210 THE ARTICULATIONS 
of the limbs, it seems advisable to employ the terms dorsal and volar or plantar 
flexion, since these joints can be “overextended.” Similarly the terms dorsal and 
ventral flexion are applied to the corresponding movements of the spinal column. 
The meaning of the term lateral flexion as applied to the vertebral column is evi- 
dent. These movements are all rotations around axes which are approximately 
either transverse or vertical. Depression, elevation, and transverse movement of 
the lower jaw fall in this category. 
3. Circumduction.—This designates movements in which the distal end of 
the limb describes a circle or a segment of one. In man such movement is easily 
performed, but in quadrupeds it is possible to a limited degree only, and is to be 
regarded usually as an indication of disease. 
4. Rotation.—As a matter of convenience, this term is reserved to indicate 
rotation of one segment around the longitudinal axis of the other segment forming 
the joint. It is seen typically in the atlanto-axial joint. 
5. Adduction and abduction designate respectively movement of a limb to- 
ward and away from the medial plane, or of a digit toward and away from the 
axis of the limb. 
Classification.—This is based on the form of the joint surfaces and the move- 
ments which occur. The following chief classes may be recognized: 
1. Arthrodia, or gliding joint. In these the surfaces are practically flat, ad- 
mitting of gliding movement. Examples: carpo-metacarpal joints; joints be- 
tween the articular processes of the cervical and thoracic vertebrie. 
2. Ginglymus, or hinge-joint. In this class the joint surfaces consist usually 
of two condyles, or of a segment of a cylinder or cone, which are received by cor- 
responding cavities. In typical cases the movements are flexion and extension, 
7. e., around a single transverse axis. Examples: atlanto-occipital and elbow 
joints. 
3. Trochoid, or pivot joint. In these the movement is limited to rotation of 
one segment around the longitudinal axis of the other. Example: atlanto-axial 
joint. 
4. Enarthrosis, or ball-and-socket joint. These are formed by a surface of 
approximately spherical curvature, received into a corresponding cavity. They 
are multiaxial, and allow of the greatest variety of movement, e. g., flexion, 
extension, rotation, abduction, adduction, cireumduction. Examples: hip and 
shoulder joints. 
AMPHIARTHROSES 
These joints, as the name indicates, share some characters with both of the 
preceding groups. In them the segments are directly united by a plate of fibro- 
cartilage, and usually by ligaments also. The amount and kind of movement are 
determined by the shape of the joint surfaces and the amount and pliability of the 
uniting medium.? These joints are all medial in position, and are best illustrated 
by the joints between the bodies of the vertebrae. There is usually no joint cavity, 
but in certain situations a rudimentary one exists. 
1 This classification makes no claims to scientific accuracy, but is simply a statement of the 
terms in general use. A grouping based on mechanical principles is desirable, but appears to be 
almost impossible on account of the great variety and irregularity of form of the articular surfaces. 
2 The movements in some of these joints are more extensive and varied than in some diar- 
throses. To illustrate this we may compare the movements of the cervical or coccygeal vertebre 
with those possible in the carpo-metacarpal or the sacro-iliac joints. 
