134. The cavity of the pelvis, together with all the organs contained in it, are lined by a continuation of the membrane that invests the abdomen and its contents. 135. The organs contained in the pelvis are the rectum (fig. LXIV. 9), which is merely the termination of the large intestines, the urinary bladder (fig. LXIV. 8), and the internal part of the apparatus of reproduction. 136. The large and strong bones of the pelvis not only afford lodgment and protection to the tender organs contained in its cavity, but sustain the entire weight of the body, the trunk resting on the sacrum as on a solid basis (fig. XLV. 5), and the lower extremities being supported in the sockets in which the heads of the thigh-bones play, in the varied movements of locomotion (fig. XXXIV. 4). 137. The last division of the body comprehends the superior and the inferior extremities. 138. The superior extremities consist of the shoulder, arm, fore-arm, and hand. 139. The soft parts of the SHOULDER are composed chiefly of muscles; its bones are two, the scapula or the _blade bone_, and the clavicle or the _collar bone_ (fig. LXV. 2, 4). [Illustration: Fig. LXV. 1. Sternum; 2. clavicle; 3. ribs; 4. anterior surface of scapula; 5. coracoid process of scapula; 6. acromion process of scapula; 7. margin of glenoid cavity of scapula; 8. body of the humerus or bone of the arm; 9. head of the humerus.] 140. The SCAPULA is placed upon the upper and back part of the thorax, and occupies the space from the second to the seventh ribs (fig. LXV. 4) [Illustration: Fig. LXVI. 1. Posterior surface of scapula; 2. margin of scapula; 3. acromion process; 4. margin of glenoid cavity; 5. clavicle; 6. body of humerus; 7. head of humerus.] Unlike that of any other bone of the body, it is embedded in muscles, without being attached to any bone of the trunk, excepting at a single point. From the bones of the thorax it is separated by a double layer of muscles, on which it is placed as upon a cushion, and over the smooth surface of which it glides. Originally, like the bones of the skull, it consisted of two tables of compact bone, with an intermediate layer of spongy bony substance (diploë); but, by the pressure of the muscles that act upon it, it gradually grows thinner and thinner, until, as age advances, it becomes in some parts quite transparent and as thin as a sheet of paper.