The projecting edge of the so-called alar ligaments offers still more marked characters. Owing to the congestion of its vessels from some unknown cause, it is frequently seen after death of a bright red colour, its surface is ntinutely rough or velvety, and its consist ence soft or almost pulpy. On examining it with the microscope, many minute and villus like processes are seen studding its border, and directed backwards towards the commis sure of the femoral and tibial articular sur faces. These processes appear to consist chiefly or entirely of two structures, viz. bloodvessels and cells. The vessels are nu merous long tortuous capillaries, which pass to the margin of the villus, and then, taking an arched or looped course, return upon themselves, and pass, with few anastomoses, into the general plexus of the fold. The cells, equally with the vessels, resemble those al ready described as existing at the border of the articular cartilage. They are of various sizes, the more numerous and larger ones are spherical, transparent, and contain a tolerably large nucleus : they are distended with fluid, and the slightest pressure on their singularly delicate cell-wall bursts the cell, and causes the fluid to exude. In this condition, the action of the surrounding water seems to impress on it something like a partial coagu lation, giving it a mottled or minutely granular appearance.
The smaller cells exhibit the same shape and general appearances, except that the nucleus is proportionally larger ; a few cyto blasts are also present, and a granular blastema completes the covering of the vessels. One would fancy this to be a favourable situation for verifying the existence of a basement membrane, did such a structure exist here; but I have been unable to detect it. On the contrary, I have often seen the curved border of a large cell seated directly on a capillary, the dark line of the wall of this tube alone separating its cavity from the delicate sphere in contact with it.
Fig. 401. represents such a villus-shaped process.
The relation of the synovial membranes to the diarthrodial cartilages, or the question of " Whether the membrane is dontinued over the articular surface of the cartilages, or not?" has been long a matter of dispute among anatomists. But a resume of the history of this discussion having been already given in an earlier part of the worls, the reader is referred to it for a statement of the arguments on both sides of this interesting question up to that date. (See ARTICULATION.) The rapid progress of histological anatomy, and the use of the microscope, have since thrown much light on the subject, yet perhaps with a less immediate effect than might have been anticipated.
From the impossibility of injecting the vessels of the synovial membrane beyond the margin of the cartilage, it had long been known that they did not extend over this articular surface ; and one might almost imagine that a looped termination of the vessels in this situation must have been sus pected. And the researches of Mr. Toynbee*
concerning the vascular arrangements of the deep or osseous surface of the cartilaginous lamina, showed a similar disposition of the vessels in this situation. Everywhere a thin plate of bone, impermeated by vessels, sepa rates them from actual contact with the car tilage ; and the capillaries themselves, as they approach this osseous lamella, appear some what dilated, and finally, taking an arched course, they return upon themselves into the neighbouring extremity of the bone. The truth of this description as a whole is readily tested and confirmed by examining any part of the substance of a diarthrodial cartilage. Such a fragment, torn up in any manner, and submitted to a sufficiently high magnifying power, evinces no trace whatever of vessels, or of their easily recognisable contents.
But although the absence of vessels is thus proved, the absence of the synovial membrane by no means necessarily follows. The less so, indeed, that modern physiological research exhibits almost all structures as essentially extra-vascular : i.e. it shows that in almost all, the characteristic substance of their tissue is separated from their vessels by an interval ; an interval which, though always minute, is nevertheless an appreciable, and often a measurable one, and which the pabulum de rived from the blood has to traverse in order to effect their nutrition.
The continuity of the synovial membrane, or the reverse, can only be settled in one way ; to wit, by an appeal to observation : and since the naked eye fails to give sufficient information, it remains to the microscope to decide its presence on, or absence froin the articular surface.
Henle* affirms the continuity of the mem brane over the cartilage, as a tesselated epi thelial covering of nucleated cells, resembling those which line the serous membranes and the other parts of the joint.
Professors Todd and _Bowman in their more recent workt, state that they have been unable to detect such a covering in the adult, but that, on the contrary, they have usually observed an irregular surface, presenting no cells beyond the ordinary scattered corpuscles of the cartilage. In the fcetus, however, they have found it readily visible.
A comparative examination of those car tilages in different genera of animals, or in the same animal at different stages of life, partly confirms, partly modifies, each of these state ments.