Bulbus glandis

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The bulbus glandis is an erectile tissue structure unique among mammals to the penis of species in the family Canidae.

After the male dog mounts and penetrates the bitch, the bulbus glandis becomes engorged with blood. It forms a spherical structure at the base of the penis, proximal to the os penis. There is a corresponding ridge of erectile tissue in the bitch's vagina; when this becomes engorged behind the engorged bulbus glandis, the penis is trapped within the vagina. This forms a seal to hold the semen within the bitch's reproductive tract, and prevents the couple from separating during copulation when the male turns away from the female.

It is known colloquially as the "knot".