b. The deep vaginal bursa are invariably found connected with tendons and with the fibrous sheaths through which these tendons are transmitted. They are somewhat more complex than the preceding, for instead of representing a simple shut sac, they form, like serous membranes, by reflexion a double sac, one of whose portions, corresponding, for ex ample, to the plura costalis, lines the interior of the fibrous sheath, while the other, answering to the plura pulmonalis, invests the surface of the tendon. There is, however, this difference between the pleune and the synovial sac, that in the latter there is no longitudinal septum, no mediastinum resulting from the reflexion of the membrane ; for the reflexion occurs not along the channel, but at either extremity of the fibrous sheath : thus the bursa, if completely detached from all surrounding structures, would represent a large tube, containing within itself a smaller one ; these two being continuous by their extremities alone.
The deep vaginal bursx generally occur in the neighbourhood of ginglymoid articulations, and by far the largest and most interesting are those connected with the flexor tendons of the vvrist and ankle. They are always of very great size, not only passing a considerable way up wards upon the forearm and leg, but likewise extending downwards into the palm of the hand and sole of the foot, and branching out at their distant extremity into several distinct sheaths for the respective tendons belonging to the different toes and fingers. Upon the phalanges the synovial sheath is firmly bound down by a dense unyielding fibrous membrane, a cir cumstance well worthy of remark; for, as we shall presently see, it modifies in a very im portant degree the characters of inflammation occurring here. Besides these, we have a re markable vaginal bursa connected with the long head of the biceps muscle; and smaller ones are found investing the tendons of the circum flexus palati, obturator internus, &c. • Having thus considered the forms and rela tions of the different sorts of bursw, we may next proceed to offer a few remarks applicable alike to all, upon their structure, contents, uses, development, and diseases. Here, however, our labour is much abridged by the fact already alluded to, and now admitted upon all hands, that the membrane forming the bursw, and the synovial membrane of joints, are anatomically and physiologically the same. They are, in fact, the same in form, being both shut sacs ; the same in structure, being both essentially composed of cellular membrane; the same in function, for they are both designed to facilitate the motion of contiguous organs; and, as we shall presently see, they are both similarly af fected by disease. Were we to enter at length into these particulars upon the present occasion, we should but anticipate details belonging pro perly to a more general head, that, namely, of synovial membrane. Hence the few remarks we
are now about to offer must be received as merely supplementary to those found under that article.
1. Structure.—The opinion of Haller, that these membranes are ultimately composed of cellular substance, though controverted by Monro and others, is, however, now universally admitted. They are, in fact, like all synovial membranes, essentially composed of cellular substance, entirely destitute of fibre, scantily supplied with vessels, and remarkable for their softness and flexibility. The vaginal bursm are, however, much more delicate than the vesicular. The fatty bundles, mistaken by Havers for glands, are frequently found in their substance. Rosenmiiller speaks of distinct synovial follicles as likewise demonstrable, but the existence of any such bodies appears to us more than doubtful.
2. Contents.—Experiments have been made by Monro and others, to shew that the fluid contained in bursw is similar to that contained in synovial membranes. These, however, may now be looked upon as superfluous, inasmuch as this question has merged in the general one, viz , the identity of the two structures. Chemistry, in fact, has proved that their fluid and that of synovial membranes are, if not completely, at least essentially the same. In the subcutaneous bursze it is scanty and thin; in the larger and deeper ones it is said to be somewhat more viscid.
3. Function.—The use of bursw is in all cases the same; they Ferve to isolate certain parts and facilitate the motions performed by them : hence they are found only in those situations which are the seat of motion. Their fluid, from its oily consistence, must of course tend considerably to diminish the effects of fric tion.
4. Development.—Bursw are developed at a very early period, and are relatively more pliant and perfect in the child than in the adult, to Jacilitate, as it would appear, the almost incessant movements natural to that period of life. They become more dense and unyielding in the adult, and in extreme old age are said to become dry and rigid. This, no doubt, is amongst the causes which render the movements of old age slow and laboured. A curious fact connected with this subject is the accidental development of bursw in cases where their presence becomes necessary. When the superficial bursa in front of the patella has been removed by operation, its place is ulti mately supplied, as Sir Benjamin Brodie has seen, by a newly formed one, similar in every respect to the original sac. In cases of club foot a large subcutaneous bursa has been found developed upon that portion of the swelling which has been the chief seat of pressure and motion : and in cases of diseased spine, at tended with considerable angular curvature, a bursa has become developed between the pro jecting spinous process and the Skill.