The various forms of uncleanness according to the Mosaic law are
enumerated in Lev. 11-15; Num. 19. The division of animals into
clean and unclean was probably founded on the practice of
sacrifice. It existed before the Flood (Gen. 7:2). The
regulations regarding such animals are recorded in Lev. 11 and
Deut. 14:1-21.

The Hebrews were prohibited from using as food certain animal
substances, such as (1) blood; (2) the fat covering the
intestines, termed the caul; (3) the fat on the intestines,
called the mesentery; (4) the fat of the kidneys; and (5) the
fat tail of certain sheep (Ex. 29:13, 22; Lev. 3:4-9; 9:19;
17:10; 19:26).

The chief design of these regulations seems to have been to
establish a system of regimen which would distinguish the Jews
from all other nations. Regarding the design and the abolition
of these regulations the reader will find all the details in
Lev. 20:24-26; Acts 10:9-16; 11:1-10; Heb. 9:9-14.