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THE INTERNAL ENVIRONMENT OF THE BODY 



Part III 



Neck vertebro- 

 Collar bone — 



Jaw bone 



Sternum 



Shoulder blade 

 Humerus — 



UIno— 

 Radius 



Cartilage of ribs 

 — Radius 



Fig. 9.15. The human skeleton. There are seven vertebrae in the human neck 

 as there are in the neck of a mouse, giraffe and every other mammal. There are 

 twelve vertebrae in the thorax, five in the small of the back (lumbar), five fused 

 in the sacrum hidden by the hipbone, and four rudimentary ones forming the 

 coccyx or tail. Comparison of right and left arms will show that (left) the radius 

 is twisted around the ulna when the hand is rotated. In a frog's forelegs these 

 bones are permanently crossed; in most mammals they are permanently straight. 

 Power to rotate the forearm has provided man, monkeys, and other primates with 

 facility in the use of their hands. (Courtesy, Etkin: College Biology. New York, 

 Thomas Y. Crowell & Co., 1950.) 



The broad thin shoulder blade (scapula) is anchored by muscles but not 

 attached to the axial skeleton (Figs. 9.15, 9.16, 9.17), At the shoulder the 

 scapula is joined by the collarbone (clavicle) extending to the breastbone 

 (sternum). The head of the humerus of the upper arm fits into a relatively 

 shallow cavity forming a ball-and-socket joint in the scapula that allows the 

 free motion of throwing a ball. When an arm is lifted the pectoral girdle is 



