SECRETION. 231 



the secretory fibres which leave the medulla by way of the facial and glosso- 

 pharyngeal nerves. Owing to the wide connections of nerve-cells in the 

 central nervous system we should expect this centre to be affected by 

 stimuli from various sources. As a matter of fact, it is known that the 

 centre and through it the glands may be called into activity by stimulation of 

 the sensory fibres of the sciatic, splanchnic, and particularly the vagus nerves. 

 So, too, various psychical acts, such as the thought of savory food and the 

 feeling of nausea preceding vomiting, may be accompanied by a flow of saliva, 

 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 iu 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). The theories bearing upon the causes of these electrical changes are 

 too intricate and speculative to enter upon here. The reader is referred to 

 an account given by Biedcrmann l for further details. 



C. Pancreas ; Glands of the Stomach and Intestines. 



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

 the abdominal cavity behiud 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-duet, 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, belong to the serous or albuminous type, and are 

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

 the pari toward the basement membrane, is composed of a clear non-glandular 



1 Eleklrophysiologie, Jena, 1895. 



