DIGESTION 



153 



their organic constituents into the lumen of the acinus, materials that col- 

 lectively constitute the saliva characteristic of any one of the glands. This 

 continues until mastication ceases, when all the structures return to their 

 former condition of relative inactivity. 



The Nerve Mechanism for the Secretion of Saliva. The nerves and 

 nerve-centers that constitute the nerve mechanism for the secretion of 

 saliva, are shown in the following table : 



Nerve-centers. 



Afferent Nerves. 



I. Taste fibers in the chorda 

 tympani. 



The nucleus saliva- 

 torius. The vaso- 

 dilatator nucleus. 

 In the floor of the 

 fourth ventricle. 



2. Taste fibers 

 pharyngeal. 



in the glosso- 



Lingual and buccal branches of 

 the trigeminal nerve. 



Efferent Nerves. 



The chorda tympani and its post- 

 ganglionic continuations for the sub- 

 maxillary and sublingual glands; 

 the glosso-pharyngeal nerve and its 

 post-ganglionic continuations con- 

 tained in the auriculo-temporal 

 branch of the trigeminal nerve for 

 the parotid gland. 



The sympathetic nerve, both pre- and 

 post-ganglionic fibers, for all the 

 glands. 



The Efferent Nerves. The efferent nerve-fibers, as stated in the fore- 

 going paragraph, that transmit nerve impulses to the submaxillary, sub- 

 lingual, and parotid glands, as well as to their associated blood-vessels, belong 

 to the autonomic system, and are contained respectively in the chorda tym- 

 pani and its post-ganglionic continuations, in the glosso-pharyngeal and its 

 post-ganglionic continuations contained in the auriculo-temporal branch 

 of the fifth nerve, and in the post-ganglionic branches of the sympa- 

 thetic nerve derived from the superior cervical ganglion. That these nerves 

 transmit the nerve impulses to the salivary apparatus is shown by the effects 

 that follow their division and stimulation. 



The Chorda Tympani Nerve. The efferent nerve-fibers for the sub- 

 maxillary and sublingual glands and blood-vessels comprise both secreto- 

 motor and vaso-dilatator fibers. They arise from cells in the " nucleus 

 saliva to rius" and a vaso-dilatator center respectively, both located in the 

 floor of the 4th ventricle. After emerging from the medulla in the nerve 

 of Wrisberg, they enter the sheath of the facial nerve, but after a short 

 course in the aqueduct of Fallopius they leave it and enter the chorda 

 tympani nerve. After crossing and emerging from the tympanic cavity 

 the chorda joins the lingual branch of the trigeminal nerve which it 

 accompanies as far as the sublingual gland. In this situation nearly all 

 the secreto-motor and vaso-dilatator fibers leave the lingual nerve by 

 four or five strands which arborize around the cells of the submaxillary 

 ganglion. From the cells of this ganglion post-ganglionic fibers are dis- 

 tributed to the gland epithelium of both the submaxillary and sublingual 

 glands and to the blood-vessels by which they are surrounded (Fig. 66). 



The effects on the secretion and flow of saliva from the submaxillary 

 gland which follow division and stimulation of the chorda tympani nerve 

 are shown in the following way: A cannula is inserted into Wharton's duct 

 and the rate of flow estimated; the nerve is then divided, after which the 

 flow ceases. The peripheral end of the nerve is then stimulated with 

 induced electric currents when a copious secretion of a thin saliva takes 

 place, accompanied by a marked dilatation of the blood-vessels of the gland. 



