8 TEXT-BOOK OF ZOOLOGY 



sumed in the combustion of the animal body. And just as our engine 

 comes to a standstill if the supply of fresh fuel ceases, in the same 

 manner the machine of the animal body would finally come to a stop if 

 fresh fuel were not supplied to it. This supply of fuel is likewise effected 

 by the blood, which contains the materials of construction and combustion in 

 a soluble form. 



These materials penetrate through the walls of the capillaries into 

 the muscles, nerves, etc., replacing therein what has been consumed. If 

 more material is introduced than has been used up, the body grows. 

 But how does the blood obtain possession of these materials ? 



This is effected by the process of feeding and 



7. Digestion. — The food is seized, and frequently also divided up, 

 by means of the jaws (in the vertebrata the jaws always move upward 

 and downward; compare insects), and passes by the mouth into the 

 gullet, or oesophagus, and thence to the stomach. Thence it is conveyed 

 into the intestine, which as a rule consists of a narrower portion (small 

 intestine) and of a wider portion (large intestine), which terminates at 

 the anus. 



It is the task of the digestion to convert the food into such a form as 

 to be capable of entering the blood, and of being carried by it over the 

 whole body. In herbivorous mammals and in man this transformation 

 commences in the mouth, where the starch present in the food is con- 

 verted into sugar by the action of the saliva, which is formed in the 

 salivary glands. The saliva, moreover, has, in addition, the important 

 function of moistening or lubricating the food, so as to render it fitter 

 for deglutition (compare, on the other hand, fishes). In the stomach, 

 by the action of the gastric juice, which is discharged from numerous 

 glands in the stomach walls, the food is not only converted into a pulpy 

 or pap-like mass, but its albuminous constituents also are dissolved. 



Prom the stomach this mass passes into the small intestine. Here a 

 fluid, the bile, is added to it, which is manufactured by the liver and 

 frequently stored in a special sac, the gall-bladder. It is the function of 

 the bile to divide the fats which are taken up with the food into infini- 

 tesimally fine droplets, which are able to penetrate the walls of the 

 intestine. The pancreatic juice, which is furnished by the pancreas, has a 

 similar function, but has the further action of converting starch into 

 sugar, sugar in solution being likewise able to penetrate through animal 

 membranes. 



By the action of all these fluids the food is converted into a condition 

 which renders it capable of being absorbed by the walls of the intestine. 

 In addition to numerous capillaries, narrow tubes, the lymphatic vessels, 

 which absorb the fluid nutriment and convey it to the blood, lie within 



