The synovial sac is lubricated by the sy novia, also called unguen articulare, axungia articularis. How is this secreted ? We believe it to be a perspiratory secretion precisely similar to that of the serum from serous membranes. Its formation cannot be imputed to a com bination of the serosity of the blood with the fat, nor to the transudation of the marrow through the extremities of the bones, nor, with Desault, to a sweating from all the parts which enter into the composition of the articulation, inas much as the chemical analysis of synovia proves that it is essentially different from any oily fluid, and does not contain a trace either of elaine and stearine.
In addition to the structures already named as entering intrinsically into the formation of joints, we find that the tendons and muscles, which lie in the immediate vicinity of or which surround the joints, contribute much to their strength and security. In joints of the hinge kind we generally see the anterior and poste rior parts protected more or less by the tendons of muscles, and even by muscles themselves passing from one segment of a limb to an other, and here it frequently happens that the tendon is bound down on the bones which form the member, by a fibrous expansion of great strength, lined by a synovial membrane of the same characters as the articular, but adapted in its form to the osseo-fibrous canal in which the tendon is placed, e.g. the tendons of the fingers. The protection and strength afforded by mus cles is particularly evinced in the case of the shoulder-joint, where the capsular ligament is closely embraced by four muscles, whose tendons become identified with the fibrous capsule as they go to be inserted into the hone. A muscular capsule, as it were, is thus provided for this joint, by which the bones are main tained much more firmly and powerfully in apposition than were they kept together by an uncontractile ligamentous capsule alone ; hence the elongation of the arm which ap pears as a consequence of paralysis, and hence also the greater liability to luxation which exists in a debilitated state of the system. Articular or capsular muscles thus placed, have also the effect, as it is said, of preventing the pinching of the capsule or synovial mem brane between the articular extremities of the bones in the different motions of the joint.
The joints are very generally copiously sup plied with blood, and are remarkable for the arterial anastomoses which take place around them. The best examples of these are met with
in each of the joints of the extremities. The parts supplied with blood are the synovial membranes, the ligaments, the fat, and the extre mities of the bones; but the cartilages cer tainly do not receive vessels carrying red blood : I believe there is no fact in anatomy, more generally admitted or better determined than this. The vascular ramifications which proceed from these vessels may be seen, par ticularly in young subjects, advancing in the subsynovial cellular tissue, and forming a vas cular net-work there, as far as the margin of the articular cartilage where they stop abruptly ; this is what W. Hunter described under the name of circulus articuli vasculosus.
Of the fbrms and classification qf the arti culations.—It is not difficult, by passing in review the various motions which take place between any two segments of a limb, to form an idea, a priori, as to the kinds and shapes of the articulations by which these segments will be united ; it is only necessary not to lose sight of the fact, that in the construction of a joint regard is had not to its mobility alone, but to its security, its durability, and the safety of the neighbouring parts. We may expect to find joints varying in the degree of motion, from the slightest perceptible quantity, to the freest that is compatible with the maintenance of the segments in their proper relation with each other, and also in extent of motion, from that which is so slight as to admit of almost no appreciable change in the position of the parts, to that which allows of the most ample variety of relation between the segments, consistent with the integrity of the articula tion.
It will appear, then, that the most simple kind of articulation is that by which two parts are so united as that the slightest appreciable degree of motion only shall exist between them. This constitutes the first great division of joints —the Synarthrosis (cvv, cum, and agOpv, arti culus)—where the parts are continuous, i. e. not separated from each other by an intervening synovial cavity. Some anatomists consider all synarthrodial joints to be immoveable; which, although not far from the truth, cannot be said to be strictly accurate. Had immobility been the object to be obtained, I imagine that that might have been more effectually accomplished by the fusion of the extremities of the segments together, as in anchylosis.