172 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



the effect in this case being due probably to stimulation of the secretion centre 

 by nervous impulses descending from the higher nerve-centres. Lastly, the 

 medullary centre may be inhibited as well as stimulated. The well-known 

 effect of fear, embarrassment, or anxiety in producing a parched throat may 

 be supposed to arise in this way by the inhibitory action of nerve-impulses 

 arising in the cerebral centres. 



Electrical Changes in the Gland during Activity. It has been shown 

 that the salivary as well as other glands suffer certain changes in electric 

 potential during activity which are comparable in a general way to the 

 " action currents " observed in muscles and nerves (see section on Muscle and 

 Nerve). Bradford ] has apparently shown that stimulation of the secretory 

 fibres proper causes the surface of the gland to become negative to the hilus, 

 while stimulation of the trophic fibres gives the reverse effect. Stimulation 

 of a mixed nerve, therefore, such as the chorda, gives a diphasic effect. The 

 theories bearing upon the causes of these electrical changes are too intricate 

 and speculative to enter upon here. The reader is referred to a recent account 

 by Biedermann 2 for further details. 



0. PANCREAS ; GLANDS OF THE STOMACH AND INTESTINES. 



Anatomical Relations of the Pancreas. The pancreas in man lies in 

 the abdominal cavity behind the stomach. It is a long, narrow gland, its 

 head lying against the curvature of the duodenum and its narrow extremity 

 or tail reaching to the spleen. The chief duct of the gland (duct of Wirsung) 

 usually opens into the duodenum, together with the common bile-duct, about 

 eight to ten centimeters below the pylorus. In some cases, at least, a smaller 

 duct may enter the duodenum separately somewhat lower down. The points at 

 which the ducts of the pancreas open into the duodenum vary considerably in 

 different animals. For instance, in the dog there are two ducts, the larger of 

 which enters the duodenum separately about six to seven centimeters below 

 the pylorus, while in the rabbit the main duct opens into the duodenum over 

 thirty centimeters below the pylorus. The nerves of the pancreas are derived 

 from the solar plexus, but physiological experiments which will be described 

 presently show that the gland receives fibres from at least two sources, through 

 the vagus nerve and through the sympathetic system. 



Histological Characters. The pancreas, like the salivary glands, belongs 

 to the compound tubular type. The cells in the secreting portions of the 

 tubules, the so-called alveoli, resemble the serous or albuminous type, and are 

 usually characterized by the fact that the outer portion of each cell, that is, 

 the part toward the basement membrane, is composed of a clear non-granular 

 substance which takes stains readily, while the inner portion turned toward 

 the lumen is filled with conspicuous granules. In addition to this type of 

 cell, which is the characteristic secreting element of the organ, the pancreas 

 contains a number of irregular masses of cells of a different character (bodies 

 of Langerhans). These latter cells are clear and small, frequently have ill- 

 1 Journal of Physiology, 1887, vol. viii. p. 86. 2 Elektrophysiologie, Jena, 1895- 



