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 axial skeleton comprising the skull, vertebral column, ribs and sternum

• The appendicular skeleton comprising the bones that form the limbs and including the pelvis, which attaches the hindlimb, and the scapula, which attaches the forelimb.

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 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 skull

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:

• Mandible – forms the lower jaw

• Maxilla – forms the upper jaw

• Incisive bone or pre-maxilla – houses the incisor teeth of the upper jaw

• Orbit – the eye socket, formed by contributions from several bones

• Frontal bone – forms the forehead

• Nasal bones – forms the nasal cavity.

Fig. 16.2 The equine skull. A Lateral view. B Dorsal view.

(With permission from Aspinall V 2006. The complete textbook of veterinary nursing. Butterworth-Heinemann, London, p 135.)

The functions of the skull are:

1 To protect the soft tissues of the head, i.e. brain, eye and inner ear

2 To provide attachment for the hyoid apparatus, which suspends the tongue and the larynx

3 To provide support for the dentition

4 To provide attachment for the muscles of facial expression

5 To form an opening for the entry of air and food.

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:

1 To form a stiff but flexible rod to support the body

2 To house and protect the spinal cord

3 To provide sites of insertion for muscles

4 To provide a means of attachment for the pelvis and ribs.

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.

Fig. 16.3 The shape of each vertebral type. A Atlas – C1. B Axis – C2. C C3–C7. D Thoracic vertebra. E Lumbar vertebra. F Sacrum.

Vertebral formula for the dog and cat: C7, T13, L7, S3, Cd 20–23

Vertebral formula for the horse: C7, T18, L6, S5, Cd15–20

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

• Cervical vertebrae (7) – found in the neck. The first two are the atlas (C1), distinguished by its large, wing-shaped processes, and the axis (C2), distinguished by a prominent cranial process known as the dens. These vertebrae have a large range of motion as they are not inhibited by spinous or transverse processes. The remaining vertebrae (C3–C7) all follow a similar basic pattern (see Ch. 3).

• Thoracic vertebrae (18) – these form the dorsal boundary of the thoracic cavity. The first 7–8 vertebrae are covered and protected by the scapula. The thoracic vertebrae have large spinous processes, of which T4–T9, make up the withers of the horse. The transverse processes have small accommodating facets allowing for the connection of the ribs. The spinous processes make dorsoventral flexion restricted and lateral flexion minimal. It is the thoracic vertebrae in combination with the muscles that lie beneath that support the rider’s weight.

• Lumbar vertebrae (5–7; normally 6) – located in the loin area and play a role in the protection of the kidneys. The vertebrae possess large transverse processes, which restrict lateral movement. The spinous processes are much smaller than those of the thoracic vertebrae; however dorsoventral flexion still remains limited.

• Sacral vertebrae (5 fused to form the sacrum) – found in the croup area at the root of the tail. Fusion of the vertebrae in this area means that movement is highly restricted. The pelvis is attached to the sacrum by an interosseous ligament, which forms the sacroiliac joint.

• Coccygeal or caudal vertebrae (15–20; average 18) – the coccygeal vertebrae have a basic shape with very small spinous and transverse processes. Towards the end of the tail the vertebrae are little more than rods, which allows great mobility of the tail.

The ribs and sternum

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:

• The first eight ribs are the ‘true’ or ‘sternal’ ribs, i.e. those that are attached directly to the sternum

• The final ten ribs are the ‘false’ or ‘asternal’ ribs, i.e. those that are attached via cartilage or through one another to the sternum. These form the costal arch.

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:

• Scapula – a large, flat, triangular bone. The scapular spine divides the bone down the middle, each side allowing for the insertion and attachment of the supraspinatus and infraspinatus muscles. A wing of cartilage lies at the proximal end, allowing for further attachment of muscle and connection with the thoracic sling. The scapula meets with the humerus to form the shoulder joint.

• Humerus – articulates with the scapula and the radius and ulna at the shoulder and elbow joint respectively. The angle at which it lies allows for great shock absorption during movement (Fig. 16.1). The humerus is one of the strongest bones in the equine body.

• Radius and ulna – in the horse these two bones are fused. All equine species bear weight on digit 3, which is the central digit of the primordial pentadactyl limb. The weight is then carried up the strong, single fused bone. The olecranon process at the proximal end of the ulna forms the point of the elbow. The radius, which lies on the medial side of the fused bone, bears most of the weight (Fig. 16.4).

• 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:

Upper row – running from the medial to lateral surfaces – radial carpal, intermediate, ulnar and accessory carpals. The accessory carpal bone lies behind the ulnar carpal and does not directly bear weight.

Lower row – these bones simply relate to the metacarpals which lie distal to them and are numbered from II to IV running from medial to lateral.

• Metacarpals – there are three metacarpal bones in the horse, which may be numbered 2–4. Of these the third or large metacarpal, also referred to as the cannon bone, is fully developed and is the weight-bearer. Metacarpals 2 and 4 are much smaller and lie on either side of the cannon bone. They are referred to as the splint bones.

• Phalanges (pasterns and pedal bone) – the horse takes its weight on the equivalent of digit 3 (the dog and cat bear weight on digits 2–5 – digit 1 being the dew claw). Three phalanges form the digit. Starting from the proximal end, the first (proximal) phalanx is known as the long pastern, the second (distal) phalanx is the short pastern and the third (middle) phalanx is the pedal bone, which is encased in the hoof. Vital ligaments and tendons run down and around these bones providing movement and support (Fig. 16.9).

Fetlock joint – lies between the cannon bone and the long pastern.

Pastern joint – lies between the long and the short pastern.

Coffin joint – lies between the short pastern and the pedal bone and incorporates the navicular bone.

• Sesamoids (proximal and distal) – there are three sesamoid bones in the lower leg. There are two proximal sesamoids located at the back of the metacarpophalangeal or fetlock joint and the distal sesamoid or navicular bone lies at the back of the pedal bone encapsulated within the hoof. The function of these sesamoids is to act as a pulley system for the tendons that pass over them, thus reducing friction and improving efficiency of movement.

Fig. 16.4 The equine forelimb.

(With permission from Aspinall V. 2006. The complete textbook of veterinary nursing. Butterworth-Heinemann, London, p 136.)

Fig. 16.5 The equine carpus.

(With permission from Aspinall V. 2006. The complete textbook of veterinary nursing. Butterworth-Heinemann, London, p 136.)

The hindlimb

The bones of the hindlimb (Fig. 16.6) are:

Fig. 16.6 The equine hindlimb.

Pelvis

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.

• Femur – a large strong bone that provides a large area for the attachment of major muscles of the hind limb. These are largely responsible for generating power and locomotion in the horse.

• Patella – the equivalent of the human kneecap. It acts as a pulley for the tendons that run over the stifle joint between the femur and the tibia. The stifle joint is formed by the distal femur, proximal tibia and patella.

• Tibia and fibula – the tibia runs down diagonally from the femur to the hock and is the larger of the two bones allowing for the attachment of the major muscles responsible for the movement of the lower leg. The fibula is much smaller and thinner and lies along the lateral border of the tibia. It tapers to a point at the lower third of the tibia where it is fused to the tibia.

• Tarsus (hock) – composed of six small bones, sometimes seven. The bones are arranged in three rows, which are tightly bound by ligaments. The bones of the upper row are the talus and the calcaneus; the central tarsus makes up the middle row; the third row is made up of the first and second tarsal bone, which are fused, and the third tarsal bone. Finally, the fourth tarsal bone is housed in spaces in the middle and lower rows. The body of the calcaneus is extended at its proximal end to form the tuber calcis or point of the hock, to which the Achilles tendon is attached.

• Metacarpals, phalanges and sesamoids – the arrangement of the bones distal to the hock follows the same pattern as in the forelimb.

Fig. 16.7 The pelvic girdle of the horse. A Right lateral view. B Dorsal view.

(With permission from Sisson S 1953 Anatomy of the domestic animals, 4th edn. WB Saunders, Philadelphia, PA, p 105.)

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.

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.

Table 16.1 Important muscles of the horse

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.

Soft tissues of the equine lower leg

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.

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:

• Deep digital flexor tendon – runs from the deep digital flexor muscle down the back of the limb, over the cannon bone and the proximal and distal sesamoids. It attaches to the third phalanx (P3 or the pedal bone). Its function is to flex the toe

• Superficial digital flexor tendon – runs from the superficial digital flexor muscle down the back of the limb, passes over the cannon bone and proximal sesamoids and then splits into two with one branch attaching to P1 and the other to P2. Its function is to flex the pastern joint

• Common digital extensor tendon – runs from the common digital extensor muscle down the front of the limb over the cannon bone and inserts on P1, 2 and 3. Its function is to extend the pastern joint and the toe

• Lateral digital extensor tendon – lies on the lateral side of the common digital extensor tendon, running down the limb over the cannon bone and attaching to P1. Its function is to assist in the extension of the pastern joint.

Fig. 16.9 Lateral view of the lower forelimb showing bones, joints, tendons and ligaments.

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.

• Suspensory ligament – this very important structure is often referred to as a modified muscle, as it is slightly more elastic than other ligaments and tendons because of the presence of some muscle tissue. The suspensory ligament runs down the back of the limb close to the cannon bone to the level of the proximal sesamoids, where it splits in to two. The two branches run either side of the fetlock joint to the front of the limb, connecting with the extensor tendon at the level of P1 (Fig. 16.9). The function of the ligament is to support and suspend the fetlock and prevent over-extension.

• Check ligaments – there are several in both the fore and hind limb. They are:

Carpal check ligament

Radial check ligament

Tarsal check ligament.

Check ligaments connect ligament to tendon and their function is to prevent strain or over-extension of the joint. The carpal check ligament lies just below the carpus and attaches the deep digital flexor tendon to the suspensory ligament, which in turn is attached to the back of the carpus (Fig. 16.9). The tarsal check ligament occupies a similar position on the tarsus. The radial check ligament lies higher up the forelimb on the back of the distal radius.

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.