Page:Encyclopædia Britannica, first edition - Volume I, A-B.pdf/334

 Part VI. 234 .A N A T O M Y The meninges or membranes are two in number. The fing in breadth in their paffage, line the whole cavity of hrfl is named dura mater; the fecond pia mater, which the orbits, at the edges of which they communicate with is again divided into two; the external lamina being the pericranium and periofteum of the face. termed araehnoides, the internal retaining the common The elongations of the dura mater which accompany the blood-veifels through the foramina of the cranium, name of pia mater. unite with the pericranium immediately afterwards. Such, fpr inftance, are the elongations which line the foflulae DURA MATER. of the foramina lacera or jugularia, and the bony or caThe dura mater inclofes the brain and all it appen- rotid canals of the apophyfis petrofa, &c. dages. It lines the infide of the cranium, and fupplies The dura mater contains in its duplicature feveral parthe place of an internal periofteum, being fpread in all ticular canals, into which the venal blood not only of the holes and depreflions, and covering all the eminences that membrane, but of the whole brain, is carried. in fuch a manner as to prevent their being hurtful to the Thefe canals are termed finufes, and. fome of them are brain. difpofed in pairs, others in uneven numbers; that is, The dura mater is made up of two lamina?, adhering fome of them are placed alone, in a middle fituation; very clofely together * the fibres of both croffing each others are difpofed laterally on each fide of the brain. other obliquely. Their texture is very clofe and llrong, The moft ancient anatomifts reckoned only four; to appearing to be partly ligamentary and partly tendinous. which we can now add four times as many. The dura mater Hicks clofely to the cranium by a great Thefe finufes are in the duplicature of the dura mater; number of filaments of the external lamina, which enter and their cavities are lined on the infide by particular the pores of-the bones chiefly at the futures both above very fine membranes. They may be enumerated in this and below ; and by penetrating thefe joints, they com- manner. The great finus of the falx, or fuperior longitudinal municate with the external periofteum. Thefe adhefions are formed intirely by the external finus, which was reckoned the firft by the aricients. lamina. The internal lamina is very fmooth and polifli- Two great lateral finufes, the fecond and third of the ed on the infide, which is alfo continually moftened by a ancients. fine fluid difcharged through its pores, much in the The finus called torcular Heropbili, the fourth of the ancients. fame manner as in the peritonaeum and pleura. The folds of the dura mater are made by the internal The fmall finus of the falx, or inferior longitudinal linus. lamina; and three of them form particular fepta; one The pofterior occipital finus, which is fometimes double. of which is fuperior, reprefenting a kind of mediaftinum Two inferior occipital finufes, which form a portion of between the two great lobes of the brain. The fecond a circle, and may likewife be called the inferior lateral is in a middle fituation, like a diaphragm between the ce- finufes. rebrum and cerebellum; the third is inferior, between the Six finus petrofi, three on each fide, one anterior, lobes of the cerebellum. The fuperior feptum is longi- one middle of angular, and one inferior. The two intudinal, in form of a fcythe, from whence it is termed the ferior, together with the occipital finufes. Complete a falx of the dura mater. The middle feptum is tranf- circular finus round the great foramen of the os occipitis. verfe, and might be called the diaphragm of the brain. The inferior tranfverfe finus. The Inferior ieptum is very fmall, and runs down be- The fuperior tranfverfe finus. tween tbe'lobes of the cerebellum.The two circular finufes of the fella fphenoidalis; one Befides thefe large folds, there are two fmall lateral fuperior, and one inferior. p-nes.on each fide of the fella turcica, each running from Two finus cavernofi, one on each fide. the pofterior to the anterior clynoid apophyfis on the Two orbitary finufes, one on each fide. fame fide. Thefe two folds, together with the anterior All thefe finufes communicate with each other, and and posterior parts of the fella turcica, form a fmall foflula with the great lateral finufes by which they difcharge themfelves into the internal jugular veins, which are onin which the pituitary gland is lodged. The elongations of the dura mater are produdtions of ly continuations of thefe lateral finufes. They likewife both laminae, which go beyond the general circumference, unload themfelves partly into the vertebral veins, which communicate with the fmall lateral of inferior occipital and pafs out of the cranium. The moft confiderable of thefe elongations pafles finufes; and partly into the external jugular veins, by through the great occipital foramen, and runs down the the orbitary finufes which communicate with the vena? common canal of the vertebrse in form of a tube, lining angulares, frontales, nafales, maxillares, &c. as the lathe infide of that canal, and inclofing the medulla fpi- teral finufes likewife communicate with the venae occipiralis/ by the name of the dura mater of that medulla. tales, &c. The other elongations accompany the nerves out of the Thus the blood which is' carried to the dura mater, cranium in form of vaginie, which are more numerous &c. by the external and internal carotid, and by the than the nervous trunks reckoned in pairs. vertebral arteries, is returned to the heart by the exterThere are two particular elongations which form the nal and internal jugular and vertebral veins ; fo that when periftoneum of the orbits, together with the vagina? of the paflage of the blood is obftruiled in any particular
 * the optic nerves. Thefe orbitary elongations go out by place, it finds another way, by virtue of thefe communithe fphenoidal or fuperior orbrtary fiflures, and, increa- cations, though not with the fame eafe.