THE HUMAN ORGANISM 



283 



The skeleton of the head region shows many adaptations to 

 the workings of the organism, as well as to external relations. 

 Besides the bony protecting case of the brain we find sockets for 

 the eyeballs and joints for the jaw. The teeth are not bones 

 but special skin structures ; 

 there are definite sockets 

 in the jaws, however, for 

 all the teeth. The spaces 

 in the face bones are very 

 intricate and are connected 

 with both the sensory and 

 the respiratory functions of 

 the nose. The whole of the 

 hearing apparatus, together 

 with the balancing organ, 

 is inclosed by the bone of 

 the skull (see page 232). 

 There are many openings 

 in the base of the brain 

 box, through which nerves 

 and blood vessels pass. In 

 a saddle-shaped depres- 

 sion in the base of the 

 skull, back of the eyes, lies 

 a two-lobed organ, the pi- 

 tuitary gland, which pro- 

 duces important internal 

 secretions (see Fig. 124 and 

 page 178). 



226. The structure and 

 growth of bones. The skeleton of vertebrates differs from the 

 exoskeletons of invertebrates in one very important respect : it 

 is made up of living tissue. Bone cells absorb from the lymph a 

 large proportion of mineral matter and deposit this as lime salts 

 outside of themselves (Fig. 137). In the development of the 

 individual from the one-celled stage there appear groups of 



Fig. 136. Development of the 

 erect form 



In relation to the upright carriage of human 

 beings, the backbone gradually changes its 

 shape from infancy .on. When the erect po- 

 sition is attained the backbone shows four 

 distinct curves 



