652 FUNCTIONS OF THE CEREBRO - SPINAL NERVOUS SYSTEM. 







shows that sensibility to irritants, such as snuff, and acuteness of smell, bear 

 no constant proportion to one another; and there is ample pathological evi- 

 dence, that the want of this sense is connected with some morbid condition 

 of the olfactory nerves or ganglia. It is well known that Magendie has 

 maintained, that the Fifth pair in some way furnishes conditions requisite 

 for the exercise of the power of smell ; asserting that, when it is cut, the 

 animal is deprived of this sense. But his experiments were made with irri- 

 tating vapors, which excite sternutation or other violent muscular actions, 

 not through the Olfactory nerve, but through the Fifth pair ; and the experi- 

 ments of Valentin, just related, fully prove that the animals are not sensitive 

 to odors, strictly so called, after the Olfactory nerve has been divided. The 

 acuteness of the true sense of smell is lessened by section of the Fifth pair ; 

 but this is because the Schueideriau membrane is then no longer duly moistened 

 by its proper secretion, and when dry it is less susceptible of the impressions 

 made by those minute particles of odoriferous substances, to which the excite- 

 ment of the sensation must be referred. A number of cases have, however, 

 been collected by Claude Bernard, and especially a well-authenticated one 

 (Marie Lemmens), in which, whilst no failure of the sense of smell was ob- 

 served during life, the complete absence of the olfactory nerve was ascertained 

 on post-mortem examination. 



516. That the Second pair, or Optic nerve, has an analogous character, 

 appears alike from anatomical and experimental evidence. No chemical or 

 mechanical stimulus of the trunk produces direct muscular motion ; nor 

 does it give rise, so far as can be ascertained, to indications of pain ; whence 

 it may be concluded that this nerve is not one of " common " sensation. 

 That the ordinary sensibility of the eyeball remains, Avhen the functions of 

 the Optic nerve are completely destroyed, is well known ; as is also the fact, 

 that division of it puts an end to the power of vision. Valentin states that, 

 although the Optic nerve may, like other nerves, be in appearance completely 

 regenerated, he has never been able to obtain any evidence that the power 

 of sight has been in the least degree recovered. He remarks that animals 

 suddenly made blind exhibit great mental disturbance, and perform many 

 unaccustomed movements; and that the complete absence of the power of 

 vision is easily ascertained. Morbid changes are sometimes observed to take 

 place in eyes whose Optic nerve has been divided ; but these are by no means 

 so constant or so extensive, as when the Fifth pair is paralyzed ; and they 

 may not improbably be attributed to the injury occasioned by the operation 

 itself, to the parts within the orbit. 



517. The Optic nerve, though analogous to the Olfactory in all the points 

 hitherto mentioned, differs from it in one important respect: that it has the 

 power of conveying impressions which excite rcjiex muscular motions. This 

 is especially the case in regard to the Iris, the ordinary actions of which are 

 regulated by the degree of light impinging on the retina. The fibres of the 

 optic nerve that reflectorially excite the branches of the third nerve dis- 

 tributed to the Iris undergo complete decussation in the chiasma optici. 1 

 When the Optic nerve is divided, contraction of the pupil takes place; but 

 this does not occur if the connection of this nerve with the third pair, through 

 the nervous centres, be in any way interrupted. After such division (if com- 

 plcte), the state of the pupil is not affected by variations in the degree of 

 light impinging on the retina; except in particular cases, in which it is in- 

 fluenced through other channels. Thus, in a patient suffering under ainau- 

 rosis of one eye, the pupil of the affected eye is often found to vary in size, 

 in accordance with that of the other eye ; but this effect is due to the action 



1 See Knoll, Contralblutt, 1872, p. 265. 



