Page:Encyclopædia Britannica, first edition - Volume I, A-B.pdf/205

 16$ Part I. ANA' r O M Y. years of age they have twenty ; and their number does thickeft upon the bafe, and gradually, as the roots turn not fmaller, becomes thinner. The hbres of this enamel increafe till they are about feven years old, when the teeth that firft made their way through the gums are are all perpendicular to the internal fubllance, and are thruft by others that have been formed deeper in the ftreight on the bafe, but at the fides are arched with a jaw, andoutfome more of the teeth begin to difeover themconvex part towards the roots; which makes the teeth felves farther back About fourteen years refill: the compreffion of any hard body between the jaws, of age, fome more inofthethemouth. firft crop are (bed, and the with lefs danger of breaking thefe fibres, than if they had been fituated tranfverfely. The fpongy fockets in which number is inereafed. This Ihedding of the teeth is of the teeth are placed fikewife ferve better tq.prevent fuch good ufe ; for if the firft had remained, they would have ftood at a great diftance one from another ; becaufe the an injury, than a more folid bafe would have done. The bony part of the teeth has its fibres running teeth are too hard in their outer cruft, to increafe fo fait ftreight, according to the length of the teeth. When it as the jaws do. Whereas both the fecond layer, and the is expofed to the air, by the breaking or falling off of teeth that come out late, meeting, while they are foft, the hard cortex, it foon corrupts. And thence carious with a confiderable refiftance to their growth in length, teeth are often all hollow within, when a veryfmall hole from thofe fituated upon them, neceffarily come out broad, and fit tp make that clofe guard to the mouth, which appears only externally. The teeth have canals formed in their middle, where- they now form. in their nerves and blood-veliels are lodged; which they The teeth are joined to the fockets by gomphofis, and certainly need, being conftantly wafted by the attrition the gums contribute to fix them there ; as is evident by they are fubjefted to in manducatipn, and for their fur- the teeth falling but when the gums are any way dether growth, not only after they firft appear, but even in ftroyed, or made too fpongy; as in the feurvy or falt■adults; as is evident when a tooth is taken out: For vations : Whence fome clals this articulation with the then the oppofite one becomes longer, and thofe on each fyffarcofis. fide of the empty focket turn broader; fo that when The ufes of the teeth are to mafticate our alithe jaws are brought together, it is fcarce obfervable ment, and to aflift us in the pronunciation of feveral letters. -where the tooth is wanting. The veffels are eafily traced fo long as they are in the Though the teeth fo far agree in their ftruffure, yet, large canal, hut can fcarce be obferved in their diftribu- .becaufe of fome things wherein they differ, they are tion from that to the fubftance of the teeth of adults. generally divided into three claffes, viz. incifores, canim, This plentiful fupply of veffels muft expofe the and niolares. teeth to the fame diforders that attack other vafcular The incifores, are the four fore-teeth in each jaw, re.ceiving their name from their office of cutting, our aliparts. Every root of each tooth has fuch a diftimft canal, ment ; for which they are excellently adapted, being each with vefiels and nerves in it. Thefe canals in the teeth formed into a lharp-cutting edge at their bafe, by their with more than one root, come nearer each other, as fore-fide turning inwards there, while* they are flopped they approach the bafe of the tooth : and at laft are down and hollowed behind ; fo that they have the form only feparated by very thin plates, which being general- of v/edges; and therefore their power of adting muft be ly incomplete, allow a communication of all the canals; confiderably increafed. and frequently one common cavity only appears within The incifores of the upper jaw, efpecially the two the bafe, in which a pulpy fubftance, compofed of nerves middle ones, are broader and longer generally than thofe of the under jaw. and veffels, is lodged. The entry of the canals for thefe veffels is a fmall Canini, from the refemblance -to dogs tulks, are one hole placed a little to a fide of the extreme point of on each fide of the incifores in each jaw. The two each rootfometimes, efpecialty in old people, this hole in the upper jaw are called eje-leeih, from the commuis entirely clofed up, and confequently the nerves and nication of nerves which is faid to be betwixt them and blood-veffels are deftroyed. the eyes. The two in the lower jaw are named anThe teeth are feen for a confiderable time in form of gular, or •wike-tt'eth, becaufe they fupport the angles of mucus contained in a membrane; afterwards a thin cor- the mouth. tical plate, and fome few offeous layers appear within The canini are broader, longer, and ftronger, than the the membrane, with a large cavity filled with mucus in incifores. Their bales are formed into a lharp edge, ■the middle; and gradually this exterior ftiell turns thicks as the incifores are ; only that the edge rifes into a point er, the cavity decreafes, the quantity of mucus is leffen- in the middle. Each of them has generally but one ed, and this induration proceeds till all the body is long root, though fometimes they have two. The roots formed ; from which the roots are afterwards produced. are crooked towards the end.——The canini of the upIn young fu^jefts, different ftamina, or rudiments of per jaw are larger, longer, and with more crooked roots, tqeth, are to be obferved. Thole next the gums hinder than thofe of the under jaw. ordinarily the deeper-feated ones from making their way The dentes molares, or grinders, which have got their out, while thefe prevent the former from fending out name becaufe they grind our food, ate generally five in roots, or from entering deep into the bony fockets of each fide of each jaw; in all twenty. Their bafes are the jaws; by which they come to be lefs fixed. broader, more fcabrous, and with a thinner cortical fubChildren are feldom born with teeth ; but at two ftance, than the other teeth. They have alfo more Vol. I. No. 7. ~ 3 T1 roots.