138 PHYSIOLOGY FOR BEGINNERS CHAP. 



about by nervous impulses. The sensory nerves of the mouth 

 or the nerves of sight or of smell carry nervous impulses set 

 up in the mouth, the eye, or the nose to the central nervous 

 system ; these eventually reach a particular part of the spinal 

 bulb, and the result is that nervous impulses are sent from the 

 spinal bulb to the glands. These nervous impulses, reaching 

 the cells of the glands, cause them to secrete saliva and at the 

 same time lead to a widening of the small arteries of the 

 glands, so that a larger quantity of blood goes to them. 



Action of Saliva. Make a very thin starch paste by 

 boiling a little starch with water and let it cool. Add to a small 

 portion of this some iodine solution. A blue colour results. This 

 is the test for starch. To some of the starch paste add some 

 of your own saliva, and put it in a warm place at about the 

 temperature of the body. In a short time the mixture will 

 become clear, thin, and watery, and a little later will give no 

 blue colour when the iodine solution is added. This shows 

 that the starch has been changed into something else. If 

 instead of adding iodine you add some caustic soda and a 

 drop or two of sulphate of copper solution and then boil, an 

 orange -red colour and precipitate will result. This is an 

 ordinary test for sugar and shows that sugar is present in the 

 mixture. The saliva has turned the starch into sugar. If a 

 little starch paste be held in the mouth for a few minutes, it 

 will taste sweet. While our ordinary food is in the mouth, some, 

 but only a little, of the starch in the food is changed to sugar. 

 The particular kind of sugar which is formed is the same that 

 exists in malt malt-sugar, as it is called. The saliva has no 

 action on the other food-stuffs. It chiefly serves to moisten 

 the food and so assist mastication. 



Saliva consists of water containing, besides certain salts in 

 solution which make it alkaline, some of the slimy sub- 

 stance, mucus, already mentioned, and a peculiar substance 

 called ptyalin. Mucin and ptyalin do not exist in the blood ; 

 they are made by the cells of the salivary glands. Ptyalin is 

 the substance in saliva which turns starch into sugar. Ptyalin 

 belongs to the class of substances called ferments. 



Ferments. Ferments are divided into two classes the 

 organised and the unorganised. The organised ferments are 

 living organisms, and an example of such is yeast, which con- 



