35.  Sutures of the Skull.  Before leaving the head we must notice the peculiar and admirable manner in which the edges of the bones of the outer shell of the skull are joined together.  These edges of the bones resemble the teeth of a saw.  In adult life these tooth-like edges fit into each other and grow together, suggesting the dovetailed joints used by the cabinet-maker.  When united these serrated edges look almost as if sewed together; hence their name, sutures.  This manner of union gives unity and strength to the skull.

In infants, the corners of the parietal bones do not yet meet, and the throbbing of the brain may be seen and felt under these “soft spots,” or fontanelles, as they are called.  Hence a slight blow to a babe’s head may cause serious injury to the brain (Fig. 14).

The Bones of the Trunk.

36.  The Trunk.  The trunk is that central part of the body which supports the head and the upper pair of limbs.  It divides itself into an upper cavity, the thorax, or chest; and a lower cavity, the abdomen.  These two cavities are separated by a movable, muscular partition called the diaphragm, or midriff (Figs. 9 and 49).

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: 

   I. Spinal Column, 26 bones: 
     7 Cervical Vertebrae.
    12 Dorsal Vertebrae.
     5 Lumbar Vertebrae.
     1 Sacrum.
     1 Coccyx.

  II.  Ribs, 24 bones: 
    14 True Ribs.
     6 False Ribs.
     4 Floating Ribs.

 III.  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 vertebrae.  The seven upper ones belong to the neck, and are called cervical vertebrae.  The next twelve are the dorsal vertebrae; these belong to the back and support the ribs.  The remaining five belong to the loins, and are called lumbar vertebrae.  On looking at the diagram of the backbone (Fig. 9) it will be seen that the vertebrae 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.