THE SKELETAL SYSTEM 



Animals possess two kinds of skeleton. The exosUelet(m, 

 found among the Arthropods, covers the outside of the 

 body, protecting the delicate internal organs, and assisting 

 in locomotion by providing surfaces for the attachment of 

 muscles. The disadvantage of an exoskeleton, like that of 

 a crayfish, is that growth must be accompanied by periodic 

 molts to permit of increase in body size. Vertebrates 

 possess an endoskeleton, which is completely inclosed by 

 the soft parts of the body. The endoskeleton supports the 

 body and thereby preserves its general form. It facili- 

 tates locomotion by providing levers which are moved by 

 the contraction of the muscles. The skull protects the 

 delicate structures of the brain, while the ribs prevent 

 injury to the organs of the thorax. One important advan- 

 tage of the endoskeleton is that it does not interfere with 

 growth, for the bones of the vertebrate body increase in 

 length, width, and thickness as the soft parts grow. 



The tt'Xial skeleton of a vertebrate comprises the skull, 

 backbone, ribs, and sternum. The appendicular skeleton 

 consists of the pectoral and pelvic girdles, the fore legs, 

 hind legs, and feet. 



THE SKULL 



The skull of the higher vertebrates is a bony case com- 

 pletely surrounding and protecting the delicate tissues of 

 the brain. It incloses and protects the olfactory organ, 



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