Chullin 93 - Fat and Blood
Some of the animal fat is prohibited by the Torah, and some is similar to it and is prohibited by the Sages. For example, the Torah prohibited " the fat that is on the flanks ." As a rule, that includes only fat that is visible when the animal is flayed, but the fat covered by the muscle is permitted. However, as Rav Yehudah taught in the name of Shmuel, the fat that is under the loins is prohibited. How could this be, if it is covered by the loins? - As Abaye explained, when an animal is alive, its limbs separate from one another, and at times that fat is uncovered.
Rav Abba said, "The blood vessels in the foreleg are prohibited." Rav Safra said to him, "Moses! Did the Merciful One say 'You shall not eat meat?'" Rava answered him, "Moses! Did the Merciful One say 'You may eat blood?'" Rather, if you are roasting the meat, the heat extracts the blood. However, if you are cooking the meat in a pot, you need to cut the vessels open first and salt them. Otherwise the prohibited blood gets reabsorbed into the meat.
Art: Esaias Boursse - Interior with Woman Cooking