I having trouble finding information regarding the actual procedure, only that it was performed. I'm curious to know the tools and medicines (if any) involved. My impression is that gelding horses was a somewhat common practice, but it's hard to tell if neutering bulls and other farm animals happened regularly. It seems easier to find information on human castration than animal. I'm mainly interested in the practices of societies during the medieval time period, but I'm not picky. Hope this is detailed enough. Thank-you!
I'm not sure what you mean by "advanced surgical knowledge"--the process of castration requires quite a bit of knowledge and skill in order to preserve urinary function. But as far as medieval-era practices, we have multiple sources from the Arab world that describe different ways of gelding. For the most part, that meant removal of the testicles themselves rather than the scrotal pouch.
In his veterinary work Kashif, Abu Bakr al-Baytar describes four methods of gelding horses:
Surgery with a white-hot iron tool. The horse is placed on its back, legs in the air. The scrotum is tourniqueted at the base and then cut lengthwise to separate the testicle from other tissue. The vet uses forceps to cut it out. Liquid tar is used to stop the bleeding. Afterwards, garlic is applied (which we know today has disinfectant properties).
This method is exactly like the first one, except the cut is deeper. Abu Bakr feels very strongly that this qualifies as a different method, and he does not like it.
Abu Bakr calls this method "Frankish," which is either a reference to learning it from the West or expressing the view that it is 'barbaric'--medieval Muslim writers are almost uniformly horrified by Frankish (Latin Christian) medical practice. This method uses heavy wooden forceps to crush the testicles, compressing them until the tissue dies off and falls off.
Used more on rams and bulls than horses, this method is like the first two, except the vet wraps the tissue and blood vessels to be removed around a stick and twists them up until they break off.
A special kind of diaper-bandage is placed around the animals hindquarters afterwards to seal off the area from "bad air" (which we now know is germs).
We know about various refinements, such as different types and sizes of white-hot forceps used, from other authors. But those are the basics.
Further reading: Housni Shehada, Mamluks and Animals: Veterinary Medicine in Medieval Islam
I grew up with horses. We had a herd of 100 that we used on a regular basis and did the castrations on them ourselves.
The testicles are attached to the body by a long cord. This cord is made up of ligaments, the sperm duct and the bloodline. If the bloodline is cut clean it is nearly impossible to stop the bleeding and the horse will most likely bleed to death.
Before we bought emasculators we would use a pair of pliers to mash the bloodline for about 4 inches before we cut off the nut. This gives the blood matter to stick to and clot, thus stopping the bleeding.
Another method I have seen used a red hot iron. You would first rub it up and down the blood line and then burn through the cord attaching the testicle. We didn't use this method because the horse puts up one hell of a fight and moves around a lot when the hot iron is burning through the cord. There was too much of a chance that we could get burnt or that the cord would get cut before the bloodline and it would end up breaking clean and kill the horse.
About 6 inches above the testicle and in the cord is a bulbous gland. If the horse is cut below this gland he is Proud Cut. He will still act much like a stud. If the bulb is cut out then the horse will be a meek and well mannered animal.
We washed the ball sack and knife off with pine tar before and after the operation. The healing properties of pine tar has been used for centuries in Europe.
Hope this helps.