The skeletal system of the

horse The horse (''Equus ferus caballus'') is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of ''Equus ferus''. The horse has evolved over the past 45 to 55 million yea ...

has three major functions in the body. It protects vital organs, provides framework, and supports soft parts of the body. Horses typically have 205 bones. The pelvic limb typically contains 19 bones, while the thoracic limb contains 20 bones.

Functions of bones

Bones serve three major functions in the skeletal system; they act as levers, they store minerals, and they are the site of

red blood cell Red blood cells (RBCs), also referred to as red cells, red blood corpuscles (in humans or other animals not having nucleus in red blood cells), haematids, erythroid cells or erythrocytes (from Greek ''erythros'' for "red" and ''kytos'' for "holl ...

formation. Bones can be classified into five categories # Long bones: aid in locomotion, store minerals, and act as levers. They are found mainly in the limbs. #

Short bone Short bones are designated as those bones that are as wide as they are long. Their primary function is to provide support and stability with little to no movement. They are one of five types of bones: short, long, flat, irregular and sesamoid ...

s: Absorb concussion. Found in joints such as the knee, hock, and fetlock. #

Flat bone Flat bones are bones whose principal function is either extensive protection or the provision of broad surfaces for muscular attachment. These bones are expanded into broad, flat plates,''Gray's Anatomy'' (1918). (See infobox) as in the cranium ...

s: Enclose body cavities containing organs. The ribs are examples of flat bones. # Irregular bones: Protect the central nervous system. The vertebral column consists of irregular bones. # Sesamoid bones: Bones embedded within a tendon. The horse's proximal digital sesamoids are simply called the "sesamoid bones" by horsemen, his distal digital sesamoid is referred to as the

navicular bone The navicular bone is a small bone found in the feet of most mammals. Human anatomy The navicular bone in humans is one of the tarsal bones, found in the foot. Its name derives from the human bone's resemblance to a small boat, caused by th ...

. Ligaments and

tendon A tendon or sinew is a tough, high-tensile-strength band of dense fibrous connective tissue that connects muscle to bone. It is able to transmit the mechanical forces of muscle contraction to the skeletal system without sacrificing its ability ...

s hold the skeletal system together. Ligaments hold bones to bones and tendons hold bones to muscles. Synovial membranes are found in joint capsules, where they contain synovial fluid, which lubricates joints. Bones are covered by a tough membrane called periosteum, which covers the entire bone excluding areas of articulation.

Ligaments

Ligaments attach bone to bone, and are vital in stabilizing joints as well as supporting structures. They are made up of fibrous material that is generally quite strong. Due to their relatively poor blood supply, ligament injuries generally take a long time to heal. Ligaments of the upper body include: * Nuchal and supraspinous ligaments: the

nuchal ligament The nuchal ligament is a ligament at the back of the neck that is continuous with the supraspinous ligament. Structure The nuchal ligament extends from the external occipital protuberance on the skull and median nuchal line to the spinous proc ...

attaches to the dorsal surface of the cervical vertebrae. Its dorsal section extends from the occipital protuberance of the skull (the poll) to the withers, then narrows to become the supraspinous ligament. It also connects the 2-7th cervical vertebrae to the 1-3rd

thoracic vertebrae In vertebrates, thoracic vertebrae compose the middle segment of the vertebral column, between the cervical vertebrae and the lumbar vertebrae. In humans, there are twelve thoracic vertebrae and they are intermediate in size between the cervical ...

. Its main purpose is to support the head and allow it to be moved upward or downward. * Intercapital ligaments: lie between the first through eleventh ribs. Help to prevent thoracic disk herniation. Ligaments of the legs include: * Suspensory ligament: runs from the back of the cannon bone (between the two splint bones), then splits into two branches and attaches to the sesamoid bones at the bottom of the fetlock. Branches continue downward and attach to the extensor tendons. The main purpose of the suspensory is to support the fetlock joint, preventing it from overextending. Injury to this ligament is an important cause of lameness in performance horses. The suspensory is a modified muscle, the equine equivalent of the interosseous muscle, which contains both tendon fibers and residual muscle fibers.The suspensory ligament
/ref> * Interosseous ligaments: connect the cannon bone to each splint bone. Injury to this ligament produces the condition known as " splints". * Proximal and distal check ligaments: The proximal check ligament originates from the radius and attaches to the superficial digital flexor tendon. The distal check originates from the palmar carpal ligament and attaches to the deep digital flexor tendon, approximately 2/3-way down the metacarpus. * Plantar ligament: in the hind leg, runs down the lateral side of the tarsus, attaches to the fibular, 4th tarsal, and 3rd metatarsal bones. Injury leads to a condition known as "

curb A curb (North American English), or kerb (Commonwealth English except Canada; see spelling differences), is the edge where a raised sidewalk or road median/central reservation meets a street or other roadway. History Although curbs have ...

." * Inter-sesamoidean ligaments: supporting ligaments, run between the two sesamoid bones. * Distal sesamoidean ligaments: run from the sesamoid bones to the two pastern bones. Important in the stay apparatus. * Impar ligament: runs between the navicular bone and the 3rd phalanx. * Annular ligament: goes around the back of the fetlock, surrounding the flexor tendons and their tendon sheath, attaching to the sesamoid bones. It helps to support the fetlock, and provides an enclosed "pulley" for the flexor tendons to run through. * Sacrosciatic ligament: Originates from the sacrum and coccygeal vertebrae, inserts into the pelvis.

Axial skeleton

The

axial skeleton The axial skeleton is the part of the skeleton that consists of the bones of the head and trunk of a vertebrate. In the human skeleton, it consists of 80 bones and is composed of six parts; the skull (22 bones), also the ossicles of the middle ...

contains the

skull The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, th ...

vertebral column The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform composition) found in all chordate ...

sternum The sternum or breastbone is a long flat bone located in the central part of the chest. It connects to the ribs via cartilage and forms the front of the rib cage, thus helping to protect the heart, lungs, and major blood vessels from injury. Sha ...

, and

ribs The rib cage, as an enclosure that comprises the ribs, vertebral column and sternum in the thorax of most vertebrates, protects vital organs such as the heart, lungs and great vessels. The sternum, together known as the thoracic cage, is a semi- ...

. The sternum consists of multiple sternebrae, which fuse to form one bone, attached to the 8 "true" pairs of ribs, out of a total of 18. The vertebral column usually contains 54 bones: 7 cervical vertebrae, including the

atlas An atlas is a collection of maps; it is typically a bundle of maps of Earth or of a region of Earth. Atlases have traditionally been bound into book form, but today many atlases are in multimedia formats. In addition to presenting geograp ...

(C1) and axis (C2) which support and help move the skull, 18 (or rarely, 19) thoracic,King, Christine, BVSc, MACVSc, and Mansmann, Richard, VMD, PhD. "Equine Lameness." ''Equine Research,'' Inc. 1997. 5-6 lumbar, 5 sacral (which fuse together to form the sacrum), and 15-25 caudal vertebrae with an average of 18. Differences in number may occur, particularly in certain breeds. For example, some, though not all,

Arabians The Arabs (singular: Arab; singular ar, عَرَبِيٌّ, DIN 31635: , , plural ar, عَرَب, DIN 31635: , Arabic pronunciation: ), also known as the Arab people, are an ethnic group mainly inhabiting the Arab world in Western Asia, No ...

, may have 5 lumbar vertebrae, opposed to the usual 6, 17 thoracic vertebrae (and ribs) instead of 18, and 16 or 17 caudal vertebrae instead of 18. The

withers The withers is the ridge between the shoulder blades of an animal, typically a quadruped. In many species, it is the tallest point of the body. In horses and dogs, it is the standard place to measure the animal's height. In contrast, cattle a ...

of the horse are made up by the dorsal spinal processes of the thoracic vertebrae numbers 5 to 9.Riegal, Ronald J. DVM, and Susan E. Hakola RN. Illustrated Atlas of Clinical Equine Anatomy and Common Disorders of the Horse Vol. II. Equistar Publication, Limited. Marysville, OH. Copyright 2000. The skull consists of 34 bones and contains four cavities: the cranial cavity, the orbital cavity, oral, and the nasal cavity. The cranial cavity encloses and protects the brain and it supports several sense organs. The orbital cavitity surrounds and protects the eye. The oral cavity is a passage way into the respiratory and digestive systems. The nasal cavity leads into the respiratory system, and includes extensive

paranasal sinuses Paranasal sinuses are a group of four paired air-filled spaces that surround the nasal cavity. The maxillary sinuses are located under the eyes; the frontal sinuses are above the eyes; the ethmoidal sinuses are between the eyes and the sphenoi ...

. The nasal cavity contains turbinate bones that protect the mucous membrane that lines the cavity from warm inspired air. The skull consists of fourteen major bones # Incisive bone (premaxillary): part of the upper jaw; where the incisors attach # Nasal bone: covers the nasal cavity # Maxillary bone: a large bone that contains the roots of the molars # Mandible: lower portion of the jaw; largest bone in the skull # Lacrimal bone: contains the nasolacrimal duct, which carries fluid from the surface of the eye, to the nose # Frontal bone: creates the forehead of the horse # Parietal bone: extends from the forehead to the back of the skull # Occipital bone: forms the joint between the skull and the first vertebrae of the neck (the atlas) # Temporal bone: contains the eternal acoustic meatus, which transmits sound from the ear to the cochlea (eardrum) # Zygomatic bone: attaches to the temporal bone to form the zygomatic arch (cheek bone) # Palatine bone: forms the back of the hard palate # Sphenoid: formed by fusion of the foetal basisphenoid and presphenoid bones, at the base of the skull. Can become fractured in horses that rear over backwards. # Vomer: forms the top of the inside of the nasal cavity # Pterygoid: small bone attached to the sphenoid that extends downward

Appendicular skeleton

The appendicular skeleton contains the fore and hindlimbs. The hindlimb attaches to the vertebral column via the pelvis, while the forelimb does not directly attach to the spine (as a horse does not have a

collar bone The clavicle, or collarbone, is a slender, S-shaped long bone approximately 6 inches (15 cm) long that serves as a strut between the shoulder blade and the sternum (breastbone). There are two clavicles, one on the left and one on the righ ...

), and is instead suspended in place by muscles and tendons. This allows great mobility in the front limb, and is partially responsible for the horse's ability to fold his legs up when jumping. Although the hindlimb supports only about 40% of the weight of the animal, it creates most of the forward movement of the horse, and is stabilized through attachments to the spine.

Important bones and joints of the forelimb

* Scapula (shoulder blade): flat bone with a large area of cartilage that partially forms the withers. The shoulder length and angle is very important to horsemen when evaluating conformation. * Humerus: lies between the scapula and the radius, making an angle of about 55 degrees down and back. (Misspelled in the picture as "Humercus") * Radius: extends from the elbow, where it articulates with the humerus, and travels downward to the carpus. It forms the "forearm" of the horse along with the ulna. * Ulna: caudal to the radius, it is usually partially fused to that bone in an adult horse. * Shoulder joint (scapulohumeral joint): usually has an angle of 120-130 degrees when the horse is standing, which can extended to 145 degrees, and flexed to 80 degrees (such as when the horse is jumping an obstacle). * Elbow joint (humeroradial joint): hinge joint that can flex 55-60 degrees. *

Carpus In human anatomy, the wrist is variously defined as (1) the carpus or carpal bones, the complex of eight bones forming the proximal skeletal segment of the hand; "The wrist contains eight bones, roughly aligned in two rows, known as the carpal ...

(knee): consists of 7-8 bones placed in 2 rows to form 3 joints. The 1st carpal bone is present only 50% of the time. Which on humans is the wrist.

Important bones and joints of the hindlimb

* Pelvis: made up of the os coxae, the largest of the flat bones in a horse. It is made up of the ilium, the ischium, and the pubis. At the junction of these three bones is a cavity called the acetabulum, which acts as the socket of the hip joint. The pelvic cavity is larger in diameter in the mare than in the stallion, providing more room for the foal during birth. *

Femur The femur (; ), or thigh bone, is the proximal bone of the hindlimb in tetrapod vertebrates. The head of the femur articulates with the acetabulum in the pelvic bone forming the hip joint, while the distal part of the femur articulates wit ...

: the largest long bone in a horse. Proximally it forms a ball-and-socket joint with the pelvis to form the hip joint, and distally it meets the tibia and patella at the stifle joint. It serves as an attachment point for the deep and middle gluteal muscles, and the accessory and round ligaments. * Patella * Tibia: runs from stifle to hock. The proximal end provides attachment for the patellar ligaments, meniscal ligaments, cruciate ligaments, and collateral ligaments of the stifle. The distal end provides attachment for the collateral ligaments of the hock. *

Fibula The fibula or calf bone is a leg bone on the lateral side of the tibia, to which it is connected above and below. It is the smaller of the two bones and, in proportion to its length, the most slender of all the long bones. Its upper extremity i ...

: completely fused to the tibia in most horses. * Hip joint : Ball-and-socket joint made up of the acetabulum of the pelvis and the femur. It is very stable. * Stifle joint (femoropatellar joint): actually composed of three joint compartments: the femoropatellar joint, the medial femorotibial joint, and the lateral femorotibial joint, which are stabilized by a network of ligaments. The stifle has an articular angle of about 150 degrees. * Tarsus (hock): consists of 6 bones (of which one is made up of the fused 1st and 2nd tarsal bones) aligned in 3 rows. The largest bone in the hock, the calcaneus or fibular tarsal bone, corresponds to the human heel, and creates the tuber calcis (point of hock). It is to this point that the tendon of the

gastrocnemius The gastrocnemius muscle (plural ''gastrocnemii'') is a superficial two-headed muscle that is in the back part of the lower leg of humans. It runs from its two heads just above the knee to the heel, a three joint muscle (knee, ankle and subtalar ...

, portions of the biceps femoris, and portions of the superficial digital flexor attach.

Bones of the lower limb

Bones of the lower limb, present in both the front and hind legs, include the

cannon bone Good conformation in the limbs leads to improved movement and decreased likelihood of injuries. Large differences in bone structure and size can be found in horses used for different activities, but correct conformation remains relatively simil ...

(3rd metacarpal/3rd metatarsal), splint bones (2nd and 4th metacarpal/metatarsal), proximal

sesamoid bones In anatomy, a sesamoid bone () is a bone embedded within a tendon or a muscle. Its name is derived from the Arabic word for 'sesame seed', indicating the small size of most sesamoids. Often, these bones form in response to strain, or can be prese ...

, long

pastern The is a part of the leg of a horse between the fetlock and the top of the hoof. It incorporates the long pastern bone (proximal phalanx) and the short pastern bone (middle phalanx), which are held together by two sets of paired ligaments to fo ...

(proximal or 1st phalanx), short pastern (middle or 2nd phalanx),

coffin bone The coffin bone, also known as the pedal bone (U.S.), is the bottommost bone in the front and rear legs of horses, cattle, pigs and other ruminants. In horses it is encased by the hoof capsule. Also known as the distal phalanx, third phalanx, ...

(distal or 3rd phalanx), and

(distal sesamoid). There are usually slight differences in these bones when comparing the front and the hind. The 3rd metatarsal is about 1/6 longer than the 3rd metacarpal. Similarly, the 2nd and 4th metatarsals are longer in length when compared to their front-end counterpart. In the hindlimb, the 1st phalanx is shorter and the 2nd phalanx is longer than in the frontlimb. In addition, the 2nd and 3rd phalanx are narrower in the hind limb. The angle created by these three bones in the hindleg is steeper by about 5 degrees, therefore making the pastern angle steeper behind than in front.

Skeletal system disorders

* Arthritis (horse) ** Degenerative joint disease (DJD), such as bone spavin, ringbone, omarthritis **Inflammatory joint disease such as Carpitis (sprained knee), osselets * Bucked shins *

Curb A curb (North American English), or kerb (Commonwealth English except Canada; see spelling differences), is the edge where a raised sidewalk or road median/central reservation meets a street or other roadway. History Although curbs have ...

* Degenerative suspensory ligament desmitis (DSLD), and sprains of the suspensory ligament * Fractures * Locked kneecap (delayed patellar release or intermittent upward fixation of the patella) * Navicular disease * Osteochondrosis (horse) * Sesamoiditis * Splints * Wry nose

Joint disease in horses

{{Main article, Treatment of equine lameness Performance horses, like human athletes, place a high amount of stress on their bones and joints. This is especially true if the horse jumps, gallops, or performs sudden turns or changes of pace, as can be seen in racehorses, show jumpers, eventers, polo ponies, reiners, and western performance horses. A high percentage of performance horses develop arthritis, especially if they are worked intensely when young or are worked on poor footing. Treatment of early joint disease often involves a combination of management and nutraceuticals. Intramuscular, intravenous, and intra-articular medications may be added as the disease progresses. Advanced therapies, such as Interleukin-1 Receptor Antagonist Protein (IRAP) and stem cell treatments, are available for acute cases.