FIFTH PAIR. 453 



The remaining portion of the third division, after giving a few 

 branches to the mylo-hyoid muscle and to the anterior belly of the 

 digastric, then enters the inferior dental canal, sends filaments to 

 the teeth of the lower jaw, emerges at the mental foramen, and is 

 finally distributed to the integument of the chin, lower lip, and 

 inferior part of the face. 



This nerve is accordingly distributed to the sensitive surfaces, 

 that is, the integument and mucous membranes about the face, and 

 to the muscles of mastication. A few of its fibres are sent also to 

 the superficial muscles of the face, such as the buccinator and the 

 orbicularis oris ; but these fibres are sensitive in their character, 

 and serve merely to impart to the muscles a certain degree of 

 sensibility. It has been ascertained by Longet that if the various 

 branches of this nerve be irritated by a galvanic current, no con- 

 vulsive movements whatever are produced in those superficial 

 muscles of the face, which it supplies with filaments; but if its 

 smaller or non-ganglionic root be irritated in the same way, con- 

 tractions instantly follow in the muscles of mastication. 



The fifth pair is the most acutely sensitive nerve in the whole 

 body. Its irritation by mechanical means always causes intense 

 pain, and even though the animal be nearly unconscious from the 

 influence of ether, any severe injury to its large root is almost 

 invariably followed by cries which indicate the extreme sensibility 

 of its fibres. 



If this nerve be completely divided, in the living animal, within 

 the cranium, at the situation of the Casserian ganglion, the operation 

 is followed by total loss of sensibility in the skin of the face and in 

 the adjacent mucous membranes. The conjunctiva, upon the affected 

 side, is then completely insensible, and may be touched with the 

 point of a needle or the blade of a knife, without exciting any un- 

 easiness, and even without the consciousness of the animal. Probes 

 and needles may be passed into the nostril, and the lips or the 

 cheek may be pinched, pierced or cut, without exciting the least 

 sign of sensibility. The animal is entirely indifferent to all me- 

 chanical injuries upon the affected side, though upon the opposite 

 side the parts retain their natural sensibility. 



Owing to the paralysis of the lingual nerve, also, after this ope- 

 ration, the tongue, in its anterior two-thirds, becomes insensible to 

 ordinary irritations, and loses, beside, the power of taste. 



Another peculiar effect of the division of the fifth pair depends 



