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 36 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.
89. Between these joints, or vertebræ, is a peculiar and highly elastic substance, which much facilitates the bending movements of the back. This compressible cushion of cartilage also serves the important purpose of diffusing and diminishing the shock in walking, running, or leaping, and tends to protect the delicate texture of the brain.
86. How are the ribs united to the sternum? 87. Describe the spinal column. 88. Give the structure of the vertebra. Where is the spinal cord placed? 89. What is placed between each vertebra? What is its use?