30 THE FUNCTIONS OF ANIMALS 



tion with the muscle proteins. When oxygen is available, 

 about one-fifth of the neutraUsed lactic acid is burnt to 

 CO2 and water, and so much energy is obtained by this 

 reaction that the rest of the lactic acid can be resynthesised 

 into glycogen. The process of contraction is therefore 

 much more economical in the presence of oxygen, and 

 fatigue is deferred. 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 : — proteins, 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 p^alin 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 proteins 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 proteins 

 into peptones and peptones into much simpler compounds such as 

 amino-acids ; it continues the work of the salivary juice, changing 



