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, which is flattened on its superior (cranial) and 

 inferior (caudal) surfaces. A neural arch arises from the dorsal side of 

 the centrum and surrounds a vertebral canal. That part of the neural 

 arch which connects with the centrum is the pedicle. A spinous process 

 extends backwards and downwards from the mid-dorsal side of the neural 

 arch. That part of the neural arch between the spinous process and the 

 pedicle is the lamina. Anterior and posterior notches or incisures con- 

 strict the pedicles so that the incisures of two successive vertebrae form 



