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ANA PI A MATER. This membrane furrounds the ■whole mafs of the brain more particularly than the dura mater. It adheres very clofely to the brain, and is connected to the dura mater only by the veins which open into the finufes. The pia mater is made up of two very fine laminae, the outermoft of which covers pretty uniformly all the convex furface of the brain, and lines in the fame manner all the concave or inner furface of the dura mater. The internal lamina forms a great number of plicae, duplicatures, and fepta, which infinuate themfelves into all the folds and circumvolutions, and between the different ftrata of the cerebrum and cerebellum. CEREBRUM. Th e cerebrum properly fo called, is a kind of medullary mafs, of a moderate confillence, and of a greyilh colour on the outer furface, filling all the fuperior portion of the cavity of the cranium, or that portion which lies above the tranfverfe feptum. The upper part of the cerebrum is of an oval figure, like half an egg cut lengthwife. It is flatted on the lower part, each lateral half of which is divided into three eminences, called lobes, one anterior, one middle, and one poilerior. The fubftance of the cerebrum is of two kinds, diftinguiflied by two different colours; one part of it, which is fofteft, being of a greyifli or afli colour; the other, which is more foiid, being very white. The afti-coloured lubftance lies chiefly on the outer part of the cerebrum like a kind of cortex, from whence it has been named fubfiantia corticalis or cinerea. The white fubftance occupies the inner part, and is named fubfiantia medullaris, or Amply fubfiantia alba. The cerebrum is divided into two lateral portions, feparated by the falx, or great longitudinal feptum of the dura mater. They are generally termed hemifpheres. Each of thefe portions is divided into two extremities, one anterior and one pofteriqr, which are termed the lobes of the cerebrum, between which there is a large inferior protuberance which goes by the fame name ; fo that in each Hemifphere there are three lobes, one anterior, one middle, and one pofterior. Thei anterior lie upon thefe parts of the os fronfts which contribute to the formation of'the orbits and of the-frontal finufes, commonly called the anterior foffae of the bafis cranii. The pofterior lobes lie on.the tranf-. verfe feptum; and the middle lobes, in .the middle or lateral foffae of the bafis cranu. Each lateral portion of the cerebrum has three fides ; one fuperior, which is convex; one inferior, which is uneven ; and one lateral, which is flat, and turned to the falx. Through the whole furface of thefe three fides we fee inequalities or windings like the circumvolutions of inteftines, formed by waving ftreaks or furrows very deep and narrow, into which the fepta or duplicatures of the pia mater infinuate themfelves, and thereby feparate thefe circumvolutions from each other. Near the furface thefe circumvolutions-are at fomediYol. I. Numb. 12. Part VI.
O M Y. 2E5 ftance from each other, reprefenting ferpentine ridges; and in the interftices between them, the luperficial veins of the cerebrum are lodged, between the two Luninas of the pia mater, from whence they pafs in the duplicature of the dura mater, and fo open into the finufes. Thefe circumvolutions are fixed through their whole depth to the fepta or duplicatures of the pia mater, by an infinite number of very fine vafcular filaments, . When they are cut tranfverfely, we obferve that the. fubftantia alba lies in the middle of each circumvolution, fo that there is the fame number of internal medullary circumvolutions as of external cortical ones. Having cut off the falx from the crifta galli, and turned it backward; if "we feparate gently the two lateral parts or hemifpheres of the cerebrum, we fee a longitudinal portion of a white convex body, which is named corpus callofum. It is a middle portion of the medullary fubftance, which under the inferior finus ofthe falx, and alfo a little toward each fide, is parted from the mafs of the cerebrum, to which it is Amply contiguous from one end of that finus to the other. The furface of the corpus .callofum is covered by the pia mater, which runs in between the lateral portions of this body, and the lower edge of each hemifphere. The corpus callofum becomes afterwards continuous on each fide with the medullary fubftance, which through all the remaining parts of its extent is intirely united with the cortical lubftance, and together with the corpus callofum forms a medullary arch or vault of an oblong or oval figure. After which we will obferve a' medullary convexity much fmaller than that which is common to the whole cerebrum, but of the fame form ; fo that it appears like a medullary nucleus of the cerebrum. Under this arch are two lateral cavities^ much longer than they are broad, and very ftiallow, feparated by a tranfparent medullary feptum. Thefe cavities are named the anterior, fuperior, or great lateral ventricles -of the cerebrum. The lateral ventricles are broad, and rounded at thefe extremities which lie next the tranfparent feptum. They go from before backward, contracting in breadth, and feparating from each other gradually in their progrefs. Afterwards they bend downward, and return obliquely from behind forward, in a courfe like the turning of a ram’s horn, and terminate almoft under their fuperior extremities. Thefe ventricles are lined with a thin membrane. The tranfparent partition or leptum fucidum, lies directly under the raphe or future of the corpus callofum, of which it is a continuation. It is made up of two medullary lamina;, more or iefs feparated from each other by a narrow medullary cavity, fometimes filled with a ferous fubftance. The feptum lucidum is united by its lower part, to the anterior portion of that medullary body, called the form's. ’with three pillars. The fornix being cut off and inverted, or quite removed, we fee firft of all a vafcular web, called plexus choroides, and feveral eminences more or lefs covered by the expanfion of that plexus. There are four pairs of 3 4 U' emiherices
