Equine Skeletal System

Spine (Vertebral Column) - Structure

Axial Skeleton
7 Cervical Vertebrae - Top 2 Atlas & Axis, 18 Thoracic Vertebrae, 6 Lumbar Vertebrae, 3 (fused) Sacral Vertebrae, Approx. 20 Coxygeal Vertebrae.

Vertebral Structure
- Body - Provides support. Bodies attached by strong Fibrocartilagenous Joints.
- Vertebral/Neural Arch - Arch over vertebral body - sum of them creates Spinal Canal, housing spinal cord. Nerves leave canal via Intervertebral Formina.
- Intervertebral Discs - Fibrous cartilage between bones - Shock Absorbers.

Vertebral Processes
Processes influence movement of parts of spine and are attachments for ligaments and muscles.
- Transverse Process - Project to the side - mainly found in Lumbar Vertebrae.
- Dorsal Spinous Process - Project dorsally, particularly high in Thoracic Spine and form withers.
- Articular Process - Form Synovial Jointswith neighbouring process and control range of motion vertebrae are capable of.

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Spine (Vertebral Column) - Function and Movement

Movement
- Flexible but stable structure.
- Spine's RoM decreases from front to back:
Cervical Spine - Extensive mobility in all directions.
Thoracic Spine - Can flex and extend in a dorsoventral and lateral direction.
Lumbar Spine - No lateral mobility but more dorsoventral flexion, especially between last lumbar and sacrum.
Sacrum - Fused, direct link between pelvis and spine.

Function
- Contains and protects spinal cord
- Allows movement
- Allows transmission of strong propulsive forces from the hind limbs.

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Skull

Axial Skeleton

34 mainly flat bones, joined by Skull Sutures.

Structure - 2 parts:

Cranial Skull 
- Surrounds Cranial Cavitiy

Facial Skull
- Forms face and jaw - Mandibular Bone (moveable lower jaw), and Maxillary Bone (upper jaw)
- Surrounds 3 Cavaties:
Oral Cavity
Nasal Cavity - contains Tubinate Bones & is connected to Paranasal Sinuses
Orbital Cavity

Function
- Houses and protects brain and other sensory organs.

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Ribs and Sternum

Axial Skeleton

18 Pairs of Ribs:
Dorsally, all ribs form a joint with small transverse processes of the thoracic vertebrae. Ventrally: 
- 8 pairs of Sternal / True Ribs Attach to Sternum via fibro-cartilagenous joint.
- 10 pairs of Asternal / Floating / False Ribs - Have no direct connection to the sternum & a larger range of motion.

Sternum
Flat bone situated ventrally in region of girth. Has an elongated segmented shape.

Function
- Ribs, Sternum & Thoracic Vertebrae form Rib Cage.
- Rib Cage surrounds Thoracic Cavity & front part of Abdominal Cavity & protects organs within.
- Movement of ribs (enabled through intercostal muscles) is responsible for expansion & contraction of rib cage - basis of breathing (inspiration & expiration).

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Forelimb - Upper & Forearm

Appendicular Skeleton.
Not directly attached to Axial Skeleton - held in place by muscle & ligament alone - large RoM. Proximal forelimb includes upper part of foreleg down to & including the Carpus.

Scapula
- Large flat bone, triangular. Broad section of cartilage on upper edge acting as shock absorber.
- On outer surface, large bony process (Spine of Scapula) - attachment for muscles which flex/extend shoulder.

Humerous
- Strong long bone at top of forelimb. Proximal end, Scapula rotates with humerous (shoulder joint - ball and socket). - Angle between Scapula & Humerous often 90degrees. Distal end connects with bone of forearm (elbow joint - hinge joint).

Radius & Ulna
- Radius - only fully developed forearm bone. Long bone extending from elbow to Carpus, forming joint with Proximal Carpal Bones.
- Ulna - Forms lower part of elbow but does not extend to Carpus (only upper 3rd of forearm). Has a large bony protuberance (Olecranon), which is attachment for muscles which extend elbow, and easily palpable.

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Forelimb - Carpus (Foreknee)

Appendicular Skeleton
Equivalent to human wrist.
Carpal Bones arranged in two rows:
- Proximal Row - 4 bones. Outermost bone is Accessory Carpal Bone & protrudes on the palmer surface which is can be palpated.
- Distal Row - 3-4 bones - in a fully grown horse, two of the bones may be fused.

Capal Joints - made up of 3 joints:
Radiocarpal Joint - Between Radius and Proximal row of Carpal bones. Incongruent synovial joint allowing flexion, extension, and a certain degree of rotation.
Intercapral Joint - Between the proximal and distal row of Carpal bones. Incongruent synovial joint allowing flexion, extension, and a certain degree of rotation.
Carpometacarpal Joint
 - Between the distal row of Carpal bones and the metacarpal bones (cannon & splint). Arthrodial Joint allowing only a small range of gliding motion.

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Hindlimb - Pelvis

Appendicular Skeleton
Hindlimb is directly connected to Axial Skeleton via Sacroiliac Joint. Proximal Hindlimb is from pelvis down to & including the Tarsus.

Bowl shaped structure surrounding pelvic cavity. Large, flat bone in two halves made up of 3 bones fused together:
Illium- Largest & most cranial part. Foremost parts of ilium form points of hip, easily palpable.
Pubic Bone - Centre on the Ventral aspect.
Ischium - Most Caudal portion, extending into palpable points of buttock (Ishcial Tuberosity).

The 3 bones join in the cavity of the hip joint known as the Acetabulum.
Left & right halves of pelvis are joined by tough fibrocartilagenous joint known as Pubic Symphis.
At the top of pelvis, the two wings of the iliac bones form a joint with the sacrum - Sacroiliac Joint:
- Synoviall joint with very limited range of motion due to strong surrounding ligaments. The firmness of the connection ensures propulsion forces generated by hind limbs are transmitted a efficiently as possible to the spine and rest of the trunk.
The many surfaces & processes of the pelvis serve as attachment & origin of several large muscles involved in extension and flexion of hind limbs.

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Hindlimb - Proximal Limb

Appendicular Skeleton

Femur
Thigh bone - long bone with thick strong shaft and round head. Together with Acetabulum of Pelvis forms the hip joint (ball & socket). Hip joint primarily allows flexion & extension with minimal ab/adduction.
Below the hip joint is the neck of femur & a large bony protuberance (Trochanter Major) which serves as a muscle attachment.

Stifle
Distal end of femur forms stifle joint with Patella & Tibia. 2 large protuberances at the end of femur (Trochanter ridges) with a groove between them that the patella glides within when the stifle is flexed. 2 Parts to stifle:
Femoropatellar Joint - between femur & patella
Femerotibial Joint - between femur & tibiaarticular surfaces of joint do not fit together well (Incongruent joint), 2 cartilagenous discs (Menisci) are in to make the joint a better fit.
Joint is also stabilised by two Cruciate Ligaments (patellar & colateral).

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Hindlimb - Proximal Limb Cont.

Appendicular Skeleton

Tibia
In the horses lower leg (Gaskin) only the tibia is fully developed.
Proximal end of tibia is part of the Stifle joint & has a strong bony protuberance at the front below the joint (Tibial Tuberosity), where the patella ligaments are attached.
The tibia is a long bone which extends down to the hock, where it articulates with the proximal layer of Tarsal Bones.

Fibula
Thin vestigal bone which runs alongside the side & back of the upper half of the Tibia. It is sometimes fused with the Tibia.

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Hindlimb - The Tarsus

Appendicular Skeleton

Tarsus/Hock joint. Tarsal bones arranged in 3 rows - Proximal, Middle Distal, with a total of 6 bones.
Proximal row contains the Talus which is a large bone with 2 large, rounded Trochlea which articulate with the distal end of the Tibia
 Tibiotarsal / Tarsocrural Joint (hinge joint) - only tarsal joint with a lot of mobility.
Behind the Talus lies Calcaneous - its large process, the Calcaneal Tuberosity is a site of attachment for tendons of hock flexor muscles. 

Middle & lower row of tarsus contains 1 & 3 bones forming following joints:
Proximal & Distal Intertarsal Joints - between the upper, middle and lower rows.
Tarsometatarsal Joint - between the distal tarsal bones & metatarsal bones (splint &
canon).
These joints are practically immobile (Amphiarthrodial Joints), allowing on minimal backward & forward gliding movement - their main function is shock absorption.
These joints can be become painfully diseased - arthritis of intertarsal & tarsometatarsal joints is known as Bone Spavin.

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The Distal Limb (Fore & Hindlimb)

Appendicular Skeleton
Bones of Canon Region
Known as Metacarpal Bones in forelimb and Metatarsal Bones in hindlimb. Only the 3rd Metacarpal/tarsal is fully developed. Splint Bones are remains or 2nd & 4th metacarpal/tarsals.

Canon Bone (3rd Metacarpal/tarsal)
Has a strong bony cortex& broad flat upper surface articulating with distal carpal/tarsal bones forming Carpometacarpal and Tarsometatarsal joints.
Lower edge is rounded with a central longitudinal ridge which fits into the corresponding groove on the upper articular surface of the Proximal Phalanx, forming the Fetlock / Metacarpophalangeal Joint (hinge joint).
Only difference between fore and hindlimbs is that the hind canon is longer than the fore, and has a more roundish cross section compared to the oval cross section of the forelimb canon.

Splint Bones
Proximal ends are well developed & form part of the Carpometacarpal/Tarsometatarsal joints with the canon bone. Splint bones end approx. 10cm above fetlocks.
They are firmly attached to the canon bone with strong bands of fibrous tissue, which can become ossified as result or trauma/overloading/age, causing Splints.

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The Distal Limb (Fore & Hindlimb) Cont.

Appendicular Skeleton

Fetlock (Metacarpophalangeal (forelimb) / Metatarsophalangeal (hindlimb))
Joints between Canon Bone & uppermost Phalangeal Bone.

Proximal Sesamoid Bones
Located at the back of the fetlock. 
Two of these approx. triangle shaped bones, with the base at the distal end & the apex at the proximal end. 
Embedded within the Suspensory Apparatus & have a smoot outer surface which forms a guide pulley for the DDFT SDFT which run over the back of the fetlock joint. 
Proximal Sesamoid Bones are subject to strong Tensile Compressive forces:
- Extension of fetlock joint causes tensile strain in Suspensory apparatus.
- Considerable compressive forces exerted on sesamoids during fetlock extension by the canon bone to the front & flexor tendons from behind.
These forces put these bones at high risk of disease & injury.

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The Distal Limb - The Digit

Appendicular Skeleton
The only digital bones present in the horse are those of the 3rd central digit. Proximal to distal, these are:
- Proximal Phalanx / Long Pastern Bone / P1
Approx. 10cm (dependent on horse size - longest of phalanges). Forms fetlock joint (metacarpophalangeal / metatarsophalangeal) at its proximal end with canon & proximal sesamoid bones. Distal end forms Pastern Joint / Proximal Interphalangeal Joint (saddle joint) with the middle phalanx.
- Middle Phalanx / Short Pastern Bone / P2 
Approx. half length of P1 (cudoidal appearance from front).Distally has a large articular surface forming Coffin Joint / Distal Interphalangeal Joint (saddle joint) with the Pedal bone, and the back articulating with the Navicular bone.
- Distal Phalanx / Pedal or Coffin Bone / P3
Wedge shaped appearance from the side. Front curved dorsal surface, towards the back is the articular surface, forming the coffin joint with P2, and towards the bottom is the solar surface. Cortex of Pedal bone is not well developed, giving it a ragged appearance - due to many holes (Foramina) for blood vessels to supply dermis of hoof.

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The Distal Limb - The Digit Cont.

Appendicular Skeleton

- Distal Sesamoid Bone / Navicular Bone (located behind the Pedal Bone)
Forms the back of the coffin joint. Small broad wedge shaped bone. 
It's back (flexor) surfaces are smooth as it's main function is as a guide pulley, guiding the DDFT round the coffin joint. 
Both Pedal & Navicular bones are subject to considerable stress therefore disease & injury are not uncommon.

Collateral Cartilages
Two colateral cartilages (Hoof & Ungular cartilages) made up of fibrous cartilage are attached to lateral & medial processes at the back of the Pedal Bone. 
Lower half of each cartilage is contained within hoof capsule, upper half extends above hoof capsule & can be felt under the skin
They act as shock absorbers during movement.

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Diseases of the Distal Limb

Distal Limb subject to huge stresses during movement.
Joints of distal limb (fetlock, pastern & coffin joints) are more likely to suffer from Chronic Inflammatory Disorders such as Arthritis & Arthrosis than most other joints.
The bones of the digit (pastern & coffin bones) are most likely to suffer from fractures, followed by the Canon Bone.
Soft tissue injuries are found most frequently in the distal limb.

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