TRANFERENCE OF FOOD MATERIALS 117 



The formula for starch is (C 6 H 10 O 6 ) n , where n is approxi- 

 mately thirty, and the formula for maltose is C^H^Ou, 

 therefore there should be about thirteen intermediate stages 

 between starch and maltose, each varying slightly in physical 

 and chemical properties from the stage on either side of it. 

 By prolonged action in vitro some of the maltose is converted 

 into glucose, but in vivo the hydrolysis is supposed to stop at 

 the stage of maltose. 



The secretion of saliva is regulated by a nervous reflex. 

 That is, the secretion is poured out in response to an impulse 

 passing down a nerve from the central nervous system. The 

 impulse from the central nervous system is the result of a 

 nerve impulse caused by stimulation of the sense organs of 

 taste, smell, sight, etc. Nervous reflexes are used when a 

 quick response is required. As the food does not remain in 

 the mouth, it is obvious that the saliva is required immediately. 



In the glands the constituents that are to form the saliva 

 exist in the form of granules. The granules are formed dur- 

 ing rest and they accumulate in the cells. As the result of 

 activity the granules decrease in number and only a few are 

 left close to the lumen of the duct.* The ptyalin is present 

 in an inactive form called ptyalinogen, which can be rendered 

 active by treatment with dilute acid. Such inactive forms 

 are called zymogens. 



TABLE XXI 

 The composition of saliva is f : 



Water ........ 99 -365 



Ottr orgamc soUi '. \ '. '. !^ } Total organic 0-4, 

 Chlorine ........ o-o$2\ 



Ash 5 insoluble in wate^ '. '. '. Total inorganic 0-225. 



Other inorganic ..... 0-080) 



In addition saliva contains a small and variable amount of Thiocyanate. 



The rate of hydrolysis of starch can be measured in various 

 ways. The time necessary to convert all the products to the 

 stage where they no longer give a colour with iodine can be 

 used for comparison! or the stage at which the blue colour of 

 starch with iodine gives place to the red colour of erythro- 

 dextrin with iodine may be used as an end point. The copper- 



* J. N. Langley, Journ. Physiol., 1879, vol. 2, p. 261. 

 f C. L. Evans, Journ. Physiol., 1912, vol. 44, p. 200. 

 I W. Roberts, Proc. Roy. Soc., 1881, vol. 32, p. 145. 

 J. Wohlgemuth, Biochem. Zeit., 1908, vol. 9, p. i. 



