(I've always been fascinated by all things equine, including their anatomy and how their various body systems work. As a project to learn more about these amazing animals, I've decided to do a series of posts on the equine digestive system. As these come along, they'll be added to the sidebar. Please keep in mind that I am neither a vet nor an expert on these topics - if you have corrections and additions or resources to direct us to, please put that in the comments. Also, you may find it helpful to visit this article, which has excellent pictures of the digestive system and the location of the various organs.)
The first section of the equine digestive system is the mouth. Horses are very selective grazers, with very flexible and adept lips. Grasses and other forage are sheared off by the incisors, which must align properly to do this job well. Then the tongue moves the food to the premolars and molars for side-to-side mastication - again, proper lateral movement of the jaw is essential to good chewing. Horses have three pairs of salivary glands, and produce up to 10 gallons of saliva a day - that's over 80 pounds - which is necessary to deal with their coarse forage diet. The saliva contains enzymes that begin the digestion process. The tongue then forms the chewed food into a bolus that can be swallowed.
Horses' teeth are truly a marvel. The mature horse has 12 incisors, 6 up and 6 down, and has 3 premolars and 3 molars on each side of the lower jaw matched by an equal set on the upper jaw, for a total of 12 premolars and 12 molars. Male horses usually have 4 canine teeth - these teeth can be very sharp and do not directly align. They are a fighting weapon in stallions, and may need rounding off. Mares sometimes have a single pair of canines. Some horses of both sexes have wolf teeth, which are small vestigial premolars located on the bars - these often have very shallow roots and are usually removed as they can cause serious problems with the bit.
Young horses have a set of baby (or deciduous) teeth consisting of the 12 incisors and 12 premolars - they do not have molars. As the horse matures, the adult teeth erupt from under the baby teeth, which continue to sit on top of the adult teeth and erode (they become what are called "caps") until they become thin enough to simply fall off. By the time a horse is 5 years old (this process may take longer in some warmblood breeds), it will have all of its adult teeth, although they may not all be evenly erupted, and only about 1/4 inch of the full tooth - which is 4 1/2 to 5 inches long - may be exposed. In an young horse, the roots of the premolars and molars occupy most of the mandible and the sinus cavities of the horse's head and add significantly to the weight of the head - studies have apparently been done that show that a horse's head may weigh 7 pounds more at age 4 than at age 15. This is why serious dental work (say removal of a cracked or abcsessed tooth) on a younger horse may require surgery rather than just work by a dentist. As a horse gets old - older than 25 - it may begin to run out of tooth surface and even have teeth fall out due to the increasingly short roots. This one reason that dental work on elderly horses must be done with extreme care. An elderly horse may have very little tooth to work with and no more root to erupt to replace tooth that is too smoothed off by aggressive dental work. And an elderly horse with teeth that are too smooth will have difficulty chewing properly.
The functioning of the temporomandibular joint - the joint that connects the jaw to the skull - is critical to proper chewing - in older horses these joints may have arthritis or severely worn cartilage, which will require the dentist to use care not to open their mouths too long or too far - a good dentist will close the speculum to give the horse a rest (any horse, not just an older one - it's just common courtesy). In all horses, if the functioning of the TMJs is impaired they may cause pain that will sometimes seriously affect performance. As discussed below, the diet many horses eat can affect how their teeth wear and how their jaws can move, which affects their TMJs as well.
Here's what a young adult horse's teeth and skull look like (it looks to me that this horse's incisors needed dental work!):
Horses do not have teeth that grow continuously from the root (like rodents), but rather have teeth that erupt continuously. The reason a horse's teeth need to continue to erupt throughout his life at a rate of about 1/8 per year is that they are designed to wear away at the biting surface due to the very abrasive, high volume diet horses in the wild eat - grass has a very high silica content and is therefore very hard on the teeth, as is the environmental grit that is also present. The horse's teeth are constructed to wear in this way - horses have what are called "hypsodont", or folded teeth. The folded, layered, multi-ridged surface of a horse's teeth is composed of dentin, which is harder than bone, and which forms about 1/2 the bulk of the tooth and forms the characteristic ridges which give the tooth its structural integrity; enamel, the hardest tooth material, in veins through the dentin, and cementum, which fills the area between the ridges and also anchors the teeth to the jaw. It is the yellow of cementum which gives the horse's teeth their characteristic yellow color - our teeth are white because we have enamel on the exterior.
As the horse's teeth wear, the harder enamel is exposed as the softer cementum and dentin are worn away by abrasive forage. It is in fact this exposed and rough surface that allows the horse to shear long-stemmed grass material - if a horse's teeth are too smooth it cannot effectively do this - some sharpness is necessary, which is at variance with much standard horse dentistry practices. (See this post for more information on this topic.) Then the exposed enamel is also gradually eroded by the extremely abrasive forage diet, as more dentin and cementum are also eroded. Here is an example of the surface of a horse's teeth (this is either a premolar or a molar):
When chewing grass or forage, the horse's jaw moves in a typical side to side shearing/grinding motion. Interestingly enough, the diet we feed our horses can adversely impact how their teeth wear. This is partly because of the structure of the horse's jaws - the lower jaw is narrower than the upper jaw:
This means that, since the teeth do not precisely align, for the teeth to wear evenly, the horse's jaw must move in a full lateral range to both sides, which only occurs when the horse chews forage. Modern pelleted feeds are easier to chew than forage, and the horse's jaw does not make the same full lateral motion when chewing pelleted feeds. This leads directly to uneven wear, which can lead to even further restriction of lateral motion when chewing, and also to strain on the TMJs. As the proportion of pelleted feeds rises, and the proportion of forage falls, in the horse's diet, and if forage is not available continuously, dental problems increase. So, once again, many of the issues our horses have with their health, and in this case their teeth, are directly attributable to how we feed and house our horses.
(Some of the illustrations and information in this post are from a couple of good sources for information about horse dentition: Tour Your Horse's Teeth, and Equine Dentistry: Anatomy. For an extremely interesting article on the evolution of the modern horse, including the very important transition from animals that browse in the forest to grazers of grasses, and the implications this has for equine dentition, see: The Evolution of the Horse: History and Techniques of Study, by Deb Bennett, Ph.D.)