BONES, MUSCLES, EXERCISE, AND REST. 257 



socket in the side bone of the pelvis. The head of the bone 

 of the upper arm is less than half a ball, and is fitted into a 

 very shallow socket in the upper corner of the shoulder blade, 

 (Fig. XXXVII. a.) 



608. The head is connected with the upper vertebra by (i 

 hinge joint) which allows it to .bend forward and backward. 

 The upper vertebra has a ring on its posterior side, in which 

 a tooth or pin from the second vertebra is inserted. This 

 allows the first vertebra to turn upon the second, as a gate 

 turns upon a hook-and-eye hinge. By this joint we turn the 

 head from side to side, and by the other we nod and lift the 

 head. 



The cartilages between the several bones of the spine allow 

 the column to bend in every direction. 



609. The head of every bone is covered with a very dense, 

 but somewhat elastic cartilage, which is sufficiently soft to 

 break the force of pressure or jars upon. the bones, but not 

 soft enough to be loose and interfere with the movements of 

 one bone upon another. These cartilaginous facings of the 

 joints are not very thick not more than a sixteenth or 

 eighteenth of an inch. They are covered with an exceed- 

 ingly smooth lining, that presents the most polished surface 

 imaginable. 



610. It is onejof the admirable provisions in regard to the 

 joints, that they never wear out. Though they are in such 

 frequent use, and exposed to so much pressure arid motion 

 from infancy to extreme old age, even eighty or ninety years, 

 yet they never wear out. The tough, cartilaginous coverings 

 of the ends of the bones, and the delicate and glairy facings 

 of the joints, are as thick and as smooth at the end as at the 

 beginning, or at any period of life. These substances, if 

 worn at all, are perpetually renewed. They pass through the 

 same changes, they are subject to the same death of particles 

 from exhaustion, and the same renewal of living particles, as 

 the other organs and textures. 



611. Not only do these joints and their parts wear well, 

 but they haw a perpetual self -oiling apparatus, that keeps 



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