FIFTH PAIR. 455 



of smell is always more or less impaired in cases of coryza and 

 influenza. The olfactory nerves become inactive in consequence 

 of the morbid alteration in their mucous membrane, and in the 

 secretions which cover it. 



The influence of this nerve over the organ of vision is still more 

 remarkable. It has been known for many years that division of 

 the fifth pair within the cranium, or of its ophthalmic branch, is fol- 

 lowed by an inflammation of the corresponding eye, which usually 

 goes on to complete and permanent destruction of the organ. 

 Immediately after the operation, the pupil becomes contracted and 

 the conjunctiva loses its sensibility. At the end of twenty -four 

 hours, the cornea begins to become opaline, and by the second 

 day the conjunctiva is already inflamed and begins to discharge a 

 purulent secretion. The inflammation, after commencing in the 

 conjunctiva, increases in intensity and soon spreads to the iris, 

 which becomes covered with a layer of inflammatory exudation. 

 The cornea grows constantly more opaque, until it is at last 

 altogether impermeable to light, and vision is consequently sus- 

 pended. Blindness, therefore, does not result in these instances 

 from any direct affection of the optic nerve or of the retina, but is 

 owing simply to opacity of the cornea. Sometimes the diseased 

 action goes on until it results in ulceration of the cornea and dis- 

 charge of the humors of the eye ; sometimes, after the lapse of 

 several days, the inflammatory appearances subside, and the eye is 

 finally restored to its natural condition. 



It has been observed, however, that although the above conse- 

 quences always follow division of the fifth pair, when performed at 

 the level of the Casserian ganglion, or between it and the eyeball, 

 they are either much diminished in intensity or altogether wanting 

 when the division is made at a point posterior to the ganglion. 

 This circumstance has led to the belief that the influence of the fifth 

 pair on the nutrition of the eyeball does not reside in its own proper 

 fibres, but in some filaments of the sympathetic nerve which join 

 the fifth pair at the level of the Casserian ganglion. If the section 

 accordingly be made at this point, or in front of it, the fibres of the 

 sympathetic will be divided with the others, and inflammation of 

 the eye will result ; but if the section be made behind the ganglion, 

 the fibres of the sympathetic will escape division, and the injurious 

 effects upon the eye will be wanting. Such is the explanation 

 usually given of the above-mentioned facts ; but the question has 

 not as yet been determined in a positive manner. 



