396 THE ANATOMY OF THE HORSE 



thirdly, amphiarthrosis (a.fJLcf>L, about, and apdpov), partaking, at the same 

 time, of the two classes of articulation above mentioned — namely, synar- 

 throsis, in the continuity established by the articular sui'faces ; and 

 diarthrosis, in the limited extent of movement it permits. 



The guide to the classification of joints is the configuration of their 

 articular sui'faces, and the movements they allow. 



DiARTHRODiAL JOINTS are arranged under three distinct classes : — 



1. Enarthrosis. This kind of joint is characterized by the reception of 

 an articular head into a cavity of appi'opriate form. It is the seat of most 

 extensive movements ; namely, flexion, extension, adduction, abduction^ 

 circumduction, and rotation. Example : Acetabulum with femur. 



2. Ginglymus. A perfect hinge-joint, the articular surfaces of which 

 are configured in a trochlear arrangement, in such a manner that two or 

 more prominences may fit into two or more excavations of appropriate form 

 for their reception. Their only movements are flexion and extension. 

 Example : Tibia with the astragalus. 



3. Arthrodia (a kind of shallow articulation), consisting almost of plain 

 surfaces. Gliding is the only possible movement. Example : the Carpo- 

 metacarpal articulation. 



Synarthrodial joints are included under four heads, all of which should 

 be examined as parts of the bony skeleton : — 



1. Hai^monia, in Avhich the bones are joined by apposition, as in the 

 nasal bones. 



2. Schindylesis, in which a ridge or keel projects into a cleft. Example : 

 Vomer with sphenoid. 



3. Gomphosis. Like a nail in its socket, as the teeth in the alveoli. 



4. Sutura. Indented, and subdivided into sutura seirata, as in the 

 frontal bones, and sutura squamosa, as in the union of the parietal and 

 temporal bones. 



The amphiarthrodial joints are often smooth, and formed after the 

 manner of diarthrodial surfaces. At other times they are more or less 

 rough. These joints are united together for the most part by fibro-cartilage. 

 Their extent of movement depends on the thickness and elasticity of the 

 interarticular fibro-cartilage. They do not glide, therefore, one over the 

 other. Only one species of amphiarthrosis exists of which the articulations 

 of the vertebrae, the ischio-pubic symphysis, and the intermetacarpal joints 

 are examples. 



MOVEMENTS OF THE JOINTS 



Thk motions permitted in the joints are four — namely, gliding, angular 

 motion, circumduction, and rotation. 



1. Gliding is the simple motion of one bone upon the other, without 

 materially altering their relations. 



2. Angular motion may be either limited to one plane, as in the trace- 

 hinge, or it may be extended to more, when the motion becomes nearly 

 allied to circumduction.- The elbow and hock are examples of the former, 

 us, indeed, are most of the horse's joints. 



3. Circumduction is a motion very little seen in the large joints of this 



