30 SECRETION 



secretion by stimulating the cerebral fibers in the gland. Nico- 

 tine causes a slight flow of saliva, followed by a paralysis. 

 The drug acts upon the end brushes of both the cranial and 

 sympathetic fibers in the superior cervical and submaxillary 

 ganglia. 



Mechanism of Saliva Secretion. The flow of saliva is normally 

 a reflex, the afferent path of which is formed by fibers of the 

 lingual and glossopharyngeal nerves. The centre is said to 

 lie in the medulla near the nuclei of origin of the seventh and 

 ninth nerves. The efferent path for the submaxillary and 

 sublingual glands is along the seventh, the lingual, and chorda 

 tympani nerves. For the parotid the efferent path lies along 

 the ninth cranial, the nerve of Jacobson, the small superficial 

 petrosal, otic ganglion and auriculotemporal branch of the 

 inferior maxillary division of the fifth cranial. Section of these 

 paths prevents the normal reflex in spite of the fact that the 

 sympathetic remains intact. No satisfactory explanation of 

 the functions of the sympathetic secretory fibers has been 

 given. 



The reflex mechanism of salivary secretion is very easily 

 set into action. The contact of food with the buccal mucous 

 membrane, the movement of the jaws, or even tasteless objects 

 like India rubber may cause a secretion. The smell, sight, 

 or thought of food may be very effective in making the " mouth 

 water/' in which case the result is a so-called psychical secretion. 

 Saliva is often reflexly excited through stimulation of the 

 gastric branches of the vagus. Likewise, through stimula- 

 tion of sensory fibers like the splanchnic or sciatic. 



It has been definitely established that a remarkable adapta- 

 tion exists between the character of the stimulation applied 

 to the mouth and the character of the resulting saliva. When 

 dry food is given to a dog there is poured out an abundant watery 

 saliva; with moist food the flow is much less. Pebbles placed 

 in a dog's mouth are rolled about and finally dropped with 

 but little secretion of saliva. The same pebbles reduced to 

 sand and placed in the dog's mouth give rise to a copious 

 secretion. In the latter case much fluid is obviously neces- 

 sary to wash the sand out of the dog's mouth. The same 

 adaptive mechanism is found in the varying activity of serous 

 and mucous salivary glands. When fresh meat is given to a 



