434 SECRETION OF PANCREATIC JUICE. [BOOK n. 



is probably brought about in this way. This effect, which however 

 is not confined to the vagus, stimulation of other afferent nerves, 

 such as the sciatic, producing the same effect, may be regarded 

 (in the absence of any proof that the result is due to reflex 

 constriction of the pancreatic blood vessels unduly checking the 

 blood-supply) as an inhibition of a reflex mechanism at its centre 

 in the medulla or in some other part of the central nervous 

 system, much in the same way as fear inhibits at the central 

 nervous system the secretion of saliva following food in the 

 mouth, 226. But if so, then we must regard the secretion of 

 pancreatic juice as closely resembling that of saliva in as much 

 as it is called forth by a reflex act. Yet it is stated that, unlike 

 the case of saliva, the secretion of pancreatic juice continues after 

 all the nerves going to the gland have been divided, an operation 

 which would do away with the possibility of reflex action. Such 

 an experiment however cannot be regarded as decisive, since it is 

 almost impossible to be sure of dividing all the nerves. 



No evidence has yet been brought forward to prove the exist- 

 ence of any double nervous mechanism similar to that of chorda 

 fibres and sympathetic fibres in the salivary gland. All that can 

 be said is that, when the gland is stimulated to secrete, the blood 

 vessels are dilated as in the salivary gland ; and we have already, 

 232, dwelt on the histological changes which accompany secre- 

 tion. We may add that when the gland is stimulated to increased 

 secretion the increase is not merely an increase of water, the 

 discharge of solids is increased even more than the discharge of 

 water, so that the percentage of solids in the juice increases. 



The quantity of pancreatic juice secreted, in the case of man, 

 in 24 hours has been calculated at 300 c.c., but such a calculation 

 is of very uncertain value. 



We have seen, 227, that in the salivary glands the pressure 

 which may be exerted by the fluid in the ducts is very considerable, 

 exceeding it may be even the blood-pressure in the carotid artery. 

 In this respect the pancreas differs from the salivary glands. 

 When, in a rabbit, a cannula connected with a vertical tube or a 

 manometer is placed in the pancreatic duct, the column of fluid 

 does not rise above a height corresponding to a pressure of about 

 17 mm. of mercury. But at this pressure the gland becomes 

 cedematous on account of the juice secreted passing back through 

 the walls of the ducts and alveoli into the connective tissue ; a 

 much higher pressure is needed to render a salivary gland 

 cedematous ; and whether the low pressure observed in the pan- 

 creas is due to the ease with which cedema takes place or to the 

 actual secretion not being able to reach a higher pressure cannot 

 be stated with certainty. 



252* The Secretion of Bile, The act of secretion of bile by 

 the liver must not be confounded with the discharge of bile from 

 the bile-duct into the duodenum. When the acid contents of the 



