466 THE NERVOUS SYSTEM. 



Longet, the membrane also assumes a fungous consistency, and is liable 

 to bleed at the slightest touch. It is owing to a similar condition that 

 the power of smell is impaired in nasal catarrh or influenza. The olfac- 

 tory nerves become inactive in consequence of the morbid alteration in 

 their mucous membrane and its secretions. 



Influence on the Sense of Sight. The anterior parts of the eyeball 

 are provided with nerves of ordinary sensibility from the fifth pair ; 

 while impressions of light are transmitted exclusively by the optic 

 nerve. The iris and cornea are furthermore supplied by filaments 

 from the ophthalmic ganglion of the sympathetic, which receives its 

 sensitive root from the fifth pair. If this nerve be divided either in 

 front of or through the Gasserian ganglion, the cornea often becomes 

 the seat of congestion and ulceration, sometimes resulting in complete 

 destruction of the eye. Immediately after the operation the pupil is 

 contracted and the conjunctiva loses its sensibility. At the end of 

 twenty-four hours the cornea is opaline, and by the second day the 

 conjunctiva is congested, and discharges a purulent secretion. As 

 the process increases in intensity, the cornea grows more opaque, 

 until it becomes quite impermeable to light, and vision is consequently 

 suspended. In some cases there is at last sloughing and perforation 

 of the cornea and discharge of the humors of the eye ; in others, after 

 a few days, the inflammatory appearances subside, and the eye is 

 gradually restored to its natural condition. 



According to Bernard, these effects are either retarded or wanting 

 when the nerve is divided behind the Gasserian ganglion. This indi- 

 cates that its influence on the nutrition of the eyeball does not reside 

 in the fibres of its own roots, but in additional filaments derived from 

 the ganglion. 



Influence on the Sense of Taste. The lingual branch of the fifth 

 pair communicates to the anterior portion of the tongue both its general 

 sensibility and the faculty of taste ; both of which are abolished by its 

 division. It is probable that these two kinds of sensibility reside in 

 different nerve fibres ; since cases have been observed in which the 

 sense of taste is diminished or lost while the tactile sensibility of the 

 tongue remains unimpaired. It has not been possible thus far to deter- 

 mine the special source or location of the two functions in the lingual 

 nerve ; but it is evident that the exercise of taste is facilitated by the 

 general sensibility of the tongue, and is influenced by the condition 

 of the local circulation and the buccal secretions. When the tongue 

 is dry and coated from febrile action the taste is either abolished or 

 replaced by morbid sensations. It depends therefore for its exercise, 

 not only on the special sensibility of the lingual nerve, but also on all 

 the conditions requisite for the integrity of the mucous membrane. 



Influence on the Sense of Hearing. The influence of the fifth pair 

 on the perception of sound is less distinct than in regard to the other 

 special senses, and is only surmised from its anatomical relations. It 

 provides for the general sensibility of the external ear by twigs from 



