72 THE HUMAN BODY. 



muscle is called two-bellied or* digastric. Banning along 

 the front of the abdomen, from the pelvis to the chest; 

 on each side of the middle line, is a long muscle, the 

 straight muscle of the abdomen (rectus abdominis) ; it is 

 polygastric, consisting of four bellies separated by short 

 tendons. Many muscles are not rounded, but form wide, 

 flat masses, as those which lie beneath the skin on the sides 

 of the abdomen. 



How the muscles are controlled. Most of the muscles 

 of the body are paired in a double sense. In the first 

 place, to nearly every one answers a corresponding muscle 

 on the opposite side of the body,* its true mate ; in addi- 

 tion, most are paired with, or rather pitted against, an 

 antagonist ; for example, to the biceps muscle (Fig. 29) 

 which lies in front of the humerus and bends the elbow 

 joint, corresponds the triceps muscle which lies behind 

 the arm bone and extends the elbow 5 when the biceps 

 contracts the triceps relaxes, and vice versa. This orderly 

 working is carried out by means of the brain and spinal 

 cord, which, through the nerves, govern the muscles and 

 regulate their activity. In convulsions these controlling 

 organs are out of gear, and the muscles are excited to con- 

 tract in all sorts of irregular and useless ways ; antagonists 

 pulling against one another at the same moment the whole 

 body is made rigid. 



A digastric. Where do we find a polygastric muscle? How is 

 the rectus ubdominis muscle constituted ? Where are flat wide 

 muscles found ? 



In what two ways are muscles paired? Give an example of antag- 

 onistic muscles. What happens to the triceps when the biceps con- 

 tracts? How is the orderly working of the muscles guided and con- 

 trolled? What parts are out of working order in a fit of convulsions? 

 Why do the limbs often become stiff in convulsions? 



'The single mnpcles cross the middle line and are made up of similar right and 

 left halves ; examples are orbicularis oris and the diaphragm. 



