546 THE CRANIAL NERVES. 



Crossed Action of the Facial Nerve. The results of minute examina- 

 tion of the mode of origin of this nerve give indications of a transverse 

 communication by decussating nerve fibres, between its nucleus at the 

 floor of the fourth ventricle and the opposite side of the tuber annulare. 

 It has not yet been possible, however, to follow with certainty the indi- 

 vidual fibres to their termination, or to decide whether the decussating 

 fibres are part of the original root fibres which have simply passed 

 through the nucleus, or whether they originate anew from the nerve 

 cells of the nucleus and thence pass to the opposite side. The opinion 

 usually adopted by anatomists from the examination of microscopic sec- 

 tions is that a part of the fibres of each cranial nerve root terminate in 

 the nucleus of the same side, and a part cross over, as decussating fibres, 

 to the opposite side. This is plainly shown in the case of the patheticus, 

 which is the only one of the cranial nerves, beside the optic, exhibiting 

 a distinct decussation of its root fibres outside their connection with 

 the nucleus. 



That the action of the facial nerve is in great part a crossed action is 

 evident from the results of pathological observation. Facial paralysis 

 is a frequent accompaniment of hemiplegia; and in the great majority 

 of instances, that is, when the cerebral lesion is situated above the tuber 

 annulare, the hemiplegia of the body and limbs and the paralysis of 

 the face are upon the same side with each other. The injury to the 

 brain, therefore, in these cases, produces both hemiplegia and facial 

 paralysis on the opposite side. When the injury is seated lower down, 

 on the contrary, in the substance of the tuber annulare, it may affect at 

 the same time the roots of the facial nerve outside its nucleus, and the 

 longitudinal tracts of the anterior pyramids above their decussation ; 

 and may cause in this way a facial paralysis on the same side and 

 hemiplegia on the opposite side. It thus appears that the facial par- 

 alysis is on the same side with the injury when this is seated externally 

 to the nucleus, and on the opposite side when it is seated above the 

 nucleus and near the central parts of the brain. This shows that for 

 a large part of its functions, the action of the facial nerve. is entirely a 

 crossed action. 



The communication, however, between the nucleus and the opposite 

 side of the brain, upon which this crossed action depends, does not affect 

 all the fibres of the nerve, nor the whole of the physiological functions 

 which are under its control. The only decussation of the nerve fibres 

 connected with the facial known to exist, is that which takes place at 

 the raphe on the floor of the fourth ventricle. If all the fibres of the 

 nerve root or their continuations crossed at this point, from right to 

 left and from left to right, then a longitudinal section at the raphe, 

 following the median line between the two nuclei, would completely 

 paralyze both sides of the face at the same time. But this effect is not 

 produced ; since, in the experiments of Yulpian, 1 who has performed this 



1 Lemons sur la Physiologic du Systeme Nerveux. Paris, 1866, p. 480. 



