THE DIGESTIVE SYSTEM. 



dilated (fig. 116). This shrinkage must be due to the first discharge of 

 material from the secretory cells. As a result of the loss of fluid of 

 lower molecular concentration, the osmotic pressure in the cells them- 

 selves is raised and water is attracted to them from the lymph. In 

 this way the molecular concentration of the lymph is also increased, 

 and therefore water is attracted from the blood. Moreover, as the 

 result of the secretory activity of the cells, large molecules are being 

 split up into a large number of smaller molecules, and the discharge of 

 these metabolites into the lymph tends still further to raise the osmotic 

 pressure and ultimately the amount of that fluid. 



If the chorda tympani has been divided, a flow of watery saliva 

 begins from one to three days later, and continues for five or six weeks 

 (paralytic secretion), when the gland atrophies and the secretion 

 ceases. If the chorda of one side only has been divided, a similar 

 secretion is said to take place in the opposite gland, an antiparalytic 

 secretion, the explanation of which is not clear. 



Division and stimulation of the nerves to the parotid and sub- 

 lingual glands give parallel results to those described above for the 

 submaxillary gland. " 



The physiological centre for the reflex mechanism of salivary secre- 

 tion is situated in the medulla oblongata. The cranial efferent fibres 

 take origin in the nucleus of the nervus intermedius. Those for the 

 submaxillary and sublingual glands join the facial nerve and leave it 

 in the chorda tympani, which subsequently joins the lingual branch of 

 the fifth nerve. The fibres for the parotid join the trunk of the glosso- 

 pharyngeal nerve, and, leaving it by its tympanic branch, reach their 

 destination after passing through the tympanic plexus, the Vidian nerve, 

 the otic ganglion, the second division of the fifth, and the auriculo- 

 temporal nerve. The cranial fibres for the salivary glands belong to 

 the autonomic nervous system, and in the case of the submaxillary and 

 sublingual glands the post-ganglionic fibres take origin in Langley's 

 ganglion and the submaxillary ganglion respectively. This can be 

 proved by painting these ganglia with nicotine, after which stimulation 

 of the chorda tympani is without effect on the glands. 



The pre-ganglionic fibres of the sympathetic nerves to the salivary 

 glands pass down the spinal cord, leaving it by the anterior roots of 

 the upper three thoracic nerves to join the sympathetic chain. They 

 run in the cervical sympathetic nerve up to the superior cervical 

 ganglion, where the post-ganglionic fibres arise. These pass on the 

 walls of the external carotid artery to the various glands. 



