Glands 107 



organs. Portions of the jejunal and ileac divisions of the small intestine 

 are united to the abdominal wall by peritoneal folds called mesenteries 

 which convey the nerves and blood vessels to the intestines. 



GLANDS OF THE ALIMENTARY CANAL. 



The principal glands opening into the human mouth-cavity are the three 

 pairs of salivary glands, viz., the parotid, the submaxillary and the 

 sublingual glands. [Fig. 79.] These are compound alveolar in struc- 

 ture. Sometimes there are vestiges of a fourth pair, the retrolingual, 

 which are fully developed in the dog, cat and pig. In addition the mucous 

 membrane of the mouth, especially of the under side of the tongue, is full 

 of small glands, mainly of the compound alveolar type. The mingled 

 secretion of all of these glands is the saliva which normally contains des- 

 quamated epithelial cells, disintegrating leucocytes and gland cells, as 

 well as inorganic salts and clots of mucus. The most important con- 

 stituent of saliva is diastase : an enzyme which converts starch into sugar. 

 The salivary digestive process begins in the mouth, and if the food be well 

 mixed with saliva, continues for some time in the stomach, until stopped 

 by the gastric juice, which penetrates slowly into the food-lump formed 

 by the act of swallowing. 



The secretory cells in these glands are of two types : mucous cells simi- 

 lar to the goblet cells already described, and serous cells, secreting a more 

 watery substance. The secretory cells of the parotid are practically all of 

 the serous type. The other glands are mixed, containing both serous and 

 mucous cells. In the mucous membrane of the esophagus there are small 

 glands similar in form to those of the oral cavity but of the pure mucous 

 type. 



The nerve fibers of the salivary glands come from both the vagus and 

 the sympathetic division of the autonomic system. [The course of the sali- 

 vary nerve fibers issuing from the vagus is complicated ; probably all issue 

 in the nervus intermedins. The fibers to the parotid glands pass by the 

 glossopharyngeal nerve to the Vidian nerve and to the otic ganglion; 

 from thence fibers run by a branch of the fifth nerve to the gland. The 

 fibers destined to the sublingual and the submaxillary glands run in the 

 facial nerve to the lingual through the chorda tympani, and end on gang- 

 lion cells near the glands, from which fibers run to the secretory cells. 

 The sympathetic fibers issue from the three upper dorsal nerve-roots, 

 pass through the stellate ganglion, and are relayed in the superior cervical 

 ganglion. From here the fibers follow branches of the external carotid 

 arteries to the glands.] 



The vagal nerve fibers excite the secretory cells (perhaps through the 



