THE SKELETON AND THE MUSCLES 



289 



But she has not wasted the space in the interior of the 

 bones; they are filled with marrow, a substance which does 

 not seem to have any particular use, but is really an es- 

 sential organ of the body, for it forms 

 the greater part of the red and white 

 corpuscles of the blood. 



We commonly have a few more than 

 200 bones in the body, the number 

 being somewhat variable because some 

 bones that are separate in youth be- 

 come fused together in later life. In 

 the skull many of the bones of the cra- 

 nium, or part enclosing the brain, are 

 united by sutures which dovetail to- 

 gether in such a way as to prevent 

 them from becoming separated while 

 at the same time affording a place where 

 the bones can grow and thus give more 

 space for the growing brain. As the 

 bones of the head are very rigid, if they 

 were fused together at the sutures the 

 head could no longer enlarge. Growth 

 takes place at the edges of the sutures. 



The central axis or backbone of the 

 skeleton (spinal or vertebral column) is 

 composed of 24 bones, the vertebra. 



' 



separated from one another by elastic column, 

 pads of cartilage. Inside of a canal 



coc.- 



. FlG> 2 8 -~ Sid< : 



view of spinal 



cv, cervi- 



_ 



running through the vertebrae is the Coc -' coccygeai ver-. 

 spinal cord which connects with the 

 brain above and sends off nerves between the vertebrae to 

 nearly all parts of the body. Projections or processes of 

 the vertebrae give attachments to ligaments and the 

 muscles of the back. 



19 



