102 PHYSIOLOGY 



as it is bathed by the blood then, and its chemical consti- 

 tution remains unchanged, the heart will continue its 

 contractions indefinitely. 



The excess of carbon dioxide that occurs at regular 

 intervals in the blood is possibly the stimulus which 

 incites the respiratory muscles to rhythmical activity. 



The movement of the muscles in the wall of the ali- 

 mentary canal by which the contents of the canal are con- 

 tinually urged onward is also apparently spontaneous, 

 but the presence of any foreign substance in the intes- 

 tines stimulates its muscular walls to activity. The 

 presence of the food then is an exciting cause. 



Influence of the Structure of Muscles. — The impor- 

 tant thing about the muscles is that through their 

 contraction they bring about the complex movements of 

 higher organisms. Each muscle is capable of a single 

 movement. It changes its shape in accordance with the 

 arrangement of its fibers, or cells, which grow shorter 

 during contraction. In the muscle that bends the arm, 

 the fibers lie side by side so that when they contract 

 the muscle grows shorter, but in the diaphragm they 

 are the radii of a circle and when they contract the 

 circle grows smaller. 



Influence of Arrangement of Muscles. — Complex 

 movements are produced by the co-operative activity of 

 a number of muscles. The character of the movement 

 depends upon the shape and the arrangement of the mus- 

 cles in the body of the animal. In an earthworm one set 

 of muscles runs lengthwise and another encircles the 

 animal. By the alternate contraction of the two sets 

 the worm creeps over the ground. In a sea anemone the 



