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exigence of fuch a cafe, as a perfect fpina bifida, fuppofes the very canal nnA fpinal marrow to divide into two branches, and the fpinal proceffes to divaricate into two rows. We have a cafe in the Philufophical Trail factions, where the fpinal marrow of an infant was found bare, without any bony covering, and which was taken for a fpina bifida. See Phil. Tranf. ^472. fed. 2. and N° 366. p. 98. See, in the Mem. dc t'Acad. Royale des Sciences anno 1730, fome curious obfervations, by Mr. Winflow, on the motions of the fpine. Cartilages of the Spine. The cartilages of all the vertebra? in general are of two kinds, one proper to each vertebra, the other common to two vert-'brse which lie next one an- other ; the firft may be termed the cartilages of articulation, the latter the cartilages of fympbyfts.
The proper articular cartilages of each vertebra of the whole fpine are thofe four which cover the furfaces of the four fmall articular apophyfes. In the natural ftate they are very white and fmooth, and much thicker than in the dried bones. Their circumference is the fame with that of the articu- lated fides of the apophyfes, except in thofe places where there are fmall fuperficial notches. The vertebra? of the back, befide the four cartilages of their fmall apophyfes, h.ave others which do not belong to their articulations with one another, viz. thofe which cover the lateral foifula? in the bodies of thefe vertebra?, and the fofluhe in their tranf- verfc apophyfes, by both of which they are articulated with the ribs.
The cartilages of the fymphyfis He between the bodies of the vertebra;, one of them being contained between, and clofely joined to the lower furface of one vertebra, and to the upper furface of the next under it ; the breadth and circumference of thefe anfwer exactly to thofe of the fur- faces between which they are contained, but their height, or thicknefs, is different in each clafs of the vertebrae. In the vertebrae of the loins they are, according to the ftature of the fubject, a quarter or a third of an inch thick, in thofe of the neck they are not fo thick, and thinneft of all m thofe of the back. Neither arc they of equal thicknefs in all their parts, thofe of the neck and loins appear to be thickeft on the forefide, and thofe of the back rather thickeft on the contrary part; but thefe differences are moft re- markable in the vertebrae that lie near the middle of each clafs.
The internal ftrudture of thefe cartilages is different from that of all the other cartilages of the body, and indeed they refemble the reft in nothing but in wlutenels and in elafti- city. When we view their circumferences only, they ap- pear to be one uniform mafs like the reft, hut when divided by an incifion, parallel to the furface of the vertebne, we fee that they are made up of a great number of concentric cartilaginous rings, contained within each other, with a fmall fpace left between them. Thefe are clofeft and thin- neft: near the center, and about the middle feem to dege- nerate into another {"after kind of Jubilance. Thefe rings do not form an entire circumference, being turned inward on the buckfide, anfwerably to the pofterior flope in the body of each vertebra. They lie horizontally, one edge being fixed to the lower fide of one vertebra, and the other to the upper fide of the vertebra next below the former. The intcrfticcs between the rings are filled with a mucila- ginous fubftance, lefs fluid than that of the joints, and their breadth, or beighth, is proportionable to the diftance of the vertebrae between which they He.
Each cartilaginous lamina taken feparately is very pliable, according to its length, but taken all together they are not fo eaJily bent, partly becaufe of their circular figure, and pattly becaufe of their proximity and multiplicity. They vield, however, in the inflexions of the fpine, and without any inflexion to the weight of the head and upper extremi- ties ; but this is done by very fmall degrees, and moft of all, when the upper part of the body is loaded with any foreign weight. Alter thefe com.pr.efllons they reftore themfelves, merely by being freed from the preffure; fo that a man is really taller after lying fome time, than after he has car- ried a burthen for a great while. Thefe lingular obferva- tions of the different heights of the fame perfon, at different times, which were firft made in England, and afterwards con- firmed at Paris by Mr. Morand, are moft naturally and ea- fdy accounted for, merely by the different ftate of the in- tervertebral cartilages.
The intervertebral cartilages of the neck lying, for the moft part, between the convex iide of one vertebra, and the con- cave fide of another, are of a greater extent, in proportion to the ilzc of thofe vertebne, than thofe of the back and loins. The os facrum has no cartilage, except that between the upper fide of the firft falle vertebra, and the la ft ver- tebra of the loins; and thofe by which it is connected to the ofla innominata, already defcribed as the cartilages of that bone. The cartilages, which join the different porti- ons of the os coccyejs, arc preferred in fome fubjects to a very great age, but in others they very foon become en- tirely bony. JVinflovji Anatomy, p. 145. Ligaments of the Spine. The vertebra? are itrongly connected Suppl. Vol. II.
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to each other by three kinds of ligaments. Each vertebra is connected to thofe above and beiow it by a great number of very fhort and Jlrong ligaments, which crols each other obliquely, and are fixed round the edges of the body of each vertebra. Thefe crucial ligaments cover the circum- ference of the intervertebral cartilages, and adhere clofely to them.
The bodies of all the vertebra?, from the fecond of the neck to the os lactam, are covered by iigamentary half vaginas on the convex fide, in which thefe vaginae arc fixed, fur- rounding all the crucial ligaments, and made up of Iiga- mentary filaments partly oblique, but for the moft part lon- gitudinal. All the vertebrse are likewife ftrongly connected by a Iigamentary tube, which lines the inner furface of the medullary canal from the occipital hole to the os facrum ; it reprefents a long flexible funnel, its cavity at the upper part being equal to that of the occipital foramen, and ending in a point at the os facrum. The firft vertebra is fixed to the os occipitis befide this, by a diftinct and very ftrong Iigamentary covering; the fecond has two ligaments pecu- liar to it, one which connects the apophyfis dentiformis to the os occipitis, and another tranfverfe, which confines this apophyfis withih the anterior portion of the cavity of the firft vertebra. The firft may be called the occipital, and the fecond the tranfverfe ligament of the odontoide apo- phyfis. Along the whole bony canal of the fpine, between the bafes of each jpinal apophyfis, lies a fiat and very elaftic ligament, of a yellowifh colour, which fills up the pofteri- or great notches of the vertebra;, adhering to their edges, and to the neighbouring portions of the inner Iigamentary tube.
Between the extremities, or apices of the fpinal apophyfes, are fmall Iigamentary ropes, which go from one fpine to the next, which are all really double, though they appear fo only in the neck, being there fixed feparately to the forked extremities of the fpines. Between all the fpinal apo- phyfes, from their apices to the middle of their hales, lies a Iigamentary membrane, going between eath two; and there is a ligament of a like kind between the two tranf- verfe apophyfes in all.
The articular ligaments of the fpina dorfi are thofe which tic the glenoide cavities of the firft vertebra to the condyles of the os occipitis; thofe that join the cartilaginous furface of the apophyfis dentiformis to the anterior cavity of the firft vertebra, and thofe by which all the oblique, or arti- cular apophyfes, are connected together. Thefe are all fmall, fhort, and ftrong. The vertebral ligaments of the ribs, or thole which connect the ribs to the vertebrae, are of the fame kind, and are inferted round the cartila- ginous foflulse in the body, and apophyfes of each vertebra. Befide all thefe ligaments of the fpine, there is one which goes in form of a membrane from the os occipitis, all the ■way to the laft two vertebra? of the -neck : this is a true in- termufcular ligament, and may be properly called the liga-* mentum cervicale pojlcrius. There are alio two lateral liga- ments of the fame kind, fixed to the tranfverfe apophyfes of the vertebra? of the neck. Wtnfilotv's Anatomy, p. 147. Spikes of echini, in natural hiftory. Thefe in their foifilc ftate make a great appearance in the cabinets of the curi- ous, and in the works of the learned, and are of an almoft infinite variety of kinds ; and many of them arc of the fame figures and dimenfions with thofe of the echini now living in our own and other feas, and well known to us. But befide thefe there are an almoft infinite variety of others, which though allowed on all hands to be truly fpines of fome echini or other, yet evidently differ from thofe of all the known recent fifli of that name, and have certainly be- longed to fpecies of" it which we have not yet the leaft knowledge of. Thefe, however different in fhape from one another, yet all agree in their texture and conftituent mat- ter, both with one another, and with the foffile remains that (apply the places of the fhells of the other fpecies io common in our chalk-pits, all being compoled of a plated, or tabulated fpar. Both thefe fhells and the fpines, though they retain every outer lineament of the bodies they owe their form to, yet have they nothing of their interior tex- ture, nor any the leaft refemblancc of it, but are compofed of plates fet edgewife, or aflant, in the fhells, and in the fpines always obliquely to the axis of the body ; fo that all the foftilc fpints of echini break regularly in an oblique di- rection, and always fhew on each part a perfectly fmooth and glolfy flanting furface.
Of the foffile fpines of echini fome are long and {lender, ta- pering from a broader bafis to' a fine point, and fometimes from a thick part, at or near the middle, to an obtufe point at each end : thefe are ufually ftriaied, ridged, or furrowed, and often elegantly granulated, though fometimes they are fmooth. Thefe moft refemble the fpines of the more com- mon fpecies of recent or living echini we are acquainted with ; others of them are of very different and very flrangc figures ; fome are of the fame length with the common long ones, but arc very flat, and are ridged more or lefs high", or covered with tubercles of different ihapes ; others are ragged, and varioufly jagged, and. knotted like a rough 3 A a a branch
