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HANDBOOK OF PHYSIOLOGY ^ CIRCULATION II 



some workers (185, 270, 299) — and denied by others 

 (84) — in the frog, rat, mouse, rabbit, and guinea 

 pig (not in the cat or man) close to the terminal 

 arborizations in the portal tract. Three-dimensional 

 reconstruction studies on the basis of injections of 

 dyes or colloids by a large number of workers now 

 seem to warrant the view that terminal branches of 

 the hepatic arteries as well as of the portal vein 

 give rise to the sinusoids in various mammals and 

 amphibia (84, 97, 102, 120, 148, 185, 205, 214, 270, 

 299). It is possible that these findings apply in general 

 to most vertebrates but additional studies of the 

 comparative anatomy of the finer hepatic vessels are 

 needed. The axial vein in the portal space appears to 

 give rise to smaller radicles which then course paral- 

 lel to the parent vessel, usually in the same direction, 

 giving off branches that penetrate a so-called "'limit- 

 ing plate" of parenchymal cells to enter the sinusoids. 

 Small branches leading directly into the sinusoid 

 may also spring from the axial portal vein. The capil- 

 lary network fed by the hepatic arterioles gives rise 

 to the sinusoids and by its link to the portal vein 

 provides an arterioportal anastomosis through which 

 the portal venous distribution may be supplied by 

 the arterial inflow; or the reverse could occur. It is 

 difficult to be certain in the welter of conflicting 

 claims, but it does seem likely that the sinusoids con- 

 stitute the most important region of terminal ar- 

 teriolar distribution rather than the portal tract and 

 supporting tissues. The hepatic arterioles may join 

 the terminal portal veins where they enter the sinus- 

 oids, or they may enter the sinusoids directly at any 

 point between the portal tract and the central veins 

 The junction of the arterial and portal venous streams, 

 therefore, apparently occurs chiefly (if not entirely) 

 under most circumstances at or within the sinusoid 

 and perhaps to some extent within the capillary net- 

 work in the portal tracts. At this level the vessels 

 appear to have very thin walls containing little if 

 any muscle tissue. Sphincters are described at the 

 point of entry into the sinusoid because closure of 

 the vessels in a manner suggesting sphincteric action 

 has been observed microscopically in transilluminated 

 livers of living animals. Apparently muscular sphinc- 

 ters have not been detected by histological techniques. 

 The point of emergence of a small capillary from a 

 larger muscular arteriole in the portal space has 

 been construed by Elias (120) as a sphincter. Certainly 

 distinct muscular sphincters do not seem to be demon- 

 strable within the sinusoidal system proper. 



The structure of the hepatic lobule and the rela- 

 tionship between the parenchyma and the capillaries 



or sinusoids has long been the subject of spirited 

 discussion, and disagreement that is not yet settled. 

 In recent years, Elias and his associates have taken 

 issue with the view that the liver is a complex tubular 

 gland modified by extensive coalescence and reorga- 

 nization to form a tightly packed mesh of cells bathed 

 on all sides by the blood in the sinusoids. They have 

 called attention to the dominance of long rows of 

 cells, one cell thick, in sections of mammalian livers 

 and the paucity of cylindrical cross sections such as 

 one might expect in a tubular organ. Careful three- 

 dimensional reconstructions indicate that the liver 

 may be considered a cell mass penetrated by a net- 

 work of tubular sinusoids separated by interconnect- 

 ing cellular sheets or plates, one cell thick in mammals 

 and certain birds, usually two cells thick in all other 

 vertebrates. A tubular layer of parenchymal cells 

 encloses the portal tracts as a limiting plate which is 

 pierced by the terminal branches of the portal vein 

 and hepatic artery. The limiting plate can be traced 

 along the tract to the surface of the liver where it 

 passes out to lie under the capsule. There may be 

 several such subcapsular (seemingly concentric) 

 limiting plates or none. The openings into the central 

 vein are so numerous that a clear-cut limiting plate is 

 not demonstrable but a similar lamina does appear 

 about the sublobular vein and layer branches of the 

 hepatic veins. These studies have not definitely ruled 

 out the possibility that the liver is basically a closely 

 conglutinated tubular structure in lower forms, with 

 flattening and realignment of the cells into single 

 cell layers in the mammals. Indeed, a tubular con- 

 struction is demonstrable when increased sinusoidal 

 pressure increases the spaces between the laminae 

 and seems to fragment them (121). Rappaport 

 (237) and his associates have attributed the usual 

 microscopic picture to the character of the basic 

 hepatic unit which they believe to be an "irregular 

 berry-like parenchymal mass situated around the 

 trio of terminal branches of portal vein, hepatic 

 artery, and bile duct, growing out from a small portal 

 triad and mainly running perpendicularly to the 

 central vein. The hepatic unit occupies adjacent 

 parts of neighboring hexagonal fields and extends 

 from the central vein of one hexagon to the central 

 vein of another." All sections of such a structure 

 would tend to be tangential and would, they claim, 

 yield a preponderance of longitudinal sections. 



Regardless of the ultimate outcome of this argu- 

 ment it is evident that sinusoids are cylindrical or 

 saccular vessels closely encased in a kind of highly 

 flexible plastic sheathing that must operate to in- 



