354 THE HORSE. 
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 vart by fibro- 
cartilage. Their extent of movement depends on the thickness and 
elasticity of the interarticular fibro-cartilage. They do not glide, there- 
fore, one over the other. Only one species of amphiarthrosis exists, of 
which the articulations of the vertebre, the ischio-pubic symphysis, and 
the intermetacarpal joints are examples. 
MOVEMENTS OF THE JOINTS. 
THE MOTIONS permitted in the joints are four—namely, gliding, angular 
motion, circumduction, and rotation. 
1. Guiprne 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 cireumduction. The elbow and hock are examples of the former, 
as, indeed, are most of the horse’s joints. 
3. CIRCUMDUCTION is a motion very little seen in the large joints of 
this animal, and is confined to the hip.and shoulder joints, in which it is 
far more limited than in the corresponding joints of the human frame. It 
is displayed when a limb is made to describe a segment of a large circle 
around the joint which connects it to the body. 
4, Roration is the movement of a bone on its own axis, and is only 
seen in the horse in the joint between the two first vertebre of the neck. 
ARTICULATIONS OF THE VERTEBRAL COLUMN. 
THE VERTEBR# are connected together by ligaments, fibro-cartilage, and 
synovial membranes ; the first two serving to retain them in position ; 
the last to facilitate motion. They correspond, firstly, by their bodies ; 
secondly, by their spines ; and, thirdly, by their oblique and transverse 
processes. It is necessary to state, that the general details into which 
this study leads us will apply only to the articulations which unite the 
six lower cervical vertebra, the dorsal and lumbar vertebre, and the 
sacrum. 
THE BODIES connect themselves by their surfaces, which in the cervical 
region represent, Ist, the anterior, or true head; 2d, the posterior, or 
glenoid cavity, which receives the head of the vertebra immediately 
behind it. In passing from the first dorsal to the sacrum, these tend to 
efface themselves, and become plainer; nevertheless, they preserve 
throughout the one its convexity, and the other its concavity. Their 
means of union are—(1) fibro-cartilages, interposed between the articular 
surfaces ; (2) a common superior vertebral ligament ; (3) a common in- 
ferior vertebral ligament. 
The intervertebral fibro-cartilages are circular or elliptical discs, convex 
before, concave behind ; firmly fixed to the surfaces of the bones which 
they separate. The fibro-cartilaginous substance which forms them is 
composed of an external laminar part, constituting the circumference of 
an internal soft or pulpy part, which occupies the centre. The laminar 
part forms more than half the whole mass, and consists of laminz, or 
