636 THE CRANIAL NERVES. 



requisite : first the peculiar sensibility of its own special nerve T and 

 secondly the integrity of the component parts of the organ itself. As 

 the nutrition of the organ is affected by injury or disease of the fifth 

 pair, this necessarily causes a derangement in its physiological action 

 and thus interferes with the exercise of the special sense belonging to it. 

 These effects seem to depend, not so much upon the division of the ordi- 

 nary sensitive fibres of the fifth nerve, as of those which are derived 

 from the nerve cells of the Gasserian ganglion, or which are supplied 

 by the fifth pair to the special sympathetic ganglia connected with the 

 organs of sense. 



Influence on the Sense of Smell. The nasal passages are supplied 

 by two different nerves derived from the cerebro-spinal S3rstem, namely, 

 the olfactory nerve distributed to their upper portions, and endowed 

 with its own special sensibility; and the nasal branches of the fifth pair, 

 distributed in the lower portions, to which they communicate the gene- 

 ral sensibility of the mucous membrane. The mucous membrane also 

 contains filaments from the spheno-palatine ganglion of the sympathetic ; 

 and this ganglion receives its sensitive root -from the superior maxillary 

 division of the fifth pair. 



The general sensibility of the nasal passages may accordingly remain 

 after the special sense of smell has been destroyed. If the fifth pair, 

 however, be divided, not only is general sensibility destroyed in the 

 Schneiclerian membrane, but a disturbance also takes place in its nutri- 

 tion, by which the power of smell is also lost* The mucous membrane 

 becomes swollen, and the nasal passage is obstructed by an accumula- 

 tion of mucus. According to Longet, the membrane also assumes a 

 fungous consistency, and is liable to bleed at the slightest touch. The 

 effect of this alteration is to blunt or destroy the sense of smell. It is 

 owing to a similar condition of the mucous membrane that the power 

 of smell is impaired in cases of influenza. The olfactory nerves become 

 inactive in consequence of the alteration in their mucous membrane and 

 its secretions. 



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

 are also supplied with nerves of ordinary sensibility from the fifth pair, 

 while the special impressions of light are transmitted exclusively by the 

 optic nerve. In addition, the iris and cornea are supplied by filaments 

 coming from the ophthalmic ganglion of the sympathetic, which re- 

 ceives its sensitive root from the fifth pair. If this nerve be divided 

 within the cranium, by a section passing in front of or through the Gas- 

 serian ganglion, a change of nutrition often follows in the cornea, by 

 which its tissue becomes the seat of vascular congestion and ulceration, 

 and which frequently goes on to complete and permanent destruction 

 of the eye. These changes may be observed in the cat, after intra- 

 cranial section of the fifth nerve by the usual method. Immediately 

 after the operation the pupil is contracted and the conjunctiva loses its 

 sensibility. At the end of twent} r -four hours the cornea begins to be- 

 come opaline, and by the second day the conjunctiva is visibly congested, 



