MUCOUS MEMBRANES. cxcv 



nov'al sac in such cases is flattened and has its two opposite sides in apposi- 

 tion by their inner surface, 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. As in the case of articular synovial membranes, 

 the bursal membrane on the rubbing surfaces may be, at parts, obliterated. 



In point of situation, the bursee may be either deep-seated or subcutaneous. 

 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 bursae situated in the neighbourhood of joints not unfre- 

 quently open into them. The subcutaneous bursae lie immediately under the 

 skin, and 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 bursse are found also over the olecranon, the malleoli, the knuckles, 

 and various other prominent parts. Tt must, however, be observed that, 

 among these subcutaneous bursae, 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 areolse, but still not bounded by an evident synovial membrane. 

 These have been looked on as examples of less developed structure, 

 forming a transition between the areolar tissue and perfect synovial mem- 

 brane. 



3. Vaginal Synovial membranes or Synovial sheaths. These are intended 

 to facilitate the motion of tendons as they 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 phalanges, where they confine 

 the long tendons of the flexor muscles. In such instances one part of the 

 pynovial membrane forms a lining to the osseo-fi^rous tube in which the 

 tendon runs, and another part is reflected at each end upon the tendon, and 

 affords it a close investment. The space between the parietal and re- 

 flected portions of the membrane is lubricated with synovia and crossed 

 obliquely by one or more folds or duplications of the membrane, in some 

 parts inclosing elastic tissue. These are named "frsena," and pass from one 

 part of the membrane to the other. 



Synovia. As already stated, this is a viscid transparent fluid; it has a yellowish 

 or faintly reddish tint, and a slightly saline taste. According to Frerichs, the synovia 

 of the ox consists of 94 - 85 water, 0'56 mucous and epithelium, 0'07 fat, 3'51 albumen 

 and extractive matter, and 0'99 salts. If a drop of synovial fluid be examined micro- 

 scopically, it is found to contain (in addition to fat-molecules and epithelium-cells) 

 small, granular corpuscles, bearing a close re-emblance to the pale corpuscles of the 

 blood. It is doubtful whether these bodies have a special nature and purpose, or 

 whether they are merely transitory forms of epithelium-particles. 



MUCOUS MEMBRANES. 



These membranes, unlike the serous, line internal passages, and other 

 cavities which open on the surface of the body, as well as various recesses, 

 sinuses, gland-ducts and receptacles of secretion, which open into such pas- 

 sages. They are habitually subject to the contact of foreign substances 

 introduced into the body, such as air and aliment, or of various secreted or 

 excreted matters, and hence their surface is coated over and protected by 

 mucus, a fluid of a more consistent and tenacious character than that which 

 moistens the serous membranes. 



