DIGESTION 139 



sists of a number of minute cells living in a fluid. Yeast has 

 tin- |X)\ver of forming alcohol from sugar, and in the process of 

 brewing the cells act on the malt-sugar in the malt, and from 

 it form alcohol. As the yeast is acting it grows, the cells 

 multiplying by budding off new ones, which themselves rapidly 

 j;nnv and bud in turn. The cells will remain alive after the 

 yeast is dried. If yeast is boiled it is killed, and then it can 

 produce no change. In the manufacture of vinegar advantage 

 is taken of the power of another minute organism to turn 

 alcohol into acetic acid. A large number of processes taking 

 place in nature, such as putrefaction, are also due to organised 

 ferments, minute organisms, or micro-organisms as they are 

 called. 



The unorganised ferments are not, like yeast, living cells or 

 micro-organisms capable of multiplying, but chemical sub- 

 stances produced by living cells, and are capable, even when 

 they are separated from the cells which have produced them, 

 of bringing about certain changes in certain substances. This 

 power they retain for a long time under favourable conditions, 

 and, in producing these changes, are not appreciably used up 

 themselves. Those found in the animal body act best at the 

 temperature of the body, and they lose their power after being 

 boiled. 1'tyalin is an unorganised ferment. It causes starch 

 to take up and unite chemically with -water in such a way as to 

 become changed into sugar. We shall see that nearly all the 

 changes which the food undergoes in the alimentary canal are 

 caused by ferments. 



Structure of the CEsophagus and Stomach. The 

 oesophagus is a tube, the walls of which consist partly of 

 striated partly of plain muscular tissue, lined internally by a 

 mucous membrane which is like that lining the mouth, and in 

 the connective tissue of which are embedded small glands 

 opening into the interior of the oesophagus. The stomach, 

 the walls of which consist of plain muscular tissue lined 

 internally by mucous membrane, may be considered as a 

 dilatation of the alimentary canal. The enlargement is greatest 

 at the left side, forming what is called the cardiac dilatation, 

 because it is near the heart. The right end of the stomat li, 

 where it becomes continuous with the duodenum, is called the 

 pylorus. The muscular fibres of its walls are here circularly 



