Glands 111 



f erent secretion ; consisting principally of mucus for lubrication, and pos- 

 sibly containing excreted material. The two most important glands of 

 the body, — the liver and the pancreas, — discharge their secretions into the 

 upper end of the small intestine. The glands of the intestines are excited 

 to activity by pressure on the intestinal lining and also by a hormone 

 called secretin which, it is believed, is formed by the epithelial cells of 

 the upper part of the small intestine, under the influence of the acid 

 chyme (product of stomach digestion). The intestines are supplied with 

 nerves from the sympathetic, the vagus and (in the case of the large intes- 

 tine, at least) from the sacral division of the autonomic system, and con- 

 tain plexuses of Auerbach and Meissner. There is not much information 

 available, however, concerning the nervous control of the glands. The 

 glandular response to pressure is probably a reflex from the local nervous 

 system. There are indications that the fibers from the vagus have an in- 

 hibitory effect. 



The liver, the largest gland in the body, has a weight in the adult in 

 the neighborhood of 1600 grams (three and a half pounds). Its structure 

 is very complex, consisting of numerous small masses (lobules) of secre- 

 tory cells between which lie the branches of the bile canal iculi (corres- 

 ponding to the tubules or alveolae of an ordinary gland), and amongst 

 which run networks of blood vessels and lymph vessels. In its develop- 

 ment the liver resembles a compound tubular gland, but the tubules anas- 

 tomose with one another, forming an intricate network, and thus losing the 

 characteristic tubular gland form. Instead of tubes whose walls are 

 formed of numerous secreting cells, the canaliculi are minute passages 

 between the solidly grouped cells, so that at most points the canaliculus 

 wall is formed by the surfaces of only two cells. The intralobular network 

 in each lobule opens externally into an interlobular bile duct which in 

 turn opens into a larger duct, these ducts finally uniting to form the 

 hepatic duct. From the hepatic duct, the bile is discharged through the 

 cystic duct into the gall bladder between periods of intestinal digestion, 

 but during digestion it passes from both the hepatic duct and the cystic 

 duct through the common bile duct into the small intestine. The gall 

 bladder has a muscular coat and there are numerous bundles of smooth 

 muscle fibers in the walls of the hepatic duct and the bile ducts, the fibers 

 being especially numerous at the orifice into the intestine. 



The pancreas is a relatively large gland weighing about 86 grams (3 

 ounces) and lying behind the stomach. In form it is sacculo-tubular, i, e., 

 the terminal sacs are elongated, giving a cylindrical shape. In the human 

 body the main duct of the pancreas (the duct of Wirsung) and the com- 

 mon bile duct empty into the duodenum through the same orifice. The 



