474 THE NERVOUS SYSTEM. 



nerve root crossed at this point, a longitudinal section at the median 

 line between the two nuclei would completely paralyze both sides of 

 the face. But this effect is not produced ; since in the experiments 

 of Yulpian,* on dogs and rabbits, the animals after this operation were 

 still capable of winking with both eyes ; only the action was no longer 

 simultaneous, each eye being closed at irregular intervals independently 

 of the other. 



It is evident, therefore, that the reflex action, in winking, takes 

 place for each eye on the same side, no doubt in the gray substance 

 of the facial nucleus ; the two nuclei habitually acting in harmony by 

 means of their commissural fibres. But the voluntary and emotional 

 impulses, which cause movement of the features, are transmitted by 

 decussating fibres from opposite sides of the brain. 



This is still further indicated by the effects of peripheral and central 

 lesions of the nerve. In man, as well as in animals, if this nerve be 

 divided or destroyed during its passage through the aqueduct of 

 Fallopius, all the facial movements are paralyzed together. But in 

 paralysis depending on a cerebral lesion above the nucleus, it is gen- 

 erally observed that the loss of motion is not complete ; but that, 

 while all other movements of the face are paralyzed, the action of 

 winking remains on the affected side. This peculiarity is used as a 

 means of diagnosis between facial paralysis from injury of the nerve 

 and that caused by a lesion in the brain. 



Sensibility of the Facial Nerve. Although this nerve is exclusively 

 motor at its origin, it subsequently receives filaments of communication 

 from the trigeminus, which give it a certain degree of sensibility. The 

 most important of these branches, given off from the inferior maxillary 

 division of the fifth nerve, joins the facial soon after its emergence from 

 the stylomastoid foramen, and thence accompanies its principal rami- 

 fications. According to the united testimony of modern experimenters, 

 the facial nerve, if examined on the side of the face, is found sensi- 

 tive to mechanical irritation, although its sensibility is much inferior 

 to that of the fifth pair. Owing to this communication, the neuralgic 

 pain of tic douloureux sometimes seems to follow the horizontal 

 branches of the facial nerve. The proof, however, that its sensitive 

 fibres are derived from anastomosis and do not originally form part 

 of its trunk, is that the sensibility of the regions to which it is distrib- 

 uted disappears completely after division of the fifth pair, notwith- 

 standing that the facial remains entire. 



Beside the communication above mentioned, this nerve contracts 

 frequent anastomoses, in the anterior part of the face, with the supra- 

 orbital, infraorbital, and mental branches of the fifth pair. 



Communications of the Facial Nerve in the Aqueduct of Fallopius. 

 While passing through its canal in the petrous portion of the temporal 

 bone, the facial nerve gives off a number of slender filaments by which 



Lepons sur la Physiologie du Systeme Nerveux. Paris, 1866, p. 480. 



