SYNOVIAL SHEATHS. 201 



various points. These are yery generally cleft into fringes at tlieir free border, 

 upon which their blood-vessels, which are numerous, are densely distributed. 

 They often contain fat. and then, when of tolerable size, are sufficiently obvious ; 

 but many of them are very small and inconsi3icu.ous. The frmged vascular folds 

 of the synovial membrane were described, by Dr. Clopton Havers (1691), under 

 the name of the inucilufiinous f/landx. and he regarded them as an apjaaratus for 

 secreting synovia. Subsequent anatomists, while admitting that, as so many 

 extensions of the secreting membrane, these folds must contribute to increase the 

 secretion, have, for the most part, denied them the special character of glands, 

 considering them rather ui the light of a mechanical provision for occupying 

 spaces which would otherwise be left ■^'oid in the motion of the joints, and this 

 view is no doubt right as regards the larger, fat-inclosing folds. The smaller and 

 less obvious fringes have, however, been found, on investigation by Eainey, to be 

 most prolmbly secreting organs as originally supposed by Havers. Rainey found 

 that the processes in question exist in the bursal and vaginal synovial membranes 

 as well as in those of joints, wherever, in short, synovia is secreted. He states 

 that their blood-vessels have a peculiar convoluted arrangement, differing from 

 that of the vessels of fat. and that the layer of cells covering them. " besides iu- 

 closing separately each packet of convoluted vessels, sends off from each tubular 

 sheath secondary processes of various shai)es, into which no blood-vessels enter."* 



2. Vesiciilar or Bursal synovial membranes, Synovial lursce, 

 Bursm mucosa.. — In these the membrane has the form of a simple sac, 

 interposed, so as to prevent friction, between two surfaces which move 

 npon each other. The synovial sac in such cases is flattened and has 

 its two opposite sides in ap})osition by their inner surfeee, which is 

 free and lubricated with synovia, whilst the outer surface is attached by 

 areolar tissue to the moving parts between which the sac is placed. 



In point of situation, the bursa? may be either deep-seated or subcu- 

 taneous. The former are for the most part placed between a muscle or 

 its tendon and a bone or the exterior of a joint, less commonly between 

 two muscles or tendons : certain of the bursa3 situated in the neigh- 

 bourhood of joints not nnfrequently open into them. The subcutaneous 

 bursffi lie immediately under the skin, aud are found in various regions 

 of the body interposed between the skin and some firm prominence 

 beneath it. The large bursa situated over the patella is a well-known 

 example of this class, but similar though smaller bursa? are found also 

 over the olecranon, the malleoli, the knuckles, and various other pro-, 

 minent parts. It must, however, be observed that, among these subcu- 

 taneous bursfe, some are reckoned which do not always present the 

 characters of true synovial sacs, but look more like mere recesses in the 

 subcutaneous areolar tissue, larger and more defined than the neigh- 

 bouring areolffi, but still not bounded by an evident synovial membrane. 

 These may be looked on as examples of less developed structure, 

 forming a transition between the areolar tissue and perfect synovial 

 membrane. 



3. Vaginal synovial membranes or Sijnovial sJ/eaflis. — These are 

 intended to facilitate the motion of tendons as thej^ glide in the fibrous 

 sheaths which bind them down against the bones in various situations. 

 The best-marked examples of such fibrous sheaths are to be seen in the 

 hand and foot, and especially on the palmar aspect of the digital pha- 

 langes, where they confine the long tendons of the flexor muscles. In 

 such instances one part of the synovial membrane forms a lining to the 

 osseo-fibrous tube in which the tendon runs, and another part affords a 



* Troceedings of the Koyal Society, J.Iay 7th, 1846. 



