SALIVA 47 



or enzymes, and to that special class of unorganised ferments which 

 are called amylolytic (starch splitting) or diastatic (resembling 

 diastase, the similar ferment in germinating barley and other grains). 

 A general description of ferments wiU be found at the end of this 

 lesson. 



The starch is^ first split into dextrin and maltose ; the dextrin is 

 subsequently converted into maltose also : this occurs more quickly 

 with erjiihro-dextrin, which gives a red colour with iodine, than with 

 the other variety of dextrin called achroo-dextrin, which gives no 

 colour with iodine. Brown and Morris give the following equa- 

 tion : — 



10(C6H,o05), + ^nR^O 



[starch] [water] 



= 47iC.2H220n + (CeH,o0.5)„ + (CgHjoOs), 



[maltose] [achroo-dextrin] [erythro-dextrin] 



Ptyalin acts in a similar way, but more slowly on glycogen ; it 

 has no action on cellulose ; hence it is inoperative on uncooked 

 starch grains, for in these the cellulose layers are intact. 



Ptyalin acts best at about the temperature of the body (35-40°), 

 and in a neutral medium ; a small amount of alkali makes but 

 little difference ; a very small amount of acid stops its activity. 

 The conversion of starch into sugar by saliva in the stomach con- 

 tinues for 15 to 30 minutes ; the hydrochloric acid which is poured 

 out by the gastric glands firet neutralises the saliva and combines 

 with the proteids in the food ; but immediately free hydrochloric 

 acid appears the ptyalin is destroyed, so that it does not resume work 

 even when the semi-digested food once more becomes alkaline in the 

 duodenum. 



FERMENTS 



The word fermentation was first appHed to the change of sugar 

 into alcohol and cai'bonic acid by means of yeast. 

 The evolution of carbonic acid causes frothing 

 and bubbling ; hence the term ' fermentation.' 

 The agent yeast which produces this is called 

 the ferment. Microscopic investigation shows 

 that yeast is composed of minute rapidly growing 

 unicellular organisms (torula?) belonging to the 

 fungus group of plants. ^"£^^^'offi 



The souring of milk, the transformation of *"«&' between which 



"^ are fome bacteria. 



urea into ammonium carbonate in decomposing (Yeo's'Phyaoiogy.';^ 

 urine, and the formation of vinegar (acetic acid) from alcohol are pro- 

 duced by the growth of verj- similar organisms. The complex series 



