CHAPTER XXIV 

 THE SKELETON AND MUSCLES 



Two very important tissues of the body are the bones and 

 muscles. 



The skeleton (fig. i66A).--The bones, of which there are 

 some two hundred or more in the body, are arranged and 

 joined in such a way as to form the skeleton. The skeleton 

 fulfills three purposes: 



(i). It is made use of by the muscles to enable the body 

 parts and the whole body to move about and handle things. 



(2). It forms a framework to protect the delicate organs 

 of other systems. 



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(3). It gives form and rigidity to the body. 



In man, as in other vertebrate animals, the axial skeleton 

 is formed of the bones of the skull, vertebral column or 

 backbone, ribs, and sternum. The bones of the skull 

 protect the brain. The vertebral column encloses the long 

 dorsal nerve cord. The long, flat, curved ribs, articulating 

 at the back with the vertebral column and in front with 

 the sternum, protect the heart and lungs in the thoracic 

 cavity (fig. 159). The large hip bones (pelvic bones, figs. 

 1 66, S and 159, P) form a sort of basket for the support 

 of the organs of the abdominal cavity. 



The bones of the legs and arms form the appendicular 

 skeleton. The leg bones articulate with or join the pelvic 

 bones, pelvic girdle, and the arm bones articulate with the 

 shoulder bones, or pectoral girdle. A comparison of the skele- 

 ton of the toad (fig. 9) with that of man (fig. 166 A) 

 shows that while the different bones vary in relative size and 

 shape, the same regions are laid down in each. 



3 2 9 



