THE DIGESTIVE SECRETIONS 943 



have been detected in the vagus and greater splanchnic nerves, but 

 no very convincing data have been obtained.^ 



The Formation of Bile. — It has been stated above that the bile 

 is secreted continuously but not at a uniform rate. Naturally, this 

 variation is dependent upon intermittent stimuli, such as result 

 whenever the stomach ejects its contents into the duodenum. This 

 fact suffices to show that the hepatic cells are under the control of a 

 mechanism which regulates their activity. The latter may be either 

 nervous or chemical in its nature. Thus, it may be assumed that the 

 liver is in possession of secretory fibers which arise in the celiac 

 ganglion and reach this organ by way of the hepatic plexus. But 

 since a reflex contraction of the gall-bladder and a more copious 

 flow of bile may also be evoked by the introduction of an acid into the 

 duodenum, and since these effects may also be obtained after the liver 

 has been isolated from the central nervous system by the division of 

 its nerves, it has been concluded that the secretion of bile is also regu- 

 lated by a specific hormone. Starling recognizes in this chemical 

 stimulant the secretin of the duodenal mucosa,' his claim being based 

 upon the fact that the injection of this agent into the blood stream 

 evokes a copious flow of bile. We shall see later on that secretin 

 also excites a secretion of intestinal juice, and hence, it may be held 

 that it serves as the initial stimulus for three of the principal digestive 

 fluids. 



Inasmuch as the hepatic cells derive the material from which they 

 form the bile from the portal vein, their activities must be adjusted 

 to a very low secretory pressure. It has been shown by Burton-Opitz^ 

 that the blood arrives in the tributaries of the portal system of the 

 dog under a pressure of about 12 mm. Hg and enters the hilum of the 

 liver under a pressure of 9 mm. Hg. In the cat, the pressure encoun- 

 tered at this point amounts to only 7 mm. Hg. Now, since the pres- 

 sure in the inferior vena cava opposite the orifices of the hepatic veins 

 is practically zero, a pressure of about 9 mm. Hg must suffice to drive 

 the blood through the portal terminals. This is the pressure under 

 which the hepatic cells perform their secretory function. The blood 

 of the hepatic artery, on the other hand, arrives at the liver, say, 

 under a pressure of 100 mm. Hg, which is used up very largely in its 

 task of overcoming the resistance in the interstitial capillaries. Both 

 types of blood then traverse the intralobular veins, that of the portal 

 vein furnishing the secretory material, and that of the hepatic artery 

 the oxygen. 



If the general blood pressure is reduced, the quantity of bile se- 

 creted is diminished, while its percentage of solids is increased. This 

 same- effect may be produced by the excitation of the vasoconstrictor 

 fibers of the liver or by the ligation of several branches of the portal 



1 Bainbridge and Dale, Jour, of Physiol., xxxiii, 1905, 138; Doyon, Archives de 

 Physiol., 1894, and Freese, Bull. Johns Hopkins Univ. Hosp., xvi, 1905. 



2 Quart. Jour, of Exp. Physiol, vii, 1913, 57. 



