Page:Encyclopædia Britannica, first edition - Volume I, A-B.pdf/204

 Part I. 1^4 A N A T O M Y. The vomer divides the noftrils, enlarges the organ of ment, extended from the ftyloid procefs of the temporal fmelling, by allowing ylace for expanding the membrane bone, is fixed. of the nc'fe on its fides, and fuftains the palate-plates of The procefles are two on each fide. The anterior fliarp thin coronoid ones have the crotaphyte mufcles inthe maxillary and palate-bpnes. ■ Maxilla Inferior, the lower jaw, confifts only of ferted into them. The pofterior proceftes, or condyles, terminate in an oblong fmooth head, fupported by one moveable bone, and fixteen teeth incafed into it. This bone, which is- fomewhat of the figure of the a cervix. The heads, whofe greateft length is tranfverfe, Greek letter v, is fituated at the lower part of the face, and whofe convexity is turned forwards, are tipped with fo as its convex middle part is forwards, and its legs are a cartilage, as the articulated parts of all other moved ftretched back. It is commonly divided into the chin, bones are.—The fore-part of the root and neck of thefe fides, and proceffes.—The chin is the middle fore-part, condyloid procefles are a little hollow and rough, where the ejctent of which to each fide is marked on the exter- the external pterygoid mufcles are inferted. nal furface by the holes obfervable there, and internally, The holes of the lower jaw are two on each fide; by the beginning ofi an oblique ridge.—Beyond thefe, one at the root of the proceftes internally, where a large the fides appear, and are continued till the bone, by branch of the third branch of the fifth pair of nerves enters with an .artery, and a vein returns: A fmall /harp bending upwards, begins to form the procefles. On the fore-part of the chin, a tranfverfe ridge appears procefs frequently jutts out backwards-from the edge at in the middle, on each fide of which the mufculi quadra-, the fore-part of this hole, to which a ligament, extended ti, or deprelfores labii inferioris, and the levatores labii ftom the temporal bone, is fixed, which faves the nerve inferioris, deprefs the bone: And below thefe prints, a and veflels from being too much prefied by the pteryfmall rifing may be obferved, where the depreflqres com- goid mufcles. From tlie lower fide of this hole, eimence.—On the back-part of the chin, fometimes three, ther a fmall fuperficial canal or a furrow defcends, where always two, fmall protuberances appear in the middle. a branch of the nerve is lodged, in its way to the myloTo the uppermoft, when it is feen, the frxnum of the hyoideus mufcle and fublingual ^tand. The other tongue is conne&ed. From the middle one, the mufculi hole is external, at the confines of the chin, where gemoglodi rife ; and from the lowed, the geniohyoidei branches of the nerve and veflels come out. The canal betwixt thefe two holes is formed in the middle of have their origin. At the lower and fore-part of the external furface of the fubftance of the bone, and is pierced by a great numof fmall holes by which the nerves and blood-veflels each fide of the lower jaw, a fmall eminence may be ber the cancelli and teeth pafs. obferved, where the depreffor labiorurn communis rifes. of The jaw generally receives the roots of fixteen Near the upper edge of the fide a ridge runs length-ways, teeth intolower fockets, by gomphofis ; and its condyloid to which the under part of the mufculus buccinator is procefles, itscovered with cartilage', are articulated with conn'efted. Internally, towards the upper edge of temporal bones. each fide, another ridge appears, from which the mylo- theThe Teeth are the hard white bodies placed in the hyoidei have their origin, and to which the internal mem- fockets of both jaws. Their number is generally fixbrane of the gums adheres. and as many below ; though fome people In the upper edge of both chin and fides are a great teen above, others have fewer. many deep pits or fockets, for receiving the roots, of the haveThemore, thick part of each tooth which appears teeth. The number and magnitude of thefe fockets are without broad the focket, is the bafe, or body. The fmallvarious, becaufe of the different number, as well of the er procefles into the maxillae, are the roots or fangs, teeth themfelves, as of their roots, in different people. which becomefunkgradually-fmaller towards the end fartheft Thefe fockets in this lower jaw, as well as in the upper from the bafe, or are nearly conical, which the furone, are lefs deep as old age comes on; when freed from face of their fides divides the preflure by on the bafes, the teeth by any means, they are fome' time after filled to prevent the foft parts, which are made at the final! points up with an offeous net-work, which at laft becomes en- of the foekets, to be hurt by fuch preflure. tirely folid, and as fmooth as any other part of the bone; Without the gums the teeth are covered with no memfo that in a great many old jaws .one cannot obferve a brane, and they are find to have no proper periofteum veftige of the fockets: But then the jaw becomes lefs, within the fockets; but that is fupplied by the reflefted and much narrower.—Hence we may know why thp chin membrane of the gums ; which, after a good injection, and nofe of edentulous people are much nearer than bebe?evideiitly feen in a young fubjeA, with the veflels fore the teeth were-loft ; while their lips either fall in may from it penetrating into the fubftance of the teeth ; and towards the mouth, or ftand prominent forwards. may be difcovered in any tooth recently pulled, by maWhen new teeth are protruded, new fockets are formed. itcerating it in water. The adhefiqn of this membrane to The lower edge of the chin'and fides is fmooth. and roots is ftrengthcned by the fmall furrov/s obferequal, and is commonly called the bafe of the lower jaw. ■ thefe on them. ./—iThe ends of the bafe, where the jaw turns upwards, vable tooth is compofed of its cortex, or enamel, and are called its angles ; the external furface of each of an Each bony fubftance. The cortex has no cavity or which has feveral inequalities upon it, where the mafle- placeinternal for marrow; and is fo folid and hard, that faws or ter mufcle is inferred; as the internal furface alfo has, files can with difficulty make impceffion on it. thicTTeft It is where the pterygoideus internus is inferted, and a liga-