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 i7o A N A T O M Y. Part I. The body of the fifth vertebra is rather thinner than os facrum are much fmaller than the former. At that of the fourth. The fpinal proc'cfs of this fifth is their back-part, near their edge, a knob and oblong flat fmaller, and the oblique procefles face more backwards furface give rife to two ftrong ligaments which are exand forwards than in any other lumbar vertebra. tended to the os ifehium ; and are therefore called facroThe False Vertebrae compofe the under pyramid fciatic. of the fpine. They are diftinguifhed from the bones al- The fpinal proceffes of the three uppermoft bones of ready defcribed juftly enough by this epithet of falfe ; the os facrum appear Ihort, lharp, and almoft eredt, becaufe, though each bone into which they can be di- while the two lower ones are open behind; and fomevided in young people, refembles the true vertebrae in times a little knob is to be feen on the fourth, though figure, yet none of them contribute to the motion of the generally it is bifurcated, without the two legs meeting trunk of the body ; they being intimately united to each into a fpine; in which condition alfo the firft is often to other in adults, except at their lower part, where they be feen. The mufculus latiflimus, and longiffimus dorfi, are moveable; whence they are commonly divided into facrolumbalis, and glutseus maximus, have part of their two bones, os facrum and eoccygh. origins from thefe fpinal proceffes. Os Sacrum, is fo called from being offered in facri- The canal between the bodies and proceffes of this fice by the ancients, is of an irregular triangular fhape, bone, for the cauda equina, is triangular ; and becomes broad above, narrow below, convex behind, for the ad- fmaller as it defeends, as the cauda alfo does. Bevantageous origin of the mufcles that move the fpine low the third bone, this paffage is no more a complete and thigh backwards; and concave behind, for enlar- bony canal, but is open behind; and is only there deging the cavity of the pelvis. Four tranfveffe lines fended by a ftrong ligamentous membrane ftretched over of a colour different from the reft of the bone, which it, which, with the mufcles that cover it, and are very are feen on its fore-part, are the marks of divifion of prominent on each fide, is a fufficient defence for the the five different bones of which it confifts in young per- bundle of nerves within. At the root of each oblique procefs of this bone, the fons. The fore-part of the os facrum is fmooth and flat, to notch is confpicuous, by which, and fuch another in the allow a larger fpace for the contained bowels, without a- laft vertebra of the loins, a paffage is left for the twentyny danger of hurting them.—The back-part of it is al- fourth fpinal nerve; and, in viewing the os facrum, moft ftreight, without fo large a cavity as the vertebrae either before or behind, four large holes appear in each have.—The bridges between the bodies and proceffes of fide, in much the fame height., as where the marks of this bone, are much thicker, and in proportion fhorter, the union of its feveral.bones remain. Some of the larthan in the former clafs of bones.—The ftrength of thefe geft nerves of the body pafs through the anterior holes; crofs-bridges is very remarkable in the three upper and fuperficial grooves running outwards from them in bones, and is well-proportioned to the incumbent weight different diredtions, (hew the courfe of thefe nerves. of the trunk of the body, which thefe bridges fuftain in From the intervals of thefe grooves, xhe pyriformis mufa tranfverfe, confequently an unfavourable, fituation, cle chiefly rifes. The holes in the back-part -of the bone are covered by membranes which allow fmall nerves when the body is eredt. There are only two oblique proceffes of the os facrum ; to pafs through them. The two uppermoft of thefe one ftanding out on each fide from the upper part of the holes, efpecially on the fore-fide, are the largeft; and firft bone. Their plain eredi furfaces face backwards, as the bone defeerids, the holes turn fmaller. Someand are articulated with the inferior oblique proceffes of times a notch is only formed at the lower part in each the laft vertebra of the loins, to which each of thefe pro- fide of this bone; and in other fubjedts there is a hole ceffes is connedted by a ftrong ligament, which rifes from common to it and the os coccygis, through which the a fcabrous cavity round their roots, where mucilaginous twenty-ninth pair of fpinal nerves paffes ; and frequently glands are alfo lodged.—Inftead of the other oblique a bony bridge is formed on the back-part of each fide by proceffes of this bone, four rough tubercles are to be a procefs.fent up from the back-part of the os coccygis, feen on each fide of its furface behind, from which the and joined to the little knobs which the laft bone of the ■mufeu!us facer has its origin-. os facrum has inftead of a fpinal procefs. Under this The tranfverfe proceffes here are a1! grown together bridge or jugum, the twenty-ninth pair of fpinal nerves into one large firong oblong procefs on'each fide ; which, runs in its courfe to the common holes juft now defo far as it anfwers to the firft three bones, is very thick, fcribed. and divided into two irregular cavities, by a long perpen- The fubftance of the os facrum is very fpongy, withdicular ridge. The foremoft of the two cavities has out any confiderable folid external plates, and is lighter commonly a thin cartilaginous Ikin covering it in the re- proportionally to its bulk than any other bone in the bocent fubjedt, and is adapted to the unequal protuberance dy; but is fecured from injuries by the thick mufdes of the os ilium, and a ftrong ligament connedts the cir- that cover it behind, and by the ftrong ligamentous cumference of thefe furfaces of the two bones. The membranes that clofely adhere to it. Cavity behind is divided by a tranfverfe ridge into two, This bone is articulated above to the laft vertebra of where ftreng ligamentous firings that go from this bone the loins, in the manner that the lumbar vertebrae are to the os ilium, with a cellular fubftance containing mu- joined; and therefore the fame motions may be performcus, are lodged. ed here. The articulation of the lower part of the os The tranfverfe proceffes of the two laft bones of the facrum to the os coccygis feems well enough adapted for allowing