ii EXTERNAL DIGESTIVE SECRETIONS 91 



contents into the duodenum, should the pancreatic secretion 

 decline and cease ? Since the duodenum continues to receive 

 uniform quantities of gastric juice, a corresponding amount of 

 secretin should be elaborated to carry on pancreatic secretion. 



Popielski (1905-7) and his pupils have recently published a 

 series of experiments and conclusions which completely refute the 

 secretin theory. Popielski states that the substance extracted 

 after the maceration of the duodenal rnucosa with hydrochloric 

 acid is not specific, but may, on the contrary, Lie obtained by 

 simple hydrolysis, from any glandular, muscular, or even nervous 

 tissue. Popielski further points out that no appreciable altera- 

 tion of blood pressure can be observed on introducing hydrochloric 

 acid into the stomach to produce an abundant pancreatic secretion ; 

 whereas the so-called secretin has no sooner been injected (even 

 in small doses, with which much less secretion is obtained) than 

 a marked diminution of blood pressure occurs. According to 

 Popielski, this proves that the substance in question acts as a 

 vaso-dilatator. 



But the following is the most cogent of Popielski's arguments. 

 On repeating the injections of secretin many times in equal doses, 

 he observed a conspicuous secretion after the first dose, less after 

 the second, less still after the third, till the substance rapidly 

 became ineffective. Now, the introduction of acid into the duo- 

 denum, however often repeated, invariably excites pancreatic 

 secretion proportional to the quantity of acid introduced. The 

 body evidently reacts to the introduction of secretin by forming 

 an anti-body capable of fixing it and annulling its action ; this 

 suggests that it is not a substance normally developed in the 

 body, but is an artificial extraneous product. 



The pancreatic, unlike the salivary, secretion ceases when 

 pressure in the excretory duct reaches the maximum of 21 mm.Hg, 

 which is far below that at which the arterial blood circulates in 

 the gland (Pawlow). During secretion there is, as in the salivary 

 glands, an acceleration of local circulation, which Klihue and Lea 

 observed directly under the microscope, in living rabbits. The 

 capillaries, which are too narrow to permit the passage of more 

 than a single erythrocyte, dilate during activity so as to allow of 

 more corpuscles passing simultaneously. The pulsation or trans- 

 mission of the blood-wave is visible in both the capillaries and 

 the small veins. These are phenomena of active vaso-dilatation, 

 from physiological excitation of the vaso-dilator nerves by paths 

 which are quite unknown to us. 



Pancreatic secretion appears to be continuous in herbivora, 

 whose gastro-intestinal tube is never empty of food, and inter- 

 mittent in carnivora, which have a shorter digestive tube and 

 intermittent digestive processes. Colin observed in calves that 

 the secretion is continuous, but that it ebbs and flows in relation 



