The first is enarthrosis, or the ball and socket, when a large head is received into a deep cavity; as the head of the os femoris is into the acetabulum coxendicis.
The second is arthrodia, when a round head is received into a superficial cavity; as in the articulation of the arm-bone and scapula. These two species of diarthrosis allow motion to all sides.
The third is ginglimus, which properly signifies the hinge of a door or window; in it the parts of the bones mutually receive and are received, and allow of motion two ways: Workmen call it charnal.
The ginglimus is generally divided into three kinds, to which some give the names of contiguous, distant, and compound.
The first kind of ginglimus is, when a bone has several protuberances and cavities, which answer to as many cavities and processes of the other bone, with which it is articulated; as in the conjunction of the femur with the tibia.
The second species is, when a bone receives another at one end, and is received by the same bone at the other end; as in the radius and ulna.
The last sort is, when a bone receives another, and is received by a third; as in the oblique processes of the vertebræ.
If the moveable bones are not connected and kept firm by some strong substance, they would be luxated at every motion of the joints: and if their hard, rough, unequal surfaces were to play on each other, their motion would not only be difficult, but the loss of substance from attrition would be great. Therefore ligaments are made to obviate the first, and cartilages to prevent the other inconveniency. But because ligaments and cartilages turn rigid, inflexible, and rough, unless they are kept moist, a sufficient quantity of proper liquor is supplied for their lubrication, and to preserve them in a flexible state. Seeing then these parts are so necessary to the articulations, we shall next consider their structure, situation, and uses, so far as they are subservient to the bones, and their motions.
Ligaments are white flexible bodies, thicker and firmer than membranes, and not so hard or firm as cartilages, without any remarkable cavity in their substance, difficultly stretched, and with little elasticity; serving to connect one part to another, or to prevent the parts to which they are fixed from being removed out of that situation which is useful and safe.
After maceration in water, the ligaments can easily be divided; and each ligamentous layer appears composed of fibres, the largest of which are disposed in a longitudinal direction.
The arteries of ligaments are very conspicuous after a tolerable injection, and the larger trunks of their veins are sometimes to be seen full of blood.
Such ligaments as form the sides of cavities, have numerous orifices of their arteries opening upon their internal surface, which keep it always moist: If we rub off that moisture, and then press the ligament, we can see the liquor ouzing out from small pores; and we can force thin liquors, injected by the arteries, into the cavities formed by ligaments.
These exhalent arteries must have corresponding absorbent veins, otherwise the cavities would soon be too full of liquor.
Ligaments then must be subject to the diseases common to other parts, where there is a circulation of fluids, allowance always being made for the size of vessels, nature of the fluids, and firmness of the texture of each part.
Some authors have alledged, that ligaments are insensible, and consequently that they have no nerves. But the violent racking pain felt on the least motion of a joint labouring under a rheumatism, the seat of which disease seems often to be in the ligaments, and the insufferable torture occasioned by incisions of ligaments, and by a collection of acrid matter in a joint, or by tophi in the gout, would persuade us, that they are abundantly supplied with nerves.
The ligaments which connect the moveable bones commonly rise from the conjunction of the epiphyses of the one bone, and are inserted into the same place of the other; or where epiphyses are not, they come out from the cervix, and beyond the supercilia of the articulated bones; and after such a manner, in both cases, as to include the articulation in a purse or bag; with this difference, depending on their different motions, that where the motion is only to be in two directions, the ligaments are strongest on those sides towards which the bones are not moved; and when a great variety of motions is designed to be allowed, the ligaments are weaker than in the former sort of articulations, and are nearly of the same strength all round.
Part of the capsular ligaments is composed of the periosteum, continued from one bone to another, and their internal layer is continued on the parts of the bone or cartilage which the ligament includes.
Besides these common capsular ligaments of the joints, there are particular ones in several places, either for the firmer connection of the articulated bones, or for restraining and confining the motion to some one side; such are the cross and lateral ligaments of the knee, the round one of the thigh, &c.
From this account of the ligaments, we may conclude, that, cæteris paribus, in whatever articulation the ligaments are few, long, and weak, the motion is more free and quick; but luxations happen frequently: And, on the contrary, where the ligaments are numerous, short, and strong, the motion is more confined; but such a joint is less exposed to luxations.—Whence we may judge how necessary it is to attend to the different ligaments, and the changes which have been made on them by a luxation, when it is to be reduced.
Ligaments also supply the place of bones in several cases to advantage: Thus the parts in the pelvis are more safely supported below by ligaments, than they could have been by bone.—The ligaments placed in the great holes of the ossa innominata, and between the bones of the fore-arm and leg, afford convenient origin to muscles.—Immoveable bones are firmly connected by them;
