Fig. 9.—Atlas of Kangaroo.... (From Parker and Haswell's Zoology.)
The second vertebra, which is known as the axis or epistropheus, is a compound structure, the anterior "odontoid process," which fits into the ring of the atlas, being in reality the detached centrum of that vertebra.[[11]] It is a curious fact about that process that it has independently become spoon-shaped in two divisions of Ungulates; that it has become so seems to be shown by the fact that in the earlier types of both it has the simple peg-like form, which is the prevailing form. The cervical
vertebrae are occasionally wholly (Right Whales) or partially (many Whales, Jerboa, certain Edentates) welded into a combined mass. Indications of this have even been recorded in the human subject.
| Fig. 10.—Side view of axis of Dog. × ⅔. o, Odontoid process; pz, posterior zygapophysis; s, spinous process; t, transverse process; v, vertebrarterial canal. (From Flower's Osteology.) | Fig. 11.—Anterior surface of axis of Red Deer. × ⅔. o, Odontoid process; pz, posterior zygapophysis; sn, foramen for second spinal nerve. (From Flower's Osteology.) |
The dorsal vertebrae vary greatly in number: nine (Hyperoodon) seems to be the lowest number existing normally; while there may be as many as nineteen, as in Centetes, or twenty-two, as in Hyrax. These vertebrae are to be defined by the fact that they carry ribs, and the first one or two lumbars are often "converted into" dorsals by the appearance of a small supernumerary rib. The spinous processes of these vertebrae are commonly long, and sometimes very long. It is only among the Glyptodons that any of these vertebrae are fused together into a mass.
The lumbar vertebrae, which follow the dorsal, vary greatly in number. There are as few as two in the whale Neobalaena, as many as seventeen in Tursiops; this group, the Cetacea, contains the extremes. Nine lumbars are found in the Lemurs Indris and Loris. As a rule the number of lumbars is to some extent dependent upon that of the dorsals. It often happens that the number of thoraco-lumbar vertebrae is constant for a given group. Thus the Artiodactyles have nineteen of these vertebrae, and the Perissodactyles as a rule twenty-three. A greater number of dorsals implies a smaller number of lumbars, and of course vice versa. The existence of a sacral region formed of a
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.
| 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
