172 STUDIES IN PHYSIOLOGY 



mandible is the only movable bone of the skull. All the 

 others are united by irregular, toothlike processes (called 

 su'tures) which fit closely into each other, like the dovetail 

 joint of the carpenter (see Fig. 71.) 



Adaptations shown in the Structure of the Skull. By its 

 rounded contour, the skull furnishes the best possible protec- 

 tion for the brain. In the first place, if a blow strikes upon 

 the head, it would be much more likely to glance off than 

 would be the case if the sides and top were flat. Again, the 

 arched form of the skull, like the arch of a bridge, gives 

 the greatest possible strength in a given amount of material 

 to resist the force of a hard blow. 



Since the end of the nose and the outside ear are the most 

 exposed portions of the head, they would, if made of bone, be 

 in constant danger of getting broken. Cartilage, however, 

 gives them sufficient permanence of form, and at the same 

 time this elastic material, if bent out of shape, at once 

 returns to its original position as soon as the pressure is 

 removed. 



The deep eye sockets, surrounded by frontal, upper jaw, 

 and cheek bones, seldom allow any blow to injure the eye. 

 The drum of the ear, the three tiny bones of the middle ear, 

 and the delicate mechanism of the inner ear are all buried 

 deep in the lower part of the temporal bone which is the 

 hardest part of the bony skeleton, and so these are out of 

 danger. 



4. DIFFERENCES BETWEEN THE SKELETON OF A CHILD 

 AND THAT OF AN ADULT 



Difference in Composition. It should be noted, in the first 

 place, that most of the bones of the skeleton are first formed 

 wholly of cartilage, and that this cartilage is gradually re- 

 placed by bone through the supply of mineral matters fur- 

 nished by the blood. This explains the fact that the bones 

 of young children are easily bent without being broken, while 

 those of an old person are brittle and break easily. Because 



