138 Basic Structure of Vertebrates 



they are the characteristic skeletal parts of the lateral body-wall 

 (Figs. 100B, 130). 



The sternum is commonly classed with the ribs because, except 

 in amphibians, it develops in close relation to the ventral ends of the 

 ribs and is possibly a derivative of the ribs. But, situated as it is, as 

 far away from the vertebral column as possible, its only claim to being 

 assigned to the vertebral axis seems to be the fact that it is attached 

 to the ribs and the ribs are attached to the vertebral column. By 

 analogy then, the hindlegs would be "axial/' for they are attached to 

 the pelvic girdle and the girdle is attached to sacral vertebrae! The 

 fact is that the sternum establishes a midventral axis of its own. While 

 it is a minor axis, it is an important one, for the sternum serves for 

 attachment of a bilaterally arranged system of muscles, including the 

 especially important pectoral muscles which operate to move the 

 pectoral appendages downward. In most mammals the sternum is a 

 longitudinal series of segmentally arranged elongated bones jointed 

 together, the whole being suggestive of a simplified and miniature 

 replica of the vertebral column (Fig. 112E). Structurally and func- 

 tionally, it constitutes a very definite ventral or sternal axis. 



If it is desirable, for descriptive purposes, to devise some scheme 

 calculated to indicate the axial relations and regional differentiation 

 of the vertebrate skeleton, the following arrangement, based on exist- 

 ing structure rather than on more or less dubious phylogenetic origins, 

 is suggested. 



1. Skeleton of the Notoehordal Axis (Dorsal Parietal) 



Vertebral Column 



Primary Cranium \ 



2. Skeleton of the Viseeral Axis [Skull 



Visceral Arches j 



3. Lateral Parietal Skeleton 



Ribs 



4. Skeleton of the Sternal Axis (Ventral Parietal) 



Sternum 



Interclavicle (Episternum) 



5. Appendicular Skeleton 



