Bulbus glandis
The bulbus glandis is an erectile tissue
Erectile tissue
Erectile tissue is tissue in the body that can become erect, usually by becoming engorged with blood.-Erectile tissue in the clitoris and penis:...

 structure on the penis of canid mammals. During mating the tissues swell up and lock (tie) the male's penis inside the female. For domestic dogs the tie may last up to half an hour, though usually less.
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