248 PRACTICAL ZOOLOGY 



liver in the gall bladder, are added to it. Both the pancreatic 

 juice and bile aid in dissolving the food, and they are helped 

 more or less by secretions from glands in the intestinal wall. The 

 movements of the food through the alimentary canal are due 

 chiefly to what are called peristaltic waves ; the circular muscles 

 in the walls relax ahead and contract behind, thus forcing the food 

 along. 



ABSORPTION. The digested food passes into the cells of the 

 intestinal wall, where it is more or less changed, and then trans- 

 ferred to the blood or lymph. This is the process of absorption. 

 Very little is known about absorption in the frog. In human 

 beings the absorbed food, part of it having passed through the 

 liver, is carried to the heart, whence it is pumped through the 

 body. 



ASSIMILATION. The extraction of the digested food from the 

 blood stream by the cells of the body, and the formation of new 

 protoplasm with it, constitute the process of assimilation. Life- 

 less things cannot grow in this way, but must increase in size by 

 the addition of substances on the outside. 



CIRCULATION. The blood which circulates throughout the 

 body carries the digested food and distributes it to the cells, 

 bears oxygen from the breathing organs to the tissues and carbon 

 dioxide away from the tissues to the breathing organs, and trans- 

 fers waste products from all parts of the body to the excretory 

 organs. 



Blood is a fluid containing red corpuscles and white corpuscles. 

 The red corpuscles owe their color to the presence of hemoglobin. 

 This substance combines with oxygen in the breathing organs 

 and gives it out again to the tissues. The white corpuscles re- 

 semble Ameba in shape and are hence said to be ameboid. They 

 act as scavengers, engulfing foreign bodies such as germs and 

 broken-down tissues that may find their way into the blood 

 stream. 



The discovery that the blood circulates was made by an English 

 physician, William Harvey, in 1621. Since then it has been 



