What an amazing athletic machine is the horse. Few other animals of similar size and bulk manage to move with such efficiency, grace, and sheer speed. But despite our having worked side by side for centuries, there's still much we don't understand about a horse's inner workings.

Take the guttural pouch, for example. It's a strange, fist-sized cavity high up in the horse's skull, slightly behind his ears. Officially, it's an "outpouching" of the eustachian tubes (the passageway that takes in soundwaves from the ears), which drains just behind the nasal septum. Technically speaking, there are two guttural pouches, separated by a thin membrane--one for each eustachian tube--but for the most part they're referred to as a single structure. All the guttural pouch seems to hold is air--about 300 to 500 ml worth on each side.

We know that all horses have a guttural pouch, and some of their distant relations have a smaller version of the same structure (rhinos, tapirs, and hyraxes--those little short-eared mountain rabbits). We know that very few other animals have anything similar. But until very recently, we didn't know why they existed.

Various fanciful functions for this big air space have been proposed over the centuries--a resonating chamber for the equine whinny, perhaps? A mechanism to help horses swallow? An air pressure regulator for the ear drum? A hearing aid? But basically, it seemed the guttural pouch's raison d'etre was to be the equivalent of the human appendix--an apocryphal structure whose main function was to cause trouble. As anyone who has struggled with respiratory disease in his barn knows, the guttural pouch is a prime site for stubborn infections, either bacterial or fungal, which can cause persistent drainage of gunk from one or both nostrils. On occasion, the guttural pouch can suffer from a condition called tympany, in which it overinflates with air. (This happens most often with foals, who end up looking like chipmunks storing nuts for the winter.) It's also prone to hemorrhages from the internal carotid artery, which runs for about 10 cm through the thin mucous membrane of the guttural pouch on its way to the brain.

Here's another mystery for you. As an animal with a lot of bulk and a proportionally small surface area, the horse generates a lot of body heat when he exercises, and has more trouble than most animals in getting rid of it. Sure he sweats, and that mechanism does a pretty good job of radiating excess heat. As the sweat evaporates on his skin, it provides a cooling effect. But in extreme conditions, or when a horse is really exerting himself to the max, sweating might not be enough.

At times like that, the temperature of his muscles can rise by more than 5° C (9° F) from their usual 38° C (100.4°F).

Most internal structures operate well only in a very narrow temperature range, and when they heat up past a certain point (a condition called hyperthermia), they cease to function at peak efficiency. The real danger is that one of the most heat-vulnerable structures is the command center, a.k.a. the brain.

An overheated brain begins to short-circuit--and when it no longer sends clear signals to all of the other body systems, they begin to fail. So, needless to say, it's important to keep the brain cool and thinking clearly.

Evolution has provided most animals with some sort of thermoregulating system to ensure that their brains stay cooler than the rest of the body when maximum exertion is called for. Cows, for example--critters with a body size roughly analogous to horses--are equipped with a "carotid rete," a system they share with many other animals. With a carotid rete, the carotid artery splits up into a complex mesh of blood vessels, which are cooled by venous blood returning from the skin surface. Arterial blood passes through major muscle groups before it heads to the brain and can pick up a lot of heat in the process, but the mesh acts like a radiator, allowing better dispersion of that heat from the blood before it reaches the nerve center.

Horses, however, don't appear to have a carotid rete. At best, they have a rudimentary version, in which the carotid artery splits into a couple of branches; it's hardly the fine and wide-spreading mesh that cattle sport. Since horses are far more likely to exert themselves than most cows, there had to be some other way that horses cooled their arterial blood so that it wouldn't cook their brains.

Jonathan Naylor, BVSc, PhD, Dipl. ACVIM, a professor in the Department of Veterinary Internal Medicine at the Western College of Veterinary Medicine, University of Saskatchewan (Canada), credits his former graduate student Dr. Keith Baptiste with putting two and two together. When Baptiste arrived at Naylor's Saskatoon lab to pursue a master's degree, he had already postulated that the guttural pouch might play a significant role in cooling a horse's arterial blood, and thus protecting his brain from hyperthermia. He'd experimented with equine cadavers, blowing air into the guttural pouch with a vacuum cleaner attachment and getting a measurable cooling of saline liquid he'd placed there. But that was with horse cadavers...the trick was to see what was happening inside a living, exercising horse. It would be no simple endeavor.

Naylor confirms, "The guttural pouch's inaccessibility probably accounts for it not being investigated very thoroughly in the past."

In fact, in order to explore whether temperature differences were occurring deep inside a horse's skull, he and Baptiste had to assemble a team of experts from across the University of Saskatchewan campus. Dr. Jeremy Bailey, BVSc, MVSc, Dipl. ACVS, Prof. of Large Animal Surgery and Assoc. Dean Academic; engineer Dr. Ernest M. Barber; physiologist Dr. Jim Thornhill; and Dr. Klass Post, DVM, M.VetSC, Prof. of Small Animal Medicine, and Head of the Dept. of Veterinary Medicine, all had a hand in the experimental design that would clear up two mysteries in one fell swoop.

Exploring Uncharted Space

The guttural pouch is home to a number of important structures, including four major cranial nerves that carry signals to and from the brain, and two major arteries (the internal carotid artery, or ICA, which carries blood to the brain, and the maxillary artery, which supplies much of the blood to the rest of the head). Naylor and his team felt that the blood carried by the internal carotid artery would be key in determining whether a cooling function was occurring in the guttural pouch. But how to measure the temperature of that blood inside an exercising horse?

Fortunately, they were able to draw on pre-existing technology. "David Freeman, MVB, MRCVS, PhD, Dipl. ACVS at the University of Illinois, had previously developed a technique for threading a catheter up the internal carotid artery--it's used to tie off dangerous hemorrhages that sometimes occur when there's a fungal infection in the guttural pouch," Naylor explains. "So we've known how to do this for at least 10 years."

In order to measure blood temperature inside the vessel, Naylor's team equipped the catheter with three tiny thermocouples, which are capable of transmitting temperature readings on an ongoing basis. They placed one so that it would rest at the point where the internal carotid enters the guttural pouch space, another about midway along the vessel in the chamber, and the third at the spot where the artery leaves the pouch on its way to the brain.

In order to make sure the catheter was properly positioned, the team threaded a fiber-optic endoscope up through the nasal passages and into the guttural pouch (a technique that is tricky, since the entrance is small). The ICA (internal carotid artery) lies so close to the internal surface of the guttural pouch "that you can pretty much see the positioning right through the wall of the artery," Naylor says.

Finally, the team inserted a fine needle with a thermocouple into the guttural pouch space in order to measure the air temperature in the chamber. For comparative purposes, the rectal temperature was taken as well.

Thus rigged up, four horses took to the treadmill at the Western College of Veterinary Medicine. Each had the temperature of his internal carotid artery blood, and the air temperature in his guttural pouch, measured when he was at rest, then during an exercise program that included up to five minutes of trotting and/or cantering in a temperature-controlled room.

Air-(Conditioned) Heads

The results were dramatic and conclusive, says Naylor. "At rest," he explains, "the air temperature in the guttural pouch was even a little warmer than the horse's core temperature. But during exercise, it maintained a nice, even temperature of about 37.5° Celsius (99.5° Fahrenheit).

In exercising horses, the blood definitely cooled as it ran through the air space of the guttural pouch--from about 39.5°C (103.1°F) at the place where the internal carotid artery enters the pouch to about 37.8°C (100.04°F) at the exit point when the horses were cantering."

Naylor notes that because his team expected the rudimentary carotid rete to contribute some small cooling effect as well, they considered the readings from the thermocouple in the middle more significant than those from the sensor at the entrance--but even then, they consistently recorded temperature differences of more than a degree Celsius (1.8° F). The difference in temperature was most significant when the horses were cantering, and it became more dramatic the longer the horse worked.

With four horses all demonstrating similar results, it was easy for the team to conclude that the mysterious guttural pouch was, indeed, fulfilling a valuable function of cooling the horse's blood so as to keep his brain operating efficiently.

"We just feel very lucky to have made this advance," says Naylor. "We found out both what the guttural pouch does, and how horses cool their brains, in a single experiment. It was definitely a team effort."

Although this information probably won't change the way you train your horses, it will certainly change the notations in future equine anatomy texts. And it might just give you a new appreciation for how cooler heads prevail at the racetrack or in the show ring.

About the Author

Karen Briggs

Karen Briggs is the author of six books, including the recently updated Understanding Equine Nutrition as well as Understanding The Pony, both published by Eclipse Press. She's written a few thousand articles on subjects ranging from guttural pouch infections to how to compost your manure. She is also a Canadian certified riding coach, an equine nutritionist, and works in media relations for the harness racing industry. She lives with her band of off-the-track Thoroughbreds on a farm near Guelph, Ontario, and dabbles in eventing.

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