Some idea may be formed of the manner in which the synovial membrane is related to the other articular structures by examining the an nexed figure, (fig. 111,) representing a vertical section of the knee-joint. The cut margin of the synovial membrane is indicated by a, which after lining the posterior surface of the patella and ligamentum patellae, is reflected upon the condyles of the femur, whence it is carried in front of the crucial ligaments to line the arti cular surface of the head of the tibia, and from that is again reflected upwards, and is con tinuous with the portion lining the posterior car tilaginous surface of the patella. This descrip tion is founded on the opinion, which I believe to be correct, that the analogy between serous and synovial membranes is accurate, in so far as their possessing in common the form of shut sacs is concerned. On this subject, how ever, anatomists are by no means likely ever to be unanimous, because of the difficulty or impossibility of tracing by the ordinary me thods of dissection the synovial membrane over the articular cartilages. The continuity of this membrane over the cartilage was first distinctly announced and described by Dr. W. Hunter, in the paper to which we have already referred in the Philosophical Trans actions : after him Soemmerring described it, and still later Bichat, who insisted more par ticularly on its analogy with serous mem branes. Bichat's description has been followed by Meckel, Beclard, and most of the anato mists of modern times ; but its accuracy has been called in question by Cruveilhier,* Gordon,t Magendie,t Blandin,§ and more re cently by Gendrin II and Velpeaulf The advocates for the continuity of the syno vial membranes over the diarthrodial cartilages, found their opinion on the following facts :— 1. Synovial membranes elsewhere, lining ten dinous sheaths or bursae mucosw, are distinctly and obviously shut sacs. 2. We do not find cartilage to present the smooth and polished aspect exhibited by the articular surfaces, ex cepting where it is connected with synovial membrane, as it evidently is to at least a cer tain extent in the moveable articulations. 3. If by an oblique cut we raise a slice from an arti cular cartilage, and turn it back so as to rupture it at its base, we shall find it still retained in connexion with the rest of the cartilage by a thin pellicle, which seems to have all the cha racters of synovial membrane. A similar mem brane may he seen by sawing a bone vertically down to the cartilage, and then breaking the cartilage by forcibly separating the segments.
4. Some observers state that they have seen the redness of inflammation affecting the synovial membranes prolonged over the cartilage, but becoming gradually less marked towards the centre—(this, I must confess, I have never seen).
5. Bands of adhesion are also said to have been met with in some cases of chronic inflammation of the synovial membrane, passing from the ar ticular surfaces, as well as from other parts of the interior of the joint. 6. In that peculiar disease of the synovial membrane described by Brodie, the pulpy substance has been seen on the articular cartilages, as well as on the menisci.t On the other hand, the opponents of this opinion deny : 1, that the membrane demon strable by slicing the cartilage in the way above described, is any thing else than a very thin lamina of cartilage ; 2, they say that by even the most successful injection the fluid cannot be made to pass beyond the margin of the car tilage; 3, they assert that inflammation always stops abruptly at the circumference of the car tilage ; 4, and that if a synovial membrane did exist on the free surface of the cartilage, there would take place a continual exhalation of sy novia from the articular surface, contrary to what was found to be the case in an experiment tried by Cruveilhier: synovia was freely exhaled from the membrane lining the ligaments, and after having been wiped off reappeared with rapidity ; but not so over the articular cartilage, the sur face of which became quite dry. I
Cruveilhier, however, relates a case which in some degree invalidates his own conclusions ; it was one in which fungous granulations sprang from the articular surfaces of the femur and tibia in the knee-joint, and by their adhesion produced anchylosis of the joint : this fact Cru veilhier § very candidly expresses his inability to explain without admitting either the exist ence of the synovial membrane, or the organi zation* of the cartilages. Velpeau,t too, al though he asserts that the synovial membrane " terminates at the circumference of the carti lages," furnishes us with an argument in oppo sition to his own views : namely, that no appre ciable line of demarcation can be detected indicating where the synovial membrane ceases. " Viewed in this way," he says, " the synovial apparatus consists of surfaces, mem branes, and glandular folds, between which there exists not the least interruption, and the use of which is to isolate the interior of the joint from the tissues which surround it." It will appear then sufficiently evident that the weight of argument preponderates in favour of the doctrine that the synovial membranes line the articular surface of the cartilages, and that maintains their analogy with the serous membmries, an analogy which receives the strongest support from the physical properties of the synovial membrane, from its obvious functions during health, and from the diseases with which it is affected ; and I apprehend, that nothing tends more fully to establish iden tity or similarity in the nature of two mem branes, than the fact of a close resemblance between their morbid conditions. We may add, what was long ago remarked by W. Hunter, that this question as to the continuity of the synovial membrane on the cartilages is very similar to that as to the continuity of the conjunctiva over the cornea of the eye ; the affirmation of which latter question, Gordon considers equally an anatomical refinement as that of the former.
Velpeau I ascribes much importance to the dense and fine cellular tissue which is sub jacent to the synovial membrane and is ana logous to the subserous cellular tissue else where. This would appear to be the seat of the vessels which in a state of inflammation give rise to the red colour of the synovial mem brane. He particularly alludes to it as afford ing a clue by which the formation of loose cartilaginous bodies in joints can be explained; these he supposes to originate in sanguineous effusions into this tissue, which subsequently become indurated and cartilaginous, and push the synovial membrane before them into the cavity of the joint. It will be remembered by many readers that this opinion is very similar to that of John Hunter regarding the origin of these bodies. § Allusion has already been made to the fatty bodies which are found in connexion with most of the joints, and in general lying behind the synovial fringes formerly described. These fatty pellets were supposed by Clopton Havers* to be the agents of the synovial secretion, and, in consequence, have obtained much celebrity under the title of Haversian glands.t The opinion of Havers and his followers as to their glandular nature was successfully combated by Bichat, who proved that they were merely composed of adipose substance, and in no way concerned in the function of synovial secretion : for 1st, the secretion of synovia takes place where no such bodies exist, as in almost all the bursae mucosw, and tendinous sheaths; and 2d, these bodies have no trace of glare dular structure, nor are they provided with any thing resembling an excretory duct; whilst, on the other hand, they possess all the properties of fat.