246 A TEXTBOOK OF PHYSIOLOGY 



with inspiration, according as the expiratory rise of vena cava pressure 

 or the inspiratory rise of arterial pressure has the greater effect. 



During Traube-Hering oscillations of arterial pressure, the volume 

 of a limb follows the rise and fall of aortic pressure. 



It has been said that an antagonism exists between the vaso-motor 

 mechanisms of the splanchnic and locomotor organs. Thus, while 

 during asphyxia the splanchnic vascular area constricts, the vessels 

 of the skin and muscles dilate. The dilatation of the latter, however, is 

 in all probability not occasioned by active dilatation, but is due to 

 the overmastering power of the splanchnic constrictors. By the rise 

 of aortic pressure the vessels in the remaining parts of the body are 

 passively dilated, and the blood-flow is thus increased through the 

 skin and muscles. That this is so is suggested by the fact that after 

 the circulation has, by ligation of the thoracic aorta and vena cava 

 inferior, been limited to the fore part of the body, either asphyxia 

 or excitation of the vaso-motor centre produces a slight rise of arterial 

 pressure, owing to the constriction of the vascular areas of the face 

 and fore-limbs. Previous to the double ligation the same excitation 

 produces splanchnic constriction, a great rise of aortic pressure, and 

 dilatation in the face and fore -limbs. Plethysmographic evidence of 

 constriction of the leg has been obtained during asphyxia. 



Whether the dilatation of the locomotor organs be active or passive 

 is of no particular importance. The fact remains that the splanchnic 

 area forms the chief seat of varying resistance, and when the splanchnic 

 vessels are constricted the blood is driven with increased velocity 

 through the locomotor organs, and is determined from the deep to the 

 superficial parts of the body. 



The Portal Circulation. The portal circulation is peculiar in that 

 the blood passes through two sets of capillaries. Arterial blood is 

 conveyed to the capillary networks of the stomach, spleen, pancreas, 

 and intestines, by branches of the abdominal aorta. The portal vein 

 is formed by the confluence of the mesenteric veins with the splenic 

 vein, which together drain these capillaries. The portal blood breaks 

 up into a second plexus of capillaries within the substance of the liver. 

 The hepatic veins carry the blood from this plexus into the inferior 

 vena cava. Ligation of the portal vein causes intense congestion of 

 the abdominal vessels, and so distensile are these that they can hold 

 nearly all the blood in the body; thus, the arterial pressure quickly 

 falls, and the animal dies just as if it had been bled to death. The 

 portal circulation is largely maintained by the action of the respiratory 

 pump, the peristaltic movements of the intestine, and the rhythmic 

 contractions of the spleen; these agencies help to drive the blood 

 through the second set of capillaries in the liver. The systole of the 

 heart may tell back on the liver and cause it to swell, for there are 

 no valves between it and the inferior vena cava, when there is 

 obstruction in the right heart or pulmonary circulation. The 

 increased respiration which results from muscular exercise greatly 

 furthers the hepatic circulation, increasing at the same time the con- 

 sumption of food material. Thus exercise relieves the overfed man. 



