"posterior fontanelle." On the top of the head, four bones come together,
leaving between them a large, four-sided opening: this is called the
"anterior fontanelle." These openings are termed the _fontanelles_,
because we can feel the pulsations of the brain through them, like the
bubbling of water in a fountain. They gradually diminish in size, owing to
the growth of the bony parts around them, and are completely closed at the
age of four years after birth.--DALTON.] Certain portions near the joints
are long delayed in this process, and by their elasticity assist in
breaking the shock of a fall. [Footnote: Frogs and toads, which move by
jumping, and consequently receive so many jars, retain these unossified
portions (epiphyses) nearly through, life, while alligators and turtles
whose position is sprawling, and whose motions are measured do not have
them at all--LEIDY] Hence the bones of children are tough, are not readily
fractured, and when broken easily heal again; [Footnote: This is only one
of the many illustrations of the Infinite care that watches over helpless
infancy, until knowledge and ability are acquired to meet the perils of
life.] while those of elderly people are liable to fracture, and do not
quickly unite.

FIG. 3.

[Illustration: _A thin slice of Bone, highly magnified showing the
lacune, the tiny tubes (canaliculi) radiating from them, and four
Haversian canals, three seen crosswise and one lengthwise._]

THE STRUCTURE OF THE BONES--When a bone is sawed lengthwise, it is found
to be a compact shell filled with a spongy substance This filling
increases in quantity, and becomes more porous at the ends of the bone,
thus giving greater size to form a strong joint, while the solid portion
increases near the middle, where strength alone is needed. Each fiber of