BONES, MUSCLES, EXERCISE, AND REST. 311 



The uppermost bend the head. They-serve also to keep the 

 head and the back in their erect position. 



717. The muscles on the opposite sides of the spine are 

 naturally of the same size and length, and equally strong. 

 They give to each side of the back the same support, so long 

 as they are accustomed to the same amount of exertion. 

 But if we allow the back to bend to one side, the muscles 

 within the curve will be shortened, and those on the outside 

 will be lengthened. The muscular action is then increased 

 on the convex, and diminished on the concave side ; for the 

 muscles on the outside of the arch are obliged to exert a 

 constant and much greater force to prevent the further curv- 

 ing of the spine than was necessary merely to keep it bal- 

 anced in the erect position ; while very little action is 

 required of those which are on the opposite side and within 

 the curve. 



718. The structure of the back-bone gives it great strength 

 as well as flexibility. Being composed of alternate bones and 

 cartilage, and held together by strong ligaments and supported 

 by many muscles, it is capable of bearing great burdens. 

 All the upper parts of the body, more than half of its weight, 

 and all the burdens that we carry on the head, the shoulders, 

 the back, and the arms, rest upon it. Porters who are long 

 trained to their business will carry upon 'their shoulders', or 

 upon their heads, a burden of some hundreds of pounds. The 

 Turkish porters in Smyrna, Asia Minor, carry e:iormous loads 

 on their backs. A friend who has been there writes to me, 

 u The porters in Smyrna have a pack on their backs, about 

 twenty inches wide, flat on the outside, so that the load lies 

 on it steady without fastening. I saw one take; a box of Ha- 

 vana sugar on his back, to carry from the boat up to the 

 warehouse, a short distance from the water. The sugar 

 weighed four hundred and fifty pounds, or more. Captain N. 

 of the navy said to him, ' You had better add a bag of coffee,' 

 which weighed one hundred and thirty pounds. The porter 

 said, ' Put it on,' which he did. He, the porter, then turned 



