Although a nazir cannot allow himself to become tameh even for his immediate relatives, a kohen is permitted – in fact, with the exception of a kohen gadol, he is required – to participate in the funeral of his immediate relatives, including his mother, father, son, daughter, brother and unmarried sister (see Vayikra 21:1-3).
Rav is quoted by Rav Ḥisda as teaching that this is true only if the father’s body is whole; if his head was removed from his body, the kohen is not allowed to become tameh. The Ramban, in his Torat ha-Adam, limits this to cases where a limb was removed at the time of death or after death. If the relative lived without a limb, upon his death we consider his body to be “complete” and the kohen is expected to participate in his burial.
The Gemara quotes a baraita that appears to contradict Rav Ḥisda’s teaching. We find that although a kohen cannot allow himself to become tameh through contact with a limb that was cut off from his father’s body while he was still alive, should his father die he will become obligated to search for every body part to bury, even an etzem ke-se’orah – a bone the size of a grain of barley. Clearly even if this bone was missing, the kohen is nevertheless involved in his father’s burial. In response, the Gemara suggests that there is a disagreement among the tannaim, and that the baraita follows the opinion of Rabbi Yehuda, while Rav Ḥisda followed the other sages.
In order to understand the concept of etzem ke-se’orah, it should be noted that most of the bones in an adult body are much larger than a barley grain. Still, there are some very small bones that are found in an adult. An example of this would be sesamoid bones, thus called because they are the size of sesame seeds, which are formed in the musculature around bone joints. There are also certainly bones this small found in the tiny bodies of infants.