The vermian group and hemispheric group of bridging veins are also termed the medial group and the lateral group, respectively (Figs. 5.1 and 5.2a, c) [14, 15]. Vermian bridging veins, which are always located beneath the torcular herophili, are mainly formed by a collection of the terminal portions of the inferior vermian and declival veins (Figs. 5.1b and 5.2a) [14]. The vermian group includes the inferior vermian veins coursing upward in the paravermian sulci, the vein of the horizontal fissure coursing in the horizontal fissure, the declival vein coursing on the posterior part of the superior vermis, and a few tributaries in the postclival fissure.

Conversely, the hemispheric group of bridging veins are formed by a collection of the terminal portions of the superior and inferior hemispheric veins and drain into the tentorial sinus (Figs. 5.1, 5.2, and 5.4) [14, 15]. Therefore, the presence and number of hemispheric bridging veins depend on the course of the superior and inferior hemispheric veins. When the inferior hemispheric veins course longitudinally upwards on the suboccipital cerebellar surface to the posterolateral margin, the hemispheric group of bridging veins usually form on the tentorial cerebellar surface. However, when the inferior hemispheric veins course diagonally in the horizontal fissure on the suboccipital cerebellar surface, the hemispheric group of bridging veins do not form on the tentorial surface (Fig. 5.2a, c). The horizontal fissure communicates between the petrosal fissure on the petrosal cerebellar surface and the upper end of the inferior vermian sulcus. There are usually one to three hemispheric groups of bridging veins near the postclival fissure on each side [14, 15]. They also comprise the origin of the Group 2 tentorial sinuses, which are described later. Therefore, a good understanding of the anatomical features of the Group 2 tentorial sinuses aids in recognizing the hemispheric group of bridging veins, which can be seen on three-dimensional computed tomography (3D CT) and cerebral angiography. Because the hemispheric group of bridging veins drains into the tentorial sinuses, classification of the tentorial sinuses, particularly Group 2, will be explained in the next section.

The tentorial sinuses are classified into four groups on the basis of their origin:

Group 4 is often related to persistent embryonic veins. Knowledge about the Group 2 tentorial sinuses and its constituent bridging veins is critical in posterior cranial fossa surgeries, particularly those employing the infratentorial supracerebellar approach.

The superior hemispheric veins running downward from the anterior part of the tentorial cerebellar surface as well as the inferior hemispheric veins running upward from the posterior part of the tentorial cerebellar surface join to form bridging veins near the postclival fissure, the fissure between the simple and superior semilunar lobules situated posteriorly on the tentorial cerebellar surface. These bridging veins form the Group 2 tentorial sinuses, which drain into the torcular herophili or the transverse or straight sinuses near it (Fig. 5.4). The Group 2 tentorial sinuses can be further divided into five subtypes on the basis of their draining veins and the direction of termination (Fig. 5.5) [15]. Our previous cadaveric study revealed that the Group 2 tentorial sinuses were present in 88.5 % specimens and were always located in posterior parts of the tentorial cerebellar surface [14, 15]. The tentorial sinuses and bridging veins can be visualized before surgery using 3D CT (Fig. 5.6a); Fig. 5.6 shows 3D CT and intraoperative findings in a patient with medulloblastoma. Majority of the Group 2 tentorial sinuses drain into the torcular herophili; therefore, they can be considered similar to the tentorial draining group of the three venous draining groups of the posterior cranial fossa, according to Huang YP and Wolf BS [4].

Close to the apex of the culmen, after passing the bridging veins beneath the torcular herophili through the midline infratentorial supracerebellar approach, we encounter bridging veins near the tentorial free edge, including the superior vermian vein and/or the vein of the cerebellomesencephalic fissure. These veins converge and join to form a large vein that finally drains into the vein of Galen as a bridging vein. These veins belong to the Galenic draining group of the three draining groups of the posterior cranial fossa, according to Huang YP and Wolf BS [4]. Coagulation and incision of one of the small bridging veins, for example, the small superior vermian vein, provides a better view of the structures around the pineal body and posterior midbrain. The basal vein of Rosenthal and internal cerebral vein appear in the operative field around the pineal body; great care should be taken to avoid injuring these veins.