Chapter 1
THE DIGESTIVE SYSTEM
The horse has a completely herbivorous (vegetable) diet and its digestive system has evolved to deal with this type of food material. For example, the large intestine contains a population of microbes which break down the vegetable cellulose. This chapter presents a brief outline of the functional anatomy of the digestive system and describes some of the more common problems affecting it.
Functional anatomy
Mouth The mouth is specialized to enable a horse to grasp grasses and other vegetation and to grind them into a digestible pulp. The adult horse has forty or forty-two teeth (compared with twenty-four temporary teeth in the foal). These are classified into incisors (twelve), which grasp and shear the grass just above its roots, and premolars and molars (twenty-four), which are the specialized grinding teeth. The canine (tushes) and wolf (first premolar) teeth appear to have no function in the modern horse and are vestiges of a more primitive ancestor.
The muscular tongue acts with the hard and soft palates to propel food material from the incisor teeth to the grinding teeth and then to the back of the mouth (oropharynx), where food material accumulates prior to swallowing. The salivary glands discharge from tiny openings into the mouth and provide fluid containing special enzymes which aid in the degradation of food material.
Oropharynx (back of the mouth) The oropharynx is a muscular compartment which is continuous with the back of the mouth, and it is here that food material is stored prior to swallowing. The roof of the oropharynx is formed by the soft palate, a horizontal muscular sheet, which separates the oropharynx from the nasopharynx, which is only involved with breathing. When a bolus of food material is swallowed, it is propelled backwards by the base of the tongue and the pharyngeal muscles from the oropharynx into the oesophagus (gullet). At this moment the soft palate is elevated to block off the back of the nose, and the larynx moves forward and closes so that food material cannot leak into the upper or lower respiratory tracts.
Swallowing is a highly coordinated activity in which the individual components or movements last only for a fraction of a second. The pharynx can be considered as the crossroads between food material and air. Any disturbance in its function can produce serious difficulties in swallowing and/or breathing.
Oesophagus (gullet) and stomach The muscular tube of the gullet transfers food material by means of wavelike movements which squeeze food in the direction of the stomach.
Food material enters the stomach through a one-way muscular valve which prevents regurgitation of food material back up the oesophagus except under the most extreme conditions (for example, when the stomach is massively distended). Consequently a horse cannot vomit. The stomach lining is partly glandular and partly non-glandular. The glandular portion provides enzymes and acids which help to digest the food material. From the stomach ingesta pass through a valve into the small intestine.
Small intestine The small intestine is a long muscular tube with a glandular lining. In the upper part enzymatic fluids secreted by the liver and pancreas, along with the enzymes produced by the intestinal glands, help to break down the ingesta into their basic constituents. These constituents -- fats, proteins and carbohydrates -- are absorbed and transported by the blood stream to be utilized by the body to produce energy and materials for growth. The small intestine lies in coils in the left side of the abdomen and is connected to the large intestine (caecum and colon) by another muscular valve (ileocaecal). Food material is transported by muscular wave movement.
Caecum and large colon (large intestine) Ingesta are passed through the ileocaecal valve into the caecum and large colon, where the resident microbial population breaks down cellulose into its basic constituents. The muscular contractions of the caecum and large colon are rather complex, but these structures, the largest in the equine abdomen, are responsible for the absorption of a considerable amount of water from the ingesta.
The muscular coat of the caecum and large colon is complicated, having bands and sacculations, but ingesta are still moved by waves as in the simpler oesophagus and small intestine. The large colon is folded on itself and narrows at one of the bends (the pelvic flexure), predisposing this site to blockage. The large colon is continuous with the transverse colon and the small colon.
Small colon, rectum and anus The small colon continues the process of electrolyte and water absorption, its muscular structure having bands and sacculations similar to the large colon and caecum. It transfers the ingesta to the rectum, a simple muscular tube through which faecal material (dung) passes to the external environment. The anus is a muscular valve which regulates defaecation.
Conditions of the Digestive System
THE TEETH
Overshot jaws (parrot mouth) are relatively common, whereas undershot jaws (sow mouth) are extremely rare. Horses affected with both types of incisor malocclusion cope well with grazing and mastication and rarely lose condition.
There is a variety of other congenital abnormalities of the mouth which involve malocclusion of the teeth. One of the more commonly encountered of these is absence of cheek teeth. This appears most frequently in ponies and usually involves the absence of one or two cheek teeth on each side of the lower jaw. This results in relative overgrowth of the teeth in the upper jaw, which develop very sharp points, which can cause oral discomfort and quidding of food.
In the normal horse the upper cheek teeth are located more widely apart than the lower cheek teeth, and this results in the development of sharp points on the outside of the upper teeth and on the inside of the lower teeth. These can be smoothed by regular rasping. Dental irregularities also develop because of the manner in which the horse masticates. Thus a sharp point or hook forms on the first upper cheek tooth and the last lower cheek tooth on each side. Although regular rasping of the teeth will remove these, the sixth tooth on the lower jaw is more difficult to rasp. Eventually, in older horses, a sharp point develops which may cause discomfort during eating.
The temporary or milk teeth (both incisors and cheek teeth) are usually shed without difficulty. However, occasionally a temporary cheek tooth may become wedged between the adjoining teeth, and this can result in difficulty with chewing, often producing a foul smell to the breath because of trapped food. Any horse which appears to salivate and drop food should be examined by a veterinarian who will remove the wedged tooth with a pair of forceps.
Dental disease in the horse is relatively uncommon and it is surprising how, when a genuine dental problem occurs, many horses appear to carry on eating without difficulty. The sign which usually attracts the owner's attention is either a swelling or a discharging wound on the lower jaw or a swelling on the side of the face. Sometimes there may be a foul-smelling discharge from one nostril, as described in sinusitis. If any of these signs develop, a vet should be called and it may be necessary to carry out a radiographic or endoscopic examination to identify the problem.
Removing permanent cheek teeth of a horse is a difficult job, requiring considerable force, and must be carried out under general anaesthesia. The one exception to this is the removal of wolf teeth, which may be indicated if there is a problem with mouthing the bit. These vestigial teeth are small, with short roots, and usually can be removed relatively easily without a general anaes
