172 THE ARTICULATIONS OR JOINTS 



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 median plane, or of a digit toward and away from the 

 axis of the liml). 



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. Gingl3rmus, 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: occipito-atlantal 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, ex- 

 tension, 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 usually no joint cavity, 

 but in certain situations a rudimentary one exists. 



THE ARTICULATIONS OF THE HORSE 

 Joints and Ligaments of the Vertebrae 



The movable vertcbrie form two sets of articulations, viz., those formed l)y the 

 bodies, and those formed by the articular processes of adjacent vertebrae; 

 the foT-mer are termed intercentral, and the latter, interneural. Associated with 

 these are ligaments uniting the arches and processes; some of these are special, 

 i. e., confined to a single joint, whil(> others are common, /. e., extend along the entire 



'This c'lassififation makes no claims to scientific accuracy, but is simplj' a statement of tlie 

 terms in general use. A f^roupinp; based on mechanical principles seems desirable, but appears to 

 be almost impossible on account of the p;reat variety and irrep;ularity 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 vertebrae 

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



