ADDITIONAL TOPICS IN HUMAN BIOLOGY 147 



■By its rounded contour, the skull furnishes the best possible pro- 

 tection 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. 



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 suffi- 

 cient 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 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 itmer ear 

 are all buried deep in the hardest 

 part of the skull, and so these are 

 out of danger. 



fibula 



Fig. 45. — Knee joint. 



214. Joints. — Thus far we have 

 considered the bones of the skele- 

 ton as though they were independent of- each other. In the Uving 

 body, however, we know that they are firmly attached to one an- 

 other by ligaments and muscles, and that thus a strong but mov- 

 able framework is formed (Fig. 45). Any region in the skeleton 

 where motion is possible between two bones is called a joint. 



215. Food and the skeleton. — In the composition of bones, 

 mineral is found to constitute about two thirds of the material, 

 and this must be supplied by the food. 



Milk is a most important article of diet in early life, since in addi- 

 tion to the other nutrients, it supplies the phosphate of lime needed 

 for bone manufacture. In the process of refining wheat flour much 

 of the mineral matter is lost ; for this reason whole wheat flour and 

 the coarser cereals like corn, rye, and oats are much more valuable 

 as bone builders, and are especially needful during the period of 



