18 THE HUMAN JiODY. 



out. The great opening at the base of the skull connects 

 its cavity with the spinal canal, and is fully an inch in 

 diameter. The other openings are very much smaller 

 than this. 



The spinal canal is also well guarded by its walls, and 

 by bony projections. It contains the spinal cord. 



2. The cavity of the trunk. This is divided by a 

 horizontal partition, called the diaphragm, into two parts. 

 The upper part is the thorax, or chest; and the lower, the 

 abdomen and pelvis. 



The thorax is a bony cage formed by the backbone 

 behind, the ribs at the side, and the breast-bone in front. 

 It contains the heart and lungs. These organs need to 

 be guarded against blows or pressure, but they are not so 

 easily injured in this way as the brain and spinal cord. 

 The thorax is, therefore, not so close a box as the skull. 

 Moreover, it is needful that the walls of the thorax should 

 be movable in order that we may breathe. The thorax 

 is so made that it gives sufficient protection to the organs 

 w T hich it contains, and at the same time it can enlarge 

 and contract. 



The abdomen is not as well guarded as the thorax. 

 There is no bony wall in front; and the intestines are 

 easily wounded, though they bear pressure and displace- 

 ment much better than the heart and lungs. But if the 

 abdomen were walled in front, like the thorax, we could 

 not bend our bodies. For purposes of motion, and to 

 permit the expansion of the intestines after a full meal, 

 they are left partially unprotected. 



The pelvis (Latin, pelvis, a basin) is formed by the hip- 

 bones and the sacrum and coccyx. Its contents are well 

 guarded by those thick bones. 



