DIGESTION. 



143 



Excretory 

 duct. 



branches of the glosso-pharyngeal, and in all probability the gustatory fibers 

 of the chorda tympani. The introduction of food into the mouth develops 

 in the peripheral terminations of these nerves, by reason of its physical and 

 chemic properties, nerve impulses which are then transmitted to the central 

 mechanism. If these nerves are divided, mastication is seriously impaired. 

 When divided and their central ends stimulated with induced electric 

 currents, the muscle will reflexly be thrown into contraction. 



INSALIVATION. 



Insalivation is the incorporation of the saliva with the food, and takes 

 place for the most part during mastication. The saliva ordinarily present 

 in the mouth is a complex fluid composed 

 of the various secretions of the parotid, 

 submaxillary, and sublingual glands and 

 the muciparous follicles of the mouth, which 

 collectively constitute the salivary ap- 

 paratus. 



The parotid gland is situated in front 

 of and partly below the external ear, where 

 it is held in position by the fascia and skin. 

 From the anterior border of the gland there 

 emerges a duct (Stenson's), which, after 

 crossing the masseter muscle to its anterior 

 border, turns inward, pierces the buccin- 

 ator muscle and opens on the surface of 

 the cheek opposite the second upper molar 

 tooth. 



The submaxillary gland is situated 

 below the jaw in the anterior part of the 

 submaxillary triangle. From the gland 

 there emerges a duct (Wharton's) which opens into the mouth by a minute 

 orifice on the surface of a papilla by the side of the tongue. 



The sublingual gland is situated just beneath the mucous membrane 

 in the anterior part of the mouth, where it forms a projection between the 

 gums and tongue. The posterior part of the gland gives origin to a duct 

 (the duct of Rivinus, described also by Bartholin) which opens into the 

 mouth with or very near to the duct of Wharton. The anterior part of the 

 gland gives origin to a varying number of ducts (the ducts of Walther) 

 which open separately along the edge of the sublingual plica of the mucous 

 membrane. 



Histologic Structure of the Salivary Glands. In their ultimate 

 structure the salivary glands bear a close resemblance to one another. 

 They are compound tubulo-alveolar glands composed of small irregularly 

 shaped lobules united by areolar tissue, and connected by branches of the 

 salivary ducts. Each lobule is made up of a number of small alveoli or 

 acini more or less tubular in shape which are the terminal expansions of the 

 smallest ducts. (See Fig. 62). The wall of the acinus is formed by a 

 reticulated basement membrane, surrounded externally by blood-vessels, 



