The horse

Chapter 16 The horse

The horse appeared in its earliest form 55 million years ago as Eohippus, a small, multitoed mammal about 30 cm high. It had four toes on the fore limb and three on the hind limb and a weight- bearing pad under the central toe on each foot. Its teeth were capable of chewing succulent leaves. During its evolution over a period of many millions of years the number of toes reduced and the central third toe became encased in a simple hoof. This species was sequentially replaced by several others with similar skeletal structures and increasingly efficient teeth suitable for eating grass.

During the Lower Pliocene period 10 million years ago, Pliohippus, a fully hoofed animal three times the size of the original Eohippus, emerged and, by the time Homo sapiens had evolved, it had become Equus, a recognisable horse. Equus appears to have originally come from North America and then migrated southwards and then spread into Asia, Africa and Europe. It became extinct in the Americas 8000 years ago and the different species of Equus developed in Asia, Africa and Europe as a result of the different climates and terrains.

The domestic horse Equus caballus is a sociable or herd-living animal, which, as its wild ancestors might have fallen prey to many different carnivorous species, still retains its primitive instinct to run. The horse has the ability to move very fast, an aspect that has been further developed by selective breeding. Much of its musculoskeletal system is adapted to speed, e.g. the single-hoofed central digit where weight is born on the toe, the well-developed muscles high up on the hind quarters and the sequences in which the feet are lifted from the ground to bring about the different speeds of locomotion. As a herd of horses must always be prepared for sudden flight they are reluctant to lie down to sleep and this has led to the evolution of the suspensory and stay apparatus.

Horses evolved to roam large areas in search of food. This was mainly poor-quality grass, which was eaten constantly (grazed) and digested slowly. The flattened table teeth, which grind the grass into boluses that can be easily swallowed, the long length of intestine providing room for the slow digestive progress of tough fibrous ingesta and the large caecum and colon designed to provide a chamber for the microbial breakdown of cellulose in the plant cell walls all enabled the horse to colonise and survive in its ecological niche.

Much of the anatomy and physiology is similar to that of the dog and this chapter is designed to highlight the differences between these two species, not to describe every aspect in repetitious detail.

The skeletal system

The equine skeleton (Fig. 16.1) consists of two separate sections:

The function of the skeleton is to provide a rigid framework for support, protection and movement. It also provides a storage facility for minerals, principally calcium and phosphorus.

The vertebral column

The vertebral column extends from the base of the skull to the tip of the tail and consists of approximately 54 individual vertebrae (Fig. 16.1).

The function of the vertebral column is:

The vertebrae in the horse (Fig. 16.3) are grouped into regions as they are in the cat and the dog, although the numbers of each vertebrae may be different.

The vertebrae Fig. 16.3 in each region vary in size and shape, which relates to their movement and function.

The appendicular skeleton

The forelimb

Like the dog, the horse has no clavicle or bony connection between the thorax and the forelimbs, which are merely attached by muscular slings, allowing for shock absorption during locomotion.

The bones of the forelimb (Fig. 16.4) are:

Carpus (Fig. 16.5) – known as the knee and made up of eight small bones arranged in two rows. This arrangement further assists in the absorption and distribution of the concussion that travels up the horse’s forelimb. The carpus is the equivalent of the human wrist and the bones are arranged as follows:


The pelvic girdle (Fig. 16.7) links the spine and the hindlimb and is composed of three large flat bones the pubis, the ischium and the ilium. The pubis forms the floor of the pelvis with the ischium lying caudal to it. The tuber ischii can be felt at the point of buttock. The ilium is the largest bone in the pelvis and is the upper, almost vertical portion. The wings or tubera sacrales of the ilium can be felt at the croup and the tuber coxae forms the points of the hip. All three bones meet to form the acetabulum or hip socket. The hip joint is formed by the head of the femur and the acetabulum.

The muscular system

The muscular system is made up of striated muscle that is attached to the skeleton and is under voluntary control. The function of muscle is to bring about movement.

The arrangement of the superficial muscles covering the neck, thorax and abdomen (Fig. 16.8) is similar to that in the dog and cat, with the obvious difference that many are better developed to bring about the rapid locomotion characteristic of the horse.

The limbs of the horse are superbly adapted for speed, although at rest both the fore- and hindlimbs support the body. The forelimbs, which should be more or less straight, carry about 60% of the weight and absorb most of the shock during locomotion, especially when landing from a jump. The hindlimbs are more angled and provide the main propulsive force. Details of the most significant muscles are shown in Table 16.1.

The hamstring group of muscles, i.e. biceps femoris, semitendinosus and semimembranosus, create the well-rounded croup area of the body. Their action is to extend the hip and flex the stifle, which provides the main forward thrust and is responsible for the speed of the animal.

Ligaments and tendons

Tendons, formed from dense connective tissue, attach muscle to bone. Their function is to harness the pull from muscle contraction that brings about movement. They are less elastic than muscle fibre and have a poor blood supply, which may have an effect on healing.

Important tendons found within the lower leg (Fig. 16.9) include:

Ligaments attach bone to bone and are similar in structure to tendons. They are relatively less elastic than tendons and, as they also have a poor blood supply, healing can be prolonged.

The ligaments and tendons in the lower fore- and hindlimbs work in conjunction with a variety of muscles to form the suspensory apparatus (Fig. 16.10). The function of the suspensory apparatus is to support and suspend the limb and fetlock joint and prevent over-extension and collapse of the limb. It consists of the suspensory, intersesamoidean, collateral sesamoidean and distal sesamoidean ligaments, which are attached to the proximal sesamoid bones.

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Jul 18, 2016 | Posted by in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off on The horse

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