It will be observed, that I do not include in the class of interarticular fibro-cartilages, the lamina which is commonly known by the name of the triangular cartilage of the wrist joint, as is done by all the systematic writers I have looked into ; for, first, it does not appear to me to be fibro-cartilaginous in its structure; it is purely cartilaginous ; and, secondly, it is not interar ticular, in the sense in which we here use that term, viz., as lying between two articular surfaces. This lamina seems to be merely an extension of the cartilaginous incrustation of the inferior ex tremity of the radius, which completes the ar ticular surface for the reception of the first row of carpal bones. The scaphoides and lunare are provided for by the radius ; but as the ulna could not be brought into the composition of the wrist-joint without interfering with the motions of the inferior radio-ulnar articulation, a structure such as the triangular cartilage, was necessary—one which would present a sufficient opposing surface to the articular portion of the os cuneiforme, and which would not impede or obstruct the necessary motions of the joint between the radius and ulna.
In the cases of the temporo-maxillary and sterno-clavicular articulations, these fibro-carti lages form, in general, complete septa between two portions of the joint : so that there are then two synovial sacs; but sometimes there is a per foration in the centre of the fibro-cartilage.
b. The second class of articular fibro-carti lages consists of those which Meckel designates fibro-cartilages of circumference, or cylindrical fibro-cartilages. They form fibro-cartilaginous brims to certain articular excavations; they are triangular in their section, attached by their basis to the osseous margin of the articular cavity, and free at their apices, lined by synovial membrane on the whole of one side, and a great part of the other. They are to be found only in two joints, namely, on the margin of the acetabulum in the hip-joint, and on the edge of the glenoid cavity in the articulation of the shoulder ; in the former, this fibro-cartilage is much larger and stronger, and is evidently intended to obviate the ill consequences which must have resulted from the violent application of the neck of the femur against the bony margin of the acetabulum : for, where the margin of that cavity is ligamentous, viz., at the notch on its inner side, this fibro-cartilage does not exist.
c. The most remarkable and beautiful variety of this structure belongs to the third class. It consists of fibro-cartilaginous lamina, generally of considerable thickness, which intervene be tween two bones and adhere intimately to each.
Examples of it are to be found between the bodies of the vertebra, (intervertebral sub stance)—between the pieces of the sacrum in early life—between the sacrum and coccyx, and between the pieces of the latter—also, be tween the ossa pubis at the joint called the symphysis pubis. In this class of fibro-car tilages too, we may place that which is situ ated between the scaphoid and lunar bones in the carpus.
It is evident that these fibro-cartilages are useful, not only as very powerful bonds of union, but also as elastic cushions placed be tween the bones to prevent the concussion which must necessarily result, did the unyield ing bony surfaces come together with any de gree of force. No where is this so beautifully exhibited as in that chain of bones which forms the spinal column in the mammiferous vertebrata, the strength and flexibility of which result from the fibro-cartilaginous discs, which, placed between the bodies of the ver tebra, are commonly called intervertebral car tilages.
As to the structure of articular fibro-cartilage, we can distinctly observe, without any process of dissection, that it is compounded of fibrous tissue as well as of cartilage. As these fibro cartilages generally assume more or less of the circular form, we find that the fibrous tissue is most abundant towards the circumference, and that the cartilage is most manifest at the centre. In the intervertebral substance, the fibrous tissue is arranged in concentric lamina, placed vertically behind one another. Each lamina is composed of a series of interlacing fibres, which have intervals between them ; these intervals, as well as those between the lamina, are filled by cartilaginous tissue; towards the centre the fibrous lamina diminish in number, the inter vals become large, and at length the fibrous tissue disappears in toto; hence the gradually diminishing density towards the centre, which characterises the intervertebral substance. In fishes, there is such a diminution of density, that the central part is fluid, but here the sur faces of the vertebrae are excavated, not plane as in the mammiferous vertebrata, and the cha racter of the articulation is thereby materially altered. The incompressible central fluid forms a ball, round which the cup-like excavations of the vertebra play, while the fibro-cartilage at the circumference is made available in the la teral motions of the spine.
Of the three varieties of fibro-cartilage above enumerated, the menisci possess the most car tilage in their structure, and the circumferential fibro-cartilages the greatest quantity of fibrous tissue.