Disclaimer: this is posted here for historical posterity only. I do not recommend following the instructions at the bottom of this writeup in real life. Basically, don't try this at home.
The Principles and the History
The method of bleeding humans to cure disease is an ancient one, possibly dating back to Ancient Egyptian culture. Although the basic premise remains the same, i.e. releasing blood from a sick body in order to help the person in question recover, there have been varying justifications or reasons for bleeding throughout history.
It is likely that the practice has its root in pagan beliefs. Almost all ancient cultures believed in evil spirits or demons that could possess a human being, and such spirits were often blamed for events such as poor harvests or illness. The demons would be present in the lifeblood of the possessed person, and so some of their blood would need to be released in order to expel the spirit and cure the person. Despite this pagan, primitive belief, the fact remains that bleeding remained standard practice right up to the discovery of micro-organisms such as bacteria, later identified as the actual causes of disease.
For example, in Ancient Greece bleeding was used as a remedy in accordance with the "Theory of the Four Humours", devised by Hippocrite. This theory stated that illness and subsequent death was caused by an imbalance in a person's "humours". The humours would be rebalanced by purging the human, by forcing them to vomit - or by bleeding them.
However, the belief in pagan spirits, or indeed Hippocrite's "humours", could not survive the spread of Christianity in the first millennium AD. The reasoning behind the "cure" of bleeding developed and evolved to suit different cultures in different parts of the world as they continued to grow. For example, in Great Britain only a couple of hundred years ago it was believed that illness was caused by simply having too much blood in the body - necessitating the release of some of the vital liquid. It is also probably likely that the "success" of the practice lay in the belief that it would actually work, and enough people must have recovered from their illnesses for the belief in bleeding to remain.
A possible explanation for this is that the "doctors" (the role played by
druids in ancient tribes; doubling as barbers more recently, hence the red-and-white striped pole) recommended bleeding for minor illnesses, such as a fever, and in combination with other (possibly more effective) remedies the person would most probably survive. If they did, then belief in the cure would grow, so the bleeding of humans to cure disease remained a standard part of medical practice for thousands of years.
A natural extension of bleeding sick humans was to bleed other sick organisms - horses. It was not practical to bleed any other animal, nor was it even desirable to do so. Good horses have always been expensive, and as they have been such an important part of human civilisations throughout history, it was seen as vitally important to look after them.
It is not known when horses were first bled, but again the method remained standard practice well into the 20th century, until the development of drugs that could heal the animals more easily. There is a very good reason for the practice of bleeding horses, and indeed humans, being so commonplace - it worked. Had there not have been a marked improvement in the horse's health after it was bled, it is unlikely that it would be recommended as highly as it has been by contemporary horse doctors. Although it may sound cruel, it was in the animal's best interests to be bled, and the process would only take a few seconds; after all, therer is a limit to the amount of blood that can be spilled before the horse is weakened to the point of death.
Examples of the afflictions that were combatted with bleeding include laminitas; fever or influenza; "founders" (?); inflamed stomach, bowels and/or kidneys; "staggers" and "distemper". Bleeding is rarely, if ever, recommended as a lone treatment. Various herbal ointments, poultices or pills are generally recommended in conjunction with it.
Points to Consider, Tools of the Trade and How To
There are several points to consider when bleeding a horse. Firstly, there are the biological considerations. Those familiar with horse anatomy will know that they have very thick skin, and so the cutting implement somehow had to be forced through this barrier. There are also several possible locations on the horses body to bleed from. The main three are: the vein in the neck (the jugular), a vein in the leg, and (unbelievably) the horse's mouth. Of the three, the neck is the most accessible and was probably the most commonly used.
Of course, when bleeding a horse the servants or grooms who were tasked with doing it took care not to actually kill the poor animal. Descriptions of various cures from the nineteenth century vary in their recommendations of how much blood to let flow: from simply "bleed freely" to exact measurements such as "one gallon" or "six to eight quarts". Any more than this, and any more frequently than only once per day, would have been taking more risks than strictly necessary. The actual process took very little time, taking barely a minute.
In 19th century Britain there were even specialised tools, specifically designed for use when bleeding a horse. These were simple objects, as the actual cutting was a fairly simple thing in itself. The blade used to cut the horse's vein was one of a dozen, all encased in a "fleam". Not unlike a penknife, the fleam held these 12 folding blades, two of which were hooked, the rest shaped like spoons. The hooks were more likely used to tend the horse's hooves rather than bleed it. To drive one of the spoon-shaped blades through the horses thick skin, a "blood stick" was used. This was simply a small wooden club, yet effective.
When spilling blood from the horse's jugular, the fleam had to be aligned along the vein, NOT across it. If the cut was made across the vein, the blood flow could not be regulated and the horse would most likely bleed out and die. The horse would be held by another servant, or (in the military) an orderly. Thus, when the blood stick drove the fleam through the horse's hair, skin and flesh and the horse reared as a result, somebody would be on hand to calm the animal down and keep it in place. The person holding the fleam inside the horse's vein would then twist the blade slightly, opening the blood vessel up (when flesh is punctured, suction closes the wound around the object, in this case the vein around the fleam) to let the required amount out. By increasing or relaxing the pressure on the fleam, the wound could be widened or decreased in size to regulate the flow of blood. Then, when enough blood was judged to have been spilled, the fleam was removed. The cut in the vein would then heal fairly quickly, perhaps with the help of a dressing, leaving a fair bit of blood over the horse, the person who bled it and the surrounding area to be cleaned up.
Although no difference may have been noticeable immediately, sometimes a horse's condition improved almost at once after it was bled. This must in part explain how the practice of bleeding horses was so common and accepted for so long and across so much of the world. The advent of modern medicine and vetinary practices have replaced older remedies such as bleeding, and there are plenty of more effective cures available for diseases once combatted by cutting.
However, the "bleeding" of horses does still occur today. Unscrupulous racehorse owners may take blood from a horse (using a syringe, mind you) several days or perhaps weeks before a race. Then, just before the race and when the horse's body has recovered, the owner will inject the blood taken earlier back into the horse. This has the effect of increasing the amount of red blood cells in the horses blood, thereby increasing the concentration of haemoglobin. This increases the horse's endurance, as more oxygen can be retained, lengthening the period of aerobic respiration, and basically keeping the horse going for longer. This process is clearly illegal, and is actually identical to human athletes doing the same thing to boost their performance (and being a form of illegal enhancement is known as "blood doping"; my thanks to Kit). In fact, the same result can be achieved by training at high altitude for long periods, as the body makes up for the lack of oxygen by increasing red blood cell production.
This highlights the link between man and horse that has existed since ancient times, and still exists today. What was applied to one was applied to the other, with more success, in fact, with horses than ourselves. From thousands of years before Christ was born to thousands of years after, there remains this (admittedly tenous) similiarity between the two species that began with the letting of blood from the body to help cure it of disease.
A big, big thanks to all the helpful horse enthusiasts at the www.newrider.com forum for replying to my thread and helping me out.
Pages 80 to 83 in Bernard Cornwell's Sharpe's Triumph were also very useful, believe it or not.
For other crazy 19th century horse remedies, see either:
http://members.tripod.com/HistoricalNovelists/hors1861.html
or:
http://www.compusmart.ab.ca/rgiokas/richard/storys/cox/notebook.htm
Lastly, for information on other modern horse "cutting", i.e. in slaughter houses, see:
http://equineprotectionnetwork.com/slaughter/render.htm