FUNCTIONS OF THE CEPHALIC NERVES. 1C5 



sign of pain when it is subjected to any kind of irritation. Neither the divi- 

 sion of the nerve, nor the destruction of the olfactive ganglia, seems to incon- 

 venience them materially. They take their food, move with their accustomed 

 agility, and exhibit the usual appetites of their kind. The common sensibility 

 of the parts contained in the olfactive organ is in no degree impaired, as is 

 shown by the effect of irritating vapours ; but the animals are destitute of the 

 sense of smell, as is shown by the way in which these vapours affect them. 

 At first they appear indifferent to their presence, and then suddenly and vehe- 

 mently avoid them, as soon as the Schneiderian membrane becomes irritated. 

 Moreover, if two dogs, with the eyes bandaged, one having the olfactory 

 nerves and ganglia sound, and the other having had them destroyed, are 

 brought into the neighbourhood of the dead body of an animal, the former 

 will examine it by its smell ; whilst the latter, even if he touches it, pays no 

 attention to it. This experiment Valentin states that he has repeated several 

 times, and always with the same results. Further, common observation shows 

 that sensibility to irritants, such as snuff, and acuteness of the power of smell, 

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

 evidence 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 main- 

 tained that the Fifth pair in some way furnishes conditions requisite for the 

 enjoyment of the sense of smell, asserting that, when it is cut, the animal is 

 deprived of this. But his experiments were made with irritating vapours, 

 which excite sternutation or other violent muscular actions, not through the 

 olfactory nerve, but through the fifth pair: and the experiments of Valentin, 

 just related, fully prove that the animals are not sensitive to odours, strictly so 

 called, after the Olfactory has been divided. It is by no means improbable, 

 however, that the acuteness of the true sense of smell may be diminished by 

 section of the Fifth pair ; since the olfactory membrane is no longer duly moist- 

 ened by its proper secretion ; and, when dry, it is not so susceptible of the 

 impressions made by those minute particles of odoriferous substances to which 

 the excitement of the sensation must be referred. 



221. That the Second pair, or Optic nerves, have an analogous character, 

 appears alike from anatomical and experimental evidence. No chemical or 

 mechanical stimulus of the nerve produces direct muscular motion, nor does it 

 give rise, as 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 ordi- 

 nary sensibility of the eyeball remains, when 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 attri- 

 buted to the injury, occasioned by the operation itself, to the parts within the 

 orbit. It is well known that, when amaurosis is produced by a morbid condi- 

 tion of the Optic nerve alone, the eye retains its usual appearance ; but, if the 

 amaurosis be complete, the texture of the Retina undergoes a remarkable 

 change, ceasing to exhibit that peculiar structure which normally characterizes 

 it. Neither primitive nervous fibrils nor nucleated globules can be distin- 

 guished in it; and the yellow spot of Soemmering becomes paler, and is at 

 last undistinguishable. But, if a very slight degree of sensibility to light 



