HEALTH AND DISEASE 



A section of several of the follicles or grape-like bodies is here shown 

 (fig. 78), and it is seen that they are lined with cells which discharge the 

 saliva they secrete into a central passage, whence it passes on into the duct. 

 This small duct and others unite to form larger ducts that discharge 

 their contents into the mouth. The saliva produced by the several glands 

 differs considerably in its character and properties, in some, as in the 

 submaxillary, being thick and slimy, whence that gland is named a mucous 

 gland, whilst in others, as in the parotid, it is thin and watery, and this 

 gland is therefore termed a serous gland. Occasionally, as in the sub- 

 maxillary gland of man, both kinds of saliva flow from one gland. In 



all instances the saliva is 

 poured forth in abundance 

 as soon as food is intro- 

 duced into the mouth; 

 and as it is a typical 

 example of the mode in 

 which secretion is effected 

 under nervous influence, 

 and that on which our 

 present knowledge of that 

 process mainly rests, it 

 may be advantageous to 

 describe in some detail, 

 but with as much sim- 

 plicity as possible, the 

 events that take place. 

 Let us suppose a (fig. 80) represents the surface of the tongue, with the 

 fungiform, and b circumvallate papillae of taste upon it; c a gland opening 

 upon the surface, L an artery ending in capillaries I) surrounding the 

 gland. There are three nerve centres implicated in the secretion of saliva, 

 each composed of many cells, here represented by one cell only. Of these 

 three b9 is the glossopharyngeal, or taste centre; A 5 is the lingual branch 

 of the fifth pair of nerves ; 7 is the facial centre, winch gives branches to 

 the gland cells and to the artery; lastly, s is the sympathetic nerve, spring- 

 ing from a centre, and supplying the gland cells and artery. "When the 

 mouth is empty, and no secretion is taking place, the sympathetic nerve 

 keeps the blood-vessels in a contracted state, and the gland is quiescent. 

 But if sugar, salt, or other sapid substance be placed on the tongue, the 

 nerves in the papillae conduct the impressions they receive in the direction 

 of the arrows to the taste centres 5 and 9. Thence a nervous impulse is 

 transmitted to the motor centre 7, as well as by other fibres to the brain. 



