Caul

CAUL (יֹתֶ֫רֶת, H3866, appendage; סְגוֹר, H6033, enclosure). Caul occurs only in the KJV. Its meaning is uncertain. The former term was used in the laws pertaining to the sacrifices, and is related to the liver. RSV trs. it, “appendage of the liver.” It is possible that it means the diaphragm, which hugs the liver like a cap; or the fatty mesentery, which extends downward like an apron to cover the small bowel. The second term appears in Hosea’s prophecy: The Lord will “rend the caul of their heart” (Hos 13:8 KJV). It prob. means the fleshy sac or pericardium in which the heart is encased. “Caul” as used in Isaiah 3:18 (KJV) refers to a decorative headband or bonnet.

International Standard Bible Encyclopedia (1915)

kol:

(2) ceghor (from the root caghar, "to enclose," "shut up"), Ho 13:8, literally the enclosure or covering of the heart, the caul or pericardium, or perhaps the chest as surrounding the heart. It must not be forgotten, however, that the expression may be taken in the sense of "mailcoat of the heart," i.e. hardened heart, which is shut to the influence of God’s grace. So Luther and many modern translators and commentators.