The entire bone is at first composed of cartilage, which gradually _ossifies_ or turns to bone. [Footnote: The ossification of the bones on the sides and upper part of the skull, for example, begins by a rounded spot in the middle of each one. From this spot the ossification extends outward in every direction, thus gradually approaching the edges of the bone. When two adjacent bones meet, there will be a line where their edges are in contact with each other, but have not yet united; but when more than two bones meet in this way, there will be an empty space between them at their point of junction. Thus, if you lay down three coins upon the table with their edges touching one another, there will be a three-sided space in the middle between them; if you lay down four coins in the same manner, the space between them will be four-sided. Now at the back part of the head there is a spot where three bones come together in this way, leaving a small, three-sided opening between them: this is called the "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 lacunæ, the tiny tubes (canaliculi) radiating from them, and four Haversian canals, three seen crosswise and one lengthwise._]