240 EXPEEIMENTAL GENERAL SCIENCE 



siderable motion in one plane ; still others are able to slide over 

 one another for a short distance. When more extended move- 

 ment is required, the ball-and-socket joint may be found, as 

 at the shoulder and hip. In the elbow and wrist, there is a 

 sort of rolling joint which enables us to turn our palms out- 

 ward. The head is mounted on the apex of the spinal column 

 by a sort of rocking joint and the vertebra supporting the head 

 turns part way around on a pivot. Great freedom of motion 

 is possible at the shoulder from the fact that the pectoral 

 girdle is attached only indirectly to the axis. Strips of carti- 

 lage attach the ribs to the breast bone and thus permit the 

 chest to be expanded in breathing. 



204. Muscles, Tendons and Ligaments. The parts of the 

 skeleton are held together by stout ligaments and the move- 

 able joints are padded with a firm substance called cartilage. 

 All of the spinal vertebrse are separated by pads of this sub- 

 stance. The muscles and tendons also aid in holding the 

 skeleton together, but their chief use is to produce motion. 

 In animals used for food, we recognize the muscles as lean 

 meat and the tendons, ligaments, and cartilage as gristle. 

 A good example of cartilage may be found in the hard parts 

 of the outer ear or at the tip of the nose. The soft and tender 

 muscle cells are bound up in small bundles by a substance 

 called connective tissue and these bundles combined into larger 

 aggregations form the muscles. In a tough piece of steak, 

 the whitish fibers running through it are sections of connec- 

 tive tissue. Toward the ends of the muscle the strands of 

 connective tissue join together forming the stout and glisten- 

 ing white tendons by means of which the pull of the muscles 

 is carried across the joints. In moving the body, the bones 

 and muscles act much like the ropes and arms of a derrick. 

 Each end of a muscle is attached to a different bone and thus 

 when the muscle contracts and shortens, motion of one bone 

 or the other is produced. It is the simultaneous contraction 



