The internal wall perpendicular to the long axis of the tooth in a Class II amalgam preparation is ______.
There are two types of internal walls. The axial wall is an internal wall that is oriented parallel to the long axis of the tooth.
The pulpal wall is an internal wall that is oriented perpendicular to the long axis of the tooth and is located occlusal to the pulp. This internal wall may also be referred to as the pulpal floor.
The external wall that is approximately horizontal (i.e., perpendicular to the occlusal forces that are directed occlusogingivally and generally parallel to the long axis of the tooth crown may also be referred to as a preparation floor (e.g., a gingival floor).
The enamel wall is that portion of a prepared external wall consisting of enamel.

Reference: Sturdevant operative dentistry. 7th edition pg no 123
Pulpal floor
Axial wall
Gingival floor
Enamel wall