. Chordate anatomy. Chordata; Anatomy, Comparative. THE SKELETAL SYSTEM 149 on both anterior and posterior sides and with the centrum of the atlas fused with the axis, as in mammals. The vertebral column of mammals shows little advance beyond that of reptiles. A few Insectivora have intercentra in the lumbar region— a diplospondylous condition reminiscent of elasmobranchs. Parapophyses are reduced to shallow pits for articulating the heads of the ribs. The human spine differs little from that of other mammals, except that the tail is reduced to a COCC50C with a few variable muscles attached. M
. Chordate anatomy. Chordata; Anatomy, Comparative. THE SKELETAL SYSTEM 149 on both anterior and posterior sides and with the centrum of the atlas fused with the axis, as in mammals. The vertebral column of mammals shows little advance beyond that of reptiles. A few Insectivora have intercentra in the lumbar region— a diplospondylous condition reminiscent of elasmobranchs. Parapophyses are reduced to shallow pits for articulating the heads of the ribs. The human spine differs little from that of other mammals, except that the tail is reduced to a COCC50C with a few variable muscles attached. Man's only distinctive feature is the sigmoidal curve, which bends his spine in two directions, instead of one only as in other creatures. In addition to the two main spinal curvatures, thoracic and lumbar, man has two lesser curvatures, cervical and sacral, in the region of the neck and sacrum respectively. The Vertebral Column in Man. In the backbone of a child there are thirty-three vertebral elements. During growth the last nine fuse to form. Fig. 139.—Diagrammatic sagittal sections of (A) amphicoelous; (B) procoelous; (C) opisthocoelous; and (D) amphiplatyan vertebrae. The head is supposed to be at the left. Cut surfaces obliquely lined. (After Kingsley modified.) two adult bones, the sacrum and the coccyx. The other twenty-four vertebrae remain separate throughout life and become differentiated into seven cervical vertebrae, twelve dorsal or thoracic, and five lumbar. These are sometimes called "true" vertebrae in contra-distinction from those of the sacrum and coccyx which are called "false" vertebrae. Although the vertebrae are separate bones, they are nevertheless so firmly fastened together by ligaments and fibrous cartilages as to make the backbone a fairly rigid column. Four curvatures appear in the adult— cervical, thoracic, lumbar, and sacral. The Structure of a Vertebra. A typical vertebra consists of a cylin- drical body, the centrum, wh
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