BRA 



its nutrition, from the dura mater, than from ihc pericra- 



nium. . , . 



The d'.ira mater, whidi is a very ftronir, dcnlc, ail "'" 

 cbllic membrane, coiilills of two layers, intimately conneaed 

 in general, but feparated from each oilier at particular parts, 

 fo as to leave vacancies between tliem called hnufes, mto 

 wliich the veins of the brain pour their blood. (For an 

 account of the ruuifeK, fee Veins, diflnbiiiion of.) Its 

 llrufturc is obvioullv fibrous ; the fibres not following any 

 re'nilardiilribution.'bnt interfefting each other m every di- 

 rcSion. After beiiii; macerated for fome time in water, it 

 is refolvcd into a iirin tendinous kind of cellular texture. 

 The fides of the membrane are covered by the ramifications 

 of the arteria mcningca media, which arc very confpicuoiis 

 on the mere removal of the flaiU cap, each branch being 

 accompanied by two veins. (For the defcription of this 

 artery, fee Arthries, (hjlribut'ion of.) In the middle ot the 

 fuperior furface of the membrane is a deprefiion, extcndjug 

 from the lower part of the os fiontis to the tentoriutn, which 

 denotes the fituation of the fuperior longitudinal finus. 



We alfo obfervc, on the upper part of the dura mater, 

 cinders of little white granular bodies, termed glandulaj 

 Pacchioni. Tliefe vary in number and iitiiation ; fomctimes 

 they rife in little heaps on the fide of the longitudinal finus, 

 and are lodged in confiderable cavities formed in the bone 

 for their reception. In otiier inlbnces the cinders of ihefe 

 glands arc fo fmall and few in number, tiiat they may efcape 

 obfcrvation. In dimfting the brain, we are in the next 

 place to cut the dura mnter round on a level with the bone 

 which we have fawn, except in front and behind, where the 

 falx is fituated. We are then enabled to rcflea it on either 

 fide from thofe membranes which immediately iiiveft the 

 brain. Its inner furface is fmooth and (hiding, and has no 

 conneftion with the pia mater, except at the upper part, 

 near to the longitudinal finus. The conneftion at this part 

 is of two kinds ; one by means of large veins, which, coming 

 from the furface of the 'brain, pafs between the layers of the 

 dura mater to terminate in the longitudinal finus : the other 

 by means of cUifters of glandulx Pacchioni, which adhere 

 equally to the under layer of the diiia mater, and furface of 

 the membranes, which more immediately inveft the brain. 

 Thefe glands are found not only in the outer and inner 

 layers of the dura mater, but frequently they communicate 

 and make one duller through an opening in the membrane. 

 They are alfo found attached to the inner furface of the 

 longitudinal finus. The ufe of thefe glands, as they are 

 called, is not known. 



The internal laver of the dura mater foiTns, by its fecef- 

 iion from the outer, proceffes for the fupport o-f the brain : 

 the principal of thefe are, one which deicends perpendicu- 

 larly between the htmifpheres of the cerebrum, and is called 

 the falx cerebri, from the fiiape of its lower edge ; and one 

 •which proceeds tranfvcrfcly, fupporting the back lobes of 

 the cerebrum, and feparating them from the cerebellum. As 

 this is ilrelchcd acrofs over the cerebellum, it is called the 

 tentorium cercbelli. The falx is a duplicature of the inter- 

 nal layer of the dura mater, wiiich begins to recede from the 

 external at the rethmoid bone, and inclofcs the criHa galli 

 of that bone. It proceeds in like manner from the fpinc, 

 and from the middle of the os frontis, froiu the faglttal 

 future ol the parrietal bones, and from the upper half of the 

 perpendicular ridge of the occipital bone, and it terminates 

 behind in the tentorium, gradually increafing in breadth to 

 its termination. 



The arteria meningea media is the principal fource of the 

 arterial iupply of the dura mater : it receives fome fmall 

 branches in front from the internal carotid, ophthalmic, and 



BRA ' 



pharyngeal arteries : behind from the occipital and vertebral 

 velfcls. Its veins open into the neareft fiiuifLS. The accu- 

 rate refearches of Lobllein and Wrifberg have fhewn that no 

 nerves can be traced to this membrane. (See Comment 

 Gottingens: an. 1777. Ludwig colled, fcriptor. neurol. 

 minor, torn, i.) 



After the connexion of the dura mater with the fub- 

 jacent membranes is broken, the front of tiic falx is to be 

 divided from the crilla galli, and that procefs is to be turned 

 back from betwe'en the hemifpheres of the brain. Then 

 the brain and its immediate invelling meinbtanes become 

 fairly expofed to our examination. Thefe membranes were 

 confidered as one, and called the pia mater, till Ruyfch 

 demoiillrated that they were two. The outer one is uni- 

 formly fpread over the vifible furface of the brain, is of a 

 pale white colour, yet in fome degree tranlparent, very 

 thin, devoid of evident vefTels, and is named the tunica arach- 

 noidea. It is fecn moll dillinClly where it paiTes between 

 the two lobes of the cerebellum : in moll other parts of the 

 brain it adheres very firmly to the pia mater. Although the 

 tunica arachnoidca has in its natural Hate no vifible vcflcls, 

 yet, when it is inflamed, its vefTels become enlarged and 

 evident, and there can be no doubt but that it contributes 

 at all times to fccrete its fhare of that liquor which prcftrves 

 it feparate from the dura mater. In inflammation of the pia 

 mater and tunica arachnoidea, it is common to have a turbid 

 and fometimes puriform fluid effufed between it and the 

 dura mater, which is doubtlels fecreted by the inflamed 

 membrane. 



The membrane which lies beneath the tnnica arachnoidea 

 is called the pia mater ; it covers the whole furface of the 

 brain, fending down procefi'es between all its convolutions. 

 It is extremely vafcular, and a great portion of the blood, 

 which the brain receives, is fpread out upon its furface in 

 minute vefTels : from thefe others proceed capable of ad- 

 mitting very fubtle injcdlions. From the great number of 

 vefTels, which ramify on this membrane, Soemmering calls 

 this membrane, membrana vafcnlofa cerebri. Its vefftls 

 are fewer and fmaller where it covers medullary parts, than 

 where it is in contact with the cortex. The proceffes which 

 pafs from its infide no where touch medullary matter. 

 The outer furface is tolerably fmooth ; the inner univcrlally 

 villous, from the torn orifices of innu.xerable vefTels which 

 entered the Tubllance of the brain. 



The membranes of the brain having been examined, may- 

 be removed ; and then the furface of the cerebrum is 

 brought 'into view, which appears convoluted, fo as to re- 

 femble the -windings of the fmall intellines. Thefe con- 

 volutions do not in general penetrate more than one inch, or 

 one inch and a half, into the fubftance of the cerebrum. A 

 very deep fiilure is however obTerved extending ilTclf obli- 

 quely upwards, and backwards, from the al:e of tlie fphenoid 

 bone ; and this is called the fifTura magna Sylvii. 



The encephalon or brain is divided into the cerebrum, 

 cerebellum, and medulla oblongata. 



The cerebrum is the uppermoit portion, and by much t!ie 

 largefT, and that which firil claims our attention in difTec- 

 tion. It rells upon the tentorium and. petrous portions of 

 the temporal bones beh.ind, and upon the Tphenoid al:e, and 

 the orbitary procefTes of the frontal bones in front. Its 

 fuperior furface prcfents a regularly convex oval, nat rower in 

 front than behind. This oval is divided into two equal parts 

 named hemifpheres, by a deep longitudinal fiiiare, into 

 which the falx cerebri defcends. The depth of this cleft 

 being greater than the breadth of the falx, the onpoftd 

 plane furfaces of the hemifpheres are loofcly united below 

 the inferior edge of the falx, by vcfTcIs anil cellular fub- 



ilancc. 



