The line where two surfaces meet Example: This tetrahedron has 6 edges. It can also be the boundary of a plane shape, such as the circumference of a circle.
1 12 thoracic vertebrae in the upper back corresponding to each pair of ribs. 2  5 lumbar vertebrae in the lower back.  5 sacral vertebrae which are fused together to form 1 bone called the sacrum and 4 coccygeal vertebrae that are also fused together to form the coccyx or tailbone.
In tetrapod anatomy, the sacrum (/ËsaekrÉm/ ËsÃ¦krÉm /or/ ; ËseÉªkrÉm: plural sacra or; Sacrums latin os) sacrum is a, large triangular bone at the base of the spine and at the, upper back part of the pelvic, cavity where it is inserted like a wedge between the two hip. boneshe floor of the groove is formed by the united laminae of the corresponding vertebrae. The laminae of the fifth sacral vertebra, and sometimes those of the fourth, do not meet at the back, resulting in a fissure known as the sacral hiatus in the posterior wall of the sacral canal.