It may be questioned whether that peculiar structure which intervenes between the base of the skull and the condyle of the lower jaw in the whalebone whale, (bairna mysticetus) be longs to the class of fibro-cartilages, although it seems to bear a nearer resemblance to that than to any of the other structures employed in the composition of joints. The following is Sir Everard Home's description of it." " Be tween the condyles of the lower jaw and the basis of the skull is interposed a thick sub stance, made up of a network of ligamentous fibres, the interstices of which are filled with oil, so that the parts move readily on each other. The condyles have neither a smooth surface nor a cartilaginous covering, but are firmly attached to the intermediate substance, which in this animal is a substitute for the double joint met with in the quadruped, and is cer tainly a substitute of the most simple kind." 4. Ligament.—The term ligament, as it is used by systematic writers on descriptive ana tomy, is by no means confined to portions of the " fibrous system" of Bichat, although all the articular ligaments (properly so called) be long to that system. Weitbrecht comprehends under this term all fibrous structures in and about joints, including the fibrous sheaths of tendons, and also all membranous folds, which are in any way concerned in maintaining soft parts or viscera in proprio situ. I apprehend, however, that a better definition of articular ligament could not be given than the following, ,which is that of Weitbrecht, the words printed in italics being added,—" Ligamentum est par ticula corporis, plerumque albicans, interdum flava, ex fibris flexilibus, interdum elasticis, plerumque parallele concretes, in substantiam tenacem fibrosam, ruptioni fortiter resistentem, et solidam compacta, eum in finem creata ut dune pluresve partes qua alias divulso2 per se subsisterent, adunentur atque in situ respectivo determinentur."-l- Most of the articular liga ments are employed to unite the bones which compose a joint; they also will be found uniting some of the interarticular cartilages within joints, or passing from one part of a bone to another (forming the " mixed" class of liga ments of Beclard); and such is the vagueness with which names are applied in descriptive anatomy, that folds of the synovial membrane often receive this appellation without the least title to it.
Articular ligaments are divisible as regards their forms into two species, the capsular and the funicular or fascicular.X Capsular ligaments are generally cylindrical in shape, or rather barrel-shaped, wider in the centre than at the extremes. Each ex tremity envelopes one of the bones that enters into the formation of the joint, so that the arti cular cavity is completely surrounded by and enclosed within the ligamentous capsule. Liga ments of this kind are composed of fibres which are closely interwoven with each other, and they sometimes receive accessions from bundles of ligamentous fibres corning from neighbouring bony prominences (these fibres being generally called accessory ligaments.) Capsular ligaments are not calculated to re strict the extent or direction of motion be tween the bones which they surround, and we consequently find them only in that kind of joint which admits of motion in all directions, viz. the enarthrosis or ball-and socket joint, of which the only examples in the human subject are to be met with in the hip and shoulder. The internal or articular surface of capsular ligaments is to a great extent lined by one lamina of the synovial membrane, which is reflected upon it from the articular portion of the most moveable of the bones which form the joint.
Funicular ligaments are found in the form of rounded cords or flattened bands : they exist generally on the exterior of joints, very rarely on the interior, and always externally to the sac of the synovial membrane. They pass from bone to bone, adherent sometimes to the syno vial membrane of the articulation, sometimes to the intervening fibro-cartilage. In ginglymoid
joints they are always placed on the sides, and are called lateral ligaments ; sometimes they cross or decussate with each other, whence the appellation crucial, and sometimes a ligament of this class assumes a nearly circular course, and forms a greater or smaller portion of the circumference of a circle, the remainder of the round being completed by the bone into which the extremities of the ligament are fixed ; a ring is in this way produced within which the head, or a special process of another bone, is enclosed, as is seen to be the case particularly with the head of the radius in the superior radio-ulnar articulation, and with the processus dentatus in the joint between the axis and atlas : the ligament in such instances is called coronary. When a ligament is concealed in the interior of a joint, although situated exter nally to the synovial sac, or, to speak more correctly, in the space between the articular surfaces, it is called an internal ligament, e. g. the ligamentum teres of the hip-joint, the mu cous ligament of the knee, or the transverse ligament of the same articulation.
Elastic ligament.—Hitherto we have been examining ligamentous structure, one of whose most prominent characteristics is the want of elasticity ; but we now come to a kind of liga ment which forms a most valuable constituent in the mechanism of some joints, and is emi nently distinguished for the great elasticity which it possesses. It differs from ordinary ligament by its yellow colour, (whence the French ap pellation tissu jaune,) as well as by its elasti city. We find it in the human subject most developed in the ligamenta sub, lava of the vertebrT. In joints, as elsewhere, this tissue is employed to restore to the position of quie scence, parts which have been previously acted upon by muscular contraction. John Hunter fully appreciated the value and utility of this structure in supplying the place of muscle, with less expense of exertion to the economy, and assigned it a place in the arrangement of his museum.* The thyro-hyoid and crico-thyroid ligaments in man are formed of this struc ture.
5. Synovial membrane.—The articular syno vial membranes, (by the older anatomists called, and confounded with, the capsular ligaments,) like all others, possess in common with serous membranes the form of a sac shut in all points; they line the whole interior of the joints, and secrete from their internal surface a peculiar fluid, obviously destined for the lubrication of the articular surfaces. These membranes are remarkable for their great tenuity ; they are transparent; in a state of inflammation, their vascularity, which is imperceptible during health, becomes very apparent by the general redness which the membrane assumes ; and their internal or secreting surface is easily dis tinguished from the external, by contrasting the smooth and glistening appearance of the former with the roughness which the latter receives from the cellular tissue and ligamentous fibres which adhere to it. The internal surface of the membrane is sometimes thrown into folds with fringe-like margins, which project into its cavity or sac. These folds contain more or less of cellular tissue and a number of pellets of fat, which being supplied with ves sels, the margin of the synovial fringe is some times tinged red. These folds are compared, and certainly with much justice, to the epi ploic folds of the abdominal serous mem branes, more especially to the appendices epiploicw of the great intestine. Beclard sup poses that these fringes are specially the seat of the synovial secretion, which being perspiratory likewise takes place, though less abundantly and manifestly, from the rest of the synovial surface. The best examples of these folds occur in the knee and hip-joints, in the former of which they have been absurdly called alar ligaments.