THE HUMAN STERNUM 43 



is that they are deficiencies in the primitive cartilaginous sternum, owing to 

 the failure of the praechondral tissue to form cartilage on account of the 

 presence or formation of a bunch of vessels with its surrounding envelope 

 of connective tissue in some part of the tissue in which the cartilage is being 

 laid down. This view is supported by the occurrence of such a condition 

 in an early foetal sternum (p. 19). One cannot ascribe the formation of 

 the foramina to the failure of the centres of ossification to unite together ; 

 nor can any special morphological significance be attached to their presence. 

 One rejects the idea of sternal foramina being associated with the 

 occurrence oi fissura slerni ; the causes of the two conditions seem to be 

 distinctly different. 



(c) Asymmetry of the Sternum (Plate IX, Fig. 61). 



This condition is not uncommon. It is shown in curvature of the 

 long axis of the bone, obliquity in the pre-mesosternal junction, inequality 

 in position and number of costal attachments, and in differences in the 

 position and form of the clavicular facets. 



The foetal sternum is itself usually straight, and is asymmetrical 

 only by reason of occasional irregularities in the position and number 

 of costal attachments. The causes of asymmetry appear to operate for 

 the most part after birth ; and the chief causes appear to be obliquity 

 in the union of the ossifying centres, right-handedness (or the reverse) ; and 

 the asymmetrical position of the thoracic and abdominal viscera. 



I. Irregularity and Obliquity in the Union of the Component Elements of 

 the sternum is one of the commonest causes of curvature, and of asymmetry 

 in the costal attachments. The most notable illustration of this condition is 

 found at the junction of the presternum with the mesosternum (Table XVI). 

 This occurs in 30*3 per cent, of all sterna examined. The obliquity of 

 junction is most often downwards and to the left (left, 17*0 per cent. ; 

 right, 1 3*2 per cent.), so as to produce a convexity to the right, and a down- 

 ward displacement of the second left costal cartilage. It is more frequent 

 with advancing age (Table XVI), showing that it is caused not merely 

 by the shape of the sternal elements, but by influences such as right- 

 handedness, operating after completion of the individual elements. 



