550 MECHANISM OF SECRETION OF PANCREATIC JUICE. 



2. By constriction of the lumen of the duct, resulting from contrac- 

 tion of its smooth muscle. 



3. By action of special nerve-fibres inhibiting secretion. 



The first hypothesis is improbable, since stimulation of the splanchnics 

 does not cause the same cessation ; and, moreover, there is reason to doubt 

 the existence of vaso-constrictor fibres in the vagus. 1 



The second hypothesis will not hold, when it is considered that 

 physostigmine produces duct constriction, but at the same time increases 

 the secretion. 



Before examining in detail the third supposition, Popielski endeavoured 

 to see how far special secretory fibres can be anatomically isolated. If 

 the larger branches of the vagi lying on the stomach, or those branches 

 which pass towards the liver, be dividedfstimulation of the vagus has the 

 same influence upon pancreatic secretion. The impulses pass therefore 

 along some of the finer nerve branches running in the subserous coat 

 towards the pyloric region of the stomach. If the duodenum be cut 

 through near the pylorus, stimulation of the vagus has no effect. If 

 the duodenum be cut across lower down, the vagus effect is apparent. 

 Stimulation of the lower cut edge of the duodenum in the first case 

 provokes secretion, and if the main mass of nerves passing with the vein 

 into the gland be stimulated (especially those lying at the upper side of 

 the vein), a secretion is evoked without marked latent period, and 

 uniform in character. This secretion is inhibited by the simultaneous 

 stimulation of the vagus in the thoracic cavity. The inhibition comes 

 about, then, either by impulses passing along nerve-fibres to the gland- 

 cells, or affecting some nerve-centre. Popielski finds that if the vagi 

 and sympathetic nerves be cut, a reflex secretion is still evoked by 

 placing hydrochloric acid in the duodenum. The reflex centre, he thinks, 

 then, must be in the abdominal cavity. He considers it probable that 

 such a centre exists in the region of the pylorus, since, if the duodenum 

 be cut through near the pylorus, the introduction of hydrochloric acid is 

 then without effect. If the pylorus be separated with the duodenum, 

 hydrochloric acid will then, however, have the usual effect of causing 

 pancreatic secretion. Popielski considers, however, that such a reflex 

 centre is not furnished by the semilunar ganglion. 



If these observations are correct, we can assume the existence of 

 secretory and inhibitory nerve-fibres, both running in the vagi, and it 

 seems probable, from the differences of latent period which result from 

 stimulation in different regions, that the inhibitory impulses passing 

 along the vagus do not act directly on the cells of the gland, but on 

 some centre which has a controlling influence on the process of secretion. 

 Popielski's reasons for regarding the semilunar ganglion as probably not 

 furnishing such a centre, seem insufficient. The fact that Bernard found 

 an increased secretion after extirpating this, can be explained, on the 

 analogy of the salivary gland, as a paralytic secretion. There is some 

 evidence that the inferior mesenteric ganglion may also act as a centre 

 for reflex action, and if so, it seems less improbable that a similar reflex 

 centre for the pancreatic secretory processes may be referred to the 

 semilunar ganglion. Should such a centre exist, it is undoubtedly 

 subject to influences proceeding from the higher centres by means of the 

 vagi. 



Though there is difficulty in admitting the existence of a controlling 



1 Francois-Franck. 



