הלכה על יבמות 139:8

The root of the commandment and all of its content is included in the previous commandment, since the reason for their prohibition is that they are considered like "foreigners," since he is not his acquired property - as "the boarder" is the perennial worker, and "the hired worker" is the annual worker (see Yevamot 70a).

From the laws of the commandment is that which they, may their memory be blessed, explained (Yevamot 70a, and see Mishneh Torah, Forbidden Intercourse 16:3), "Who is [considered] 'one whose testes are crushed?' Anyone whose testicles were wounded [...]. And [who is considered] 'one whose member is cut off?' One whose penis was cut off." And there are three organs that the male reproduces with: with the penis, and with the testicles and with the paths in which the seed matures - and they are called the testicular ducts. And therefore from when one of these three organs is wounded or cut or crushed, behold, he is disqualified. And the rest of its details are elucidated in the eighth chapter of Yevamot.