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 vertebrae. 



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, 

 i. 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, circumduction. 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 usuallj^ no joint cavity, 

 but in certain situations a rudimentary one exists. 



^ 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. 



- 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 vertebrse 

 with those possible in the carpo-metacarpal or the sacro-iliac joints. 



