BRA 



BRA 



itfelf, tTie feptum hicidum, fornix, and pfcjlterium, are parts 

 of one continuous medullary body. To underllaiid clearly 

 the relative pofition and connections of tilde parts, it is ab- 

 fohitely neccfTary to relort to the proper fource of all anato- 

 mical dcfcription, difi'cdion of the parts., wliicli wdl (liew 

 how inadequate a notion of thefc circumilances is ottaincd 

 from the belt defcription. 



An oval opening is feen between the optic thalami in 

 front, named the foramen commune anttrius, which leads 

 into the tiiird ventricle. As it is fituatcd under that part of 

 the fornix, which is not adherent below, it opens by means 

 of the flit above mentioned, as forming the communication 

 between the two lateral ventricles, into each of thcfe cavities. 

 Hence they communicate with the third by means, of this 

 aperture. The foramen commune poitcrius, which opens 

 alio into the third ventricle, is a triangular fpace left between 

 the optic thalami at the poilerior part, and only expofed 

 when the choroid plexus and velum have been reflefted. 



On drawing afunder the thalami, and fcparating their foft 

 commiifure, we expofe a conliderable oblong cavity, which 

 is na;aed the third ventricle. In front of this a round me- 

 dullary cord appears ju(t under the anterior crura of the 

 fornix, which paffes traiilverfely between the two hemifpheres, 

 and IS continued in a dillinfl form, for the fpace of an inch, 

 iTito the fnbllance of the brain on each fide, as will be bell 

 feen by a tranfverie leclion. A fimilar body is leen at the 

 back part of the ventricle, which is not however produced 

 into the hemifpheres like the anterior one ; thefc are the an- 

 terior and poilerior commiffnres of the brain. There is a 

 round aperture under the anterior conimiffure, bej'ond which 

 t^ie ventricle terminates by a pointed and conical extremity, 

 from which a fmall and Ihort cylindrical procefs is contiiiued 

 to the pituitary gland, under the name of the infundibulura. 

 It is a matter of difpute among anatomifts, whether or not 

 this procefs be hollow. The foramen commune anterius leads 

 through the third ventiicle into this opening. Jull before the 

 poilerior commiifure a round opening is found leading through 

 a ihort canal, in front of the tubtrcula quadrigemina, into the 

 fourth ventricle. The canal of communication is named ca- 

 nahs medius, iter ad quartum ventriculum, or aqua:du£tu3 

 Sylvii. 



The pineal gland, or conarium, is found at the back of 

 the optic thalami. Its fize is about that of a fmall horfe- 

 bean ; its colour a. reddiih grey ; its iliapc moft frequently 

 conical. Two fmall meuvillary cords connefl it to the poile- 

 rior commiflure and to the optic thalami ; they are inferted 

 in the convex part of the thalamus, and form a diftinguifh- 

 able prominent line, extending quite to the front of that 

 body, as is very beautifully reprelented in the 8th plate of 

 Vicq d'Azyr. This gland is covered by the velum, and 

 lias immediately over it the two fmall plexufes, which have 

 been already delcribed under the name of plexus de la glande 

 pineale. In the fubllance of the pineal gland, or in the me- 

 dullary matter, which connedls it to the optic thalami, there 

 is found a fmall quantity of a gritty matter nearly refembling ^ 

 fand. It conliils of a number of little lemitianfparent and 

 light yellow grains. Soemmerring, who firft dilcovered that 

 this was a part of the healthy llruclure of the brain, calls it 

 the acervulus of the pineal gland. (Diifertatio de lapillis 

 intra vel prope glandulam pinealemfitis, in Ludwig. colledl. 

 Icriptor. neurol. minor, torn, iii.) 



The pineal gland relts on a iquare portion of the brain, 

 which is feen behind and below the optic thalami, and below 

 the level of the tentorium. The furface of this is divided 

 into four projeclions or tubercles, and hence it has acquired 

 liie name of corpora quadrigemina. Of thefe bodies the 

 two fuperior are larger and more plane ; the inferior more 



convex". From the lower bodies a medullary produc^lion ex- 

 tends to the upper part of the crura certbelli. VieuPeiis de- 

 fcribed this part by the name of the v.ilvula magna cerebri, 

 fuppofing it to perform the oflice of a valve to the fourth 

 ventricle. Vicq d'Azyr, whofe in<:omparable plates thriiv/ 

 new light on this as well as on every other part of the fiib- 

 jeft, calk it the " lame medullaire du cervclet." The late- 

 ral portions of this produtlion conllitute pretty ftrong me- 

 dullary bodies, which arc the " procelfus a cerebello ad 

 t'jftes" of Haller, " les pedoncales ou color.ncs de la lame 

 medullaire" of Vicq d'A/.yr. The interval between thefe is 

 filled by a very thin medullary plate, the velum interjciTtuin 

 of Haller, which is the part commonly known as the valve 

 of the brain. This valve forms the roof of the fourth 

 ventricle ; therefore, if air be impelled into that cavity from 

 the canalis medius, the valvula cerebri is elevated, and wc 

 can perceive the fourth pair of nerves arifing from it ; the 

 origin of this nerve evidently projccling from the furface of 

 the valve. 



On lifting up the poilerior lobes of the cerebrum, a 

 tranfverfe produtlion of the dura mater, called the ten- 

 torium, comes into view. This is attached to the internal 

 traniverfe ridge of the occiput, and pollerio.- anglg of the 

 parietal bones behind, and to the (liarp margin of the pttixius 

 portion of thetemporal bone in front, as far foi-wards as its con- 

 nection to the body ofthefphencid bone, where it terminates in 

 a pointed form. This procefs therefore feparates the cavity, 

 which holds the cerebellum, from that part of the cranium 

 which contains the cerebrum. The vacancy left in its ante- 

 rior part, oppofite to the baulary procefs cf the occiput, 

 tranlmits the commencement of the medulla oblongata. 

 The middle of the tentorium, which has the termination 

 of the falx fixed to it, is elevated ; the lateral portions of 

 the procels are rather Hoping. 



The tentorium fhould be divided on either fide, in order 

 to expofe the cerebellum, v.'hich then appears lying in the 

 poilerior foffx of the occiput. This is divided into a right 

 and left lobe by the falx cerebelli, which is a fmall procefs 

 of dura mater, attached to the lower part of the perpendi- 

 cular ridge of the occiput. 



Tiie re!l of the encephalon fliould now be removed from 

 the cranium, that the fourth ventricle and bafis of the brain 

 may be confidered. In doing this, we lift up the anterior 

 lobes, and detach the different nerves as they go out of the 

 cranium, cutting them near to the brain on one fidr, and 

 near to the apertures by which they quit the flcull on the 

 other. The pituitary gland is to be fcooped out of the 

 fella turcica, and the medulla fpinalis to be cut acrofs as low 

 down as pofTible. > 



The part which has been t:.ken out fhould be laid on a 

 plate, ill the fame fituation which it held in the cranium, 

 that the fourth ventricle may be demonftrated. This cavity 

 is formed between the upper and poilerior part of the me- 

 dulla oblongata, and the middle anterior part of the cere- 

 bellum. It extends laterally to a confiderable dillancc in 

 the crura cerebeili ; a groove runs along the middle of the 

 medulla oblongata, which conflitutes the front of the ven- 

 tricle, and terminates in the end of the cavity in a point. 

 From the lateral produftions, and the pointed termination 

 of the cavity, it has been named the cahmus fcriptorius. 

 Its fhape indeed bears confiderable refemblance to a pen, ex- 

 cept that it is much larger. To defcribe its boundaries 

 more accurately, we may Itate, that it has four fides or pa- 

 rietes. The fuperior is formed by the valvula cerebri, the 

 lower and anterior by the back of the medulla oblongata, 

 the two lateral by the crura cerebeili, and in part by the 

 thick portions of tlie valve, v.-liich unite tb« crura cerebeili 



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