26 THE FUNCTIONS OF ANIMALS. 



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. 



(<:) 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 ; 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 

 said to have slight power of converting starch into sugar ; by its alka- 

 linity it helps the action of the trypsin of the pancreas (which, unlike 

 pepsin, acts in an alkaline fluid) ; it affects cell membranes, so that they 

 allow the passage of small drops of fat and oil ; and it is said to have 

 various other qualities. 



(e) In addition to the liver and the pancreas, there are on the walls 

 of the small intestine a great number of small glands, which secrete a 

 juice which probably seconds the pancreatic juice. The digested 

 material is in part absorbed into the blood, and the mass of food, still 

 being digested, is passed along the small intestine by means of the 

 muscular contraction of the walls known as peristaltic action. It 

 reaches the large intestine, and its reaction is now distinctly acid by 

 reason of the acid fermentation of the contents. The walls of the 

 large intestine contain glands similar to those of the small intestine, 

 and the digestive processes are completed, while absorption also goes 

 on ; so that by the time the mass has reached the rectum, it is semi- 

 solid, and is known as faeces. These contain the indigestible and 

 undigested remnants of the food and the useless products of the chemical 

 digestive processes. 



Absorption. — But the food must not only be rendered 

 soluble and diffusible, it must be carried to the different 

 parts of the body, and there incorporated into the hungry 

 cells. It is carried by the blood stream, and in part also 

 by what are called lymph vessels, which contain a clear 

 fluid resembling blood minus red blood corpuscles. 



Absorption begins in the stomach by direct osmosis into the capillaries 

 or fine branches of blood vessels in its walls, and a similar absorption, 



