VARIOUS ARTICULATIONS 177 



eluding them, as vceW as the eushions or pads of fatty tissue situate -within and the 

 tendons which perforate the fibrous capsule, from the articular cavity. It is a thin 

 delicate membrane, frequently forming folds and fringes Avhich project into the 

 cavity of the joint; or, as in the knee, stretches across the cavity, forming a so- 

 called synovial ligament. In these folds are often found pads of fatty tissue, which 

 fill up interstices, and form soft cushions between the contiguous bones. Some- 

 times these fringes become villous and pedunculated, and cause pain on movement 

 of the joints. They contain fibro-fatty tissue, Avith an isolated cartilage cell or two. 

 The synovial membrane is Avell supplied with blood, especially near the margins of 

 the articular cartilages and in the fringes. It secretes a thick glairy fluid like Avhite 

 of egg, called synovia, which lubricates the joint. Another variety of s^movial 

 membrane is seen in the bursae, which are interposed l)etween various moving 

 surfaces. In some instances bursse in the neighbourhood of a joint may communi- 

 cate with the synovial cavity of that joint. 



THE VARIOUS KIXDS OF ARTICULATIONS 



There are three chief varieties of joints — viz., synarthrodial, or immovable; 

 anqihiarthrodial. or yielding; and diarthrodial, or moval)le joints. 



Synarthrosis is the term applied to all immovable joints in which the apposed 

 surfaces or edges of the bones are in direct contact, as in the face bones^ except the 

 mandible, and all those of the skull; or where bone and cartilage are in immediate 

 union, as in the case of the first rib and the sternum, and the costal cartilages and 

 the ribs. The unions of the bones of the skull and face are usually called sutures, 

 of which there are three chief varieties — 



(1) True sutures, where the edges of bones are firmly implanted into one 

 another by means of projecting processes, as in the sagittal, lambdoid, and coronal 

 sutures. 



(2) False sutures, Avhere the rough edges of the bones are in simple contact 

 without interlocking, as in the intermaxillary suture; or where they overlap one 

 another, as in the squamous suture. 



(3) Grooved sutures, where the edge or plate of one bone is received into a 

 corresponding groove in the other, as in the rostrum of the si:)henoid and vomer, or 

 vomer and palatine processes of the maxilla-, and the horizontal plates of the palate 

 bones. 



Amphiarthrosis is the term applied to mixed joints which permit of slight 

 movements, the opposed bony surfaces being firmly united by a plate or disc of 

 fibro-cartilage. There is sometimes a partial synovial membrane. Examples are 

 seen in the spine, sacro-iliac, and pubic joints. 



Diarthrosis is the term applied to all movable joints in which the bones have 

 smooth cartilage-covered surfaces, lubricated by synovia, and bound together by 

 more or less perfect capsules. This class is subdivided into the following 

 varieties : — 



1. Enarthrosis, or ball-and-socket joint, where the more or Icv^s spherical head 

 of one bone is adjusted to a socket on the other, as in the hip- and shoulder-joints. 

 They are the most movable of all articulations, combining angular movements in 

 all directions with axial rotation. 



2. Condylarthrosis, or an articulation having on one bone an elongated surface 

 called " condyle " and on the other a glenoid surface. They permit all the move- 

 ments of a ball-and-socket joint except axial rotation. 



3. Ginglymus, trochlearthrosis, or hinge-joint, where there is a pulley or troch- 

 lea on one bone and a surface adapted for moving round it on the other. The 

 movement is principally in two directions, namely, flexion and extension, though 

 some slight amount of lateral movement is also permitted. The most perfect 



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