IIO ATLAS AND TEXT-BOOK OF HUMAN ANATOMY. 



2. BIAXIAL JOINTS. 



This class of joints includes the ellipsoidal or condyloid joint and the saddle joint. 



In the ellipsoidal joint the convex surface of an ellipsoid of rotation articulates with a cor- 

 responding concavity, and the two axes of motion, the lesser and the greater axis, are placed 

 at right angles to one another, but both pass through the same bone. An example is found in 

 the atlanto-occipital articulation. 



The saddle joint is formed by the approximation of two saddle-shaped surfaces, i. e., sur- 

 faces which are concave in one direction and convex in the other. Each surface alternately 

 forms a head and a socket, and the two axes are at right angles to one another but are situated 

 in different bones. The most typical saddle joint of the human body is the carpometacarpal 

 articulation of the thumb. 



In reality the motions in the ellipsoidal and saddle joints are not strictly biaxial, but may 

 occur in any intervening axis between the two, and one of the two motions is frequently so lim- 

 ited that the joint practically becomes a ginglymus. 



3. POLYAXIAL JOINTS. 



The only polyaxial joints are those with spherical surfaces, the spheroid joints. These are 

 subdivided into two varieties, the gliding joints or arthrodia and the ball-and-socket joint or 

 enarthrosis. 



In the arthrodia a spherical head moves in a spherical socket, but the articulating surfaces 

 are segments of very large spheres, and may seem in some cases to be almost .planes. Com- 

 paratively little motion can take place between the surfaces, and what does occur is more or 

 less of a gliding character. Good examples of arthrodia are to be seen between the articulating 

 processes of the vertebrae. 



In the enarthroses the articulating surfaces are more extensive segments of smaller spheres 

 and the capsular ligaments are roomy and relaxed, so that a considerable range of motion is 

 possible between the two bones. Typical examples of these ball-and-socket joints are to be 

 found in the shoulder-joint and hip-joint. 



SPECIAL SYNDESMOLOGY. 

 JOINTS AND LIGAMENTS OF THE VERTEBRAL COLUMN. 



THE CONNECTIONS OF THE VERTEBRAL BODIES. 



The bodies of the true vertebrae are connected by intervertebral fibrocartilages (Figs. 172 

 to 175), each of which (Fig. 174) consists of an external firm fibrous ring composed of concentric 

 and interwoven bundles of connective tissue, and of a central gelatinous or pulpy nucleus, the 

 latter being firmly compressed within the fibrous ring and between the adjacent vertebral sur- 

 faces so that it rises above the level of a horizontal section of the disc. Among other substances 

 it contains true* cartilage and the remains of the chorda dorsalis, an embryonic structure which 

 indicates the future position of the vertebral column and is composed of a substance resembling 



