STRUCTURE OF JOINTS. 249 
cartilage coating each of the opposing surfaces of the bones concerned. All the 
_ joints belonging to this group occur in the mesial plane of the body. It includes 
the symphysis pubis, the joints between the bodies of the vertebre, and the joint 
between the manubrium sterni and the gladiolus. 
A diarthrosis (Fig. 196) is the most elaborate as well as the most complete form 
of articulation. It is characterised by (a) capability of movement which is more or 
less free in its range; (0) a reduction of the 
uniting structures to a series of retaining liga- 
ments; (¢) a joint cavity which is only 
limited by the surrounding ligaments; (d) Articular 
the constant presence of synovial membrane ; conlaee 
(¢) hyaline encrusting cartilage which clothes, Joint_} Feat 
the opposed surfaces of the articulating bones. “yy... | | capsule 
The majority of the joints in the adult belongs synovialis __ Synovial 
membrane 
to this group. This series of joints has been Synovial 
Reon : q : : membrane 
subdivided into a number of minor sections, 
in order to emphasise the occurrence of 
certain well-marked structural features, or a iticutar/ 
_ because of the particular nature of the move- cartilage 
ment by which they are characterised. 
Although in all diarthroses there is a certain 
amount of gliding movement between the 
opposed surfaces of the bones which enter into their formation, yet, when this gliding 
movement becomes their prominent feature, as in most of the joints of the carpus and 
tarsus, they are termed arthrodia. But bones may be articulated together so as to 
permit of movement in one, two, or more fixed axes of movement, or in modifications 
of these axes. Thus in uniaxial joints the axis of movement may le in the longi- 
tudinal axis of the joint, in which case the rotatory form of joint results, as in 
the superior and inferior radio-ulnar articulations; or it may correspond with the 
_ transverse axis of the articulation, as in the elbow-joint and knee-joint, when the 
ginglymus or hinge variety results. If movement takes place about two principal 
axes situated at right angles to each other, as in the radio-carpal joint, the terms 
biaxial or condyloid are applied. Movements occurring about three principal axes 
placed at right angles to each other, or in modifications of these positions, con- 
stitute multiaxial joints, in which the associated structural peculiarities provide the 
alternative terms of enarthrodial or ball-and-socket joints, 
Fic. 196.—DIAGRAM OF A DIARTHRODIAL 
JOINT. 
STRUCTURES WHICH ENTER INTO THE FORMATION OF JOINTS. 
The structures which enter into the formation of joints vary with the nature 
of the articulation. In every instance there are two or more skeletal elements, 
whether bones or cartilages, and in addition there are the uniting media, which are 
either simple or elaborate according to the provision made for rendering the joint 
more or less rigid or capable of movement. We have already seen that the uniting 
medium in synarthrodial joints is a remnant of the common matrix, whether fibro- 
vascular membrane or hyaline cartilage, in which ossification has extended from 
separate centres. Among the amphiarthroses there is still extensive union between 
the opposing surfaces of the articulating bones, but the character of the uniting 
medium has advanced from the primitive embryonic tissue to fibrous and fibro- 
cartilaginous material, as well as hyaline cartilage. These, with very few exceptions, 
are permanent, non-ossifying substances, such as may be seen between the opposing 
osseous surfaces of two vertebral bodies. The joint cavity, more or less rudimentary, 
is confined to the centre of the fibro-cartilaginous plate, and may result from the 
softening or imperfect cleavage of the central tissue. It may also present rudiments 
of a synovial membrane. 
In the diarthrodial group the extensive cavity has produced great interruption 
in the continuity of the uniting structures which originally existed between the 
bones forming such a joint. Ligaments have therefore additional importance in 
this group, for not only do they constitute the uniting media which bind the 
