132
ANATOMY.
extend the ancle and raise the body upon the toes. . These twobones have a considerable motion between themselves, and theastragalus also with the os navioiilare. All the rest have an obscuremotion one with another, and with the bones of the metatarsus;the principal share of these motions falling towards the great toe,where lies the greatest stress of action.
The Os Cai.cis, is of a very irregular figure, and is the largestbor.e of the foot. Behind, it is formed into a considerable tube-rosity, called the heel; without this tuberosity, which supports us;in an erect posture, and when we walk, we should be apt to fallbackwards. On the internal surface of this bone there is a con-siderable sinuosity, which affords a passage to the tendon of amuscle: and to the posterior part of it, a strong tendinous cord,called tendo achillis , is attached, which is formed by the tendonsof several muscles united together. The articulations of all thesebones are secured by ligaments.
Tile Os Naviculare, is so called on account of its resemblanceto a little bark. At its posterior part, which is concave, it receivestire astragalus; anteriorly it is articulated with the cuneiformbones, and laterally it is connected with the os cuboides, whichforms an irregular cube, and is articulated with the os calcis,supports the two last bones of the metatarsus, and joins the thirdcuneiform bone and the os naviculare.
Each of the Ossa Cuneiformia, resembles a wedge,.andfrom this similitude their name is derived. They are placed nextto the metatarsus, are united to the os naviculare, and support thethree first metatarsal bones. When these seven bones of the tarsusare viewed together in the skeleton, they appear convex above,where they help to form the upper part of the foot; and concaveunderneath, where they form the hollow of the foot, in whichthe vessels, tendons, and nerves of it are placed secure from pressure.
The Metatarsus is made up of five bones. The first of thesesupports the great toe, and is much larger than the vest, whichnearly resemble each other in size. These bones are articulatedby one extremity with the cuneiform bones and the os cuboides,and by the other with the toes.
The Toes, like the fingers, consist of three bones each, exceptthe great toe, w hich is formed of tivo. Those of the other fourare distinguished into three phalanges. In walking, the toes bringthe centre of gravity perpendicular to the advanced foot: and as.the soles of the feet are naturally concave, we can at pleasure in-crease this concavity, and form a kind of vault, which adjustsitself to the different inequalities that occur to us in walking; andwhich, without this mode of arrangement, would incommodeus. exceedingly.
There are several small bones besides these above describedthat are met with only in the adult skeleton; which, from theirresemblance to the seeds of the sesamum, are called the OssaSesamoidea. —'[’hey are commonly seen at the first joint of thegreat toe. and sometimes at the joints of the thumb; they arelikewise found at the lower extremity of the fibula, upon the con-dyles of the thigh-bone, under the os cuboides of the tarsus, andin other parts of the body. .. v
Cartilages and Ligaments are so necessarily connected with thebones, (all bones indeed in their first stage being only cartilages,)that they are always described under osteology. If the moveablebones were not connected and kept firm by some strong substance,they would be luxated at every motion of the joints ; and if theirhard rough unequal surfaces were to play on each other, their mo-'lion would not only be difficult, but tl:« loss of substance from af-irition would be great. -Therefore ligaments are made to obviatethe first, and cartilages to prevent the other inconveniency. Butbecause ligaments and cartilages turn rigid, inflexible, and rough,ur.le.ss they are kept moist, a sufficient quantity of a proper liquortermed synovia, is supplied for their lubrication. Those parts of abone, therefore, which is articulated to another bone for motion,afe covered or lined with a cartilage, as far as it moves upon, oris itself moved upon by another bone in any action; for cartilagebeing smoother and softer than bone, renders motion more easythandt would otherwise be, and prevents the bones from injuringeach other by the friction of the joint. In each articulation of thelower jaw, there is a loose cartilage, upon which the condyloidprocess moves on one side, while the jaw is thrust forward: ’Thisstructure renders the motions of the jaw more firm, as that with in-
tervening cartilages makes it more loose and voluble. Thereare also cartilages of this kind between the clavicles and thesternum. In the joint of the knee are too loose, almost annularcartilages, which being thick at their outer edges, and thin at theirinner ones, make the greatest parts of the two sockets in this joint.The use of these cartilages is to make variable sockets to suit thedifferent parts of the lower end of the osfemoris; for none but around head and a round cavity can suit in motion, unless the shapeof one or the other alters ; and it is plainly necessary, that thislower end of the os femoris should be flatfish, and projected back-ward, to give advantage to the muscles that extend the tibia, bysetting the centre of motion backwards: which mechanism, thoughit equally lessens the powers of those muscles which bend thisjoint, is yet of great service, because the extending muscles movethis joint under the weight of the whole body, but the flexors onlyraise file legs ; and as no bead or socket moves so easily as roundones, there seems to be some provision made against the inconve-nience of a flatfish head and cavity, by having the friction madeupon two surfaces; the os femoris mo.ving upon (he loose carti-lages, and the loose cartilages, upon the tibia. This contrivanceis practised by mechanics, where the friction of the joints of anyof their machines is great, as between the parts of hook hinges ofheavy gates, and between the male and female screws of largevices, where they usually place a loose ring. There are othercartilages which serve to give shape to parts. Of this sort are theciliary cartilages at the edge of the eye-lids, the cartilages of theouter ears, and those which compose the nostrils, which have thisparticular advantage in these places, that they support and shapethe parts as well as the bones do, and without being liable to bebroken. Bones that are articulated for motion are tied together. by very strong ligaments, to prevent their dislocating, and theseligaments also surround the joints, and preserve their lubricatingmucus. The thickness and strength of those ligaments are propor-tioned to the actions of the several joints, and their lengths are fiomore than sufficient to allow a proper degree of motion ; but theforms of them are different, according as the actions of the severaljoints are to be performed. To the bones of those limbs thatmove in every direction, are-attached ligaments like purses, whicharise from or near the edges of the sockets of the receiving bones,
, -and are inserted all round the received bones a little below theirheads. The beginnings of these ligaments, from the edges of thesockets of the scapula and os innominatum, are of a nature almostcartilaginous, which serves in the scapula to make a larger socket,and such a one as will alter its figure as the bone moves, for thereason given with regard to the loose cartilage of the knee. In-deed the head of the os humeri not being an exact portion of asphere, requires such a socket, and the hard part of this ligament,in the acetabulum of the os innominatum, makes that cavity deep-er than the semi-diameter of the socket, by which means the arti-culation is made stronger without any hindrance to motion. Itwill give way to the neck of the os femoris when it presses againstit; and the thigh-bone being more disposed to be dislocated up-wards than any other way, the upper side of this bursal ligamentis made exceedingly strong to prevent such an accident. Fromthe lower edge of the acetahuluw or socket of the os innominatumarises a ligament about an inch long, called teres or rotunduni; ithinders the os fi moris from dislocating upwards, though down-wards it will suffgr it to go far out of (lie socket. This ligament isabo thought to assist in pressing the gland in the bottom of theacetabulum, so as to compel it to yield its secretions. The liga-ments in those joints which admit only of llexion and extension,differ from the foregoing in this, that they are much shorter andstronger at the sides of the joints, and thinner backward and for-aavard. Besides these there are in the middle ami back-part of thejoint of the knee, two very strong ligaments, w hich arise from aprocess at the end of the tibia. They cross each other in such amanner, as is best calculated to secure the joint from being dis-placed in any direction. They also hinder the extensors of thetibia from pulling that hone too far forwards, and are so connectedto the semilunar cartilages, as to move (hem us the joint moves.But besides these, there exists in this joint, another small ligament,which arises from the os femoris, anil ends in the fatty membranethat it supports. From this arrangement of parts, it seems evident,that the knee cannot be completely dislocated, without breakingthe cross ligaments. The bones of the carpus and tarsus' are con-nected together by the ligaments running promiscuously upon
their