THE HUMAN STERNUM 5 



to expand the chest well and keep the halves separate. The expansion of 

 the thoracic viscera, and the tendency of the curved ribs to straighten them- 

 selves, should oiFer every inducement to the occurrence of a cleft sternum. 

 On the other hand, fissura sterni may possibly be explained on other 

 grounds. May it not be caused by an abnormal ratio of growth of the 

 thoracic viscera and the chest walls at an early period, which may cause a 

 stretching of the embryonic pre-chondral tissue out of which the sternum is 

 normally produced ? This in consequence does not chondrify or ossify, 

 but instead gives rise to the membrane described as connecting together the 

 two hemisterna or the ventral ends of the sternal ribs, in cases of fissured 

 sternum. In the embryo chick, where the sternum is in two halves at first, 

 there is a genuine ectopia cordis, and the two halves of the sternum are 

 united by a thin layer of embryonic mesenchyme. 



