286 



HUMAN EMBRYOLOGY AND MORPHOLOGY. 



six or more sternebrae being then present. In man the number 

 has been reduced to seven pairs, the sternal ends of the seventh 

 pair lying in front of the base of the ensiform process. It is not 

 uncommon to find the 8 th rib reaching the sternum, especially on 

 the right side; it is rare to find the 7th pair fail to reach the 

 sternum. The more frequent presence of an 8 th sternal rib on 

 the right side is due to right-handedness (Cunningham) or the 

 pressure of the underlying liver requiring support (Tredgold). 



Development of the Sternum. — The sternum is developed in 

 the mesoblast of the ventral median line between the first 8 or 9 

 dorsal segments (Paterson). It is developed in two parts — a 

 right from the right somatopleure, a left from the left somato- 

 pleure. These two halves, the right and left fibrous sternal bars, 

 fuse gradually in the middle line, the process of fusion commencing 

 at the presternum and spreading back (Fig. 232). The bifurcated 

 end of the ensiform process represents the posterior extremities 

 of the sternal bars. 



epi coracoid element 



basis of clavicle 



;~S$k:^^^lis^-2nd rib (chondrified) 



r-^centres of chondrification 



sternal bar 



ventral median line 



Fio. 232.— The Sternal Bars in a human embryo of six weeks (after Paterson). 



The sternum is described here as a structure arising in- 

 dependently in the median ventral line. This, however, is not 

 the commonly accepted view. Euge's researches led him to the 

 conclusion that the segments of the sternal bars were produced as 



