678 PHYSIOLOGY OF DIGESTION AND SECRETION. 



arborizations around sympathetic nerve cells placed somewhere along 

 their course, in the sublingual ganglion, for instance, or within the 

 gland substance itself. It is the axons from these second nerve units 

 that end around the secreting cells. Langley has accumulated some 

 facts to show that within the period of continuance of the paralytic 

 secretion (five to six weeks) the fibers of the sympathetic cells are 

 still irritable to stimulation. He is inclined to believe, therefore, that 

 the continuous secretion is due to a continuous excitation, from some 

 cause, of the local nervous mechanism in the gland. On the histo- 

 logical side it is stated * that after section of the chorda the resulting 

 degenerative changes affect only the cytoplasm, while after the 

 section of the sympathetic the nuclei of the cells are affected, and 

 indeed to some extent on the sound as well as on the injured side. 



Normal Mechanism of Salivary Secretion. Under normal 

 conditions the flow of saliva from the salivary glands is the result 

 of a reflex stimulation of the secretory nerves. The sensory fibers 

 concerned in this reflex must be chiefly fibers of the glossopharyngeal 

 and lingual nerves supplying the mouth and tongue. Sapid bodies 

 and various other chemical or mechanical stimuli applied to the 

 tongue or mucous membrane of the mouth produce a flow of saliva. 

 The normal flow during mastication must be effected by a reflex of 

 this kind, the sensory impulse being carried to a center and thence 

 transmitted through the efferent nerves to the glands. It is found 

 that section of the chorda prevents the reflex, in spite of the fact that 

 the sympathetic fibers are still intact. No satisfactory explanation 

 of the normal functions of the secretory fibers in the sympathetic 

 has yet been given. Various authors have suggested that possibly 

 the three large salivary glands respond normally to different stimuli. 

 This view has lately been supported by Pawlow, who reports that in 

 the dog at least the parotid and the submaxillary may react quite 

 differently. When fistulas were made of the ducts of these glands it 

 was found that the submaxillary responded readily to a great number 

 of stimuli, such as the sight of food, chewing of meats, acids, etc. 

 The parotid, on the contrary, seemed to react only when dry food, 

 dry powdered meat, or bread was placed in the mouth. Dryness in 

 this case appeared to be the efficient stimulus. 



Pawlow lays great stress upon the adaptability of the secretion of saliva 

 to the character of the material chewed. Dry, solid food stimulates a large 

 flow of saliva, such as is necessary in order to chew it properly and to form it 

 into a bolus for swallowing. Foods containing much water, on the contrary, 

 excite but little flow of saliva. If one places a handful of clean stones in 

 the mouth of a dog he will move them around with his tongue for a while 

 and then drop them from his mouth; but little or no saliva is secreted. 

 If the same material is given in the form of fine sand a rich flow of saliva 

 is produced, and the necessity for the reflex is evident in this case, since 

 otherwise the material could not be conveniently removed from the mouth. 



*Gerhardt, "Archiv f. die gesammte Physiologic, " 97, 317, 1903. 



