118 1'HILOSOPHY OF ZOOLOGY. 



which is fixed, describing the segment of a circle, whose 

 centre is in the joint. In twisting, the bone which is mov- 

 ed turns round its own axis, or round an imaginary axis, 

 passing through the articulation. In sliding, 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, is exceedingly various. When a large sphe- 

 rical shaped head, called a candy le, is received into a deep 

 cavity, there is formed the ball and socket joint, or the En- 

 urthrosis 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 likewise 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 



