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ANATOMY.

tiguous to the cranium ; the other is very thin, and immediatelytouches the brain. The 1 ’ia Mater is again divided into two; theexternal lamina bding termed arachnoides, the internal retainingthe common name ol pi a mater.

The Dura Matf.r is a very compact strong membrane, liningthe inside of the skull, iinnly adhering at its basis, though butslightly at the upper part, except at the sutures. It lias three pro-cesses'; the first named Falx, (or the falciform process, from itsresemblance to a scythe) begins at the crista galli, and runs back-wards under the sutura sagittalis to the cerebellum, dividing thecerebrum into two hemispheres.

The dura mater lias in it several Sinuses, which answer theend of large veins to receive the blood from the sinallerveins of thebrain. Their number is uncertain. The first that presents it-elfis the longiludinalis superior, running from a blind hole a littleabove the crista galli, all along the upper edge of the falx. On thelower edge of this process is generally another very small one, calledlongitudinalis inferior. This runs into the rectus, and when want-ing, is supplied by a vein. The Rectus runs between the twofirst processes of the dura mater, and along with the sinus lougitu-dinalis superior pours into the two lateral sinuses; but for the mostpart the longitudinal sinus goes more directly into one of the lateralsinuses, and the straight sinus into the other. There is sometimesa small one in the third process, which empties in the same placewith the former. From the terminations of the longitudinal andstraight sinuses, begin the two lateral sinuses, which, when theycome to the os petrosum, dip down and pass through tne eighthforamina into the internal jugular vein. Another, named circu-luris, runs round the fore-part only of the cella turcica, i hetwo extremities of this empty into lour sinuses, one on the top oteach os petrosum, which pass into the sinus lateralis, and one at(lie inferior parts of the same bones, which pass indifferently intoboth the lateral and cervical sinuses. These two last sinuses haveah' ays communicating branches.

The Cervical Sinuses run from the basis of the skull throughthe great foramen on both sides of the medulla spinalis colli, andthrough the transverse processes of the cervical vertebral; the lastof these have sometimes proper foramina running from the eighthforamina to the back part of the condyle of the occipital bone.There are also two more of these vessels, which run from the cir-cular sinus, between the os sphenoides and the fore-part of the ospetrosum, directly into the internal jugular veins.

Tiie Pia Mater is an exceedingly tine membrane immediatelyinvesting the brain, even between its lobes, hemispheres, andfolds. It serves to contain the brain, and support its blood-ves-sels, which run here in great numbers, and are divided into aninfinite degree of minuteness, in order that the blood may notenter the brain too impetuously. Between the dura and piamater there is a membrane, ealled Tunica Arachnoides, whichis easily seen at the back part of the cerebrum, upon the cere-helium and back part of the medulla spinalis.

'Fhe Cerebrum is that part of the brain which occupies all theupper and fore-part of the cranium; being separated from the ce-rebellum by the second process of the dura mater. Its upper sideis divided into two hemispheres, and its lower side into four lobes,two anterior and two posterior, which latter are much the largest.At the meeting of the four lobes appears the infundibulum, whichseems to he a lymphatic, running from the ventricles ol the braininto the glandula pituitaria: this gland is seated in the cella turci-ca. Immediately' behind the infundibulum appear two smallbodies, named protuberantiie dux alba; pone infimdidulum. Be-tween the two hemispheres of the cerebrum, lower than the cir-cumvolutions, appears a white body, named corpus callosum.Under the corpus callosum appear the two lateral or superiorventricles, which are divided into right and left by a very thinmembrane, named septum lucidum, which is extended betweenthe corpus callosum and fornix.

The Fornix is a me Hullary body beginning from the fore-partof these ventricles, with two small roots which soon unite; thenrunning towards the hack part, they divide into parts, called crurafornicis. In the basis cf these two ventricles are four prominences:the two anterior are called, from their inner textme, corporastriata; the other two, thalami nervorum opticorum. Beyondthese are two other processes, called nates; and under them,Dearer the cerebellum, two called testes. -

Above the nates is situated the Glandula Pineai.is, famous

for being supposed, by Des Cartes , the seat of the soul. Uponfhe thalami nervorum opticorum are a number of blood-vessels,glands, and lymphatic', called plexus choroides. Under thebeginning of tire lornix is a small hoie, called foramen ad radicesfornicis, or iter ad infundibulum; and under the middle of tlmfornix, one railed foramen posterius, which is covered v illi avalve, named membrana, or valvu’.a major ; and the space underthe two anterior ventricles, between the foramina and the cere-bellum, is the third ventricle.

Ihe Cerebellum is situated under-the second process of thedura mater. Bv dividing this part of the brain lengthways wediscover more plainly the fourth ventricle, whose extremity iscalled calamus scriptorius. _ Here also appear two medullary bo-dies, called peduncwli, which are the basis of the cerebellum".

I he Medulla Ojilon data is a medullary continuation of (he.under part of the cerebrum ami cerebellum. It first appears intwo bodies from the anterior part of tiie posterior lobes ot the cere-brum, called crura medulla; oblongata-. The union of these crurabetween the cerebrum and cerebellum is called isthmus; andimmediately beyond this is an eminence named processus annu-laris.

The Medulla Spinalis, or Sbinai. Marrow, is a pro-duction of tiie medulla oblongata through the great foramen ofthe skull, and through the channel of the spine. It enlarges aboutthe la it vertebra; ol the back and first of the neck, where tiie largenerves, are given olf to the arms. 11 again enlarges in the loins,where tiie crural nerves begin; and the lower end of it, joinedwitli those and other nerves, is called, from its resemblance tothe tail of a horse, cauda equina. 'File coats of this pari are thesame with those of the brain; but the membrane here, which isanalogous to the dura mater, is thinner ami more connected to thebones, and the tunica arachnoides more conspicuous.

From these parts ot the brain the nerves proceed in a vast num-ber ot small medullary while fibres, which, at their first egress,seem easily to separate; but as they pass forward are somewhatmore, but still loosely connected, by the coal which they obtainfrom the pia mater, and at last piercing the dura mater, are closely-embraced by that membrane, which covers them in their progress.These being diffused through every- part of the body, are con-stituted as the instruments of the senses, so that without themthere can L-e no sensation. Although the nerves, if all joinedtogether, would hardly make a cord of an inch diameter, yet,from their existing in almost every part, they would appear to bedistributed with the utmost degree of minuteness. In their courseto the places for which they are destined they generally run asstraight as the part over which they- are to pass, and their own se-curity from external injuries, will allow, sending off their branchesat very acute angles, and consequently running more parallel thanthe blood-vessels, Their distribution is seldom different in theopposite sides of the same subject, nor indeed in any two subjectsis there any considerable variety to be found, Frequently nerveswhich come out distinct or separate, afterwards conjoin into onefasciculus, under the same common covering; and though thenervous fibrils probably do not communicate, yet because thecoats at the conjoined part are common, and these strong coatsmay have great effects on the soft pulpy nerves, it is evident allsuch will have a considerable sympathy- with one another. Insome parts where these conjunctions take place, the size of thenerves seems much increased,and those knotty oval bodies, namedganglions, are formed. The coats of these knots are stronger,thicker, and more muscular than the whole nerves which enterinto them would seem to constitute; while the nervous fibrilspass through w ithout any great change.

The nerves are distinguished into two classes, viz. those of theENCETHALON,-or Medulla Oblongata, and those proceedingfrom tiie medulla spinalis. Of the first there are nine pairs; of thelast thirty.

Of the nerves proceeding from the encephalon, the first are theOlfactory Nerves, rising from that part of the brain wherethe carotid arteries are about to enter, and running under the an-terior lobes of the brain, they become a little larger, till theyreach the os cribriforme, into the foramina of which the smaltfilaments insinuate themselves, and are immediately spread on themembrana narium. The second pair are the optic nerves, whicharise single from the thalami ncr\ ovum opticorum, and then uniteing at the fore-part of the cella turcica, seem, to be pretty much

' blended-