316 PHYSIOLOGY AND HEALTH. 



727. This erect position is particularly attended to by 

 those who wish to exert the most effective force, and to strike 

 the heaviest blows. Soldiers, who endure fatiguing marches 

 and fight with the greatest energy, are especially directed to 

 maintain the erect attitude. So the prize-fighters are taught 

 to stand in the struggle; their back is straight, their shoul- 

 ders thrown back, and breast forward ; then their arms are 

 completely under their control. The best public speak- 

 ers and readers stand in the same position. Their chests 

 are free to expand, and they inhale large portions of air; the 

 head is erect, and the windpipe is not compressed ; then the 

 air can be thrown forcibly from the lungs, the voice is full, 

 and the articulation easy and effective. 



728. The healthy spine, in its natural position, curves 

 from front to back and from back to front, but not from side 

 to side. When left to itself, it assumes this shape ; yet its 

 structure of alternate bones and cartilages allows the column 

 a great variety of motions and positions; but the elasticity of 

 the cartilages tends tocestore it to its natural direction, after 

 having been bent to either side. So that, after leaning and 

 bending the spine to the left or to the right, by force of the 

 muscles, when these cease to act, the side of the cartilages 

 which was flattened springs upward, and throws the spine 

 again into its natural position. But if the column be turned 

 to one side frequently, and continued for a long period, and at 

 the same time be pressed down by a weight, the compressed 

 side of the cartilage wanting opportunity to regain its usual 

 form, will become permanently thinner, and the opposite side 

 thicker; and then the whole pile would, without any external 

 aid, remain curved. 



729. This great flexibility of the spinal column allows it 

 to bend in any direction , and, for a short period, without 

 danger of permanent curvature, if a/terwards the upright pos- 

 ture be assumed and maintained. But, if we continue these 

 positions for a long time, the spine does not easily recover its 

 proper shape. If the student bends his back and leans his 

 head down, to bring the eyes nearer his book, if the seam- 



