The bones of the trunk are variously related to each other, and some of them become united during adult life into bony masses which at earlier periods are quite distinct. For example, the sacrum is in early life made up of five distinct bones which later unite into one. The upper cavity, or chest, is a bony enclosure formed by the breastbone, the ribs, and the spine. It contains the heart and the lungs (Fig. 86). The lower cavity, or abdomen, holds the stomach, liver, intestines, spleen, kidneys, and some other organs (Fig. 59). The bones of the trunk may be subdivided into those of the spine, the ribs, and the hips. The trunk includes 54 bones usually thus arranged: Spinal Column, 26 bones: 7 Cervical Vertebræ. 12 Dorsal Vertebræ. 5 Lumbar Vertebræ. 1 Sacrum. 1 Coccyx. Ribs, 24 bones: 14 True Ribs. 6 False Ribs. 4 Floating Ribs. Sternum. IV. Two Hip Bones. V. Hyoid Bone. 37. The Spinal Column. The spinal column, or backbone, is a marvelous piece of mechanism, combining offices which nothing short of perfection in adaptation and arrangement could enable it to perform. It is the central structure to which all the other parts of the skeleton are adapted. It consists of numerous separate bones, called vertebræ. The seven upper ones belong to the neck, and are called cervical vertebræ. The next twelve are the dorsal vertebræ; these belong to the back and support the ribs. The remaining five belong to the loins, and are called lumbar vertebræ. On looking at the diagram of the backbone (Fig. 9) it will be seen that the vertebræ increase in size and strength downward, because of the greater burden they have to bear, thus clearly indicating that an erect position is the one natural to man. Illustration: Fig. 16.—The Spinal Column. This column supports the head, encloses and protects the spinal cord, and forms the basis for the attachment of many muscles, especially those which maintain the body in an erect position. Each vertebra has an opening through its center, and the separate bones so rest, one upon another, that these openings form a continuous canal from the head to the lower part of the spine. The great nerve, known as the spinal cord, extends from the cranium through the entire length of this canal. All along the spinal column, and between each two adjoining bones, are openings on each side, through which nerves pass out to be distributed to various parts of the body. Between the vertebræ are pads or cushions of cartilage. These act as “buffers,” and serve to give the spine strength and elasticity and to prevent friction of one bone on another. Each vertebra consists of a body, the solid central portion, and a number of projections called processes. Those which spring from the posterior of each arch are the spinous processes. In the dorsal region they are plainly seen and felt in thin persons.