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shape and that of the interlocking mechanism on its surfaces; a rotatory movement, however, is produced by which the anterior segment is tilted downward and the posterior upward; the axis of this rotation passes through the dorsal part of the middle segment. The movement of the anterior segment is slightly limited by its wedge form, but chiefly by the posterior and interosseous sacroiliac ligaments; that of the posterior segment is checked to a slight extent by its wedge form, but the chief limiting factors are the sacrotuberous and sacrospinous ligaments. In all these movements the effect of the sacroiliac and iliolumbar ligaments and the ligaments of the symphysis pubis in resisting the separation of the iliac bones must be recognized.

During pregnancy the pelvic joints and ligaments are relaxed, and capable therefore of more extensive movements. When the fetus is being expelled the force is applied to the front of the sacrum. Upward dislocation is again prevented by the interlocking mechanism of the middle segment. As the fetal head passes the anterior segment the latter is carried upward, enlarging the antero-posterior diameter of the pelvic inlet; when the head reaches the posterior segment this also is pressed upward against the resistance of its wedge, the movement only being possible by the laxity of the joints and the stretching of the sacrotuberous and sacrospinous ligaments.

6. Articulations of the Upper Extremity. a. Sternoclavicular Articulation

The articulations of the Upper Extremity may be arranged as follows:

I. Sternoclavicular.		VI. Wrist.
II. Acromioclavicular.		VII. Intercarpal.
III. Shoulder.		VIII. Carpometacarpal.
IV. Elbow.		IX. Intermetacarpal.
V. Radioulnar.		X. Metacarpophalangeal.
XI. Articulations of the Digits.

Sternoclavicular Articulation (Articulatio Sternoclavicularis) (Fig. 325)

The sternoclavicular articulation is a double arthrodial joint. The parts entering into its formation are the sternal end of the clavicle, the upper and lateral part of the manubrium sterni, and the cartilage of the first rib. The articular surface of the clavicle is much larger than that of the sternum, and is invested with a layer of cartilage, 1 which is considerably thicker than that on the latter bone. The ligaments of this joint are:

The Articular Capsule.		The Interclavicular.
The Anterior Sternoclavicular.		The Costoclavicular.
The Posterior Sternoclavicular.		The Articular Disk.

The Articular Capsule (capsula articularis; capsular ligament).—The articular capsule surrounds the articulation and varies in thickness and strength. In front and behind it is of considerable thickness, and forms the anterior and posterior sternoclavicular ligaments; but above, and especially below, it is thin and partakes more of the character of areolar than of true fibrous tissue.

The Anterior Sternoclavicular Ligament (ligamentum sternoclaviculare anterior).—The anterior sternoclavicular ligament is a broad band of fibers, covering the anterior surface of the articulation; it is attached above to the upper and front part of the sternal end of the clavicle, and, passing obliquely downward and medialward, is attached below to the front of the upper part of the manubrium sterni. This ligament is covered by the sternal portion of the Sternocleidomastoideus and the integument; behind, it is in relation with the capsule, the articular disk, and the two synovial membranes.

The Posterior Sternoclavicular Ligament (ligamentum sternoclaviculare posterius).—The posterior sternoclavicular ligament is a similar band of fibers, covering the posterior surface of the articulation; it is attached above to the upper and back

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