FIFTH PAIR OF NERVES. 



307 



how will it appear after the qualification by 

 which it is followed has been perused ? " I 

 owe it to truth," he says, " to add that these 

 last statements were not collected till after the 

 dissection, and that they were gathered from 

 the patients of the ward." Such an admission 

 manifestly destroys the value of the case : evi- 

 dence obtained only after the individual's death, 

 so little marked as during his lifetime to have 

 been overlooked, and relating to a question at 

 once so obscure and delicate, can hardly fail to 

 be imperfect; but admitting that the patient 

 did relish and distinguish between different 

 kinds of snuff, and that he was disagreeably 

 affected by his neighbour's ailment, what then ? 

 The chief property of common, if not of every 

 snuff, is pungency and not odour, and the per- 

 ception of pungency is not the function of the 

 olfactory nerve; and one may be as disagree- 

 ably affected by a disgusting sight as by a dis- 

 gusting smell, and the patients of the ward not 

 make any distinction between the senses af- 

 fected, until taught by the inquiries made that 

 it must have been that of smell. And if the 

 case just quoted prove the existence in the 

 organ of smell of a sensibility to the impres- 

 sion of volatile agents independent of the ol- 

 factory, and conferred by the fifth nerve, the 

 existence of another equally independent of 

 the latter is satisfactorily established by the 

 continuance of smell in those cases in which 

 the faculty conferred by the fifth nerve has been 

 lost through disease. This may be seen from 

 reference to the case furnished by Beclard, and 

 advanced by the very advocates of Magendie's 

 doctrine in support of it; but the fact is still 

 more strongly established by the case some 

 time since published by Mr. Bishop, in which, 

 though the fifth nerve was completely destroyed 

 by the pressure of a tumour within the cra- 

 nium, and both the ordinary and tactile sensi- 

 bility of the same side of the face and its cavi- 

 ties was in consequence altogether lost, the 

 sense of smell continued unimpaired. In a 

 case of disease of the fifth nerve which the 

 writer has witnessed, the patient did acknow- 

 ledge the perception of certain odoriferous 

 agents ; but judging from it alone, he could 

 not say that smell was not impaired; on the 

 contrary it seemed very much so, inasmuch as 

 the patient denied at first any perception of 

 the impression of several agents accounted 

 odorous, and when he did say that he smelt 

 these, it was not of himself, nor until he had 

 b^en particularly questioned, and then he said 

 it was up in his head' that he felt the sensa- 

 tion, and positive must take precedence of nega- 

 tive evidence. Further, it is very likely that in 

 the case of sensations, themselves neither dis- 

 agreeable nor acute, the vividness of which 

 may depend very much upon association with 

 other and more acute ones, the former may be 

 disregarded where the latter have been lost, 

 and hence the rashness of inferring that brutes 

 have lost certain faculties, because in the course 

 of experiments they do not by the exercise of 

 these give evidence of their existence. The 

 fact of the absence of olfactory nerves in the 

 Cetacea as established by Cuvier, has also led 



some to the conclusion that the proper faculty 

 of smell may be capable of being transferred 

 at least to the fifth ; but until the faculty has 

 been proved to exist in such case, the inference 

 is manifestly not warranted by the premises. 



It appears then that there is a distinct per- 

 ceptive faculty enjoyed by the nostrils, inde- 

 pendent of the fifth and dependent on the 

 olfactory nerve; that we possess no positive 

 evidence of the latter nerves being in any case 

 the media by which this peculiar perception is 

 recognized, but that they serve for the recogni- 

 tion only of impressions of contact, pungency, 

 or irritation. 



2. Relation of the fifth nerves to vision. 

 That in all animals having at once the faculty 

 of vision and an optic nerve, the latter is in- 

 dispensably necessary to the exercise of the 

 former cannot be denied : disease or division 

 of the nerve is uniformly attended by loss of 

 the function ; but some circumstances coun- 

 tenance the opinion that the fifth nerve pos- 

 sesses a more important connection with vision 

 than may at first appear. 1. Injury of the 

 frontal and certain other branches of the fifth 

 nerve has been long accounted among the 

 causes of amaurosis. 2. Magendie has found 

 that " on division of the two ' fifth ' nerves 

 upon an animal it seems blind." 3. The fact 

 which countenances most strongly the opinion 

 that the fifth nerve is concerned directly in 

 the function of vision is derived from com- 

 parative anatomy. It has been stated that in 

 certain animals a special optic nerve is wanting, 

 and the ocular nerve is derived from the fifth 

 pair. Of this it appears universally admitted 

 that the proteus anguinus is an instance ; its 

 eyes are situate immediately beneath the epi- 

 dermis, which is transparent* in front of them ; 

 the optic nerve is wanting,f and the only nerve 

 received by the eye is a branch of the second 

 division of the fifth.J Whatever vision, there- 

 fore, may be enjoyed by this animal, and 

 according to Carus it is considerable, must 

 be exerted through the medium of the fifth 

 nerve. Among the mammalia also are several 

 animals which appear to be in the same, or 

 nearly the same state ; but anatomists are not 

 agreed on the point : the absence of a special 

 optic nerve in the mole was announced by 

 Zinn, || who shewed that its place was taken by 

 a branch of the fifth. Carus and Treviranus, 

 however, maintain that the optic does exist 

 in the animal, but that it is very minute, grey, 

 and capillary ; that in the same proportion 

 the fifth nerve is large, and that its second 

 division at its exit from the cranium gives 

 off a branch, which enters the globe of the 

 eye, and according to the former concurs in 

 forming the retina.ir Serres again positively 

 denies the existence of the optic nerve in the 

 mole, and maintains that these anatomists are 

 mistaken ; he states that he has sought the 



* Serres. 



t Treviranus, Serres. 

 $ Ibid. 



$ Comparative Anatomy. 



\\ De differentia fabrics octili human! et brutorum. 

 f Journal Complementaire, vol. xv. 



x 2 



