392 LYMPHATIC SYSTEM. 



SYNOVIAL MEMBRANES. 



These are connective tissue membranes which are found surrounding closed 

 cavities in connection with moveable structures in certain parts, such as the joints, 

 the elongated sheaths in which some tendons glide, and at various situations between 

 the skin and bony prominences below it. Although they resemble serous membranes 

 in some respects, the synovial membranes are distinguished by the nature of their 

 secretion, which is a viscid glairy fluid resembling the white of an egg and named 

 synovia. From its nature it is well adapted for diminishing friction, and thereby 

 facilitating motion. 



If a drop of synovial fluid is examined microscopically, it is found to contain 

 (in addition to fat-molecules) a few amoaboid corpuscles, as well as cells similar to 

 those which occur on the projections of the membrane. 



The different synovial membranes of the body are referred to three classes, viz., 

 articular, vaginal, and vesicular. 



1. Articular synovial membranes, or Synovial capsules of joints. These by 

 their secretion lubricate the cavities of the diarthrodial articulations, that is, those 

 articulations in which the opposed surfaces glide on each other. In these cases the 

 membrane may be readily seen covering internally the surface of the capsular and 

 other ligaments which bound the cavity of the joint, and affording also an invest- 

 ment to any tendons or ligaments which pass through the articular cavity, as in the 

 instance of the long tendon of the biceps muscle in the shoulder- joint. On 

 approaching the articular cartilages the membrane does not pass over them, but 

 terminates after advancing but a little way on their surface, with which it is here 

 firmly adherent. The synovial membranes, therefore, do not form closed bags lying 

 between the articular cartilages as was supposed by the older anatomists, for the 

 main part of the surfaces of the joints is not covered at all by the membrane, 

 nor even by a layer of epithelium-cells, prolonged from the membrane, as some have 

 described. 



In several of the joints, folds of the synovial membrane pass across the cavity ; 

 these have been called synovial ligaments. Other processes of the membrane simply 

 project into the cavity at various points. These are very generally cleft into fringes 

 at their free border, upon which their blood-vessels, which are numerous, are densely 

 distributed. The larger folds and processes often contain fat, and then are sufficiently 

 obvious ; but many of the folds are small and inconspicuous. 



The frinsred vascular folds of the synovial membrane were described by Havers 

 in 1691, under the name of the mucilaginous glands, and he regarded them as an 

 apparatus for secreting synovia. Rainey found that these Haver sian fringes, as they 

 are sometimes called, may exist in all kinds of synovial membranes, and that from 

 the primary vascular fringes other smaller secondary processes are sent off, into which 

 no blood-vessels enter. 



2. Vaginal synovial membranes, or Synovial slieaths. 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 synovial membrane forms a lining 

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

 investment to the tendon. The space between these portions of the membrane is 

 lubricated with synovia and crossed obliquely by one or more folds or duplications 

 of the membrane named " frsena," in some parts inclosing a considerable amount of 

 elastic tissue (J. Marshall). 



