The three liver segments are each separated into smaller areas according to the branching pattern of the tertiary branch. The area fed by each of the tertiary branches is called a "cone unit". The base of each cone unit lies on the surface of the liver and the apex lies in the origin of these tertiary branches.
Each segment is composed of six to eight cone units. One cone unit is the smallest unit for which a tertiary branch can be transected selectively.
The exact number of cone units in each segment differs from individual to individual, because the branched structure of the Glissonean pedicle (tertiary branching) is rather complicated, with no regularity.
The distribution of cone units in each segment is not always constant, and it is often impossible to determine, because the number of cone units and their distribution differ from person to person. With no fixed rule to follow, a case-by-case approach is required.
Therefore, for convenience, the right and middle segments are each subdivided into two parts: superior and inferior segments, and the left area is subdivided into medial and lateral areas. Usually each part consists of three or four cone units. Figure 4.1 shows a schema of a core unit.
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