162 THE STERNUM. 



dimensions of breadth or tliickness. In the sides of it are com- 

 plete pits for the third, fourth, fifth, and sixth ribs, and one 

 half of the pits for the second and seventh ; the lines, which 

 are marks of the former division of this bone, beins extended 

 from the middle of the pits of one side, to the middle of the 

 corresponding pits of the other side. Near its middle an un- 

 ossified part of the bone has sometimes been found ; which, 

 freed of the ligamentous membrane or cartilage that fills it, is 

 described as a hole. When the cartilage between this and the 

 first bone is not ossified, a manifest motion of this upon the 

 first may be observed in respiration ; or in raising the sternum, 

 by pulling the ribs upwards ; or distending the lungs with air, 

 in a recent subject. 



The third bone is much less than the other two, and has 

 only one half of the pit for the seventh rib formed in it ; where- 

 fore it might be reckoned only an appendix of the sternum. 

 In young subjects it is always cartilaginous, and is better known 

 by the name of cartilago-xijphoides or ensiformis, than any 

 other. This third bone is seldom of the same figure, mag- 

 nitude, or situation, in any two subjects ; for, sometimes, it is 

 triangular ; with one of the angles below, and perpendicular to 

 the middle of the upper side, by which it is connected to 

 the second bone. In other persons, the point is turned to 

 one side ; or obliquely forwards or backwards. Frequently it 

 Is nearly of an equal breadth, and often it is bifurcated ; some- 

 times, also, it is unossified in the middle. In the greatest number 

 of adults, it is ossified, and tipped with a cartilage ; in some, 

 one half of it is cartilaginous ; and in others, it is all in a car- 

 tilaginous state. 



The sternum is joined by cartilages to the seven upper ribs, 

 except when the first coalesce with it. It is also articulated 

 with the clavicles. 



It contributes to the formation of the cavity of the thorax, 

 and supports the mediastinum. As a movable fulcrum for the 

 ribs, it assists in respiration ; and it affords origin and insertion 

 to several muscles. 



