THE BACK-BONE. 



limbs. Its central part, which bears 

 all the rest, is a stout, bony pillar, the 

 back-bone, c, e, on the top of which is the 

 skull. 



7. The Back-Bone, Vertebral Column, 

 or Spine, is represented in side view in 

 Fig. 4. Its upper part is made of 

 twenty-four short thick bones piled 

 one upon another, and each called a 

 vertebra. Between each pair of verte- 

 brae there is placed during life an elas- 

 tic cartilaginous cushion. The lower 

 part of the spine consists of two 

 bones; a large one, the sacrum, extend- 

 ing from Si to Coi; and a much small- 

 er, the coccyx, reaching from Coi to the 

 end. 



Projecting from the back of each 

 vertebra (to the right in the figure) 

 is a bony bar, called its spinous process. 

 Through the skin along the middle of 

 the back we can feel the tips of these 

 processes, and it is their presence which 

 has given the name spinal column to 

 the whole. 



A canal runs through the whole back- 

 FIG. 4 . -The spinal bone except the coccyx, and opens into 



column viewed from the , T i 



left side. Ci-7, the ver- the skull-chamber above. It is the 



tebraeof the neck; Z>i-ia, ,./** \ 

 the vertebrae behind the lower part of the dorsal cavity (a, ^ Ig. l), 

 thorax. L 1-5, the verte- 

 brae of the loins; Si to and, as we have already learned, con- 



Coi, the sacrum; 01-4, 



the coccyx. tains the spinal cord. 



7. What other names has the back-bone ? Divisions of its upper 

 part ? Lower part ? What is the spinous process ? The dorsal cavity ? 



