II.

Physical Characteristics

As a result of deliberate breeding by humans, horses display a remarkable variation in size, body shape, and coat color. Traditionally, a horse’s size is measured at the withers—an elevated part of the spine between the neck and the back. The measurement is made in hands; one hand equals about 10 cm (4 in). Typical riding horses stand 14 to 16 hands high and weigh 400 to 500 kg (900 to 1,100 lb). The smallest horse on record, a Falabella miniature horse, stood 48 cm (19 in), or just under 5 hands, and weighed 14 kg (30 lb). The largest horse on record was a Belgian that stood 1.8 m (6 ft) tall, or 18 hands, and weighed 1,450 kg (3,200 lb).

The horse has a hairy coat and a long mane and tail. A heavy winter coat grows in the fall and sheds in the spring. Typical coat colors include black, brown, gray, cream, gold, and white. The mane and tail can be the same or different from the body color, and many variations in color can result from inherited traits that cause spotting, dilution of the basic coat colors, or a sprinkling of white hairs in the coat. Many color patterns have specific names, such as bay (brown with black mane and tail), chestnut (reddish brown with mane and tail of the same or lighter color), and palomino (gold with a creamy white mane and tail).

A horse’s head is composed of the cranium, which encloses the animal’s large, complex brain, and the face, distinguished by a long muzzle consisting of the nose and lips. The muzzle provides enough distance between the horse’s mouth and its eyes so that it can graze and watch for danger at the same time.

Horses have the largest eyes of any land mammal. The large eyes protrude from the sides of the head, enabling horses to see almost directly behind themselves, even while facing forward. Their night vision is excellent. Horses have limited color vision, which appears to be similar to one of the less common forms of color blindness in humans—they perceive red and blue, but they cannot distinguish between green and shades of gray.

Horses have powerful teeth and jaws to grind and break down plant fibers. Their teeth grow continuously as their surfaces wear down. Male horses usually have 40 teeth and females have 36. Between the front incisors and the rear molars is a gap called the diastema, where the bit is placed. Horses can close their wide nostrils against dusty winds, and they can move their large ears to detect sounds from various directions.

A horse’s head is held up by its long, flexible neck, which lets the horse reach down to the ground to feed, rise to a high vantage point to sight danger, and bite itches on the front part of its body. The horse’s body has a wide chest, which holds its enormous lungs and heart; and a muscular back, beneath which lie the horse’s internal organs for digesting food and reproducing. A horse’s long, flowing tail helps keep its hindquarters warm and is used to swish away insects.

The specialized structures of the horse’s legs make the animal a very efficient runner. What we think of as the horse’s knee is actually the equivalent of a human’s ankle, so from the knee down the leg is really a highly elongated foot. The lowest part of the foot is the tip of a single toe, which corresponds to the tip of a person’s middle toe. This large, strong toe tip is well protected by a tough, curved hoof. By “standing on its toes,” the horse has a very long leg for an animal of its size, but also a very light leg, since toes are lightweight structures, carrying a minimum of bone and tendon and no muscle at all. Like a person’s foot, a horse’s foot has a sole. In the horse, the sole includes a rubbery, V-shaped structure called the frog, which helps absorb the impact of the foot against the ground.

Many of the joints in horses’ legs are comparable to hinges that permit forward and backward motion only. This type of joint requires fewer muscles than are needed for the kind of ball-and-socket joint that occurs in the human hip, which can rotate in any direction. This yields a further savings in weight. Long, light legs allow a horse to move very efficiently. A long leg produces a long stride, and a light leg allows the horse to swing its limbs back and forth quickly with a minimal expenditure of energy. The top speed of the horse is about 70 km/h (45 mph).

A.

Internal Organs

The horse has very efficient respiratory and circulatory systems that enable it to race at high speeds without running short of air. While walking, a horse consumes only 1 liter (about 0.25 gallon) of oxygen a minute, but at a racing gallop, its oxygen consumption can approach 60 liters (nearly 15 gallons) per minute. At the gallop, the horse’s head and neck move up and down in rhythm with each stride. This motion tends to squeeze and expand the lungs, so that a galloping horse automatically takes exactly one breath per stride. This mechanism ensures that the faster the horse gallops, the more air it takes in.

The horse has a single stomach and a large digestive organ called the cecum, which forms a dead-end alley at the junction of the large and small intestines. Microorganisms that live in the cecum break down cellulose, a tough substance within the walls of plant cells, making it possible for the horse to digest grasses. The cecum has a comparable role to the rumen, a specialized stomach chamber present in ruminants, or cud-chewing animals, such as cows and sheep. Horses cannot extract as much energy out of food as ruminants do, but they are able to digest food more quickly. As a result, a horse can eat more food each day than a cow of the same size. Due to this difference, horses can survive on stemmy, high-fiber roughage that would not sustain a cow.