THE HEPATIC CIRCULATION 



1415 



clearly innervate the smooth muscle of gastrointestinal 

 tract, biliary tract, and pancreatic ducts (including 

 secretory cells), but none has been traced to the blood 

 vessels (8, 190, 249, 253). The innervation of the blood 

 vessels within the splanchnic viscera appears to be 

 derived exclusively from the sympathetic venous 

 system. Moreover, all the sympathetic efferent path- 

 ways are now believed to be vasoconstrictive in ac- 

 tivity. 



Recent work (80) strongly supports the view that 

 neither sympathetic nor dorsal root vasodilator fibers 

 run to the splanchnic vasculature. The appearance of 

 vasodilation evident in a rise in splanchnic blood flow 

 with no blood pressure change, or in the face of a 

 reduction in blood pressure, is therefore to be referred 

 to ''diminished vasoconstrictive tone." Although this 

 conclusion is not universally acceptable, it must be 

 admitted that a great weight of evidence gives it 

 strong support. Direct stimulation of the splanchnic 

 nerves, the hepatic plexus, or splenic nerve by a 

 tetanizing current induces only a vasoconstrictive 

 response in vivo or in situ which may be expressed by 

 diminished blood flow, by a tendency for the liver 

 and spleen to contract, by diminished cross section of 

 intrahepatic vessels under direct observation, and by 

 peripheral ischemia of the liver evident in micro- 

 radiographic studies (11, 20, 25, 102, 104, 132, 161, 

 204, 270, 299). Variation in the extent of this response 

 appears to be referable to differences in species studied 

 and in the techniques employed, but the general 

 agreement upon its qualitative features is unmistak- 

 able. In contrast, stimulation of the vagus produces 

 little or no obvious change in intrahepatic or splanch- 

 nic resistance under similar circumstances (11, 104, 

 161, 270). Richins (248) has claimed that vasodilation 

 may occur in the pancreas during stimulation of the 

 celiac plexus after cutting the splanchnic nerves in 

 the cat, because "quick-freezing" the pancreas during 

 this period and careful preparation of microscopic 

 sections of the tissue reveal larger cross sections of the 

 arterioles and veins. This method is obviously open to 

 question because it requires the assumption that 

 fixation and preparation of the tissues for study do 

 not affect the state of the vessels which obtains at the 

 moment of freezing. Somewhat stronger support for 

 active cholinergic hepatic vasodilation under special 

 conditions has been put forward by Grayson and his 

 associates (147, 157). These workers have attributed 

 increments in hepatic blood flow, measured by inter- 

 nal calorimetry in intact unanesthetized rats and 

 rabbits, during increments in arterial pressure pro- 

 duced by infusion of epinephrine or by transfusion of 



rat blood, to reflex vasodilation because the response 

 could be blocked by section of the right vagus, celiac 

 neurectomy, atropine, and hexamethonium. They 

 could not compute the changes in hepatic vascular 

 resistance, however, and it seems more likely that the 

 failure for blood flow to change after neural blockade 

 during the rise in blood pressure is the result of active 

 vasoconstriction. In this view, an intact innervation 

 maintains flow by minimizing or blocking the funda- 

 mental vasoconstrictive response rather than by 

 inducing vasodilation. In fact, Grayson and Ginsburg, 

 like manv other workers, have found that stimulation 

 of the cut distal end of the vagus has no effect upon the 

 hepatic circulation. 



The undeniable fact of abdominal pain clearly 

 indicates the presence of afferent pathways mediating 

 visceral perception. Within and about the vessels of 

 the splanchnic bed, myelinated and nonmyelinated 

 fibers may be found ending freely in a fine meshwork 

 or in Pacinian corpuscles. The first seem to accompany 

 the vessels closely, branching dichotomously at each 

 bifurcation and losing their myelin sheaths distal to 

 the last branching. A filmy plexus of nonmyelinated 

 nerves about the vessels extends into the avascular 

 portions of the mesenterv, onto the visceral perito- 

 neum covering the intestines and bladder, and into the 

 substance of the liver and kidney. Sheehan (271) 

 found that small nerve ganglion cells appear in this 

 network at wide intervals and concluded that single 

 fibers "branch and anastomose in a true network 

 arrangement." Fine twigs may be seen occasionally 

 issuing from the plexus to end freely among the en- 

 dothelial cells. Since they are demonstrable after 

 removal of the splanchnic sympathetic chain and the 

 vagi, they are presumably somatic in origin and pos- 

 sibly responsible for visceral sensations. Perhaps the 

 most prominent and clearly definable afferent nerve 

 endings associated with the splanchnic vasculature 

 are the Pacinian corpuscles. These structures vary 

 considerably in size and shape, ranging from easily 

 visible ovoid bulbs (1.0 by 0.6 mm in diameter) to 

 end bulbs measuring only 8 by 4 n. Typically the 

 Pacinian corpuscle is composed of a relatively thick, 

 laminated capsule with a central core through which 

 the main afferent nerve runs to its termination. It 

 lies embedded in the vessel walls particularly in the 

 pancreas, lymph nodes, and mesenteries or in the 

 surrounding connective tissue and fat, usually ar- 

 ranged with the long axis parallel to the vessel. The 

 number of corpuscles varies from species to species, 

 widely distributed and common in the cat, but almost 

 absent in the mesentery of the dog, rabbit, mouse. 



