CHAPTER XVII 



HUMAN STRUCTURE AND LIFE-ACTIVITIES 



370. General Plan of Human Body. Like the frog, 

 already studied, the human body consists of head, trunk, 

 and limbs. The trunk is composed of the chest or thorax, 

 and the belly or abdomen. The arms are the upper or an- 

 terior limbs ; the legs are the lower or posterior limbs. Note 

 that the thigh of a leg corresponds in structure to the upper 

 arm, the shank to the forearm, the ankle to the wrist, the 

 toes to the fingers. Note also that externally the human 

 body is bilaterally symmetrical. 



371. Skeleton. Like the frog and all the other back- 

 boned animals, the human body is supported by an internal 

 framework or skeleton composed of bone and cartilage, which 

 is chiefly at the ends of bones. There are more than 200 

 bones in the human skeleton ; 33 vertebrae or segments of 

 the spinal column, about 25 bones in the skull of an adult, 

 24 ribs, 30 bones in each arm and leg, and the bones in the 

 shoulder-girdle and pelvis. The number of bones varies 

 with the age ; for example, the skull bones are more numer- 

 ous in young children, but they grow or fuse together as the 

 individual becomes older. Each half of the pelvis is com- 

 posed of three bones which have fused together. Nine bones 

 (vertebrae) at the posterior end of the spinal column are 

 fused together in connection with the pelvis, leaving 24 



separate vertebrae. 



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