The various forms of uncleanness according to the Mosaic law are enumerated in Leviticus 11-15; Numbers 19. The division of animals into clean and unclean was probably founded on the practice of sacrifice. It existed before the Flood (Genesis 7:2). The regulations regarding such animals are recorded in Leviticus 11 and Deuteronomy 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 (Exodus 29:13, 22; Leviticus 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 Leviticus 20:24-26; Acts 10:9-16; 11:1-10; Hebrews 9:9-14.