BONES, JOINTS, MUSCLES. ,205 



joint, the elbow-joint, the wrist-joint, the knee- 

 joint, and others. 



The number of bones in the human body is 

 about 260. They are divided into the bones of 

 the head, of the trunk or body, of the upper ex- 

 tremities or arms, and of inferior extremities or 

 legs. Their shapes are very various, some being 

 long, round, and hollow, as those of the thigh, the 

 leg, and the arm ; others are flat, as the shoulder- 

 blade, the breast-bone, and the bones of the head ; 

 and others are in small rounded pieces, as those of 

 the hands and feet. 



Bones are very soft in infancy, only part of them 

 being ossified, or made into bone. As we grow 

 older, and have occasion to use our limbs for mo- 

 tion and support, they harden; and about the 

 fifteenth year of our age, are perfect. 



Muscles are the organs or parts by which we 

 move our bodies. What we call flesh is muscle, 

 and this consists of a number of fibres, or little 

 threads, bound together in bundles, by the cellular 

 membrane, and a strong sheath on the outside. 

 One of these bundles forms a muscle. Muscles are 

 fastened to the bones by a tough strong substance, 

 which seems to grow into it. 



Muscles have the power of shortening and 

 lengthening themselves, and thus produce motion. 

 For example, stretch out your arm : this is done 

 by the muscles at the back of the upper arm, which 

 arise above the elbow-joint, and are fixed into the 

 bone below it. When these contract or shorten 

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