THREE SOURCES OF FORCE IN THE PORTAL CIRCULATION 39 



the capillaries of the portal veins, but also to give it a pressure in a direction towards 

 the hepatic veins. For any pressure which arises between the arterial blood of the 

 hepatic and its corresponding venous blood, must give rise to motion towards the he- 

 patic veins ; no regurgitation can take place backward through the portal vein upon the 

 blood arriving from the chylopoietic viscera, because along that channel there is a pres- 

 sure propagated in the opposite direction, arising from the arterial blood of the aortic 

 branches. The pressure, therefore, arising from the relations of the hepatic arterial 

 blood, conspires with that arising from the pressure of the portal blood, and both to- 

 gether join in giving rise to motion towards the ascending cava. 



127. So great are the forces which arise under these circumstances, that there is no 

 doubt that the blood of the hepatic artery alone could, of itself, control the circulation 

 of the liver without any re-enforcing aid from the portal blood. So when, from acci- 

 dent or otherwise, the portal vein is shut, or when from malformation it opens directly 

 into the ascending cava, the hepatic artery takes charge of the functions of the liver, 

 and directs its circulator}- conditions. 



12S. These views, therefore, lead us to understand that there are three sources of 

 force engaged, under normal circumstances, in directing the portal circulation. One 

 of these is found in the aortic capillaries, when they are spread on the chylopoietic 

 viscera, the mode of action being precisely analogous to that w ; hich obtains in the gen- 

 eral svsteuiic circulation. The other two are found in the liver itself; the first is a 

 pressure exerted by the portal blood on that of the hepatic veinlets ; the second bv the 

 blood of the hepatic artery, which, conspiring with the former, urges the resulting mix- 

 ture along the hepatic veins into the ascending cava. 



129. I might now proceed to show with what clearness these doctrines explain the 

 circulation of the blood in other tribes of life ; for example, in the case of the model 

 which is adopted in fishes, the aorta of which has long ago been recognised as bearing 

 a strong resemblance to the portal vein of the mammalia ; but, as throughout this chap- 

 ter these latter have been constantly referred to, I shall continue what is here to be of- 

 fered farther by using their type of construction for illustration. To any one who re- 

 flects on the principles which have been laid down, there will arise no difficulty in ex- 

 plaining the mechanical causes of the circulation in any particular case, more especially 

 if this plain precept is kept constantly in mind, that, for the physical reasons which have 

 been aligned (88), a pressure will always be exerted in every one of these instances by the 

 jluid which is ready to undergo a change upon that which has already undergone it; a 

 pressure which, as there is no force to resist it, will always gite rise to motion in a direc- 

 tion from the changing to the changed fluid. 



130. As we have said, it is the character of a true theory to be applicable to all 

 cases, and to render a clear account of every circumstance that may arise. A true the- 

 ory is like a window of crystal glass, through which we can see all objects in their 

 proper positions, and colours, and relations, no matter whether they are such as are 

 near, or those that are at a distance ; no matter whether they are directly before us, or 

 enter only obliquely into the field of view. A fictitious theory is like a Venitian blind, 

 which has to be set in a certain position with respect to the observer, and only shows 



