Breaking the amniotic sac?

[Putangitangi]

If you're going in to help a cow calve and the calf's sac isn't out yet, how do you (do you?) break it before putting ropes/chains on the calf's legs to get it out?
Often when I've gone to help, the bag is already broken, so I am in direct contact with the legs and can work with them more easily but the other night, couldn't quite work out what to do when I couldn't break the bag. Didn't want to try going in there with something pointy!

 

[wbvs58]

Usually I am able to poke my finger through it then tear it. Those knives that fit over your a finger and sit in the palm of your hand and cut in reverse, used to skin out emphysematous calves, would be a help.

Ken