83. The THORAX is formed by the sternum in front; the ribs, at the sides; and the twelve dorsal bones of the spinal column, posteriorly. The natural form of the chest is a cone, with its apex above; but fashion, in many instances, has nearly inverted this order. This cavity contains the lungs, heart, and large blood-vessels. [Illustration: Fig. 9. 1, The first bone of the sternum, (breast-bone.) 2. The second bone of the sternum. 3, The cartilage of the sternum. 4, The first dorsal vertebra, (a bone of the spinal column.) 5, The last dorsal vertebra. 6, The first rib. 7, Its head. 8, Its neck. 9, Its tubercle. 10, The seventh, or last true rib. 11, The cartilage of the third rib. 12, The floating ribs.] 84. The STERNUM is composed of eight pieces in the child. These unite and form but three parts in the adult. In youth, the two upper portions are converted into bone, while the lower portion remains cartilaginous and flexible until extreme old age, when it is often converted into bone. 85. The RIBS are connected with the spinal column, and increase in length as far as the seventh. From this they successively become shorter. The direction of the ribs from above, downward, is oblique, and their curve diminishes from the first to the twelfth. The external surface of each rib is convex; the internal, concave. The inferior, or lower ribs, are, however, very flat. -=-=-=-=-=-=-=-=-=-=-=-= 83. Describe the thorax. Explain fig. 9. 84. Describe the sternum. 85. Describe the ribs. -=-=-=-=-=-=-=-=-=-=-=-= 86. The seven upper ribs are united to the sternum, through the medium of cartilages, and are called the _true ribs_. The cartilages of the next three are united with each other, and are not attached to the sternum; these are called _false ribs_. The lowest two are called _floating ribs_, as they are not connected either with the sternum or the other ribs. 87. The SPINAL COLUMN is composed of twenty-four pieces of bone. Each piece is called a _vert´e-bra_. On examining one of the bones, we find seven projections, called _processes_; four of these, that are employed in binding the bones together, are called _articulating_ processes; two of the remaining are called the _transverse_; and the other, the _spinous_. The last three give attachment to the muscles of the back. 88. The large part of the vertebra, called the body, is round and spongy in its texture, like the extremity of the round bones. The processes are of a more dense character. The projections are so arranged that a tube, or canal, is formed immediately behind the bodies of the vertebræ, in which is placed the _me-dul´la spi-na´lis_, (spinal cord,) sometimes called the pith of the back-bone.