118 PHILOSOPHY OF ZOOLOGY. 
which is fixed, describing the segment of a circle, whose 
centre is in the joint. In ¢wisting, the bone which is moy- 
ed turns round its own axis, or round an imaginary axis, 
passing through the articulation. In sdiding, the free ex- 
tremity of the bone moved, approaches the bone which is 
fixed, in a straight line. 
The form of the joints, in order to admit of these diffe- 
rent motions, 1s exceedingly various. When a large sphe- 
rical shaped head, called a condyle, is received into a deep 
cavity, there is formed the ball and socket joint, or the En- 
arthrosis of anatomists. In this articulation, both flexion 
and twisting are permitted, and it is the kind of joint em- 
ployed in those members where great freedom of action is 
required. When the cavity is shallow, and the head flat, 
or when both surfaces are nearly plain, the articulation is 
termed by anatomists Arthrodia. Flexion cannot be per- 
formed by this kind of joint to any extent, and the motion 
by twisting is hkewise limited. It is principally employed 
where a great freedom of action is not necessary, but where 
flexion, twisting, and sliding are all in a small degree requi- 
site, as is the case with the bones of the hand and the foot. 
Where sliding only is required, the articular surfaces are 
either flat, as is the case with the bones which join the or- 
bital septum in ducks, and unite the central branches of the 
palatine arch with the os quadratum, or, there is a groove 
formed by the one bone to slide along a ridge on another. 
This appears to be the case with the central branches of the 
palatine arch in the solan-goose, which are ossified, and 
cover the inferior edge of the orbital ridge like a saddle. 
These articulations are necessary to regulate the motions 
of the upper mandible in birds. 
When the articulating surfaces are semicylindrical, the 
one convex and the other concave, or when both are 
partly convex and partly concave, the joint is called 
