22 THE FUNCTIONS OF ANIMALS. 



Digestion. 



The energy expended in doing work or in growth is 

 balanced by the potential energy of the food stuffs taken 

 into the body. These consist of proteids, carbohydrates, 

 fats, water, and salts in varying proportions according to the 

 diet of the animal. 1 Oxygen may also be regarded as form- 

 ing part of the food. 



In some of the lower animals, such as sponges, the food 

 particles are directly engulfed by some of the cells with 

 which they come in contact. Within these cells they are 

 dissolved ; this is known as intracellular digestion. In most 

 cases, however, the food is rendered soluble and diffusible 

 within the food-canal by the action of certain ferments made 

 by the cells which line the gut or form the associated 

 glands. The great peculiarity of these fermenting substances 

 is that a small quantity can act upon a large mass of 

 material without itself undergoing any apparent change. But 

 however digestion be effected, it means making the food 

 soluble and diffusible. In a higher vertebrate, there are 

 many steps in the process. 



(a) The first ferment to affect the food, masticated by the teeth and 

 moistened by the saliva, is ihzptyalin of the salivary juice, which changes 

 starch into sugar. The juice is formed or secreted by various salivary 

 glands around the mouth. 



(b) The food is swallowed, and passes down the gullet to the stomach, 

 where it is mixed with the gastric juice secreted by glands situated in 

 the walls. These walls are also muscular, and their contractions churn 

 the food and mix it with the juice. In the juice there is some free hydro- 

 chloric acid and a ferment called pepsin ; these act together in turning 

 proteids into peptones. The juice has also a slight solvent effect on fat, 

 and the acid on the carbohydrates. 



(c) The semi-digested food, as it passes from the stomach into the 

 small intestines, is called chyme, and on this other juices act. Of these 

 the most important is the secretion of the pancreas, which contains 

 various ferments, e.g.) trypsin, and affects all the different kinds of 

 organic food. It continues the work of the stomach, changing proteids 

 into peptones ; it continues the work of the salivary juice, changing starch 

 into sugar ; it also emulsifies the fat, dividing the globules into extremely 

 small drops, which it tends to split into fatty acids and glycerine. 



(d) Into the beginning of the small intestine, the bile from the liver 

 also flows, but this is not of great digestive importance, being rather of 

 the nature of a waste product. It seems to have a slight solvent, emul- 

 sifying, and saponifying action on the fats ; in some animals it has a 

 slight power of converting starch into sugar ; by its alkalinity it helps 

 the action of the trypsin of the pancreas (which, unlike pepsin, acts in a 



