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Part VI. A N A T O M T. mater; which common bafts lies immediately on into two lobes, feparated by the occipital feptum of the dura that portion of the dura mater which lines the bafis dufa mater. of the cranium. The medulla oblongata is therefore It is made up, like the cerebrum, of two fubftances, juftly to be a third general part of the whole but it has no circumvolutions on its furface. Its fulci mafs ofefteemed the brain, or as the common produftion or united are pretty deep, and difpofed in fuch a manner as to of the whole medullary fubftance of the cereform thin flat ilrata, more or lefs horizontal, between elongation and cerebellum. which the internal lamina of the pia mater infinuates it- brum is extremely difficult, if not altogether impoflible, felf by a number of fepta equal to that of the ftrata. to Itexamine or demonftrate it as we ougnt, in its natural Under the tranfverfe feptum, it is covered by a vafcular texture, which Communicates with the plexus cho- fituation; but we are obliged to do both on a brain inroides. It has two middle eminences called appendices verted. venniformes ; one anterior and luperior, which is turned The lower fide of the medulla oblongata in an inverted prefents to our view feveral parts which are in forward; the other pofterior and inferior, which goes back- fituation, either medullary produftions, trunks of nerves, ward. There are likewife two lateral appendices, both general or trunks of blood-veffels. turned outward. chief medullary produftions are thefe : The large Befides the divifion of the cerebellum into lateral por- or The anterior branches of the medulla oblongata; which tions or into two lobes, each of thefe lobes feems to be have been named crura anteriora, femora, and likewife fubdivided int® three protuberances, one anterior, brachialikewife medullae oblongatae, and pedunculi cerebri: The one middle or lateral, and one pofterior. When we feparate the two lateral portions or lobes, tranfverfe protuberance, called likewife proceflus annulahaving firft made a pretty deep incifion, we difcover flri't ris, or pons varolii: The fmall or pofterior branches, called . of all the pofterior portion of the medulla oblongata; and pedunculi cerebelli, or crura pofteriora medulke cblpngaextremity or cauda of the medulla oblongata,. .in the pofterior furface of this portion, frohi the tubcr- tjeWith: The two pairs of tubercles, one of which is named corcula quadfigemina, all the way to the pofterior notch in the body of the cerebellum, and a little below that pora olivaria, the other corpora pyramidalia; and to all notch, we obferve an oblong cavity which terminates thefe produftions we muft add a produftion of the infunbackward like the point of a writing pen. This cavity is dibulum and two medullary papiibe. The great branches of the medulla oblongata are two what is called the fourth ventricle. At the beginning of this cavity, immediately behind the very confiderable medullary fafciculi, the anterior extrefmr.ll csmmon cahal which lies under the tubercles, we mities of which ase feparated, and the poftericr unimeet with a thin medullary lamina, which is looked up- ted, fo-that, taken both together, they reprefent a Roon as a valve between that canal and the fourth ventricle. man V. A little behind this lamina, the cavity grows wider to- The tranfverfe, annular, or rather femi-annular prowards both hands, and then contrafts again to its firft tuberance, is a medullary produftion, which feems at firft: lize. It is lined interiorly by a thin membrane, and fight to furround the pofterior extremities of the great feems oftentimes to be diftinguiftied into two lateral parts, branches; but the medullary lubftance of this protubeby a kind of fmAl groove, from the valvular lamina to rance is in reality intimately mixed with that of the two former. VaroHus, an ancient Italian author, viewing the point of the calamus fcriptorius. This membrane is a continuation of that which lines the thofe parts in an inverted fituation, compared the two fmall canal, the third ventricle, infundibulum, and the branches to two rivers, ancl the protuberance to a bridge over them both, and from thence it has the name of ions two great ventricles. On each fide of this ventricle the medullary fubftance Varolii. forms a trunk which expands itfelf in form of laminse The fmall branches of the medulla oblongata are lathrough the cortical ftrata. When one lobe of the cere- teral produftions of the tranfverfe protuberance, which bellum is cut vertically from above downward, the me- by their roots feern to encompafs that medullary pordullary fubftance will appear to be difperfed in ramifica- tion in which the fourth ventricle or calamus fcriptorius tions through the cortical fubftance. Thefe ramifica- is formed.. tions have been named arbor vita ; and the two trunks The extremity is no-more than the medulla oblongata from whence thefe different laminae arife, are called pe- contrafted in its paffage backward to the anterior edge of the great foramea of the os occipitis,. where it termidunculi cerebelli. nates-in the medulla fpinalis; and in this part of it feveral things are to be taken notice of.. We fee firft of all, MEDULLA OBLONGATA. four eminences, two-named corpora olivaria, and the oThe medulla oblongata is a medullary fubftance fitua- ther two corpora pyramidalia. Immediately afterwards* ted from before backward in the middle part of the bafes it is divided into two lateral portions by - two narrow of the cerebrum and cerebellum without any difcontinua- grooves, one on the upper fide, the other on the lower. tion, between the lateral parts of both thefe bafes; and- They both run into the fubftanee of the medulla, as betherefore it may be looked upon as one middle medullary tween two-cylinders, flatted on that fide by which they bafts common to both cerebrum and cerebellum, by the are joined together. reciprocal continuity of their medullary fubftances, When we feparate thefe ridges with the fingers, we through the great notch in the tranfverfe feptum. of the oblerve a crucial intertexturc of fevent! final] medullary cords*.
