root of a tooth 

 may produce not only toothache but also referred pain in the 

 external acoustic (auditory) meatns (auriculo-temporal nerve, 



P- 75)- 



Referred pains may be excited not only by pathological 



processes affecting structures supplied by cerebro-spinal nerves, 

 but also in the case of structures which receive their nerve- 

 supply from the sympathetic system. For example, in attacks 

 of angina pectoris the pain frequently spreads from the prae- 

 cordia to the medial side of the upper limb, and, in rare 

 cases, it may commence in the upper limb and spread to the 

 chest. The pain in the upper limb is unquestionably a 

 referred pain and it may be accounted for by the " overflow " 

 hypothesis. Afferent impulses from the heart pass via the 

 sympathetic to the spinal medulla, where they terminate in 

 connexion with nerve-cells in the upper thoracic segments 

 (p. 307). These segments also contain the cells which 

 receive afferent sensory impressions from the medial side of 

 the arm. Under normal conditions the afferent impulses from 

 the heart do not "overflow," but, in angina pectoris, the 

 impulses are abnormal in character and they " overflow " so 

 as to stimulate those neighbouring nerve-cells which are con- 

 cerned in the cutaneous supply of the medial side of the 

 upper limb. To this sequence of events Mackenzie has given 

 the name of " viscero-sensory reflex." 



It is clear that viscera may give rise to referred pains, and 

 it is therefore important to differentiate, if possible, between 

 referred pains and pains actually experienced in the viscera 

 themselves, but attempts to do so meet with numerous 



