Which of the following is the least effective barrier?

The vital dental pulp has a slight positive pressure that results in continual dentinal fluid flow toward the external surface of the tooth. Enamel and cementum, though semipermeable, provide an effective layer serving to protect the underlying dentin and limit tubular fluid flow. Dentin permeability is not uniform throughout the tooth. Coronal dentin is much more permeable than root dentin. Dentin permeability primarily depends on the remaining dentin thickness (i.e., length of the tubules) and the diameter of the tubules. Because the tubules are shorter, more numerous, and larger in diameter closer to the pulp, deep dentin is a less effective pulpal barrier compared with superficial dentin. Reference: Sturdevant operative dentistry, 7th edition page no 9