number of vertebrae fused together and supported by the pelvic girdle is characteristic of the mammals, but is not found in the Cetacea and the Sirenia, where functional hind-limbs are wanting. Strictly speaking, the sacrum is limited to the two or three vertebrae whose expanded transverse processes meet the ilia. But to these are or may be added a variable number of vertebrae withdrawn from both the lumbar and the caudal series, which unite with each other to form the massive piece of bone which constitutes the sacrum of the adult.
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Fig. 12.—Lepus cuniculus. Innominate bones and sacrum, ventral aspect. acet, Acetabulum; il, ilium; isch, ischium; obt, obturator foramen; pub, pubis; sacr, sacrum; sy, symphysis. (From Parker and Haswell's Zoology.) |
Fig. 13.—Anterior surface of fourth caudal vertebra of Porpoise (Phocoena communis), × ½. h, Chevron bone; m, metapophysis; s, spinous process; t, transverse process. (From Flower's Osteology.) |
The caudal vertebrae complete the series. They begin in as fully developed a condition as the lumbars, with well-marked transverse processes, etc.; but they end as no more than centra, from which sometimes tiny outgrowths represent in a rudimentary way the neural arches, etc. Very often the caudal vertebrae are furnished with ventral, generally V-shaped, appendages, the chevron bones or intercentra.[1] These are
- ↑ Intercentra are but rarely met with anterior to the caudal series. Mr. Parsons has, however, recorded their occurrence in the lumbar vertebrae of Atherura.
