THE CRANIAL NERVES. 463 



the ear, which is supplied by fibres from the third division of the fifth 

 nerve, no effect is produced. But if the same irritation be applied to 

 the back part of the ear, which is supplied by the great auricular 

 nerve from the cervical plexus, a twitching movement is at once 

 excited. According to Longet, the most violent injuries, such as 

 exsection of the eyeball, evulsion of the hairs about the lips, extraction 

 of the teeth, or destruction of the integument by the actual cautery, 

 may be performed, after complete division of the fifth nerve, without 

 causing any painful sensation. 



The fifth pair is accordingly the exclusive source of sensibility in 

 the superficial regions of the face, and all parts of the na&al and buccai 

 cavities to which it is distributed. 



Painful Affections of the Fifth Pair. This nerve is also the seat 

 of neuralgic affections about the head and face. The most common of 

 these is headache ; which may be general, extending over the whole 

 forehead and vertex, or confined to one side. It often seems to be 

 located in the branches supplying the periosteum, especially of that 

 lining the orbit and the frontal sinuses. Where the pain is deep-seated, 

 its location may 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 a slender recurrent branch to the dura 

 mater and the cranial bones. That from the ophthalmic division may 

 be traced into the tentorium, in which it ramifies as far as the sinuses 

 bordering its attached edge. 



Toothache, from irritation of the dental filaments of the fifth pair, is 

 generally due to decay of the dentine, and consequent exposure of the 

 tooth pulp to mechanical injury. Neuralgia of the teeth may also be 

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

 existing simultaneously in several teeth, without morbid alteration of 

 their structure. 



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

 tic douloureux ; habitually located in one of its three principal divisions 

 as they emerge upon the face. The pain is usually intermittent, recur- 

 ring in great severity at various intervals, and lasting but a few min- 

 utes at a time. It is most frequently seated in the upper and middle 

 regions of the face, corresponding with the distribution of the supra 

 and infraorbital nerves. 



Lingual Branch of the Fifth Pair. This branch, known as the 

 "lingual nerve," communicates to the mucous membrane of the 

 tongue its tactile sensibility. This sensibility is highly developed in 

 the anterior two-thirds of the tongue, and at its tip is more acute than 

 in any other region of the body. It disappears completely on the oper- 

 ated side, when the fifth nerve has been divided within the cranium ; 

 and after section of both lingual nerves, according to Longet, it is lost 

 in the whole anterior two-thirds of the organ. The tactile sensibility 

 of the tongue is of great importance in man and many animals, as an 

 aid in mastication, for appreciating the physical qualities of the food, 



