
I have to admit none of the books on breeding mention this aspect. The bulbous glandis is mentioned in relation to mating.
I have ownerd bitches for nearly 20 years, and bred my first litter 11 years ago, but had never lived with a dog.
I have to say I didn't realise that the gland could swell whilst sheathed until I was grooming our imported boy about three years ago, and cupped my hand over the sheath (to protect it) while brushing his belly with a slicker. I realised straight away what it was.
The first time I had ever seen the Bulbous glandis of a dog was when I was trying to mate my awkward bitch and the dog tied himself outside the bitch who had whipped away at the crucial moment. I was gob-smacked as it looked for all the world like the dog had an extra set of balls