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.
Fig. 16.1 The skeleton of the horse.
(With permission from Aspinall V 2006. The complete textbook of veterinary nursing. Butterworth- Heinemann, London, p 134.)
The equine skull (Fig. 16.2) is made up of approximately 37 fused bones providing a rigid structure with minimal movement; the only moving part is the temporo-mandibular joint, which is essential for chewing. The major components of the skull are:
(With permission from Aspinall V 2006. The complete textbook of veterinary nursing. Butterworth-Heinemann, London, p 135.)
The horse has 18 ribs, although this may vary in some individuals (Fig. 16.1). The ribs play an essential role in housing and protecting the vital internal organs of the thorax. The ribs may be separated into two groups:
Fig. 16.4 The equine forelimb.
(With permission from Aspinall V. 2006. The complete textbook of veterinary nursing. Butterworth-Heinemann, London, p 136.)
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 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.
Fig. 16.8 Superficial muscles of the horse. 1, Rhomboideus; 2, splenius; 3, sternocephalicus; 3′, jugular vein; 4, brachiocephalicus; 5, cutaneous colli; 6, omotransversarius; 7, serratus ventralis; 8, trapezius; 9, subclavius; 10, deltoideus; 11, pectoralis descendens; 11′, pectoralis ascendens; 11″, superficial thoracic vein; 12, triceps; 13, latissimus dorsi; 14, cephalic vein; 15, external abdominal oblique; 16, stump of cutaneous trunci forming flank fold; 17, sheath; 18, medial saphenous vein; 19, tensor fasciae latae; 20, gluteus superficialis; 21, biceps femoris; 22, semitendinosus.
(With permission from Dyce KM, Sack WO, Wensing CJG 2002 Textbook of veterinary anatomy, 3rd edn. WB Saunders, Philadelphia, PA, p 570.)
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.
The horse is a prey animal and during its evolution it has developed the turn of speed necessary to escape predators. To reduce weight and bulk and improve manoeuvrability there is little or no muscle below the knee (carpus) and hock (tarsus). The lack of muscle as a protective layer suggests that these structures will be vulnerable to stress and trauma.
The horse has evolved from a three- to four-toed, dog-like animal into the large animal taking its weight on one digit that we recognise today. The central digit is encased in a hoof while the outer toes are reduced to vestigial appendages that no longer reach the ground. This arrangement adds to the horse’s ability to run fast.
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.
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.
(With permission from Aspinall V 2006. The complete textbook of veterinary nursing. Butterworth-Heinemann, London, p 141.)