CHIEF FUNCTIONS OF THE ANIMAL BODY. 27 



liberation of CO 2 . Besides the chemical change and the 

 change of shape, there are also changes of " electric poten- 

 tial " associated with each contraction. Beside muscular 

 movement we must rank ciliary, amoeboid, and epithelial 

 movement. Under the last heading are included active non- 

 amceboid contractions and expansions of covering cells. 



Digestion. The energy expended in work or in growth is 

 balanced by the energy of the food-stuffs : proteids, carbo- 

 hydrates, fats, water, and salts, in varying proportions. 



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

 particles are engulfed by certain cells with which they come 

 in contact, and digested within these cells (intracellular 

 digestion). In most cases, however, the food is digested 

 within the food canal^ by ferments made by the secretory 

 cells of the gut or of associated glands. The peculiarity 

 of these ferments is that a small quantity can act upon 

 a large mass of material without itself undergoing any 

 apparent change. However digestion be effected, it means 

 dissolving the food and making it diffusible. In a higher 

 vertebrate there are many steps. 



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

 moistened by the saliva, is the ptyalin 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 

 hydrochloric 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 intestine, 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 and peptones into much simpler compounds such as 

 ammo-acids ; 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 saponify or split into fatty 

 acids and glycerine. 



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

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

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

 emulsifying, and saponifying action on the fats ; in some animals it is 



