It's left open so it drains properly. Infection can set in if the wound is closed up and is not allowed to drain freely. For those interested in knowing more about castrating a horse: http://www.thehorse.com/articles/10024/castration-in-the-horse
A friend has just got a job as an office admin at a local vet. She told me that it is popular in this area that dog owners request prosthetics (silicon testicles) inserted after castration. She also said that if the husbands select the prosthetics, they go for the bigger size! (This particular vet has made it her life's ambition to castrate every dog she can, hence another reason I don't go to her.)
"The last step is veterinary preference; I remove the skin between the two sites, leaving one large incision to drain. Other veterinarians leave the two sites open without removing the skin in between. The horse will stand up within 10-20 minutes, sometimes sooner. The open incisions are left to heal on their own (otherwise called second intention healing)." From the link provided by Oberon. Bold is mine. This appears to be more common here. I am not involved with horses now but I don't remember cleaning and grooming any geldings with anything other than smooth skin. The one stallion I rode was sold to a private owner who had him gelded at age 7 and it was one heck of mess, but complicated by another stallion getting out and the two fighting, so the area where the skin was removed might have ruptured. It was a sin to geld him, I rode that horse in shows with mares in heat flagging right in front of him and he was perfectly controllable. I see lots of neutered doggy friends who have a smooth tummy. It actually looks a bit strange to me to see them with nothing between their penis and bum, not even a bit of shrivelled up skin. My Vet says it is the norm at her practice to only leave the skin of the scrotum on cats and maybe young small dogs as anything bigger is not likely to disappear. We did a cat in November and he still looks like a Tom.
This isn't removing the whole scrotal sac though, which is very large. After the op the sac reduces dramatically in size over time, which is why you don't see much in a gelding. If you have a closer look though, the scrotal sac is there (just small and very drawn up) and so is the scar from the incisions.
After a human vasectomy there is normally a need to check the semen to make sure there are no sperm remaining. This is because there may be remaining live sperm in the tubes at a point AFTER the "snip" that can still be ejaculated to achieve pregnancy and they only dwindle slowly following repeat ejaculations. Also there can be more than one tube carrying the sperm and they can be darned small to spot and clip/tie or destroy. And yes it is possible they can regrow hence I think current practice for mens tubes and women's tubes is to remove a short length and then turn both ends back on themselves to reduce joining potential. So....I am guessing vasectomy is TECHNICALLY more difficult. And has a higher failure rate. Although the in humans it can be and normally IS done under a local rather than general so safer but guess that doesn't apply to our furry friends! Don't know if that is any help....
