vi.] GENERAL VIEW OF INTERNAL SKELETON. 221 



processes of this kind descend from the vertebral centra and 

 constitute a large proportion of the whole axial skeleton. 



FIG. 189. PART OF THE VERTEBRAL COLUMN OF A SOLE. 



ii, the summit of one of the epaxial arches ; h, the apex of one of the hypaxial 

 arches ; p, one of the paraxial elements. 



These elements are developed beneath the skeletal axis (the 

 centra), and therefore in the line of tissue by which are sus- 

 pended the internal plates of the primitive ventral laminae. 

 They have been therefore called hypaxial. l 



FIG. 190. DIAGRAM OF THE FURTHER DEVELOPMENT OF THE TRUNK AS 



SHOWN IN A SECTION SIMILAR TO THE LAST. 



a, alimentary canal supported by a mesentery 2 formed of the dorsal portion of the 

 inner parts of the split wall of the embryonic ventral laminae ; e, epaxial arch ; 

 /i, hypaxial arch descending in the median line in the root of the inner part 

 of the split wall of the ventral laminae ; /, rib, bifurcating proximally and 

 abutting ventrally against the sternum, which thus completes the paraxial 

 arch ; m, peritoneum, bounding on all sides//, the pleuro- peritoneal space. 



Hypaxial processes may also be developed beneath ver- 

 tebrae to which complete paraxial arches are annexed, as e.g. 

 in the thoracic region of many Birds. 



ii. Are there yet other elements of this hypaxial system 

 present in man's skeleton ? 



1 From ITTO, under (the axial skeleton). 



2 For the explanation of a "mesentery" see Lesson XI. 



