102 



MECHANISM OF THE BONES. 



Fig. 15. 



The spine t ribs and breast bone, or sternum, which constitute the 

 frame work of the chest or thorax. Referring, however, to 

 the plate, or to nature, we observe, that the ribs are not con- 

 tinued throughout from the spine to the sternum, but they 

 are eked out and joined to the breast bone by means of pieces 

 of gristle of a form corresponding to that of the ribs, being 

 as it were a completion of the arch of the rib by a substance 

 more adapted to yield in every shock or motion of the body. 

 A severe blow upon the ribs does not break them, because 

 their extremities are tipped with this elastic or springy sub- 

 stance, which recoils and yields to the violence. It will also 

 be noticed how much the same construction must assist the 

 play of the chest in the operation of breathing. The muscles 

 of respiration enlarge the capacity of the chest by elevating 

 the ribs ; and during the momentary interval of muscular ac- 

 tion, the gristly parts of the ribs, from their great elasticity, 

 restore them to their former position. 



