BONES, MUSCLES, EXERCISE, AND REST. 249 



bar vertebra. These are in the hollow of the back, between 

 the pelvis and the chest. They are very thick, and have 

 short, club-shaped spinous processes, that extend directly 

 backward. These processes can be felt through the skin. 



597. Between these vertebrae are layers of very tough and 

 elastic cartilage, which adhere very firmly to the bones, and 

 hold them together. These layers are very thick in the loins, 

 and thin in the neck. They are capable of compression and 

 expansion, like India rubber. They may be compressed on 

 one side and stretched on the other at the same time. When 

 we bow, these cartilages are pressed and flattened on the front 

 side, and stretched and thickened on the opposite side. In this 

 way, we bend the back in any direction, and the spinal column 

 is made exceedingly flexible with this succession of joints. 

 This cartilage is very strong, and capable of sustaining great 

 weights. The head, the arms, the chest, and most of the ab- 

 domen rest upon, and are supported by, the spine. All this 

 weight resting upon the back-bone from morning till night, 

 while the body is in an erect position, brings so much pres- 

 sure upon these intefvertebral cartilages, especially those at 

 the lower part of the spine, that they become somewhat flat- 

 tened in the course of the day, and thereby the length of the 

 spine is diminished, and a man is from a half to a whole inch 

 shorter at night than he is in the morning. But as soon as 

 the body is placed in a horizontal position, and the pressure 

 taken from the cartilages, they begin again to expand, and the 

 column recovers its length, and the man regains his ordinary 

 height by morning. Nevertheless, the continued pressure of 

 the weight of the upper part of the body overcomes, in some 

 degree, the elasticity of the cartilages in the course of a long 

 series of years, and a man is consequently somewhat shorter 

 in old age than in his youth. 



598. The bones of the spine are arranged so as to form a 

 column with a double curve, somewhat like the Italic f. At 

 its lower end it is curved oujtward ; as it ^ascends it is curved 

 inward at the loins, and forms the hollow of the back ; again, 

 it is bent outward at the upper part of the back to enlarge 



