532 THE CRANIAL NERVES. 



especially that lining the orbit of the eye, or the frontal sinuses. Where 

 the pain is deep seated, its location may even be in the dura mater or the 

 bones of the skull ; since each division of the fifth pair, either before or 

 immediately after leaving the cavity of the cranium, sends backward a 

 slender recurrent branch, destined for the dura mater and the cranial 

 bones. That from the ophthalmic division is traced backward into the 

 tentorium, in the substance of which it ramifies as far as the sinuses 

 bordering its attached edge. 



In cases of toothache, which depends upon irritation of the dental 

 filaments of the fifth pair, the cause of the neuralgia is usually the 

 decay of the bony substance of the tooth, and consequent exposure of 

 the tooth pulp to external injury or inflammation. It is usually con- 

 fined to the single tooth affected by decay ; but in severe cases the pain 

 may radiate to other teeth in the immediate neighborhood, or may even 

 spread over the entire corresponding side of the maxilla. Neuralgia of 

 the teeth may also be wholly sympathetic in its origin, as where it is 

 caused, like headache, by indigestion, exposure, or fatigue; the pain 

 existing simultaneously in several teeth, without any morbid alteration 

 of their structure. 



The most severe and persistent form of neuralgia in this nerve is that 

 known as tic douloureux ; in which the pain is habitually located in one 

 of its three principal divisions as they emerge upon the face. Here 

 also the pain is not constant, but intermittent, recurring in great 

 severity at longer or shorter intervals, and usually lasting but a few 

 minutes at a time. It is more frequently seated in the upper or middle 

 region of the face, corresponding with the distribution of the supra or 

 infra-orbital nerves. 



Lingual Branch of the Fifth Pair. This branch, which is designated 

 by the special name of the " lingual nerve," possesses an especial interest 

 because it communicates to the mucous membrane of the tongue both 

 the property of tactile sensibility and the special sense of taste. The 

 general sensibility of the tongue is highly developed over the whole 

 of its anterior two-thirds, where it is supplied by the lingual nerve ; and 

 at its tip is more acute than in any other region of the body. This 

 sensibility disappears completely on the operated side, together with 

 that of the external portions of the face, when the fifth nerve has been 

 divided in animals in the interior of the cranium ; and Longet has found 

 that after section of both lingual nerves, the surface of the anterior two- 

 thirds of the tongue may be cauterized with potassium hydrate or the 

 red-hot iron, without producing any indication of pain. The tactile 

 sensibility of the tongue is of great importance in man, and also in 

 some of the lower animals, as an aid in the process of mastication, by 

 enabling this organ to appreciate the simple physical qualities of the 

 food introduced into the mouth, to perceive when it is uniformly re- 

 duced to the proper consistency for swallowing, and to detect any 

 remnants left among folds or crevices of the mucous membrane. These 



