SYNOVIAL ME^[BRANES 395 



SYNOVIAL MEMBRANES 



The synovial membrane is a thin layer, which invests tne articular 

 cartilages of opposite bones, and is continued from one to the other by 

 being reflected beneath the ligaments which connect ttiem. It resembles 

 ;he serous membrane in being a shut sac or bladder, and a synovial capsule 

 may be compared to a small bladder, containing only as much fluid as will 

 adhere to its interior, placed between the opposite ends of two bones 

 forming a joint. The secretion formed by it, synovia (avv, together, u)uv, an 

 tgg), is alkaline, and contains albumen, which is coagulable by boiling- 

 Heale has ascertained, by the aid of the microscope, that this membrane is 

 actually reflected over the articular cartilage, a point which has been long 

 disputed. Besides the joints, the synovial membranes also form smaller 

 sacs which lubricate the tendons as they pass over the ends of the bones, 

 and which are called hursce mucosce. The epithelium lining these mem- 

 branes is of the kind called tesselated ; it is developed in the same manner 

 with that of other free surfaces, being continually reproduced as it is worn 

 away. Synovial membranes, in many situations, are closely and completely 

 invested externally by fibrous layers — the fibrous capsules, as they ai'e 

 termed. These fibrous coats are met with especially in situations where 

 the articulation is either wholly unprotected or but thinly covered by 

 8oft parts ; or where a very firm connection is required, as in the hip 

 joint. The}' are absent where muscles or ligaments rest upon the articu- 

 lation ; or where, for special purposes, the synovial membrane is exposed 

 to more considerable movements, as in the knees. 



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 wdiich 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 

 olood. 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 od account of the presence of 

 albumen, is coagulable by heat. 



CLASSIFICATION OF THE JOINTS 



The skeleton lias already been described as composed of diiferent 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 iimnobility, 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 (5ia, through, and apdpov, a 

 limb) ; in the second, synarthrosis {(tvv, together, and &p0pov, a limb) ; and 



