AND PHYSIOLOGY. 



77 



reason the jar of any blow is greatly lessened by the same 

 cause. 



169. Use of the Ribs. — Again we see the bones of the 

 thorax arranged for the protection of the enclosed organs. 

 Within this cavity are organs delicate and easily destroyed, 

 but which require elastic and movable walls. The elasticity 

 is easily gained by the cartilaginous portion of the ribs at- 

 tached to the sternum, which yields considerably upon pres- 

 sure ; and the motion and consequent enlargement is effected 

 by the oblique position of the ribs, as they run downwards 

 and forwards from their articulation with the vertebrae. As 

 the vertebral extremity is the fixed point, of course the eleva- 

 tion of the sternal end will enlarge the cavity of the thorax 

 antero-posteriorly. Another use of the cartilaginous extrem- 

 ity of the ribs is to lessen the chance of fracture. The tho- 

 rax is exposed to blows and falls more than many other por- 

 tions of the body, and therefore more exposed to fracture. 

 For instance, if a person suddenly falls to the ground, the 

 head by an instinctive movement is raised, while the trunk or 

 extremities receive the force of the shock. Also the head or 

 extremities can by rapid movements be suddenly removed 

 from the contact with missiles, while the body, comparatively 

 unwieldy, must meet the blow. 



170. Use of the Innominata. — The expanded condition 

 of the Innominatum affords service and protection in different 

 ways. A depression, or cup-shaped cavity is thus made for 

 giving a firm support to the organs contained in the abdomen, 

 as well as a solid foundation to the spinal column. It how- 

 ever renders especial service by furnishing a powerful point 

 of attachment for many of the muscles both above and below : 

 those which form the walls of the abdomen, and many of 

 those which move the extremities. 



171. Why there are so many Bones in the Spinal 



169. Describe the uses of the ribs, and the reason why they are partly made up of car- 

 tilage. Why do they run obliquely from the points of attachment? 170. Explain why 

 the Innominata are so broadly expanded. 



