THE CRANIAL NERVES. 471 



facial nerve. In the normal condition it expands in inspiration, and 

 partially collapses in expiration. But after section of the nerve it col- 

 lapses in inspiration, and partially opens in expiration; moving pas- 

 sively, like an inert valve, with the changing direction of the air 

 current. 



Effect on the Lips. In animals, and especially in the herbivora, the 

 movements of the lips serve mainly for prehension of the food ; and if 

 they be paralyzed on both sides, the consequent incapacity to introduce 

 food into the mouth may be sufficient to cause death by inanition. In 

 the carnivora the retraction and elevation of the lips, by which the 

 canine teeth are uncovered, have a marked effect on the expression of 

 the face ; and in most animals, after division of the facial nerve, the 

 change of appearance in the corresponding side, even in the quiescent 

 condition, is distinctly perceptible. The lips are inactive, and the 

 corner of the mouth hangs down partly open, owing to paralysis of 

 the orbicularis oris. 



Effect on the Ears. In many quadrupeds the external ears are 

 more important than in man, owing to their greater development and 

 superior mobility. Their varying position has great influence in 

 modifying the expression; and their rapid and extensive movements 

 are of essential aid in the sense of hearing. When the facial nerve 

 has been divided, the ear on the corresponding side becomes motion- 

 less ; and if long and narrow, as in the rabbit, it can no longer 

 maintain the erect position. 



Facial Paralysis in Man. Facial paralysis, from disease involving 

 the trunk of the nerve, or its sources in the brain, is not an uncom- 

 mon affection in man. It is usually confined to one side, being limited 

 by the median line, and producing a difference of expression on the two 

 sides of the face. Where the difficulty is located in particular branches 

 of the nerve, certain portions of the face may be affected to the exclu- 

 sion of others. The lips may be paralyzed without loss of motion in 

 the parts above, and vice versa ; or the affection may be fully developed 

 in one region, and only partial in the remainder. But when the dis- 

 ease is seated on the trunk of the nerve, within the aqueduct of Fal- 

 lopius, or involves its central origin, its consequences extend uniformly 

 over one side, forming a complete unilateral facial paralysis. 



The signs of facial paralysis in man are, in general, those which fol- 

 low experimental division of this nerve in animals. Its main peculiar- 

 ity depends on the greater development, in man, of the facial muscles 

 as organs of expression ; and its most marked effect is consequently 

 loss of expression on the paralyzed side. All the features have a col- 

 lapsed appearance. The eyelids are motionless, the eye remains con- 

 stantly open, and the lower lid sinks below the level of the cornea ; 

 thus giving to the eye a staring, vacant appearance. The act of wink- 

 ing is no longer performed on the affected side. Owing to the paralyzed 

 condition of the frontalis and superciliary muscles, all the characteristic 

 lines and wrinkles on this side disappear, and the forehead and eyebrow 



