254 



ADVANCED PHYSIOLOGY 



connected indirectly with the sternum by short pieces of 

 cartilage; Fig. 101. This material is more flexible and elastic 



than bone, thus making it 

 possible for the ribs to move 

 freely without danger of 

 breaking. Two of the lower 

 ribs on each side, called float- 

 ing ribs, are very short and 

 are not attached to the 

 sternum at all. The thorax, 

 which contains the heart and 

 lungs, is thus protected on 

 all sides by a framework of 

 bones. 



The sternum, or " breast 



FIG. 124. THE SACRUM 



Consisting of five fused vertebrae at the 

 lower end of the spinal column. 



bone " (Fig. 120), is composed 



of three flat, elongate bones, placed end to end; the uppermost 

 is roughly shield-shaped, the middle bone elongate rectangu- 

 lar, and the lower one triangular, with the point downward. 

 They are so closely joined that little movement occurs be- 

 tween them, and the three are generally spoken of as though 

 a single piece. 



The Skull. The skull is balanced on the spinal column and 

 is attached to the top vertebra in such a way that it may be 

 nodded backward and forward. The joint involved in 

 turning the head from side to side is that between the first 

 and second vertebrae. The skull is a complicated arrangement 

 of bones, twenty-two in all (Fig. 125), so rigidly fitted 

 together that there is no motion between them, except that 

 of the jaw bone. Teeth are not a part of the skeleton, since 

 they arise from the lining of the mouth and because this lining 

 is essentially the same as the outer skin. 



We may consider the skull as made up of three parts. 



1. The cranium, a large, rounded box of bones firmly 



