294 ESSENTIALS OF PHYSIOLOGY. 



Or it may be epitomised in the formula 



(Ci 2 H 20 10 ) 100 + 80H 2 = 80C 12 H 22 O n + (C 12 H 20 10 ) 20 . 

 Starch Maltose Achroo-dextrin. 



Evans' method of determining the amylolytic power of saliva is based 

 upon a calculation of the amount of maltose formed from a given quantity of 

 starch in a definite time. 5 c.c. of mixed saliva are diluted to 50 c.c. with 

 distilled water, and the mixture is filtered. 3 c.c. of the diluted saliva are 

 added to 50 c.c. of a 3 per cent, solution of neutral soluble starch, which is at 

 a temperature of 46 C. Digestion is allowed to proceed at 46 C. for ten 

 minutes, and is then stopped by the addition of a little sodium hydrate. The 

 copper-reducing power is then determined, and the amount of maltose formed 

 is calculated. 



The digestive action of ptyalin on starch is most energetic in a 

 neutral medium. Hence it is favoured by the addition of a trace of 

 acid to normal alkaline saliva. Like other ferment processes, it is 

 arrested by a high temperature, and ptyalin itself is destroyed by the 

 slightest excess of hydrochloric acid, even less than is contained in 

 gastric juice. The optimum, temperature for the action of ptyalin is 

 46 C. When a meal is taken, it forms a fairly compact mass in the 

 stomach, but is gradually penetrated by the gastric juice; as this 

 penetration takes place, the ptyalin is gradually destroyed, but about 

 half an hour elapses before salivary digestion in the centre of the 

 mass is finally terminated. 



THE SECRETION OF SALIVA. 



There is a constant production of saliva in sufficient quantity to 

 keep the buccal mucous membrane moist, but in certain circumstances 

 the flow is largely increased. The conditions which are followed by 

 more profuse production of saliva are (1) the presence of food in the 

 mouth, and (2) the sight, smell, or thought of food. Food in the mouth 

 is followed so promptly by the increased salivary flow that it must 

 necessarily produce its effect through the nervous system. The second 

 group of conditions likewise obviously produce their effect by means of 

 a nervous mechanism. It is clear, therefore, that the secretion of 

 saliva is brought about by a reflex mechanism, the afferent nerves 

 usually being those connected with the buccal mucous membrane, and 

 the efferent nerves those passing to the various salivary glands. 



Each salivary gland has a double nerve supply. The chorda 

 tympani nerve is distributed to the submaxillary and sublingual glands 

 (fig. 115), and the parotid receives a branch from the auriculo-temporal 

 nerve. In addition, each gland receives fibres from the sympathetic 

 system. If a cannula is placed in ; the duct of the submaxillary gland 

 and the chorda tympani is divided, no flow of saliva is observed. But 



