The eyes are situated deep within the orbits, offering them some degree of protection. They each sit in a pad of fat which provides a cushioning effect. When horses are ill or starving this fat disappears and the eyes appear more sunken.
The skin which covers the horse’s face and neck is thinner than the skin anywhere else on the body.
The horse’s head evolved to be relatively large in relation to the overall size of the animal. This was due to changes in dietary habits, which developed with time from shrub browsing to a trickle-feeding herbivore. The horse needed large teeth for grinding herbage and consequently a large jawbone to accommodate them. All the lower teeth are situated in the mandible. The upper teeth are situated in sockets in the incisive (premaxillary) and maxillary bones.
The horse’s teeth are designed to cope with a diet mainly consisting of grass, grinding away at tough stems and leaves. To compensate for the continual wear that occurs at their grinding (occlusal) surfaces, the horse’s teeth possess a very long crown, most of which is embedded in a socket. As the tooth is worn away the crown gradually emerges from the socket, compensating for the wear. The horse’s jaws need to be long to house this battery of grinding teeth; they also need to be deep to encase the long, embedded crowns. This gives rise to the characteristic shape of the horse’s head.
To assist the grinding process, the upper jaw is wider than the bottom jaw and the surfaces of the teeth comprise complex folds of enamel. The side-to-side chewing action of the jaws means that the teeth glide across each other, with continual eruption compensating for wear. Through generations of selective breeding, horse’s heads have become more refined, with the Thoroughbred and Arab clearly possessing a wide forehead and top jaw and a narrower lower jaw. When the upper jaw is wider than the bottom jaw, the chewing action does not involve the entire occlusal surface of the tooth. The inside of the upper molars is worn smooth while the outside is not sufficiently worn away, leaving sharp edges that can damage the horse’s cheeks. The outside edge of the lower teeth is worn but the inside edges become sharp and can lacerate the tongue. The situation is made even worse when horses are fed lower levels of fibrous roughage and larger amounts of easily chewed concentrate feed. The horse’s teeth should be checked twice a year by a horse dentist or veterinary surgeon, and if necessary rasped to remove these sharp edges.
The skeleton of the horse’s neck consists of seven cervical vertebrae (Fig. 2.3), the first of which is the atlas (Fig. 2.4) and the second the axis (Fig. 2.5). The long strong ligament of the neck, the nuchal ligament (Fig. 2.7), is attached to the axis. It helps to support the horse’s heavy head and neck, and allows them to be raised and lowered. The joints between the other cervical vertebrae enable the horse to stretch the neck downwards, bend it sideways and arch it. This allows the horse to graze and to move its head towards strange sounds or sights which may indicate danger. The curves formed by the vertebrae are deep in the neck and do not follow the crest.
The first two bones of the horse’s neck, the atlas and axis, are different anatomically from the other neck vertebrae and also from each other. The hollow space within these vertebrae, which allows the vital spinal cord to run through their middle, is relatively large to allow for the great amount of movement which takes place in this part of the neck.
Atlas (Fig. 2.4)
The atlas consists of a short tube of bone with large wings; it lacks the body of the axis and other cervical vertebrae. It articulates with the skull at the occiput, allowing the horse to nod its head. The wings of the atlas can be felt on either side of the horse’s neck below the poll and behind the jawbone.