ARTICULATIONS OF THE VERTEBRAL COLUMN 



287 



the apex is in the articular cavity; this kind of motion is best seen in the shoulder- 

 and hip-joints. 



Rotation. Rotation is a form of movement in which a bone moves around a 

 central axis without undergoing any displacement from this axis; the axis of rota- 

 tion may lie in a separate bone, as in the case of the 'pivot formed by the odontoid 

 process of the axis vertebrae around which the atlas turns; or a bone may rotate 

 around its own longitudinal axis, as in the rotation of the humerus at the shoulder- 

 joint; or the axis of rotation may not be quite parallel to the long axis of the 

 bone, as in the movement of the radius on the ulna during pronation and supina- 

 tion of the hand, where it is represented by a line connecting the center of the 

 head of the radius above with the center of the head of the ulna below. 



Ligamentous Action of Muscles. The movements of the different joints of a limb are combined 

 by means of the long muscles passing over more than one joint. These, when relaxed and stretched 

 to their greatest extent, act as elastic ligaments in restraining certain movements of one joint, 

 except when combined with corresponding movements of the other the latter movements being 

 usually in the opposite direction. Thus the shortness of the hamstring muscles prevents com- 

 plete flexion of the hip, unless the knee-joint is also flexed so as to bring their attachments nearer 

 together. The uses of this arrangement are threefold: (1) It coordinates the kinds of move- 

 ments which are the most habitual and necessary, and enables them to be performed with the 

 least expenditure of power. (2) It enables the short muscles which pass over only one joint to 

 act upon more than one. (3) It provides the joints with ligaments which, while they are of very 

 great power in resisting movements to an extent incompatible with the mechanism of the joint, 

 at the same time spontaneously yield when necessary. 



The articulations may be grouped into those of the trunk, and those of the upper 

 and lower extremities. 





ARTICULATIONS OF THE TRUNK. 



These may be divided into the following groups, viz.: 





I. Of the Vertebral Column. 

 II. Of the Atlas with the Axis. 



III. Of the Vertebral Column with 



the Cranium. 



IV. Of the Mandible. 



V. Of the Ribs with the Vertebrae. 



VI. 



Of the Cartilages of the Ribs with the 



Sternum, and with Each Other. 

 VII. Of the Sternum. 



Of the Vertebral Column with the 



Pelvis. 

 Of the Pelvis. 



VIII. 



IX. 



I. Articulations of the Vertebral Column. 



(1) 



a series of amphi- 

 of diathrodial 



The articulations of the vertebral column consist of 

 arthrodial joints between the vertebral bodies, and (2) a series 

 joints between the vertebral arches. 



1. Articulations of Vertebral Bodies (intercentral ligaments}. The articulations 

 between the bodies of the vertebras are amphiarthrodial joints, and the individual 

 vertebrae move only slightly on each other. When, however, this slight degree 

 of movement between the pairs of bones takes place in all the joints of the vertebral 

 column, the total range of movement is very considerable. The ligaments of these 

 articulations are the following: 



The Anterior Longitudinal. The Posterior Longitudinal. 

 The Intervertebral Fibrocartilages. 



The Anterior Longitudinal Ligament (ligamentum longitudinale anterius; anterior 

 common ligament) (Figs. 301, 312). The anterior longitudinal ligament is a broad 

 and strong band of fibers, which extends along the anterior surfaces of the bodies 

 of the vertebrae, from the axis to the sacrum. It is broader below than above, 



