Siman 74

The spleen, even though it has a reddish appearance and looks like an abundance of blood, nevertheless, its law is only like that of other meat, whether for roasting or for a pot, and it is permitted to cook it with meat after salting, because it is not blood but rather meat that resembles blood [the Rambam chapter 6], and its appearance is due to its fat [Chullin 111a], and it does not require cutting into it, because it does not have large vessels as the liver does [Tosafot ibid.], and it only requires the removal of the chelev and the membranes as written in siman 64, and on the head of the spleen there is chelev for which one is liable for karet as written there. [In the Hagahot Mordechai, chapter Kol HaBasar, it is found that it is forbidden to cook a spleen with meat, see there, and it appears to me that it is a printing error, and it should say liver, for regarding the spleen there is no reason to forbid, and furthermore, above he himself wrote the opposite, see there]:

There was an incident involving a spleen that was roasted without nikur, and the great authorities permitted it, because the chelev of the spleen was nullified by the spleen itself and the kosher fat within it, and they estimated that the spleen and the kosher fat within it contain sixty times the chelev [Bach and Taz in the name of Hagahot Sha'arei Dura]. And there is one who wishes to say that in an ordinary spleen this is the case, that it contains sixty times the chelev [Rema in Torat Chatat]. And if so, in our case as well one should permit it if such an incident occurred, and this is like the heart in siman 72 (Shulchan Aruch Yoreh Deah 72) where it was explained that there is no bird that does not have sixty times the heart, and here too it is similar. And some say that since the poskim did not explain regarding the spleen as they did regarding the heart, it implies that it is not a general rule; rather, in that specific incident they estimated that there was sixty, and therefore if such an incident occurs, one must estimate if there is sixty or not [Bach, and Taz, and Pri Chadash]. And one of the great authorities decided according to the first opinion, that even ordinarily there is sixty, and the reason the poskim did not mention it regarding the spleen as they did regarding the heart is because in the spleen this is no novelty at all, for it is a case of "kind in its own kind," which according to the Torah is nullified by a majority, and therefore even if there is a doubt whether there is sixty, it is permitted, as in the case of "spilled" in siman 98 (Shulchan Aruch Yoreh Deah 98). This is not the case regarding the heart, whose prohibition is due to blood, making it "kind in a different kind," which according to the Torah requires sixty; and even though cooked blood is a rabbinic prohibition, nevertheless, according to the opinion of Rashi and the Rambam, it contains a Torah prohibition [Kreiti U'Pleiti].

And we learned from this incident that the fat of the tahol permeates through the entire piece, for if it were not so, how would sixty be relevant here? For the incident occurred with roasting and not with boiling, and it should have only required the removal of the place. But if it permeates through the entire piece, it correctly requires sixty as in boiling, as will be explained in siman 105 with the help of Heaven, and it is a simple matter that they also required the removal of the place as is written there. And we also learned from this that if a whole tahol was cooked without nikur, certainly one should be lenient, for even according to the one who says that ordinarily there is not sixty here, nevertheless, in combination with the cooked food there is certainly sixty. And in this case, one should not be stringent because of the prohibition of a "clinging" forbidden substance, since there are those who say that there is always sixty in the tahol itself as I have written, and the prohibition of a "clinging" forbidden substance is not agreed upon by all the poskim; therefore, one should not be stringent in this. Furthermore, it appears to me that regarding a tahol, where the fat is on the tahol from the outside, the prohibition of a "clinging" forbidden substance is not relevant at all, for the reason for the prohibition of a "clinging" forbidden substance is because this piece absorbs first and becomes a neveilah, but here the senses testify that as soon as they placed the tahol in the pot, the fat was immediately absorbed in the water and in everything in the pot just as in the tahol itself.

Some say that it is the custom not to cook the kidneys or the testicles even after they have been de-veined, because they contain a majority of blood, but post facto one need not be concerned. Our teacher, the Beit Yosef, in his great book wrote regarding this that we practice the permission to cook them and no one objects, see there. And it seems to me that even in our time we cook the kidneys after salting, and it is permitted to salt all of these with other meat, even though they contain a majority of blood, provided that they remove the membranes and the chelev from them as was written in siman 64, and it is not similar to liver on top of meat which is forbidden ab initio as was written in siman 73, for the liver is entirely blood.