VARIOUS FORMS OF JOINTS. 



149 



coverings and all other parts of the articular cavity except those formed by cartilage. 

 This membrane is continuous with the margin of the articular cartilages, and along 

 with them completely encloses the joint-cavity. The bones are united by fibrous 

 tissue in the various forms of ligaments, such as membranous capsules, flat bands, 

 or rounded cords. These ligaments, it is true, are not always so tight as to main- 

 tain the bones in close contact in all positions of the joint, but are rather tightened 

 in some positions and relaxed in others, so that in many cases they are to be looked 



EPHJST!1UM 



SYNOVIAL FOLC 



JOINT-CAVITY 



Fig. 173. SCHEMKS OF DIARTHRODIAL JOINTS ; A, SIMPLE; B, COMPOUND. (GK D. T. ) 



The synovial membrane is represented by a dotted line. In the natural condition the oppose 1 

 surfaces are in contact. 



upon chiefly as controllers of movements. The bones are likewise held together in 

 diarthrodial joints by atmospheric pressure, and by the surrounding muscles. The 

 following forms of diarthrodial joint are distinguished : 



1. Gliding joint (Arthrodia). The articular surfaces are nearly flat and admit 

 of only a limited amount of gliding movement, as in most of the articulations of 

 the carpus and tarsus, and in the joints between the articular processes of the 

 vertebras. 



2. Hinge-joint (Ginglymus). The movements are only those of flexion and 

 extension, the articular surfaces being either approximately cylindrical, or more 

 frequently the one pulley-shaped (trochlear) and the other correspondingly ridged. 

 To this group belong the humero-ulnar articulation, the ankle-joint, and the inter- 

 phalangeal articulations. 



3. Condyloid joint (Condylarthrosis). The articular surfaces are spheroidal 

 and allow of all varieties of angular movement, together with circumduction. a? 

 jn the metacarpo- and metatarso-phalangeal articulations. 



4. Saddle-joint. The movements allowed are similar to those in a condyloid 

 joint, but the articular surfaces are reciprocally saddle-shaped, the convexity being 

 directed from within out on the one bone and from before back on the other, as in 

 the articulation of the first metacarpal bone with the trapezium. 



5. Ball and socket joint (Bnarthrosis). 1 The articular surfaces are approxi- 

 mately spherical, and movement can take place freely in any direction, as in the 

 shoulder and hip-joints. 



6. Pivot-joint (Trochoides). Eotation only is allowed, the articular surfaces 

 being cylindrical, or portions of a cone, as in the atlanto-axial and radio-ulnar 

 articulations. 



The hinge and pivot- joints are uniaxial articulations, the movements taking place around 

 #n axis which is essentially transverse in the hinge- joint, although never precisely at right 



1 The name arthrodia, is sometimes given to this form of articulation. 



L* 2 



