504 SALIYA [GIL xxxn. 



the sympathetic is mainly vasoconstrictor, but in some animals it 

 does contain a few secretory fibres also. 



Reflex Secretion. Under ordinary circumstances the secretion 

 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, irrita- 

 tion of the stomach has a similar effect. These sensory nerves stimu- 

 late a centre in the medulla from which efferent secretory impulses 

 are reflected along the secretory nerves (chorda tympani, etc.) to the 

 glands. 



Pawlow has recorded some interesting observations on the salivary 

 glands. He made an external fistula of the submaxillary duct in the 

 dog, and found that the sight of food, the smell of food, or the 

 administration of any kind of food, caused secretion; acid or even 

 sand introduced into the mouth produced a similar effect. By means 

 of similar experiments on the parotid secretion, very different results 

 were obtained. If the dog was shown meat, or the meat was given 

 to it to eat, there was practically no secretion. If, however, the meat 

 was given as a dry powder, a copious secretion followed ; dry bread 

 produced a similar effect ; in fact, the parotid secretion flows freely 

 if dry food is simply shown to the animal; of course, in all such 

 experiments the dog must be hungry. 



Such observations emphasize the psychical element involved in 

 secretion, and point out also the adaptation of the secretory process 

 to the needs of the animal ; thus the submaxillary saliva, which is 

 mainly a lubricant in virtue of its mucin, flows whatever the food 

 may be, whereas moist food requiring no watery saliva from the 

 parotid excites the flow of none. 



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 are mixed in 

 the mouth ; the secretion of the minute mucous glands of the mouth 

 and a certain number of epithelial scales and the so-called " salivary 

 corpuscles " derived from the tonsils are added to it. The liquid is 

 transparent, slightly opalescent, of slimy consistency, and may con- 

 tain 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 (21 to 2'5 per cent.). The 



