Reference for Chullin 221:39
רב אשי איקלע לבי רמי בר אבא חמוה חזייה לבריה דרמי בר אבא דקא
only with the liver, but as to the spleen it contains merely a fatty juice.<span class="x" onmousemove="('comment',' It does not contain much blood, and so does not need to be cut up.');"><sup>18</sup></span> Thus on the day when Samuel was bled they prepared for him spleen broth. It was stated: [To roast] the liver on top of meat, is permitted, for the blood glides off;<span class="x" onmousemove="('comment',' The blood that is drawn out of the liver will not be absorbed by the meat but will run of the meat and drip on to the fire, and so the meat is permitted.');"><sup>19</sup></span> [to roast] the udder on top of meat is forbidden because the milk clings [to and penetrates into the meat]. R'Dimi of Nehardea reports this just the reverse thus, [To roast] the udder on top of meat, is permitted, because the milk of a slaughtered animal is but a Rabbinic prohibition; liver on top of meat is forbidden because the blood is a Biblical prohibition. Meremar declared in a public exposition: The law is, both with regard to the liver and the udder: under meat, it is permitted; on top of meat, it is permitted only after the act, but one may not do so in the first instance. R'Ashi once visited the house of Rami B'Abba his father-in-law when he saw the son of Rami B'Abba