SYNOVIAL MEMBRANES. 353 
but thinly covered by soft parts; or where a very firm connexion is 
required, as in the hip joint. They are absent where muscles or liga- 
ments rest upon the articulation; or where, for special purposes, ths 
synovial membrane is exposed to more considerable movements, as in 
the knee. 
The synovial capsule is attached, either simply to the cartilaginous 
surface, extending thence directly to the other bone, or it may, in the first 
place, besides the cartilage, also invest a larger or smaller extent of the 
surface of the bone itself, and then pass to the second bone, with which it 
is connected in the one way or the other. 
SynoviA is a viscid transparent fluid, of a pale straw colour, slightly 
alkaline. In chemical and general characters, it is like the serum of the 
blood. A drop of synovia is found to contain—fat molecules, epithelial 
cells, and small granular corpuscles, bearing a close resemblance to the 
white corpuscles of the blood. This fluid, on account of the presence of 
albumen, is coagulable by heat. 
CLASSIFICATION OF THE JOINTS. 
Tue skeleton has already been described as composed of different pieces 
of bone, united to each other in various manners: from this union result 
the articulations, which are sometimes very movable ; sometimes joined 
to each other through the medium of long digitations or teeth, which 
fix them, if not to immobility, at least to a very constrained movement ; 
and, lastly, united together through the medium of cartilage, the elasticity 
of which permits latitude of movement. In the first case, the articulations 
take the name of diarthrosis or movable articulations (6:4, through, and 
apOpov, a limb); in the second, synarthrosis (ctv, together, and apOpor, a 
limb); and thirdly, amphiarthrosis (du¢t, about, and <pOpov), partaking, 
at the same time, of the two classes of articulation above mentioned— 
namely, synarthrosis, in the continuity established by the articular sur- 
faces ; and diarthrosis, in the limited extent of movement it permits. 
The guide to the classification of joints is the configuration of their 
articular surfaces, and the movements they allow. 
D1aRTHRODIAL JOINTS are arranged under three distinct classes :— 
1. Enarthrosis. This kind of joint is characterized by the reception of 
an articular head into a cavity of appropriate form. It is the seat of most 
extensive movements ; namely, flexion, extension, adduction, abduction, 
circumduction, and rotation. Example: Acetabulum with femur. 
2. Ginglymus. A perfect hinge-joint, the articular surfaces of which 
are configured in a trochlear arrangement, in such a manner that two or 
more prominences may fit into two or more excavations of appropriate 
form for their reception. Their only movements are flexion and ex- 
tension. Example: Tibia with the astragalus. 
3. Arthrodia (a kind of shallow articulation), consisting almost of plain 
surfaces. Gliding is the only possible movement. Example: the Carpo- 
metacarpal articulation. 
SYNARTHRODIAL JOINTS are included under four heads, all of which 
should be examined as parts of the bony skeleton :— 
1. Harmonia, in which the bones are joined by apposition, as in the 
nasal hones. 
2. Schindylesis, in which a ridge or keel projects into a cleft. Example: 
Vomer with sphenoid. 
3. Gomphosis, Like a nail in its socket, as the teeth in the alveoli. ® 
4. Sutura. Indented, and sabdivided into sutura serrata, as in the 
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