Large inferior right hepatic vein. Clinical implications

Extended left hepatectomy associated with resection of the vena cava and suprahepatic veins by in situ perfusion to treat intrahepatic cholangiocarcinoma

Cholangiocarcinoma is a heterogeneous group of aggressive tumors that correspond to the second most common primary liver tumor. They can be classified according to their anatomical position concerning the biliary tree, and each subtype demonstrates different behavior and treatment. A 38-year-old male patient presenting solely right lumbar pain was diagnosed with a 7 cm hepatic tumor involving segments I, Iva, and VIII associated with involvement of the hepatic veins. He underwent a bloc resection of hepatic segments I, II, III, IV, partial V, partial VII, and VIII; right, middle, and left hepatic veins; and inferior vena cava segment, with perfusion of the remaining liver in situ with a preservation solution. As the patient had a large accessory inferior right hepatic vein draining the remaining liver, no reimplantation of hepatic veins was necessary. He remained clinically stable in outpatient follow-up, with excellent performance status-current survival of 2 years 6 months after surgical treatment.

Relationship of the Presence of the Inferior Right Hepatic Vein with the Right Hepatic Vein Diameter and CT Liver Volumetry

Background  Right hepatic venous anatomy, right lobe volume, and percentage of remnant liver are issues to be considered in preoperative planning especially transplantation. Objectives  The aim of this study was to investigate the relationship of the presence of the inferior right hepatic vein (IRHV) with the right hepatic vein (RHV) diameter, right lobe volume, and percentage of remnant liver. Materials and Methods  In t his cross-sectional study, the computed tomography (CT) images of 90 patients who underwent triphasic CT for being living liver donation were evaluated retrospectively. The number and diameter of IRHVs and the diameter of main RHV were recorded. For the liver volume analysis, a deep learning-based automatic liver segmentation (Hepatic VCAR) program was used. A virtual hepatectomy plane was drawn, where the right and left liver volumes were found and the percentage of the left lobe to the total liver volume was calculated. Pearson's correlation analysis was used for correlation analysis and Student's t -test was used to compare parameters. Results  A total of 74 IRHVs were detected in 53 (58.88%) of 90 patients. There were no differences in the percentage of remnant left lobe volume, right lobe volume, and RHV diameter between the IRHV (+) and (-) groups. The RHV diameter had a weak negative correlation with the IRHV diameter, and a weak positive correlation with the right lobe volume. Conclusions  The percentage of remnant left lobe volume, right lobe volume, and RHV diameter did not differ in liver donors with and without an IRHV. The RHV diameter had a weak negative correlation with the IRHV diameter and a weak positive correlation with the right lobe volume.

Surgical relevance of anatomic variations of the right hepatic vein

BACKGROUND

Variations in the anatomy of hepatic veins are of interest to transplant surgeons, interventional radiologists, and other medical practitioners who treat liver diseases. The drainage patterns of the right hepatic veins (RHVs) are particularly relevant to transplantation services.

AIM

The aim was to identify variations of the patterns of venous drainage from the right side of the liver. To the best of our knowledge, there have been no reports on RHV variations in in a Caribbean population.

METHODS

Two radiologists independently reviewed 230 contrast-enhanced computed tomography scans performed in 1 year at a hepatobiliary referral center. Venous outflow patterns were observed and RHV variants were described as: (1) Tributaries of the RHV; (2) Variations at the hepatocaval junction (HCJ); and (3) Accessory RHVs.

RESULTS

A total of 118 scans met the inclusion criteria. Only 39% of the scans found conventional anatomy of the main hepatic veins. Accessory RHVs were present 49.2% and included a well-defined inferior RHV draining segment VI (45%) and a middle RHV (4%). At the HCJ, 83 of the 118 (70.3%) had a superior RHV that received no tributaries within 1 cm of the junction (Nakamura and Tsuzuki type I). In 35 individuals (29.7%) there was a short superior RHV with at least one variant tributary. According to the Nakamura and Tsuzuki classification, there were 24 type II variants (20.3%), six type III variants (5.1%) and, five type IV variants (4.2%).

CONCLUSION

There was significant variation in RHV patterns in this population, each with important relevance to liver surgery. Interventional radiologists and hepatobiliary surgeons practicing in the Caribbean must be cognizant of these differences in order to minimize morbidity during invasive procedures.

[Resection of liver segments VII-VIII: is right hepatic vein reconstruction advisable?]

OBJECTIVE

To improve the treatment outcomes in patients with primary and metastatic liver tumors localized in segments VII-VIII involving the right hepatic vein and its branches.

MATERIAL AND METHODS

There were 16 surgical interventions including resection of liver segment VII and/or VIII with resection of the right hepatic vein and its branches without reconstruction. All procedures were carried out at the Department of Liver and Pancreatic Tumors of the Blokhin National Medical Cancer Research Center for the period 2016-2020. The cause of surgery was colorectal cancer liver metastases in 8 patients, hepatocellular carcinoma in 2 cases, angiomyolipoma in 1 case and metastases of uterine cancer in 1 patient. Minor liver resection was additionally performed in 5 cases.

RESULTS

Median surgery time was 150 (80-220) min, intraoperative blood loss - 400 (100-2000) ml. Afferent blood flow was blocked in 4 patients for 14 (12-25) min. None patient had intraoperative signs of impaired venous outflow. Biliary fistula in postoperative period occurred in 1 patient. No complications were noted in other cases. Median postoperative hospital-stay was 13 (9-19) days. There were no specific complications in long-term postoperative period that could be associated with venous outflow blockade through the right hepatic vein.

CONCLUSION

Existing vessels and intrahepatic collaterals de novo can provide adequate venous outflow into the middle hepatic vein and short hepatic veins during resection of liver segments VII and/or VIII with resection of the right hepatic vein and its branches without reconstruction and the absence of inferior right hepatic vein.

Correlation of clinical features with inferior right hepatic vein incidence: a three-dimensional reconstruction-based study

PURPOSE

The correlation between right hepatic vein (RHV) diameter and inferior RHV (IRHV) incidence and that between IRHV incidence and other clinical features remain unclear. We investigated factors correlated with IRHV incidence as well as provide a simple and reliable method for predicting IRHV presence preoperatively.

METHODS

We obtained computed tomography (CT) imaging data of 1980 patients from the Department of Radiology, Qingdao Municipal Hospital, from July 1, 2016, to July 1, 2017. We excluded patients with heart disease, inferior vena cava (IVC) disease, history of liver surgery or trauma, space-occupying lesions in the liver, and other diseases, which can cause hepatic hemodynamic changes. CT images of patients were three-dimensionally reconstructed. We measured RHV and IRHV diameter as well as the angle between the RHV and the IVC.

RESULTS

Data on 299 patients were included in this study; the incidence of IRHV was 34.44%. Sex, age, and the angle between the RHV and IVC did not correlate with IRHV incidence. RHV diameter negatively correlated with IRHV incidence (P < 0.05). The area under the receiver-operating characteristic curve for IRHV incidence was 0.878. The diagnostic threshold value of RHV diameter was 8.86 mm.

CONCLUSION

A negative correlation was found between RHV diameter and IRHV incidence, suggesting that IRHV is absent with RHV diameter > 8.86 mm, but is present with RHV diameter < 8.86 mm. This suggests that measuring only RHV diameter can predict the presence of an IRHV when IRHV-related hepatectomy and IRHV preserved living donor liver transplantation are needed.

Surgical impact of an inferior right hepatic vein on right anterior sectionectomy and right posterior sectionectomy

BACKGROUND

In hepatocellular carcinoma, anatomical resection is important because of portal spread. In right anterior sectionectomy (RAS) and right posterior sectionectomy (RPS), the right hepatic vein (RHV) may not correspond with the intersectional plane if an inferior RHV (IRHV) is present. The aim of this study was to evaluate the influence of the IRHV on the exposure of the RHV retrospectively.

METHODS

One hundred ninety-one patients underwent RAS or RPS by the Glissonean pedicle transection method. The calibres of the RHV and IRHV were measured and assessed the extent of exposure of RHV.

RESULTS

One hundred seventeen patients underwent RAS and 74 underwent RPS. The calibre of the RHV averaged 8.0 mm and that of the IRHV, 6.2 mm. Exposure of the RHV was divided into three groups: no exposure 31 (16.2%) (with IRHV, 20 patients; without IRHV, 11 patients), upper half exposure 49 (25.7%; with IRHV, 24; without IRHV, 25) and full exposure 111 (58.1%) (with IRHV, 16; without IRHV, 95). The effect of the IRHV on exposure of the RHV was substantial (P < 0.001).

CONCLUSIONS

The IRHV can affect the course of the RHV and its exposure. Therefore, in RAS and RPS, it is important to evaluate the existence of the IRHV.

Clinical anatomy related to the hepatic veins for right lobe living donor liver transplantation

The complexity of liver reconstruction has limited partial right lobe living donor liver transplantation. It is largely due to the difficulty of dealing with the middle hepatic vein. We sought to define the anatomic features of hepatic veins. Forty-one fresh adult livers, 43 formalin-fixed adult cadaver livers, and 91 adult liver corrosion casts were used for the study. We determined the number of branches, the maximum diameter, the whole length, the extrahepatic length of the hepatic veins, and the deviation of the middle hepatic vein from the main portal fissure. Nakamura and Tsuzuki's classification of hepatic vein types was used. Type A, B, and C accounted for 59.4, 27.8, and 12.8% of all specimens in this study, respectively. The middle and left hepatic veins formed a common trunk in 60.3% of the specimens, and the length of the common trunk was 1.12 ± 0.62 cm. The degree of deviation to the right of the middle hepatic vein from the main portal fissure was 14.11° ± 12.65°. The frequency of hepatic vein types and the degree of deviation to the right of the middle hepatic vein in this study is markedly different from that reported in other literature. The anatomic features of the hepatic veins in this study suggest that right lobe living donor liver transplantation is more suitable for Chinese.

Anatomical basis for clamping of the right hepatic vein outside the liver during right hepatectomy

The possibility and value of clamping the right hepatic vein (HV) outside the liver during right hepatectomy remain a matter of debate. We carried out an anatomical study on ten fresh cadaveric subjects with no abdominal scarring or hepatic lesions, to determine the biometry of the extraparenchymatous segment of the right HV. One or several accessory right HVs were found in 90% of cases on release of the right edge of the inferior vena cava (IVC). These accessory right HVs had a diameter greater than that of the superior right HV in 10% of cases. In 70% of cases, the extraparenchymatous segment of the vein was free of collateral branches, and in 30% of cases, it was joined by a branch close to its point of exit from the hepatic parenchyma. The length of the vein that can be clamped (length between the point of exit from the hepatic parenchyma and the point of entry of the right HV into the IVC) was 8.6 +/- 1.8 mm (6-12). The right HV entered the vena cava, at an acute angle, in 100% of cases. Clamping of the right HV was possible in all cases. Knowledge of these anatomical points makes it possible to isolate an extraparenchymatous segment of the right HV more safely. The right HV can be isolated and clamped outside the liver in more than 80% of cases, making it possible to carry out right hepatectomy on an exsanguinous liver.

Venous variants and anomalies on routine abdominal multi-detector row CT

OBJECTIVE

This study aims to determine the types and prevalence rates of anatomic variations of the hepatic veins, portal vein, inferior vena cava and renal veins, and to establish statistical correlations between various anomalies and frequency differences between male and female using multi-detector row computed tomography (CT).

MATERIALS AND METHODS

One thousand one hundred and twenty patients (588 men, 532 women) were evaluated with routine abdominal CT. Frequencies of different variants were noted and compared, and correlations between three categories of variation were tested.

RESULTS

In total, 1261 abdominal vein variants and anomalies were identified in 756 (67.5%) of 1120 patients. Six hundred and forty-two hepatic vein variants were detected in 468 (41.8%) patients. One or more inferior right hepatic veins were identified in 356 (31.8%) individuals, and tributary hepatic veins were detected in 147 (13.1%) patients. Portal vein variations and anomalies were observed in 307 (27.4%) cases. The most frequent of these was trifurcation (139 patients, 12.4%). A total of 311 inferior vena cava and renal vein variants were identified in 258 (23%) cases. Six patients (0.5%) exhibited inferior vena cava anomalies, 62 (5.5%) had circumaortic renal veins, 53 (4.7%) had retroaortic renal veins, and 210 (18.8%) had multiple renal veins.

CONCLUSION

The prevalence of abdominal vein variations is high, and routine abdominal CT demonstrates these abnormalities very well. The data suggest that hepatic vein variants and multiple right renal veins are more frequent in women than in men, and that hepatic vein variation is correlated with portal vein variation.

[Retrohepatic veins of the posterior section of the right hepatic lobe--terminology and surgical significance]

Three main hepatic veins: right, middle and left are constant, but there is a variable number of retrohepatic vessels called accessory or minor hepatic veins. The most important of them are veins reffered to as middle right hepatic vein (MRHV) draining segment VII and inferior right hepatic vein (IRHV) draining segment VI. The incidence of large MRHV and IRHV reaching or exceeding a caliber of 5mm, their arrangement in the liver and drainage territories were investigated in our collection of 142 injection-corrosion specimens of the liver. In 1/5 of the cases with large IRHV this vein drains small part of segment VI, sometimes its insignificant marginal part so it couldn't be used for segment VI preservation when it is necessary. A precise knowledge of the vein anatomy of right posterior sector of the liver and its vein drainage territories is very important during complex dissections of the retrohepatic areas, resections and preservation liver parenchima.

The prevalence and variations of inferior right hepatic veins on contrast-enhanced helical CT scanning

PURPOSE

To present the prevalence and variations of inferior right hepatic veins (IRHVs) on contrast-enhanced helical computed tomography (CEHCT) scans.

MATERIALS AND METHODS

The routine abdominal CEHCT scans of 349 patients were reviewed. Three hundred and eight patients (88.2%) were included in the study. Of the 349 patients, 41 (11.8%) were excluded from the study because of improper opacification of hepatic veins and right hepatic lobe lesions which made difficult the optimal visualization and assessment of IRHVs. The mean age of 308 patients was 43 years (range 3-97 years). One hundred and forty-three patients (46.4%) were men and 165 (53.6%) women. Scans were examined whether the IRHVs were demonstrated or not and classified according to their numbers, levels, diameters, and joinings to inferior vena cava (IVC).

RESULTS

Of the 308 patients, 65 (21.1%) had one or two IRHVs. Fifty-four patients (83.1%) had only one IRHV and 11 (16.9%) patients had two. More than two IRHVs were not seen in any patient. Eight (72.7%) of 11 double IRHVs joined the IVC at the same level and others (27.3%) did not. There was no truncal opening to the IVC. In five patients (7.7%) the IRHV were large (> or =0.5 cm).

CONCLUSION

The presence of IRHVs is common and routine CEHCT scanning is efficacious in assessment of IRHVs.

A exposição do contorno lateral direito da veia cava inferior na cirurgia hepática

O conhecimento da relação entre a veia cava inferior e o tecido hepático é fundamental para a sua abordagem durante a cirurgia hepática. Observa-se na literatura dados contraditórios. Objetivo: Pesquisar a sintopia da veia cava inferior, o número de veias confluentes para o contorno direito e a distância do segmento retro-hepático da veia cava inferior. Métodos: Foram estudados 38 fígados humanos, entre 13 e 98 anos de ambos sexos. Obtida a peça anatômica, era dissecada a veia cava inferior, observando-se a sua sintopia com o parênquima hepático. Foram obtidas medidas biométricas da veia cava inferior, como a medida do segmento retro-hepático e anotado o número de veias confluentes para o contorno lateral direito da veia cava inferior. Foi realizado estudo estatístico comparando os dados em relação ao sexo. Resultados: a sintopia do lobo caudado foi determinada como incompleta em 37 (97,4%) casos. A distância total da veia cava inferior em seu segmento retro-hepático foi em média 59,66 mm. O número de veias confluentes foi de 3,44 significantemente maior no sexo masculino (p = 0,027). Conclusões: A sintopia incompleta do lobo caudado é encontrada na maioria dos casos. Este fato facilita o acesso cirúrgico a veia cava inferior retro-hepática quando exposta pelo seu contorno lateral direito. Esta veia apresenta um segmento relativamente curto ocupando um sulco na parte posterior do fígado. Um pequeno número de veias confluem para o contorno direito, significantemente maior no sexo masculino. Deve o cirurgião preocupar-se com a dissecção da região devido a presença destes vasos e a possibilidade de sangramentos volumosos.

Functional venous anatomy for right-lobe grafting and techniques to optimize outflow

Right-lobe living donor liver transplantation has emerged as an alternative to cadaveric transplantation. An appreciation of the unique anatomy and behavior of the right lobe has emerged and has precipitated technical modifications. Living donors underwent right lobectomy, including preservation of significant inferior hepatic veins. The parenchyma was divided following a plane approximating the right border of the posterior two thirds of the midhepatic vein (MHV), but deviating anteriorly to include the distal one third of the MHV with the graft. Large venous tributaries from segment VIII were preserved. Anastomosis in the recipient was accomplished by means of complete cavoplasty. Significant inferior veins, tributaries to the MHV, and the distal portion of the MHV were reconstructed when technically possible. Forty-eight right-lobe resections and transplantations were performed in the manner described. There were no donor complications attributable to the technique. Forty-six of the 48 recipients are alive, and 44 of the 46 surviving patients have their original graft. Venous tributaries from segment VIII and/or the distal portion of the MHV were reconstructed in only 3 patients. Outflow obstruction was recognized intraoperatively in 2 patients; 1 patient had a caval web excised and the other patient required revision of the main anastomosis. Neither organ was lost. There were no other significant venous complications. The incidence of ascites was the same as that in recipients of whole organs. These methods of parenchymal transection and venous reconstruction resulted in a low rate of complications. The wide anastomosis and collateral pathways between the MHV and right hepatic vein seem to be more critical than reconstruction of tributaries from segment VIII or the distal MHV.

The minor hepatic veins: anatomy and classification

A detailed description of the distribution and drainage pattern of the minor hepatic veins is presented in this paper. A classification based on the segmentation of the liver divides these veins into four main groups: 1) veins of Segment I which includes the veins of the caudate lobe and the veins of the caudate process; 2) veins of Segment VI; 3) veins of Segment VII; and 4) veins of Segment IX. A knowledge of the anatomy of the minor hepatic veins becomes more clinically valuable as the number of complex dissections of the retrohepatic areas, hepatectomies. and hepatic transplantations grow.

Model of complete separation of the hepatic veins from the systemic venous system

BACKGROUND

In patients undergoing a Fontan operation, partial diversion of the hepatic veins to the pulmonary venous atrium has been tried with various techniques. They failed because of the development of intrahepatic collaterals leading to an unacceptable right-to-left shunting. We postulate that to avoid the formation of intrahepatic collaterals, the totality of the liver has to be drained into the same pressure compartment. We have designed a model of cavopulmonary anastomosis in which a prosthetic conduit reproduces an azygos continuation, associated with the diversion of the totality of the hepatic venous return. This article reports on the early hemodynamics and the fate of the separation of the two venous compartments in long-term survivors.

METHODS

Eighteen goats were operated on; the pulmonary artery and hepatic vein pressures were recorded. During month 2, an opacification of the inferior vena cava and the cavopulmonary connection was performed. Between months 6 and 14, another opacification was performed, together with pressure recording at both ends of the conduit.

RESULTS

Postoperatively the pulmonary artery pressure was pulsatile with a mean of 10 mm Hg and the hepatic vein pressure was 0 mm Hg. The first angiogram showed patent tubes with fast progression of the contrast. Throughout the inferior vena cava injection, there was no opacification of the portal or hepatic veins. The late study showed a narrowed conduit in all animals. During the injection, a collateral was injected, feeding into the inferior mesenteric vein. No collateral circulation could be seen draining directly into the liver. The median gradient between the two ends of the conduit was 11 mm Hg.

CONCLUSIONS

The isolation of the entire hepatic venous drainage is feasible and efficient for the separation of two pressure compartments. No intrahepatic collaterals are observed with this model at short- or long-term follow-up. The separation of the hepatic venous drainage should persist without collateral circulation as long as the inferior vena cava pressure stays at the levels observed in Fontan circulation.

Anatomical variations in the pattern of the right hepatic veins: possibilities for type classification

A morphological study of the right hepatic veins (RHVv) was conducted based on the shape and the confluence pattern of the superior right hepatic vein (SRHV) and the presence of accessory right hepatic veins. The study was performed in 110 undamaged, randomly selected, cadaveric human livers prepared using the corrosion cast methodology. The principles for classifying the RHVv into types were as follows: the length of the vein trunk, the confluence of 2 or 3 main tributaries that form a trunk, and the accessory right hepatic veins that modify the venous drainage of the right side of the liver. Four types of SRHV were identified. Type 1 (20 %), type 2 (40 %) and type 3 (25 %) were the most common, while type 4 (15 %) was linked to the accessory right hepatic veins in cases where they drain a surgically important part of the liver. Accessory right hepatic veins were found in a total of 31 casts (28 %). The hepatocaval confluence was studied and the tributary-free part of the SRHV trunk before it entered the inferior vena cava was measured. The tributary-free part of the SRHV was longer than 1 cm in 77 % of the casts. Anastomoses between the terminal tributaries of the veins involved in the drainage of the right side of the liver were also investigated.

Anatomic basis of hepatic surgery

Modern hepatic surgery is based on precise anatomic foundations. The importance of this information applies to all levels of the diagnostic and therapeutic chain. Modern methods of imaging--CT scanning, MR imaging, and preoperative sonography--help physicians to detect variations and plan surgical excision.

Identification of segments VI and VII of the liver based on the ramification patterns of the intrahepatic portal and hepatic veins

We describe the pattern of intrahepatic vessel ramification in the right posterior hepatic sector in a population of 197 adults. Each specimen was dissected from its visceral (inferior) surface in order to demonstrate variations in the distribution of the portal vein branches to the hepatic segments of the right lobe, especially to segments VI (S6) and VII (S7) as described by Couinaud. We also examine whether three hepatic veins, i.e., the right hepatic vein (RHV), middle hepatic vein (MHV), and the short hepatic vein (SHV), aid the identification of segmental portal branches in the lower posterior sector. Four major patterns of branching of the posterior sectorial trunk of the portal vein system are described. In group A (32.0%) a single posterior trunk formed an arch-like pattern sending multiple branches to S6 and S7 (P6 and P7). We named the multiple branches to the apparent S6 the inferoposterior portal branches. It was difficult to identify which of these branches were equivalent to P6. In group B (27.9%), the posterior sectorial trunk bifurcated to form P6 and P7. In most of the specimens in this group, therefore, we were able clearly to identify both S6 and S7 based on the portal vein system. In group C (6.6%), the trunk trifurcated to form P6, P7, and an intermediate branch, which supplied both segments or a gray zone between them. Group D (33.5%) included variations of the anterior segmental branches, and in specimens of this group, the anteromedial border of the sector was difficult to identify. Notably, the three-dimensional interdigitating topographical relationship of the hepatic veins and the portal branches was not evident in the lower posterior sector, since tributaries of the RHV and the portal branches followed similar courses and paralleled each other in the region and since the territory of the SHV was usually restricted to the superficial parenchyma near the inferior surface. In group A, tributaries of the RHV/SHV (>3 mm in diameter) passed between the inferoposterior portal branches in only 22.2%/14.3% of the specimens. Thus the hepatic veins often did not reveal which of the multiple inferoposterior branches was P6. Moreover, in the subset of Group B in which the segments were identified based on the portal vein ramification, tributaries of the RHV/SHV (>3 mm in diameter) showed the intersegmental interdigitating arrangement in only 32.0%/6.0% of the specimens. In addition, a thick tributary of the MHV, sometimes arising from S6, did not run along, but penetrated the S5/S6 border plane from the lateral to the medial side. Therefore, the three hepatic veins (RHV, SHV, MHV) often did not aid the identification of the liver segments in the region. Consequently, the less than ideal combinations of irregular configurations of the portal and hepatic venous systems suggest that the right posterior segments cannot be conclusively identified anatomically in 30-40% of cases. Other means of identification, such as the conventional proportional manner (the upper and lower halves of the posterior sector roughly correspond to S6 and S7) may be required.

Functional anatomy of the retro- and suprahepatic portions of the human inferior vena cava and their main affluents

The arrangement of muscle, collagen and elastic fibers was studied in the retro- and suprahepatic (subdiaphragmatic) portions of the inferior vena cava, the hepatic veins and their main affluents. Distinctive features of the longitudinal and transverse muscle bundles are described. In these portions of the vena cava, both bundle systems are clearly separate and any continuity was observed only at the entrances of the hepatic veins. A musculo-venulolymphatic complex was noted in spurs formed by the vascular junctions. The hepatic veins and their main affluents exhibit an elliptical contour in transverse section, which apparently results from cranial and caudal thickenings of the longitudinal muscle layer. Many of these bundles are in continuity with those of the transverse muscle layer. Terminal elastic tendons were rarely observed in connection with muscle fibers of the inferior vena cava and are not present in the hepatic veins and their main affluents. In terms of form and function, the relatively thin muscular layer has a dilating action on the hepatic venous system because of the external fixed insertion point of the muscle bundles. Such an arrangement and a "polar" disposition of the muscle bundles in the hepatic venous system may assists in "suction" of the blood toward the heart. A sphincteric control of the ostia by means of crossed muscular loops supported by venulo-lymphatic micropads is a possibility.