CHIEF FUNCTIONS OF THE ANIMAL BODY. 27 
changes of “electric potential” associated with each con- 
traction. Beside muscular movement we must rank ciliary, 
amoeboid, and epithelial movement. Under the last head- 
ing 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, 
carbohydrates, fats, water, and salts, in varying pro- 
portions. 
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 (éxtracellular 
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 Atyalim of the salivary juice, which 
changes starch into sugar. The juice is formed or secreted by various 
salivary glands around the mouth. 
(6) 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, ¢.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 
amino-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 
