THE DIGESTIVE SECRETIONS 913 



the Glaserian fissure, they attach themselves to the Ungual branch of 

 the fifth cranial nerve. Having followed this highway for a short 

 distance, they again pursue a separate course along Wharton's duct to 

 the hilum of this gland. .A small ganglion, known as the submaxillary 

 ganglion, marks the point where these fibers diverge from the lingual 

 nerve (Fig. 492). Langley, however, believes that the fibers destined 

 for the submaxillary gland do not form synapses here, but merely 

 skirt its border, while those fibers which innervate the sublingual 

 gland actually terminate in this structure to be continued as secondary 

 fibers. This ganglion, therefore, should really be called the sublingual 

 ganglion. 



Those fibers, which first enter the sympathetic system, traverse the 

 thoracic ganglia and become a part of the cervical sympathetic nerve. 

 They form synapses in the superior cervical ganglion, whence their 

 postganglionic fibers continue onward along the arteries supplying 

 the different salivary glands. 



On the afferent or sensory side, the salivary center is connected 

 with a number of external as well as internal receptors. Under or- 

 dinary conditions, however, the former are of greater value, because a 

 flow of saliva may be evoked by impressions received from the retina 

 and olfactory cells as well as by stimuli produced by the food as it 

 traverses the oral cavity. Thus, it is a matter of common exper- 

 ience that a flow of saliva may be elicited by the mere smell and sight 

 of food as well as by psychic stimuli. The opposite effect, in the form 

 of a dry mouth and parched throat, is incited by fear, embarrassment 

 and anxiety. Consequently, the salivary center must be connected 

 reflexly with the olfactory cells, the retina and some of the higher 

 association centers. Lastly, this center must be in reflex communica- 

 tion with various general interoceptors, because the feeling of nausea, 

 visceral pain, and other internal sensations frequently give rise to a 

 copious flow of saliva. 



The Mechanism of Salivary Secretion. — The secretion of saliva 

 is a reflex act which may be evoked by the stimulation of any one of the 

 receptors just mentioned. It should not be assumed, however, that 

 these stimulations affect the different glands in a perfectly uniform 

 manner, giving rise to a definite quality of saliva under all circum- 

 stances. The truth seems to be that the quality of the secretion varies 

 with the quality of the stimulation. Thus, a more specific excitation 

 of the parotid yields a serous saliva, and a more specific stimulation 

 of the sublingual a mucous saliva. Such variations are not at all 

 uncommon, and their occurrence may easily be demonstrated experi- 

 mentally. Thus, if a fistulous communication is established between 

 the duct of the submaxillary gland and the outside, a copious flow of 

 saHva may be produced by the introduction into the mouth of either 

 a 0.25 per cent, solution of hydrochloric acid or of powdered meat. 

 On analysis it will then be found that the type of saliva secreted after 

 the ingestion of meat, contains approximately twice as much solid ma- 

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