THE BODY WALL, RIBS AND STEENUM. 



285 



posterior. The sternum also becomes wider and shorter. To 

 understand its true nature it is necessary to note the characters 

 of the sternum of a pronograde mammal, such as the dog or ape 

 (Fig. 230). In such the sternum is typically made up of seven 

 segments : 



1. A modified anterior segment, the pre-sternum. 



2. Five narrow, cylindrical segments or sternebrae, forming the 

 body of the sternum. 



3. The ensiform process, a hind segment, complex in nature 

 and ending in the middle ventral line. The ensiform process 

 usually bifurcates and probably belongs to several segments. 



supra-sternal 



"~~ r mterart. cart. 



7th rib 

 8th rib 

 9th rib 



ensiform 



6th seg. „< 



centres of ossific 

 - of 3rd seg. 



(vestigial of '5th 

 & 6th segments) 



ensiform 



Fig. 231. 



Fig. 230. 



Fig. 230.— The Form of Sternum in a Pronograde (quadrupedal) Mammal. 

 Fig. 231.— The Form of Sternum in a Mammal adapted to the orthograde (upright) 

 Posture. The Points of Ossification are also shown. 



The chief changes in the human sternum are : 



1. Each segment has become flat and wide ; 



2. The segments of the body fuse together during the years of 

 adolescence, the fusion beginning behind and passing forwards ; 



3. The 4th sternebra of the body is usually vestigial; is probably 

 made up of two or more fused segments. 



In low primates 8 or 9 pairs of ribs may reach the sternum, 



