192 NERVOUS SYSTEM. 



ever, the operator feels, at a certain depth, that the bony 

 resistance ceases : he is then on the fifth pair, and the cries 

 of the animal give evidence that the nerve is pressed 

 upon. 



5. l< It is at this moment that it is necessary to hold 

 firmly the instrument and the head of the animal ; then the 

 cutting edge is turned so as to be directed downward and 

 backward, at the same time pressing in this direction so as 

 to divide the nerve on the extremity of the petrous portion, 

 behind the ganglion of Gasser, if possible, or at least on the 

 ganglion itself. 



6. " The instrument is then drawn back, pressing upon 

 the bone so as to accomplish completely the section of the 

 trunk of the fifth pair ; then it is withdrawn by passing over 

 the same course on the anterior face of the petrous portion 

 so as not to lacerate the cerebral substance. 



" The accident to be feared in the operation is section of 

 the carotid when the instrument has penetrated too far, or 

 lesion of the cavernous sinus when it is pressed too far for- 

 ward." x 



When this operation has been performed without acci- 

 dent, its immediate effects are very striking. The cornea 

 and the integument and mucous membrane on that side of 

 the head are instantaneously deprived of sensibility, and 

 may be pricked, lacerated, or burned without the slightest 

 evidence of pain on the part of the animal. Almost always 

 the small root of the fifth is divided as well as the large 

 root, and the muscles of mastication are paralyzed upon one 

 side ; but, with this exception, there is no paralysis of mo- 

 tion, sensation alone being destroyed upon one side. 



Immediate Effects of Division of the Trifacial. It is 

 hardly necessary to discuss the functions of the trifacial, af- 

 ter the statement of the effects which instantly follow upon 



1 BERNARD, Lemons sur la physiologic et la pathologic du systeme nerveux, 

 Paris, 1858, .tome ii., p. 53. 



