I find myself wondering if I should be embarassed that I know the answer to this, but it's from veterinary suppliers that sell to farming areas. When you are running a meat or dairy farm, you will have a lot of pregnant cows. And since you can't ultrasound a cow very easily, you check the foetus is growing well using a shoulder length rubber glove and a bucket of lube. Cows are big, bottles just don't cut it. Also very helpful for when you have an entire herd of cows giving birth and inevitably one gets stuck.