78 THE NERVOUS SYSTEM 



The lesion in tic douloureux is not definitely known, but 

 it presumably affects either the semilunar (Gasserian) ganglion 

 or the sensory root of the nerve. In this case the pain 

 starts in one particular branch and, later, spreads to affect 

 other branches of the same division. It is important to 

 observe that the pain is restricted to the areas of peripheral 

 distribution (Fig. 41). After a time, hyperaesthetic areas 

 develop in the skin of the face or head, and their appearance 

 is due to the establishment of a "focus of irritation" (p. 195) 

 in the sensory nucleus of V. 



Pain of a similar nature may be caused by an intracranial 

 tumour in its early stages. 



Referred pain (p. 190) occurs in the trigeminal area with 

 great frequency. Head has pointed out that two distinct 

 varieties of referred pain occur in this area. In one the pain 

 is radiating or neuralgic in type and affects the area of 

 distribution of a definite branch or branches of the trigeminal. 

 Pyorrhoea alveolaris gives rise to such a condition and may 

 be accompanied by the development of localised areas of 

 hyperaesthesia, which are restricted to the same regions. 



In the other variety, the pain does not radiate and is 

 constantly referred to a definite area, which does not corre- 

 spond to the peripheral distribution of any one branch of 

 the fifth. Hyperaesthetic areas may develop, and they are 

 localised to the same " segmental " regions. This variety 

 of referred pain is met with in iritis, glaucoma and inflamma- 

 tion of the tooth pulp. The lower molars are intimately 

 related to the skin over the posterior part of the ramus of 

 the mandible and to the skin lining the external acoustic 

 meatus. In irritation of the pulp of a lower molar, there 

 may be no local pain and yet the ear pain may be very acute. 

 These cases may readily lead to wrong diagnosis. 1 



When a "focus of irritation" (p. 195) is established in the 



1 For a full description of the "segmental" areas and their relation to 

 the teeth, the reader is referred to Head's article in Allbutt and Rolleston's 

 System of Medicine. 



