55o 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, Pi ipielski 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, 1 >e divided, stimulation 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. 
