FIFTH PAIR OF NERVES. 



277 



Yet it is asserted by M. Serres,* who has 

 founded his opinion upon the observations 

 which he has made upon the successive de- 

 velopement of the brain and nerves in the 

 embryo of vertebrate animals, that in the 

 Mammalia the nerve is implanted upon the 

 trapezium. Such is the form of expression by 

 which he intends, as the author understands, 

 the ultimate connection of the nerve with the 

 brain. Now, in the first place, we have al- 

 ready seen where that connection is in those 

 animals in which the trapezium does not exist, 

 and it appears to the author reasonable to con- 

 clude that similar nerves have similar or ana- 

 logous attachments in the several classes of 

 animals, however the parts with which they 

 are connected may be complicated or ob- 

 scured by superadded structures. In the second 

 place the trapezium can be regarded only as a 

 superadded structure, and is not among those 

 parts from which nerves are likely to arise, 

 being itself but a commissure : and, thirdly, 

 the situation and connections of the part to 

 which the nerve is attached, are altogether in- 

 compatible with the opinion that it is the tra- 

 pezium, inasmuch as the latter is situate be- 

 fore the cords, which ascend from the anterior 

 columns of the spinal cord to the crura cerebri, 

 while the structure with which the nerve is 

 connected is posterior to them. For these 

 reasons the author concludes that M. Serres 

 has mistaken the place of the nerve's attach- 

 ment in the Mammalia. 



In conclusion, the representation of the ori- 

 gin of the nerve, which appears to the writer to 

 be the most remote of all from the real one, is 

 that given by Swan, in his plates of the nerves 

 lately published, in which the fifth is re- 

 flected into the auditory nerve : such a con- 

 nection is merely artificial and does not really 

 exist ; it can be produced only by stopping short 

 in the pursuit of the fifth nerve, and mould- 

 ing it into the anterior root of the auditory, 

 which is in contact with it. 



This view of its encephalic attachment has 

 probably originated in the intimate connection 

 known to exist between the two nerves in in- 

 ferior animals. The complication of the cere- 

 bral connection of the nerve in the higher 

 animals may be now better understood. In 

 those, in which the pons and trapezium do not 

 exist, the nerve emerges directly from the 

 spinal bulb, in a manner similar to the ad- 

 joining nerves ; but in those, in which the 

 bodies alluded to are present, inasmuch as the 

 attachment of the nerve is behind them, it can 

 reach the surface only by either passing be- 

 tween them, or traversing their substance. 

 Hence, if the nerve simply traverse them, it 

 ought not to receive any accession of fibres from 

 them, and such, according to the writer's 

 experience, is the case. As it emerges from 

 the pons, the lesser packet receives an epithe- 

 lium from its surface; but he has not been 

 able to detect any fibres originating within the 

 substance of that part. 



The structural arrangement, which the ence- 



* Op. cit. 



phalic portion of the nerve presents within the 

 brain, is different from that, for which it is 

 remarkable, while superficial to it. Exter- 

 nally it is, as has been stated, of a fascicnlar 

 texture ; but, within, that appearance is not to 

 be observed : there the larger portion is a 

 white, soft, homogeneous, flattened cord, the 

 delicacy of which, in the natural state, forbids 

 the separation of it into distinct parts; but 

 when sufficiently hardened, it may be divided 

 into numerous thin strata, and these again into 

 delicate fibrils. That such an arrangement is 

 a natural, and not an artificial appearance, is 

 manifest from the circumstance, that the sepa- 

 ration into fibrils can be effected only in one 

 direction, the length of the nerve, and that 

 they break off when it is attempted in the 

 other. The nerve retains those characters as 

 far as its attachment behind the crus, but there 

 they cease ; the pure white colour suddenly 

 disappears ; the point of attachment and the 

 cords descending from it present a cineritious 

 tint ; and they are not absolutely distinct from 

 the surrounding substance, as the nerve had 

 previously been, but immersed in it ; they are, 

 however, still manifestly composed of fila- 

 ments, which may be rent either toward or 

 from the point of attachment ; and after im- 

 mersion in spirit they become nearly white. 

 The course of the nerve, from its attachment 

 to the surface of the brain, is forward and out- 

 ward toward the internal anterior extremity of 

 the petrous portion of the temporal bone ; it 

 next passes over the superior margin of that 

 portion, and descends upon its anterior surface 

 into the middle fossa of the base of the cra- 

 nium, where it reaches the Gasserian ganglion. 

 During its short course, from its attachment to 

 the brain, to the ganglion, it is at first contained 

 within the proper cerebral cavity, by the side 

 of the pons Varolii, and beneath the internal 

 anterior angle of the tentoriura cerebelli ; in 

 the second place, in the middle fossa, it is 

 not within the cerebral cavity of the cranium, 

 but beneath it, separated fro n it by a lamina 

 of dura mater; it is there contained in a canal' 

 or chamber, formed by a separation of the 

 dura mater into two layers, between which the: 

 nerve and its ganglion are inclosed, one be- 

 neath them attached to the bone, another above 

 separating them from the brain. This chamber 

 is situate immediately external to, and lower 

 than the cavernous sinus, but separated from 

 it by the inferior lamina of the dura mater just 

 described, which ascends from the bone to 

 join the superior, and in so doing forms a 

 septum between the two chambers ; it is about 

 three-fourths of an inch long, reaching from 

 the superior margin of the petrous bone to the 

 anterior margin of the depression upon its. 

 anterior surface, in which the ganglion rests. 

 In front this chamber is wide, containing abthat 

 part the ganglion, anoVsends fibrous offsets upon 

 the nervous trunks proceeding from it ; poste- 

 riorly it is narrow, and presents an oval aperture, 

 about one-third of an inch long, situate ex- 

 ternal and inferior to the posterior clinoid pro- 

 cess of the sphenoid bone beneath the attach- 

 ment of the tentorium cerebelli to that process, 



