20 THE REASON WHY: 



&quot; Each night we die, 



Each morn arc born anew : each day a life 

 And shall .ve kilt each day 1 If trifling kills, 

 Sure vice must butcher.&quot; YOUNU. 



AO. How are the strength and lightness of the human body 

 preserved independently of the bones ? 



By the pressure of the atmosphere, consequent on the air-tight 

 character of the bag formed by the synovial membrane (which 

 secretes the oily fluid of the joints), and which is of itself more than 

 sufficient to keep the articulating surfaces of the bones in contract. 



51. This admirable fact is most readily demonstrated by the hip-joint. The 

 round head of the thigh-bone is received into a socket, thus constituting what is 

 familiarly known as a ball-and-socket joint; and all communication between 

 the cavity of the joint and external fluids is cut off by the synovial membrane. 

 The power thus exercised by the atmosphere is about one-fifth greater than would 

 be necessary to support a limb weighing thirty pounds, and the barometer would 

 require to fall twenty-five inches to place the limb and the atmosphere in exact 

 equilibrium. The pressure of the atmosphere on the shoulder joint is capable of 

 supporting a weight nearly twice that of the arm, and the force thus exercised upon 

 the elbow-joint, knee-joint, and highest joint of the fore-finger are respectively 

 six times, nine times, and thirty-five times greater than are requisite for the 

 support of the fore-arm, leg, and finger. 



52. What is the structure of the rib&? 



The ribs are a frame of bones which enclose 

 a hollow space. The lungs and heart are 

 within them. The ribs are fastened in front 

 to a bone called the breast-bone, and are 

 jouied at the oack to the backbone. In front, 

 the rib-bones are joined to the breast-bone by 

 gristle, and this gives them a certain amount 

 of flexibility, and enables them to move more 

 easily when the lungs fill with air. 



53. Why is the skull the only cavity in the body that is not 

 enclosed by a membrane ? 



Because the importance of the brain to life, and the extreme 

 tenderness of its substance, make a solid case more necessary for it 

 than is required for any other part. The skull also completely 

 aurrounding its contents, is calculated not for motion, but solely for 

 defence. 



