of which the conjunction of the os sacrum and innominatum is an example.—They afford a socket for moveable bones to play in, as we see part of the astragalus does on the ligament stretched from the heel-bone to the scaphoid.
Numerous inconveniencies may arise from too long or short, strong or weak, lax or rigid ligaments.
Cartilages are solid, smooth, white elastic substances, between the hardness of bones and ligaments, and covered with a membrane, named perichondrium, which is of the same structure and use to them as the periosteum is to the bones.
Cartilages are composed of plates, which are formed of fibres, disposed much in the same way as those of bones are; as might be reasonably concluded from observing bones in a cartilaginous state before they ossify, and from seeing, on the other hand, so many cartilages become bony. This may be still further confirmed, by the exfoliation which cartilages are subject to, as well as bones.
The perichondrium of several cartilages, for example, those of the ribs and larynx, has arteries which can be equally well injected with those of the periosteum.
The granulated flesh which rises from the ends of metacarpal or metatarsal bones, when the cartilage exfoliates, after a finger or toe has been taken off at the first joint, is very sensible, from which the existence of nerves in cartilages may be inferred.
While cartilages are in a natural state, it is to be remarked, first, That they have no cavity in their middle for marrow. Secondly, That their outer surface is softest, which renders them more flexible. Thirdly, That they do not appear to change their texture near so much by acids as bones do. And, lastly, That as the specific gravity of cartilages is near a third less than that of bones, so the cohesion of their several plates is not so strong as in bones; whence cartilages laid bare in wounds or ulcers, are not only more liable to corrupt, but exfoliate much sooner than bones do.
Cartilages seem to be principally kept from ossifying, either by being subjected to alternate motions of flexion and extension, the effects of which are very different from any kind of simple pressure, or by being constantly moistened: Thus, the cartilages on the articulated ends of the great bones of the limbs, and the moveable ones placed between the moving bones in some articulations, which are obliged to suffer many and different flexions, and are plentifully moistened, scarce ever change into bone; while those of the ribs and larynx are often ossified.
The cartilages subservient to bones, are sometimes found on the ends of bones which are joined to no other; but are never wanting on the ends, and in the cavities of such bones as are designed for motion.
The uses of cartilages, so far as they regard bones, are, to allow, by their smoothness, such bones as are designed for motion, to slide easily without detrition, while, by their flexibility, they accommodate themselves to the several figures necessary in different motions, and, by their elasticity, they recover their natural position and shape as soon as the pressure is removed.This springy force may also assist the motion of the joint to be more expeditious, and may render shocks in running, jumping, &c. less.To these cartilages we chiefly owe the security of the moveable articulations: For without them the bony fibres would sprout out, and intimately coalesce with the adjoining bone; whence a true anchylosis must necessarily follow; which never fails to happen when the cartilages are eroded by acrid matter, or ossified from want of motion or defect of liquor, as we see often happens after wounds of the joints, paidarthrocace, scrophula, and spina ventosa, or from old age, and long immobility of joints. The moveable cartilages interposed in joints, serve to make the motions both freer and more safe than they would otherwise be.Those placed on the ends of bones that are not articulated, as on the spine of the os illium, base of the scapula, &c. serve to prevent the bony fibres from growing out too far.Cartilages sometimes serve as ligaments, either to fasten together bones that are immoveably joined, such are the cartilages between the ossa sacrum and ossa illium, the ossa pubis, &c. or to connect bones that enjoy manifest motion, as those do which are placed between the bodies of the true vertebræ, &c.—Cartilages very often do the office of bones to greater advantage, than these last could, as in the cartilages of the ribs, those which supply brims to cavities, &c.
Too great thickness or thinness, length or shortness, hardness or suppleness of cartilages, may therefore cause great disorders in the body.
The liquor, which principally serves to moisten the ligaments and cartilages of the articulations, is supplied by glands, which are commonly situated in the joint, after such a manner as to be gently pressed, but not destroyed by its motion. By this means, when there is the greatest necessity for this liquor, that is, when the most frequent motions are performed, the greatest quantity of it must be separated. These glands are soft and pappy, but not friable: In some of the large joints they are of the conglomerate kind, or a great number of small glandules are wrapt up in one common membrane. Their excretory ducts are long, and hang loose, like so many fringes, within the articulation; which, by its motion and pressure, prevents obstructions in the body of the gland or its excretories, and promotes the return of this liquor, when fit to be taken up by the absorbent vessels, which must be in the joints, as well as in the other cavities of the body; and, at the same time, the pressure on the excretory ducts hinders a superfluous unnecessary secretion, while the fimbriated disposition of these excretories does not allow any of the secreted liquor to be pushed back again by these canals towards the glands.
Very often these fountains of slimy liquor appear only as a net-work of vessels.Frequently they are almost concealed by cellular membranes containing the fat;—and sometimes small simple mucous folliculi may be seen.
The different joints have these organs in different numbers and sizes; the conglomerate ones don't vary much, especially as to situation, in the similar joints of different bodies; but the others are more uncertain.
Upon pressing any of these glands with the finger, one
