CH. xxx.] SECRETORY NERVES. 477 



When secretory nerves are stimulated, the main result is 

 secretion leading to a diminution of the granules in the cells. 

 The accompanying vascular condition determines the quantity of 

 saliva secreted. Electrical changes also accompany secretory 

 activity. A rise of temperature is stated by some to occur, but 

 if this is the case it is very slight, so that some observers have 

 not been able to detect it. 



Reflex Secretion. Under ordinary circumstances the secre- 

 tion of saliva is a reflex action. The principal afferent nerves are 

 those of taste; but the smell or sight of food will also cause "the 

 mouth to water ; " and under certain circumstances, as before 

 vomiting, irritation of the stomach has a similar effect. These 

 sensory nerves stimulate a centre in the medulla from which 

 efferent secretory impulses are reflected along the secretory nerves 

 (chorda tympani, &c.) to the glands. 



Extirpation of the Salivary Glands. These may be 

 removed without any harmful effects in the lower animals. 



The Saliva. 



The saliva is the first digestive juice to come in contact with 

 the food. The secretions from the different salivary glands differ 

 somewhat in composition, but they are mixed in the mouth ; the 

 secretion of the minute mucous glands of the mouth and a certain 

 number of epithelial cells and debris are added to it. The so- 

 called ' salivary corpuscles ' are derived from the glands them- 

 selves or from the tonsils. 



On microscopic examination of mixed saliva a few epithelial 

 scales from the mouth and salivary corpuscles from the salivary 

 glands are seen. The liquid is transparent, slightly opalescent, 

 of slimy consistency, and may contain lumps of nearly pure 

 mucin. On standing it becomes cloudy owing to the precipitation 

 of calcium carbonate, the carbonic acid, which held it in solution 

 as bicarbonate, escaping. 



Of the three forms of saliva which contribute to the mixture 

 found in the mouth the sublingual is richest in solids (2-75 per 

 cent.). The submaxillary saliva comes next (2-1 to 2^5 per cent.). 

 When artificially obtained by stimulation of nerves in the dog the 

 saliva obtained by stimulation of the sympathetic is richer in 

 solids than that obtained by stimulation of the chorda tympani. 

 The parotid saliva is poorest in total solids (0*3 to 0-5 per cent.), 

 and contains no mucin. Mixed saliva contains in man an average 

 of about 0*5 per cent, of solids : it is alkaline in reaction, due to 

 the salts in it; and has a specific gravity of 1,002 to 1,006. 



