THE HUMAN STERNUM 47 



These somewhat minute difFerences in the form of the bone, and its 

 relations to the costal cartilages and the clavicles — difFerences that appear for 

 the most part to be caused by influences operating after birth — by their very 

 minuteness serve to emphasize the essential character of the sternum as a bone 

 of a fixed and determined type not subject to large variations or much 

 affected by changes in correlated structures. It particularly impresses 

 one with the idea of the secondary relation of the ribs to the sternum, the 

 former being subject to very considerable variations without producing any 

 appreciable alteration in the sternum. 



(d?) Shape of the Metasternum. 



The shape of the metasternum is extremely variable. Without 

 describing in detail these variations, it is enough to point out that it is 

 normally median in position, and is usually not bifurcated at the extremity 

 (four hundred and twenty-nine cases == 57-8 per cent.) It is bifid in a 

 minority of pases (three hundred and thirty-four cases = 42 per cent.) 

 The constancy of its median position, and the constant occurrence of a 

 single median centre of ossification, lend support to the idea of the formation 

 of the sternum in the middle line, rather than by fusion of lateral structures 

 derived from costal cartilages. 



Summary 

 Surveying the foregoing facts, one has a clear picture of a series of 

 conditions caused by various influences (of which right-handedness is 

 probably the most important), and giving rise, in many cases, to a slightly 

 asymmetrical form of sternum. The right costal catilarges have a firmer 

 adhesion to the sternum than the left. At the ends of the series this is 

 most obvious : the first right costal cartilage has oftener a larger and 

 deeper connexion with the presternum than that on the left side ; the 

 left seventh costal cartilage is more often detached from the sternum than 

 the right ; and the right eighth costal cartilage is more often adherent than 

 the left. Coincidently there is a greater tendency on the left than on the 

 right side for a downward displacement of the costal attachments. These 

 conditions are, probably, the cause of lateral curvature of the sternum, with 



