BRA 



fiirface of the tl-.alami nervorum i);)\i.-o: im. It ftparatfs 

 behind into two polUrior crura, wliich are thin, bro:\J, and 

 flat, and pali into tlie rcflcAcd horns of ihc vtiitiiik':, wliire 

 they run alonp; the front of the hippocampus major, and foon 

 terminate. Tlicir polU-rior margin is conncflcd to the hip- 

 pocampus ; thc.ai.tirior is loofe and floatin,-;, and forms the 

 bandelcttc dc I'liippocampe of Vicqd'A/.yr. The triangular 

 fpace kft between the poflerior crura fornicis, where ihey 

 diverge to enter the reflcflcd horns of the vcntritles, is filled 

 up by medullary matter markid on its under furface by fevc- 

 ral oblique lines'; it is called triponum pfaltcrium, or corpus 

 pfaltoides. It may be regarded as the under furface of the 

 corpus callofiim. The origin of the fornix in the front of 

 the brain is fituatcd at fomc diftance from the corpus callo- 

 fum } it rifos gradually in an arched manner, until it reaches 

 the under furface of the corpu;* callofum, before its fepara- 

 tion into the pofterior crura. On this ciicumllancc depends 

 the different breadth of the feptum lucidimi in the front and 

 back part of the ventricks. Under the anterior part of the 

 fornix is a fmall flit-like opening, by which the two lateral 

 ventiScles communicate. This aperture is juft in front of 

 the thalami nervorum opticorum, and precifely over the fo- 

 ramen commune anterius. It becomes cor.fideiably enlarged, 

 in cafes of grt;at accumulation of fluid in the ventricles. 

 The communication between tliefe cavities would be mucli 

 •more cxtenfive, did not the fornix behind this aperture ad- 

 here to the choroid plexus and velum, which intervene be- 

 tween its under furface and the optic thalami. The thin 

 edge of the fornix, and of its pofterior crus, receives veffels 

 from this vafc.ilar membrane. The fornix fliould now be 

 cut'through oppofite to the communication juil defcribed. 

 Uy turning up the anterior part of this body, its origina- 

 tion by two diftinft crura becomes clearly feen, which would 

 rot be fatisfaflorily made out on tlie upper furface. By re- 

 ftcfting the pofterior part, the pfalteriutn, the feparation of 

 the pollerior crura, their courfe, flat fhape, extenuated mar- 

 gin, and pointed termination on the hippocampus, will be 

 clearly expofcd. 



In this Itage of the didedlion, the fituation and connec- 

 tions of the choroid plexus may be traced. This is a pro- 

 duftion of the pia mater, containing a vaft number of tor- 

 tuous, arterial, and venous ramifications, entering the re- 

 flected horn of the ventricle, and afcending into the fiipeiior 

 part of the cavity. It is broader and larger below, dimi- 

 nifhes in fize, as it proceeds along the upper part of the 

 ■ventricle, and terminates jult at the opening of communica- 

 tion between the two literal ventricles, by joining the oppo- 

 fite plexus. The right and left plexiiles are joined together 

 by a middle expanlion under the fornix, which is tlie velum, 

 five plexus choroideis interpofitus of Haller, or the rete,- or 

 tela choroidesof Vicq d'Azyr, who has illuilrated thefe parts 

 by the moll beautiful coloured plates which have ever been 

 publilhed on any anatomical fubjeft. (See particularly Tab. 

 7, alfo Tub. 5, and 8.) The two plexufes, with the inter- 

 vening vtlum, cover the hippocampi majorts, the pineal 

 gland, and upper furface of the oplic thalami. They fup- 

 port the fornix and plakcrium, intervening between them 

 and the optic thalami, and giving blood-vedels to all the 

 parts juft enumerated. When fuccefsfully injeftcd, thefe 

 plexufcs refemble a mere net-work of blood-velfels. They 

 contain numerous little glandular bodks, which are often 

 enlarged, fo as to refemble hydatids. The veins of the 

 choroid plexus form a large trunk htuated in the middle of 

 the velum, and named the vena magna Gakni. This, pafTuig 

 in the angle of conneftion of the falx and tentopiiim, 

 conftitutes the torcular Heropliili. The choroid pkxufes 

 und vtlum fhould now be reflcittd, to expofe ckarly the 

 4 



ERA 



eminences which form the fides of the lateral ve:itricle5. 

 We now fee, in the middle of the under furface of the velum, 

 a fmall and narrow portion of plexus, which was firft de- 

 fcribed and reprcfented by Vicq d'Azyr under the name of 

 plexus de la glande pineale ou du troifieroe ventricule. (Tab. 

 -. ff'.x. p. zi.) The anterior and fuperior eminence in 

 eaclAatcral ventricle is oblong, fmoolh and grey on its fur- 

 face, but compofcd internally of a ftriated mixture of cor- 

 tical and medullary fubftance, whence it derives the name of 

 corpus ftriatum. The more deeply we cut into this body, 

 the more complicated does its ftriated texture appear. The 

 corpora ftiiata are large, convex, and near to each other in 

 front ; they diminidi in fize, and diverge behind,' being fe- 

 patatcd by the optic thalami, and prefenting on the whole a 

 ])yritorm appearance. The pofterior eminence of the lateral 

 ventricle is called the thalamus, or coUiculus ncrvi optici. 

 It is found behind, within, and rather below the former. 

 Its furface is white, but the interior of the body is formed 

 of a ftriated intermixture of cortical and medullary matter; 

 in Ikape, it is nearly Uemifphevical, except that it is fur- 

 mounted above by a more or lefs prominent o\al tubercle, 

 which extends from before backwards. The convexities i-,: 

 the optic thalami are turned towards each other, and are con- 

 ncded by a cineriticus union, which is called the middle 

 commifl'ure of the brain, or foft commiflure of the optic 

 thalami. This union, which i5 naturally of a foft and deli- 

 cate nature, is torn by the mere weight of the parts, wh»u 

 the brain has been kept a few days before it is ufed for dif-- 

 fetlion. Hence fome anatomifts have denied its exiftence 

 altogether. This is the opinion of Sabatier, derived, as he 

 fays, from the moft exaft obfcrvation and very numerous 

 diffeftions. CTraite d'Anatomie, torn, ii, p. 32. ) The care- 

 ful examination of a recent brain will flicw the error of thij 

 celebrated anatomil^ ; and if authority be required on a point 

 which can be made the fubjecl of demonftration, it will be 

 fufhcient to cite the names of Vieuffens, Winflovi', Mor- 

 gagni, Soemmerring, and Vicq d'Azyr. Thefe bodies fe- 

 parate from each other below, and iorm a cavity, which is 

 the third ventricle of the brain. The thalamus ncrvi optici 

 is feparated from the corpus ftriatum, by a whililh, fibrous, 

 and flat cord, which is broader in front than behind. This 

 arifes from the front of the third ventricle, and following 

 the pofterior margin of the corpus ftriatum, was named by 

 Willis limbus pofterior corporis tliinti. It dcfcends into the 

 inferior horn of the ventricle, and terminates on the upper 

 furface of that cavity. This part is the centrum gcminum 

 femicirci'.lare of VieufTens, the txnia femicircularis of Haller, 

 bandektte ftriee of Vicq d'Azyr, and ftria cornea of Soem- 

 merring. The pofterior and reflefted horns of the ventricle 

 contain ceitain eminences, which are named hippocampi. 

 The pofterior marain of the corpus callofum mixes on cither 

 fide with the medullary matter of the brain, and produces 'v\ 

 the digital cavity one or more fmall projeftions, correfpond- 

 ing in direction to the couife of the cavity, and named coU 

 liculus, or unguis caves pofterioris ventriculi lateralis (I'er- 

 got, or hippocampus minor of Vicq d'Azyr). 



In the defcending horn a very large and confpicuous me- 

 dullary body is feen, extending to the termination of the 

 ventricle, where it forms a bulbous extremity divided by 

 three or four fillures, in fuch a manner as to bear an obfcure 

 refemblance to the foot of an animal. Tiie eminence is 

 named hippocampus major, or cornu Ammonis (proceffus 

 cerebri lateralis Soemmerring de corporis humani fab. tom. 

 iv. jj .36.), and its termination pe's hippocampi. 



It is nectflary to remark in this place, that the hippo- 

 campi are continuations of the medullary matter, which 

 forms the corpus callofum ; and that the corpus callolum 



itfclfj 



