exceedingly vascular, fecreting a fluid of a clear cle; the bone of the arm or os humeri; the mucilaginous nature, which ferves to lubricate bones of the fore-arm, and those of the hand. the joints. They are placed in fmall cavities in The lower extremity on each fide of the trunk the articulations, fo as to be capable of being confifts of the thigh bone and the bones of the gently comprefied by the motion of the joint, leg and foot. which expreffes their juice in proportion to the degree of friction. When the fynovia is wanting, or is of too thick a confiftence, the joint becomes tif, and incapable of flexion or extenfion. This what is termed anchylofis. 113.) The LIGAMENTS of the JOINTS are either round or burfal. The round ligaments are white, tendinous, and inclaftic. They are frong, Bexible, and are found only in the joint of the knee, and in the articulation of the os femoris with the os innominatum. The burial, or capfular ligaments, furround the whole joint like a purfe, and are to be found in the articulations which allow motion every way, as in the articulation of the arm with the fcapula. (114.) A few of thofe facs, called BURSA MUcose, were known to former anatomists, but by much the greater number have been fince difcovered by Dr Monro, who observes, that they are to be met with in the extremities of the body only; that many of them are placed entirely on the inner fides of the tendons, between these and the bones. Many others cover not only the inner, but the outer fides of the tendons, or are interpofed between the tendons and external parts, as well as between those and the bones. A few fuch facs are obferved where the proceffes of bones play upon the ligament, or where one bone plays upon another. Their proper membrane is thin and transparent, but very denfe, and capable of confining air or any other fluid. It is joined to the neighbouring parts by the common cellular fubftance. Between the burfa and the hard fubftance of a bone, a thin layer of cartilage or of tough membrane is very generally interpofed. To the cellular fubftance on the outside of the burfa, the adipofe fubftance is connected; except where the burfa covers a tendon, cartilage, or bone, much expofed to preffure or friction. In feveral places a mafs of fat, covered with the continuation of the membrane of the burfa, projects into its cavity. The edges of this are divided into fringes. The inner fide of the membrane is smooth, and is extremely flippery from the liquor fecreted in it. The ftructure of the burfæ bears a ftrong resemblance to the capfular ligaments of the joints. (15.) It would be improper to conclude this fection without mentioning the SKELETON. This word Ext, which by its etymology implies fimply a dry preparation, is ufually applied to an femblage of all the bones of an animal united together in their natural order. It is faid to be a natural skeleton, when the bones are connected together by their own proper ligaments; and an artificial one, when they are joned by any other fubftance, as wire, &c. The fkeleton is generally divided into the head, trunk, and extremities. The first divifion includes the bones of the cranum and face. The bones of the trunk are the ipme, ribs, fternum, and bores of the pelvis. The upper extremity on each fide confifts of the two butes of the thoulder, viz. the feapula and clavi SECT. II. Of the SUTURES and BONES of the CRANIUM. (116.) The mutual indentation of one bone with another, forms what is called a SUTURE. Those which have proper names we shall describe; thofe which have not, will be found to derive their names from the bones which they furround. 1. The coronal future runs across the fkull, from one upper edge of the fphenoidal bone to the other, and joins the parietal to the frontal bones. 2. The jagittal future joins the parietal bones; begins at the os occipitis, and is continued to the os frontis. In children it defcends to the nofe, the os frontis in them being two bones; and fometimes it is found in adult fubjects. 3. The lambdoidal future joins the back part of the offa bregmatis, or parietal bones, to the upper part of the occipital. In this future are frequently obferved fmall bones called offa triquetra, and fometimes they occur in other futures. 4. The fquamous future is formed by the upper part of the temporal and sphenoidal bones wrapping over the lower edges of the parietal bones. 5. The tranfverfe future runs across the face through the bottoms of the orbits of the eyes. It joins the lower edge of the frontal bones to the os fphenoides, maxillæ fuperioris, ofla nafi, unguis, plana, palati, and malarum. (17.) An evident advantage, arifing from the fkull being divided into fo many bones is, that it is neither fo liable to be fractured, nor to have fractures extended fo far as would happen were it compofed of one bone only. This ftructure is alfo convenient to the process of offification, as has been already fhewn; and it is ftill more important to infants in the birth, because there bones not being perfect at that time, admit of being preffed together, fo as to make the head conform to the shape and dimenfions of the parts through which it is to pass. (118.) Ten of the bones of the head compofe the CRANIUM, the use of which is, to contain the brain, and defend it from external injuries. These bones merit particular defcription. (119.) 1. &. 2. The OSSA PARIETALIA are two large bones which compofe the fuperior and lateral parts of the skull. On their infide they are remarkably imprinted by the arteries of the du ra mater, exhibiting somewhat the appearance of the branches of a tree. (120.) 3. The OS FRONTIS forms the upper and fore part of the cranium. Its inferior parts compofe the fuperior portions of the orbit of the eyes, where, on its infides, are imprefled the valvuli of the brain. In its middle above the os ethmoides ufually arifes a thin fpine, which ftrengthens that part of the bone, it being otherwife weak from its flatnefs. In fome fkulls this fpine is wanting; but then the bone is ufually thicker in that place, and from its middie, externally, goes a procefs which fupports the bones of the note. Immediately above the os eth moides in this bone is a small blind hole, through which runs a vein into the beginning of the longitudinal finus of the dura mater; and on the upper edge of each orbit, a small perforation, or notch, through which nerves and an artery pass to the forehead; it has also a small hole in each orbit, near the os planum, through which passes a branch of the fifth pair of nerves. In the fubftance of this bone near the nose are two, three, four, and fometimes five finuses, which open into the nofe; they differ very much in different perfons, and are very rarely found in children. Thofe finufes, and the spine in this bone, make it very dangerous, if not impracticable, to apply a trephine on the middle and lower part of the fore-head. (121.) 4. The Os ETHMOIDES, or fieve-like bone, is about two inches in circumference, and is fealed in the anterior part of the bafis of the fkull, being almost surrounded by the bone laft defcribed. It is pierced with a number of holes, through which the olfactory nerves país. From its middle arifes a large procefs named crista galli; and oppofite to this a thin one which in part divides the nofe. The greater part of the laminæ, fpongiofæ in the nofe belong to this bone. (122.) 5. The OS SPHENOIDES is of fo very irregular a thape as fcarcely to allow of verbal defcription. It is feated in the middle of the bafis of the skull, and is bounded by the os frontis, ethmoides, vomer, occipitis, maxillæ fuperioris, olla parietalia, palati, malarum, temporum, and petrofa, which are parts of the former bones. In its infide, next the brain, is a cavity named cella turcica, which is bounded by four processes called clinoides; under the two foremost of which pass the internal carotid arteries, and from their outfides are continued two thin long proceffes upon that part of the frontal bone, which feparates the anterior lobes of the brain from the pofterior. Oppofite to the cella turcica is a procefs which makes part of the feptum narium. On the outside of the skull adjoining to the upper jaw, are two proceffes of this bone on each fide, named pterygoides, from which arise one on each fide near the palate, which have no name. Over thefe pafs the tendons of the pterygoftaphalina externi muícles; and nearer towards the occiput, between thefe and the ftyloid proceffes of the offa petrofa, arife two more fmall rugged proceffes. Under the cella turcica, in this bone, is a finus or two, moftly found in adults, but in children, confifting only of fuch a spongy fubftance as is feen in the ends of fome of the bones. According to fome writers, this finus belongs properly to the os ethmoides. At the infide of the bafis of the two anterior clinoid proceffes are two round holes, which are the first foramina of the fkull; through thefe the optic nerves pafs. Almoft under thefe, towards the fides of the fkull, are two irregular flits, named foramina lacera, or the fecond foramina of the fkull, through which pats nerves and blood veffels into the o bits of the eyes. Under thefe again, towards the occiput, are two round holes, which make the third foramina, and through which pafs nerves to the face. About half an inch nearer the occiput are two more, of an oval figure, which are the fourth foramina, and through which pafs the A largest branches of the fifth pair of nerves. ftraw's breadth farther are two very small ones, called the fifth foramina, through which thofe branches of the carotid arteries enter that are bestowed upon the dura mater. Between this laft defcribed bone and the offa petrofa are two large rough holes, in which are fometimes feen large veins; and from thefe holes, through part of the os fphenoides, under the pterygoid proceffes proceed the fmall apertures, through which pass arteries to the back part of the nofe. (123.) 6 & 7. The two OSSA TEMPORUM are fituated below the parietal bones, at the middle and lower parts of the fides of the fcull. They have each, at their back parts, one large fpongy process called mammillaris, or maftoideus; and from the lower and middle parts of each, a procefs iffues, which joins the offa malarum, and which is named zygomalis or zygomaticus. (124.) 8 & 9. The two OSSA PETROSA lie between the former bones and the occipital bones, or rather are portions of the former, as they are never found feparate in adult bodies. Each on its outfide has one long flender process called styliformis, and near the fide of this procefs a foramen, running obliquely forwards into the fcull, and through which the carotid arteries pafs to the brain. Thefe are the fixth foramina, and two foramina in the infide of the fcull leading to the organs of hearing, which are the feventh foramina. The ridges on the upper parts of each of thefe bones help to keep the brain fteady, and are ftrong fupports to the thin and flat parts of the fcull. What remains to be said of this bone belongs properly to the defcription of the organs of hearing, to which we refer. Between the laft defcribed bones and the following bone are two large holes, which are the 8th foramina. Through these pass the 8th pair of nerves and the lateral finufes. Sometimes there are two on each fide, one for the nerve and one for the finus. To thefe foramina we may add another very fmall one on each fide, through which pafs the portiones duraz of the auditory nerves; and fometimes there is another for the passage of an artery. (125.) 10. The OS OCCIPITIS forms all the back part of the fcull. It is bounded by the fphenoidal, temporal, petrofal, and parietal bones. It has two fmall apophytes, by which it is articulated to the fpine. Near thofe apophyses are two fmall foramina, which are the ninth of the fcuil; and through them pafs the ninth pair of nerves. Be tween thefe is the great or tenth foramen, through which the medulla oblongata defcends into the spine, the cervical arteries enter, and the cervical veins país out. In the inde of this bone is a crucial spine impreffed by the longitudinal and lateral finutes; and on the outfide, opposite to the middle of this' fpine, in fome bodies, is an apophyfis, and from that down to the great foramen a mall thin fpine. The fpines in this bone are of the fame ufe with thofe in the os frontis and other bones, viz. to ftrengthen it. The thinner parts of this bone are alfo defended by the mufcles that cover them; a kind of provifion which is very neceflary, fince we can leaft defend this part, and blows here are often attended with the moft fatal confequences. There exifts in molt foults ! fculls a foramen behind each apophyfis of the ocpital bone. Through thefe pafs finufes from the lateral finufes to the external cervical veins: and by means of thefe pallages, as in all other communications of the fame fort, the blood paffes from thofe that happen to be furcharged by rry pofture of the head, into thofe that from the fre pofture would have been almost empty. Such fculls as want thefe foramina have two finuis which answer the fame purpofe. Any perton acquainted with each bone of the cranium, can, without difficulty, examine them as they ftand nited, fo as to know the fhapes, fizes, diftances, &c. of their several parts, and the forms, capacitres, &c. of the cavities formed by them. This is of great ufe towards understanding the anatomy of the parts contiguous to, contained within, or connected with them. SICT. III. Of the BONES of the FACE, and MOUTH. 126.) THE face is that irregular pile of bones Mich compofe the fore and under part of the head. These are divided by authors into the upper and lower maxilla, or jaws. The upper jaw confifts of 13 bones, exclusive of the teeth. Of thefe 6 are placed on each fide of the maxilla fit perior, and one in the middle. The bones, which are in pairs, are the offa nafi, offa malarum, offa unguis, offa maxillaria, offa palati, and olla fpongiofa inferiora. The fingle bone is the vomer. (127.) 1. The OSSA NASI make the upper part of the nofe, and form that kind of arch, which is Ettett to sustain such injuries as the nofe is moft expofed to, as blows, bruifes, &c. (128.) 2. The OSSA MALARUM Ccompofe the anterior, lower, and outer parts of the orbits of the eyes. They have each à short procefs, which procefs joins the proceffus jugales of the temporal bones, and form arches which have been called offa jugalia. (129.) 3. OSSA UNGUIS are fituated immedi ately below the os frontis towards the nofe, and within the orbits of the eyes, whofe anterior and inner parts they help to compofe. Between each of thefe and the upper jaw is a foramen as large as a goofe quill, for the paffage of the lachrymal duct into the nofe. Through this the puncta lachrymalia carry off any fuperfluous moisture from the eyes. (130). 4. The OS MAXILLARE SUPERIOR is of ten defcribed as fingle, though it evidently confifts of two bones joined by a future, which is fcarce ever obliterated.. It has two proceffes, which join the os frontis, and make part of the nofe; and a third, which joins to the cartilage of the feptum nafi. Its upper and outer parts compofe the lowet portions of the orbits of the eyes; its lower fide, all that part of the face under the cheeks, eyes, and nofe to the mouth, and two thirds of the roof of the mouth. A little below the orbits of the eyes, in this bone, are two holes, and behind the dentes incifores one more, which divides into two, as it opens into the nofe, on each fide of the feptum nafi. Between the pofterior grinding teeth and the orbits of the eyes are two great finuses, called, antra maxillæ fuperioris, which open in the upper part of the nofe. In the VOL. II. PART I. lower edge of this jaw are the alveoli, or fockets for the teeth. All the body of the maxillary bone is hollow, and leaves a large finus, which is commonly called antrum highmorianum. When the os maxillare is feparated from all the other bones of the skeleton, its antrum appears to have a large aperture into the noftrils; but, in a recent fubject, it is fo covered at its back part by the palate bone, in the middle by the os fpongiofum inferius, and before by a trong membrane; that one or fometimes two holes, fcarcely fo large as a crow quill, are only left at the upper part; which, after a thort winding procefs, open into the noftrils between the two ofia fpongiofa. At the bot tom of this cavity, we may often obferve tome protuberances, in which the fmall points of the roots of the teeth are contained. This cavern and the fockets of the teeth are often divided by the interpofition of only a very thin bony plate, which is liable to be eroded by matter, or to be broke in drawing a tooth. (131.) 5. The OSSA PALATI are two finall bones that form the back part of the roof of the mouth, and a small part of the bottom of each orbit. Between the ofla palati and os maxillare, near the pterygoid proceffes of the spheroidal bone, are two finall foramina, through which ar teries and nerves pass to the palate. (132.) 6. The Os SPONGIOSUM is ufually treated as a diftinét bone, though it is only the fpongy laminæ in the nofe, of the os ethmoides and ofa plana, but chiefly of the os ethmoides, to which it usually adheres. The OSSA TURBINATA, or SPONGIOSA INFERIORA, resemble the fuperior offa fpongiofa in fhape and fubftance, but have their anterior and upper edges contiguous to the traniverfe ridges of the nafal proceffes of the maxillary and palate bones. From their upper ftraight edge, two fmall proceffes ftand out; the pofterior, which is the broadeft, descends to cover fome of the antrum highmorianum; the anterior rifes up to join the os unguis, and to make part of the lachrymal duct. (133.) 3. The VOMER, fo called from its refemblance to a plow-fhare, is feated between the bones of the palate, and the fphenoidal bone. It is alfo joined to the procefs of the ethmoides, and part of the lower jaw. Its fore part is spongy, and is continued to the middle cartilage of the nofe. This bone and cartilage form the feptum na. (134.) 8. The OSSA PLANA are feated imme diately beyond the foregoing bones, in the orbits of the eyes, and are near thrice as big. They are rather smooth furfaces of the os fpongiofum, than diftinct bones, and are very often imperfect. (135.) 9. The MAXILLA INFERIOR, or Lower Far, is articulated with loose cartilages to the temporal bones, by two proceffes, named condyloides. Near thefe arife two more, called coronales, and at the infide of the chin a small rough proceffus innominatus. In the infide of this bone under each proceffus coronalis, is a large foramen, which runs under the teeth, and paffes out near the chin. Through this foramen, the veffels pafs that belong to the teeth; and in the upper edge of this jaw are the ALVEOLI or fockets for the TEETH, which feldom exceed 6 in each jaw. The 4 firft C in in each are called incifores, the two next canini, the rest molares. The 4 laft of thefe are named dantes fapientia, because they do not appear till men arrive at years of difcretion. The incifores and canini have only one fingle root, but the molares more; the eight first, two; and the reft, fome 3, fome 4, especially in the upper jaw; where also they are spread wider, because that jaw being more spongy than the other, the teeth need more space to fix them. Each of thefe roots has a foramen, through which pass an artery, vein, and nerve, which are expanded in a fine membrane that lines the cavity in each tooth. These veffels and membrane are the feat of the tooch-ach. The teeth of children caft off in confequence of the preflure of the new teeth, which caufes the roots of the firft teeth to be abforbed. When teeth are drawn from the jaws of adult perfons, the fockets are abforbed, and new teeth very rarely, if ever fupply the place of thofe that have been extracted. All the teeth are covered, above the focket, with a very hard, white vitreous fubItance called enamel, which is equally useful and ornamental. It is thickeft on the grinding furface, and becoming gradually thinner terminates infenfibly at the neck of the tooth. It ferves to preferve the teeth, and while the enamel remains found there is no danger of tooth-acb; but when once it is broken or injured, it is impoflible to reflore it. (136.) We cannot close this section without taking notice of the OS HYOIDES, which, thugh it is not one of the bones of the face, cannot be ranked any where fo properly, as after the bones of the mouth. It is placed at the root of the tongue, and was fo called by the ancients, on account of its fuppofed resemblance to the Greek letter v. The body, which is the middle and broadeft part of the bone, is fo placed that it may be eaily felt at the fore part of the throat. Anteriorly it is irregularly convex, and is inner furface is unequally concave. Its cornua, or horns, which are flat and a little bent, being much longer than the body, may be described as forming the fides of the . The appendices, or little horns, are two proceffes which rife up from the articulations of the cornua with the body, and are ufually connected with the ftyloid procefs on each fide by means of a ligament. The ufes of this bone are to fupport the tongue, and afford at tachment to a great number of mufcles; fome of which perform the motions of the tongue, while others act on the larynx and fauces. SECT. IV. Of the BONES of the TRUNK. (137) THE TRUNK confifts of the spine, pelvis, and thorax. 1. The SPINE is compofed of 24 vertebræ, befides thofe of the os facrum and coccygis. Seven of thefe belong to the neck, and the first of them is called atlas, becaufe it immediately fupports the head. Its upper fide has two cavities, into which the apophyies of the os occipitis are received; but the two cavities together, unlike all other joints, are laterally portions of concentric circles, by which means they are but as one joint, and fo fuffer the head to move eally fide ways, which otherwife it could no more do than the knee, which alfo has two heads and two cavities. The under fide of this bone has a very flat articulation with the next; by which means it is fitted for a rotatory motion. The fecond vertebra is called dentata, or axis, from a procefs which paffes through the former bone, and is the axis upon which it turns; neverthelefs all the vertebræ of the neck contribute fomething to the rotatory motion of the head. The proceffus dentatus is ftrongly tied to the os occipitis, and to the atlas by ligaments, to prevent its hurting the spinal marrow. Of the vertebræ 12 belong to the back, and 5 to the loins. The os facrum conlifts fometimes of 5, fometimes of 6 bones, and the os coccygis of 4. If the spine had been compofed of fewer bones, they must have either not been capable of bending fo much and fo variously as they do, or have bent more in each joint, which would have endangered the fpinal marrow, that runs through them. (138.) The fuperior VERTEBRAE of the neck be ing fixed behind the head's centre of gravity, the neck is fo far bent forward, as that the last of thefe vertebræ, which has a firm bearing upon thofe of the thorax, falls exactly under the centre of gravity. Those of the thorax are bent backwards, behind the centre of motion, to make room for the parts contained in the thorax; and that they might not be made too weak by this ftructure, they are formed for less motion than the other vertebræ. Thofe in particular which are bent farthest from the centre of gravity have the leaft motion. The middle vertebræ of the loins are again bent forwards under the centre of gravity, or near it; and from thence they go backward to the os facrum, where being fixed to the offa innominata behind the centre of gravity, the articulation is therefore firm and without motion; and from thence the ofla innominata are fo formed, as that their fockets, into which the thigh bones are admitted and enjoy a free motion, are exactly under the centre of gravity. (139) In all thefe vertebræ, except the first, there is a middle anterior spongy body, by which they are firmly articulated with a very strong intervening ligament; from the middle of the hind part of each, except the firft, ftands a procefs named spinalis; and from every one a proccfs on each fide, called tranfverfalis, and two fuperior, and two inferior thort ones, by which the back parts of the vertebræ are articulated, named obliqui fuperiores, and inferiores. The forepart of the 7 vertebræ of the neck, and two upper of the back, are flat forwards, to make roomfor the trachea. The third and fourth of the back are acute, to give way to the veffels of the lungs and heart, and they are directed to the right side for the better fituation of the heart, which makes that fide of the breaft more convex than the other. The spinal procentes of the 2d, 3d, 4th, and 5th vertebræ of the neck are forked; the two laft long and horizontal, the three or four upper ones of the back like them, only a little declining. The middle ones of the back run obliquely downwards, and the procelles of the remaining vertebræ become fucceflively thicker, stronger, and lefs declining; thofe of the loins being horizontal, like the laft of the neck. The mufcles, that are inferted into the fpinal proceffes of the vertebræ of the the neck and loins act with more strength than trove of the back, because their proceffes beng perpendicular to the fpine, they are longer kies. Besides, thofe of the back almoft touch one another, to prevent much motion, becaufe it would interrupt refpiration; but more motion beng neceflary in the neck and loins, their proces are contrived accordingly. The tranfverfe pocelles of the vertebre of the neck are perforated, for the admittion of the cervical blood veffels, and bowed downwards, and hollowed, for the paffage of the cervical nerves. The eight or Line upper ones of the back receive the upper abs; and the reft, with thofe of the loins, ferve only for the origin and infertion of mufcles. (140) 2. The OS SACRUM has two upper obique proceffes, fome fmall spinal proceffes, and two foramira in each interftice of the bones which compofe it, both before and behind. (141.) 3. The Os CoccYGIS, or rump-bone, is that triangular chain of bones depending from the os facrum; each bone becoming fmaller as they defcend, till the last ends almost in a point. The os coccygis is convex behind, and concave bere; from which crooked pyramidal figure, which was thought to refemble a cuckow's beak, it has got it's name. This bone confifts of four pieces in people of middle age. In children, alEof the whole of it is cartilage. In old subjects, all the bones are united, and become frequently one continued bone with the os facrum. (142.) There is a large foramen through every boue of the fpine, the offa coccygis excepted, Thefe foramina together make a channel through the spine, defigned to contain the medulla fpinalis; and in each space between the vertebre are fwo large holes for the nerves to pafs out. Nothing can be more worthy of admiration, than the provifion made to prevent luxations in this chain of bones, fuch luxations being worfe than any other, because of the fpinal marrow that is contained within. The bodies of the vertebræ are all in the fame manner connected by strong intervening ligaments or cartilages. In the neck the oblique proceffes of the received bone are wrapped over thofe of the receiving bone, which forbids their luxating forwards. The tranfverfe proceffes, with a fmall apophyfis of the body of the fame bone, in like manner, fecure them from Lipping backwards; and an apophyfis on each fide of the body of the receiving bone, hinders them from flipping to either fide. The vertebræ of the back are hindered from diflocating forwards by the fame provifion with thofe of the neck; and from luxating backwards, by the ribs which are faftened to the tranfverfe procefles of the inferior vertebre, and against the back part of the body of the next fuperior. They alfo hinder them from diflocating to either fide; but the Laft ribs are not fixed to the tranfverfe proceffes of the vertebræ of the back, and therefore it is that luxations are moft frequently feen in this part. The vertebrae of the loins, however, are received int deep cavities, and are tied with much frong laments for their fecurity. Each joint of the vertebre, except the two uppermoft, has two contres of motion, one upon the bodies of the vertebrae, when the trunk is bowed forward; and the other at the articulations of the oblique procefles, when the body is inclined backwards. From this structure the extenfor mufcles have about twice the lever to act with, and confequent. ly twice the power to raife the trunk into an erec pofture, that they have to carry it beyond that pofture; for then the oblique proceffes begin to be the centre of motion, and give a like advantage to the flexors. Without this contrivance it would be difficult to keep the body erect, or to recover an erect pofture after the body has been bent forwards. (143.) 4. The RIBS are 12 in number on each fide. The feven uppermoft are called true ribs, becaufe their cartilages reach the sternum; and the five lowest are named falje ribs. They are articula ted to the twelve vertebræ of the back, and all, except the two or three laft, are articulated to their tranfverie proceffes, and the under fide of the middle ribs are hollowed for the paffage of the intercoftal veffels. They defend the parts con. tained in the breaft, and when they are drawn upwards, the cavity of the breast is enlarged for infpiration, and vice verfa. The fubflance of the ribs is fpongy, cellular, and only covered with a very thin external lamellated furface, which increafes in thickness and ftrength as it approaches the vertebra. To the fore end of each rib a long, broad, and ftrong cartilage is fixed, and reaches thence to the fernum, or is joined to the cartilage of the next rib. This courfe, however, is not in a ftraight line with the rib; for the car. tilages generally make a confiderable curve, the concave part of which is upwards; therefore, at their infertion into the sternum, they make an obtufe angle above, and an acute one below. These cartilages are of fuch a length as never to allow the ribs to come to a right angle with the fpine; but they keep them fituated fo obliquely as to make the angle very confiderably obtufe, above, till a force exceeding the elafticity of the cartilage is applied. Thefe cartilages, like all others, are firmer and harder internally than they are on their external furface; and fometimes in old people, all their middle fubftance becomes bony, while a thin cartilaginous lamella appears externally. The offification, however, begins frequently at the external furface. (144) 5. The STERNUM, or breaft-bone is generally made up of three fpongy bones, fometimes more. To this the two ribs are articulated by their cartilages, which fometimes in robust men have moveable joints, fuch as are seen in oxen and other quadrupeds. At the end of the fternum is the cartilago enfiformis, fo called from its fhape, (the form of a fword,) but it is very often double. There is alfo frequently a variety in the form of the cartilages which join the ribs and fternum; fometimes one cartilage ferving two ribs, and fometimes a cartilage not being joined to any rib. It is feldom that we find fewer than 24 vertebræ in the fpine, befides the os facrum, but often more. Sometimes there are 13 of the back, with as many ribs on each fide: fometimes there exift 6 in the loins, and in fome bodies 2 ribs from the firft vertebræ of the loins; but then the transverse proceflès are generally wanting. (145.) 6. The bone called OS INNOMINATUM, OF C 2 the |