THE PHYSIOLOGY OF AN ANIMAL 205 



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changes under the influence of the separate digestive fluids, 

 those secreted by the stomach producing a different kind of 

 digestion from those of the pancreas; but all aid in rendering 

 the food soluble. 



Absorption. The food is driven through the alimentary canal 

 by the muscular contractions of its walls. These muscles are in 

 two sets, one extending lengthwise and the other running around 

 the intestine in a circular direction. By their contraction waves 

 of constriction pass along the intestine, forcing the food slowly 

 along. This peculiar writhing motion of the intestine is spoken 

 of as peristalsis (Gr. peri = around + stalsis = a compression). 



As the food is pushed through the intestine its digestion and 

 solution is completed and it begins to pass through the walls 

 of the intestine into the surrounding blood vessels. As the 

 intestinal contents pass onward more and more of the nutriment 

 contained in the food is absorbed from it and enters the blood. 

 The undigested and useless portions of the food pass on and even- 

 tually, in the form of faeces, are voided through the cloacal 

 opening. 



Circulation. The food absorbed into the blood is now carried 

 over the body in the blood. The liquid part of the blood, the 

 plasma, is the circulating medium, the red and white corpuscles 

 having special functions. The red corpuscles (erythrocytes) , 

 which are by far the most numerous, give the blood its red color 

 and are associated with respiration. The white corpuscles (leu- 

 cocytes), of which there are several kinds, have various functions, 

 one of which is the removing of foreign bodies from the body 

 and protecting it from the attacks of microscopic germs, or other 

 irritating substances that may enter the tissues. The white 

 corpuscles with this power are called phagocytes (Gr. phagein = 

 to eat + cytos) ; they are able to leave the blood vessels, by 

 forcing their way through the walls of the capillaries; Fig. 

 101 leu. They then migrate among the tissues and collect at 

 any part of the body to guard it from an attack. 



The blood is kept circulating through the vessels by means of 



