Part 4 (2/2)

78. The bones of the cranium are united by ragged edges, called _sutures_. The edges of each bone interlock with each other, producing a union, styled, in carpentry, _dovetailing_. They interrupt, in a measure, the vibrations produced by external blows, and also prevent fractures from extending as far as they otherwise would, in one continued bone. From infancy to the twelfth year, the sutures are imperfect; but, from that time to thirty-five or forty, they are distinctly marked; in old age, they are nearly obliterated.

[Ill.u.s.tration: Fig. 7. 1, 1, The coronal suture at the front and upper part of the skull, or cranium. 2, The sagittal suture on the top of the skull. 3, 3, The lambdoidal suture at the back part of the cranium.]

79. We find as great a diversity in the form and texture of the skull-bone, as in the expression of the face. The head of the New Hollander is small; that of the African is compressed; while the Caucasian is distinguished for the beautiful oval form of the head.

The Greek skulls, in texture, are close and fine, while the Swiss are softer and more open.

78. How are the bones of the skull united? What are the uses of the sutures? Mention the appearance of the sutures at different ages. What does fig. 7 represent? 79. What is said respecting the form and texture of the skull in different nations?

80. In each EAR are four very small bones. They aid in hearing.

81. In the FACE are fourteen bones, some of which serve for the attachment of powerful muscles, which are more or less called into action in masticating food; others retain in place the soft parts of the face.

[Ill.u.s.tration: Fig. 8. 1, The frontal, or bone of the forehead. 2. The parietal bone. 3, The temporal bone. 4, The zygomatic process of the temporal bone. 5, The malar (cheek) bone. 6, The superior maxillary bone, (upper jaw.) 7, The vomer, that separates the cavities of the nose. 8, The inferior maxillary bone, (lower jaw.) 9. The cavity for the eye.]

82. The TRUNK has fifty-four bones--twenty-four _Ribs_; twenty-four bones in the _Spinal Column_, (back-bone;) four in the _Pelvis_; the _Sternum_, (breast-bone;) and the _Os hy-oides_, (the bone at the base of the tongue.) They are so arranged as to form, with the soft parts attached to them, two cavities, called the _Thorax_ (chest) and _Ab-domen_.

80. How many bones in the ear? 81. How many bones in the face? What is their use? Explain fig. 8. 82-94. _Give the anatomy of the bones of the trunk._ 82. How many bones in the trunk? Name them. What do they form by their arrangement?

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 fas.h.i.+on, in many instances, has nearly inverted this order. This cavity contains the lungs, heart, and large blood-vessels.

[Ill.u.s.tration: 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 _verte-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 ca.n.a.l, is formed immediately behind the bodies of the vertebr, in which is placed the _me-dulla spi-nalis_, (spinal cord,) sometimes called the pith of the back-bone.

89. Between these joints, or vertebr, is a peculiar and highly elastic substance, which much facilitates the bending movements of the back. This compressible cus.h.i.+on of cartilage also serves the important purpose of diffusing and diminis.h.i.+ng the shock in walking, running, or leaping, and tends to protect the delicate texture of the brain.

<script>