314 THE VERTEBRATE BODY 



The pelvic girdle and legs of man are very similar to those of the 

 frog except that the tibia and fibula which are fused in the frog are 

 separate in man. The development of the heel in the foot is an im- 

 portant factor in enabling man to maintain an erect position. There is 

 a powerful tendon, the tendon of Achilles, which holds the foot flat and 

 prevents us from falling forward. Savage races sometimes cut this on 

 their captives to prevent them from escaping. The tendon received its 

 name from the story that the mother of Achilles held her son by this 

 region when she immersed him in the river Styx. 



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Photo by Winchester 



Fig. 22.6. Photomicrograph of thin section of compact bone from the femur. Note 

 the Haversian canal as a large circle and the small lacunae arranged concentrically 

 around the canal. The small processes extending out from the lacunae are the 



canaliculi. 



In spite of popular opinion, bones are not immutable structures, but 

 are actually very plastic — their parts are constantly being torn down 

 and replaced from within. Many primitive races change the shape of 

 the heads of their children by binding them. Damage to the nerve cells 

 in the spinal cord by poliomyelitis will result in small, weak bones on 

 the side of the body afflicted, while extensive use of an arm or leg on 

 one side will result in larger bones on that side. 



The Muscular System. Since the muscular system is so closely re- 

 lated to the skeletal system it is logical to study this system next. Mus- 

 cle cells are highly specialized tissues that do only one thing; that is, 

 they make themselves shorter, or contract, upon stimulation. It is by 



