22 THE ENGINES OF THE HUMAN BODY 



know the reason why it is so very tiring to sit bolt upright, 

 and to do that we must know something of the wonderful 

 machinery of the backbone or spine. We shall see that 

 it is provided with scores of levers, and each lever has 

 two or more little living engines which move and 

 work it. 



In the walking skeleton (Plate I.) the backbone is seen 

 to rest on the pelvis ; above, it has the skull mounted on 

 it ; the ribs are also attached to it, twelve on each side. 

 In a skeleton, the backbone appears to keep upright 

 without the help of muscles, but that is only because a 

 stiff iron rod has been thrust along it, thus fixing together 

 the twenty-four blocks or vertebrae, which are built up 

 on it, one above another. In life each segment or verte- 

 bra is free to move, and hence has to be balanced by 

 means of muscles. That is why our bodies can be bent 

 backwards and forwards or from side to side or twisted ; 

 in old age the vertebrae often become joined together, and 

 then we walk stiffly and move with great difficulty. 



We can easily imagine how the backbone or spinal 

 column is made up if we take from a child's play-box 

 twenty-four wooden cubes and build them up in a single 

 pile or column. We must place the larger blocks at the 

 foot of the column and the smaller ones at the top, but even 

 then we shall have a difficulty in preventing the pile from 

 toppling over. To a column thus built up we must add 

 something more if it is to resemble the spine of our back. 

 It has to be buffered. Railway engineers soon discovered 

 that when they joined railway coaches together to form 

 trains they bumped so hard against each other when the 

 engine slowed down or stopped that the passengers who 

 rode in those primitive trains were badly shaken. Hence 

 they invented buffers which take up the shock when 

 railway coaches are forced against each other, and thus 

 passengers are able to ride in comfort. That is what 

 Nature did to our spines. She set between each pair of 

 blocks or vertebrae which make up the train of the spine 

 a very wonderful kind of buffer, which not only absorbs 

 shocks but binds the vertebrae so stoutly together that 



