AN INTRODUCTION TO ANIMAL BIOLOGY 47 



In the third lesson we found that an animal body contains 

 water, carbon, gaseous substances, and mineral substances. 

 In order that food may serve for making new body-substance 

 for growth or repair it must contain all these; in fact, all 

 the chemical elements found in the body. 



Dissolved Food. The food which is taken into the 

 mouth of the frog and thence passes through the esophagus 

 into the stomach is largely solid, such as worms and insects. 

 These must be reduced to a liquid condition before they can 

 " soak through " or be absorbed through the stomach lining 

 into the blood. Solid food retained in the stomach cannot 

 fulfil its purpose of supplying the energy and materials for 

 repair, for the reason that energy is being expended and re- 

 pair is necessary, not only in the cells composing the stomach, 

 but also in all living cells of all the organs of the body, even 

 in the remotest parts, such as the fingers and toes. It is 

 evident that solid food cannot be distributed so widely; 

 but a solution (like sugar dissolved in water) can " soak 

 through " the lining of the stomach into tubes (blood-tubes) 

 through which it can flow to distant organs and then "soak" 

 into the cells. Hence we see the necessity of rendering solid 

 food soluble in the water which is taken with food. 



(D) A drop of milk, when examined with the microscope, is 

 found to contain numerous fat droplets. A drop of water containing 

 some starch shows the small particles mixed with the water. Exam- 

 ining with the microscope a drop of water in which sugar or salt has 

 been dissolved, no particles of these substances are evident. 



Milk and the mixture of water and starch are examples 

 of fluids or liquids, but they are not solutions. The particles 

 of starch or fat are not dissolved (i.e., not soluble) in the 

 water. On the other hand, the water-and-sugar mixture 

 is a liquid or fluid, and it is also a solution. It is clear that 

 all liquids are not solutions. This distinction should be kept 

 in mind for use in connection with the changes which foods 

 undergo in the alimentary organs, for in them foods are 



