· The skull of Rabbit consists of two regions:
    (A) Posterior Cranial region &
    (B) Anterior Facial region

1. Cranial Region

· The cranial part is shorter than the facial part.
· Cranium, which forms the braincase, encloses the brain.
· Cranium is divided into three segments which are Occipital segments, Parietal segments and Frontal segments.

A. Occipital segment:
· This segment includes four bones that surround the foramen magnum.
· They are supra-occipital lies just above the foramen (1), exoccipitals (2) located at both of the lateral sides of foramen and basioccipital (1) that lies on the floor of the foramen.
· The skull of a rabbit is dicondylic because it has two occipital condyles.

B. Parietal segments:
· This segment consists of five bones.
· They are parietal (2) in the roof, alisphenoids (2) in the lateral sides, and basisphenoid (1) on the floor.
· Basisphenoid has a depression called sella turcica where the pituitary gland is lodged.
· An inter-parietal bone is sandwiched between the two parietal bones.

C. Frontal segments:
· They form the side and frontal region of the cranium.
· There are five bones in this segment.
· They are frontal 2 in the roof, orbitosphenoid 2 on sides and presphenoid 1 on the floor.
· An ethmoid bone or cribriform plate perforated with many small holes closes the cranial cavity in front.

D. Auditory capsule:
· These are attached posterior-lateral regions of the cranium.
· Each auditory capsule encloses the membranous labyrinth of the internal ear which has periotic bone and tympanic bulla.
· Periotic is a compound bone that is formed by fusion of pro-otic, epiotic and opisthotic bones.
· is located between the squamosal and occipital ring.
· Tympanic bulla is the flask-shaped bone applied to the outside of periotic bone in between the basisphenoid and squamosal.
· Tympanic bulla encloses the tympanic cavity of the middle ear containing tympanic membrane and chain of three ear ossicles i.e. malleus, incus and stapes.
· Stapes is the smallest bone of the body.
· Ear ossicles are concerned with hearing.
· Periotic consists of two parts-internal hard bony part called the petrous part enclosing the internal ear and the posterior light and porous mastoid part.

E. Orbits:
1. These are situated on the sides of the frontal segments of the cranial region.
2. Two orbits are separated by a median bony interorbital septum.
· So, the skull of a rabbit is tropibasic or an interorbital septum is present.
3. Each orbit has a small bone called lacrymal, in front of which a notch for tear duct is present.

2. Facial Region
· This region contains the bones of the olfactory capsule and jaws.
A. Olfactory capsule
· This capsule consists of nasal bone on the roof, vomer bone on the floor of premaxillae and maxillae bones on the sides.
· Mesethmoid or internasal septum separates two nasal chambers.
· Each olfactory or nasal chamber encloses an irregular mass or turbinal or scroll bones.
· Scroll bones help in increasing the sensory surface of the olfactory chamber.

Skull of Rabbit (Dorsal view)

B. Upper jaw

· The upper jaw consists of the following bones:

a. Premaxilla:
· It is the anteriormost bone with two incisor teeth.
· It is thick and about triangular in shape.

b. Maxilla:
· It is an irregular bone that forms the major part of the upper jaw and the side of the face.
· Its middle region has cheek teeth. (3 premolars, 3 molars and a zygomatic process.) and palatine process.

c. Palatine:
· It is located behind the maxilla.
· A secondary process is formed from the processes of premaxillae, maxilla and palatines.

d. Pterygoid: a small bone that articulates with the palatine.

e. Squamosal:
· It is a rectangular plate-like bone.
· It is located between parietal and frontal dorsally, periodic and tympanic bull posteriorly.
· It also bears the zygomatic process. on its outer surface.

f. Jugal:
· It is a narrow wavy bone.
· It contacts the zygomatic processes of the squamosal and maxilla forming a zygomatic arch.
· The zygomatic arch surrounds the eye orbits from the outside.

C. Lower jaw
· Lower jaw comprises two rami (dentaries which are fused together in front of mandibular symphysis).
· Each dentary has a conspicuous elongated condyle, coronoid process and rounded angular process at the posterior end.
· Teeth in each dentary are incisor (1), premolar (2) and molar (3).
· Diastema is found in between incisor and premolar in each ramous (dentary).
· The bony socket of the jaw in which teeth are implanted are called alveoli.
· The jaw super suspensorium is craniostylic i.e the lower jaw is articulated with the upper jaw by squamosal.
· Inferior dental foramen is present on the outer anterior end of each dentary for supplying blood vessels and nerve fibres.

D. Hyoid Apparatus
· Its main part is located beneath and supports the root of the tongue called basihyal.
· Anterior cornu is long and 4 segmented. These segments are ceratohyal, epihyal, stylohyal and tympanohyal.
· Posterior cornu consists of a single bone called thyrohyal bone.

Mandible of Rabbit (Lateral view)


Vertebral Column

· It is acoelous or amphiplatyan in mammals.
· Cartilaginous intervertebral discs are present between the two centra of adjacent vertebrae.
· Vertebral column is divisible into 5 regions: 
  1. Cervical, 
  2. Thoracic, 
  3. Lumbar, 
  4. Sacral &
  5. Caudal.

1. Cervical Vertebrae

· There are 7 cervical vertebrae in the rabbit.
· They have reduced head of cervical ribs.
· All cervical vertebrae except for the 7th possess a vertebrarterial canal for the passage of cervical blood vessels and nerves.
· The total number of vertebrae ranges from 45-47.
· Vertebral formula is C7Th12-13L6S4Cd16

a. Atlas

· It is the first cervical vertebra.
· It is articulated with the skull through condyles.
· Odontoid fossa is present.
· It has a rudimentary neural spine but the Neural canal is wide and long.
· It is a bony circle like signet ring particularly formed by neural arch.
· It has no centrum and zygapophysis.
· It has a well-developed wing-like cervical rib called transverse processes. These processes are perforated basally by inter vertebrarterial canals.
· Anteriorly, the atlas bears a pair of large shallow concave facets for occipital condyles and posteriorly, it has two stout lateral facets and a small-mid ventral facet for the odontoid process of axis vertebra.

Atlas of Rabbit (Dorsal view)

Atlas of Rabbit (Dorsal view)

b. Axis or Epistropheus
· It is the second cervical vertebra.
· Its neural spine is sharp and laterally compressed.
· Transverse processes are small, directed posteriorly and perforated by vertebrarterial canals.
· Anteriorly, it has a long, pointed peg-like odontoid process which is fitted into the posterior concavity of the Atlas canal of an atlas.
· Therefore head and atlas can freely rotate on the odontoid process.
· Post-zygapophyses are present but pre-zygapophyses are absent.

Axis of Rabbit (Lateral view)


c. Typical cervical vertebrae

· These are the cervical vertebrae from the third to the seventh which are similar in shape. So, they are called typical cervical vertebrae.
· It has a large neural arch, small, pointed, backwardly directed neural spine and short flattened acoelous centrum.
· Pre and postzygapophyses are present.
· Small transverse processes are basally perforated by vertebrarterial canals and transverse processes are bifurcated.

Typical cervical vertebra of Rabbit (Anterior view)

2. Thoracic Vertebrae

· There are 12-13 thoracic vertebrae in rabbits.
· They are divided into two types:
    a. Anterior thoracic vertebrae
    b. Posterior thoracic vertebrae

A. Anterior thoracic vertebrae
· These are the first six to seven thoracic vertebrae.
· Each bears a very long slender thoracic neural spine directed backwardly and obliquely.
· Neural arch has pre-zygapophyses facing outwardly and postzygapophyses are facing inwards and outwards.
· Transverse processes are short, stout and each bears a facet for the tuberculum of the rib.
· Centrum has a small concave demi-facet.

Anterior thoracic vertebra of Rabbit (Lateral view)

Anterior thoracic vertebra of Rabbit (Dorsal view

B. Posterior Thoracic Vertebrae
· These are posterior 4-5 vertebrae of the thoracic region which are somewhat different from anterior thoracic vertebrae.
· They have shorter, small, vertical and laterally compressed neural spines and reduced transverse processes without tubercular facets.
· Neural spines and zygapophyses are increasing posteriorly.
· Pre-zygapophyses have metapophyses just above them and post-zygapophyses have anapophyses just below them.

Posterior thoracic vertebra of Rabbit (Lateral view)

3. Lumbar Vertebrae

· They are divided into two types:
    a. Anterior Lumbar vertebrae
    b. Posterior Lumbar vertebrae

A. Anterior Lumbar vertebrae
· The number of lumbar vertebrae ranges from 6-7 out of which the first two are recognized as anterior lumbar vertebrae.
· Each bears median ventral process of centrum called hypapophysis.
· Neural spines are crest-like and slope anteriorly.
· Transverse processes are long, large, flattened and are directed downwardly and forwardly.
· Anterior end of the neural arch has a large forward sloping process called metapophysis bearing a zygapophysis on its medial aspect.
· Small, backwardly directed anapophysis is also developed from the posterior end of the neural arch, just below each post-zygapophysis.
· Lumbar vertebrae possess additional processes like metapophysis and anapophysis.

Anterior lumbar vertebra of Rabbit (Anterior view)

B. Posterior Lumbar vertebrae
· The third to the seventh vertebrae are called posterior lumbar vertebrae.
· They are similar to the anterior lumbar vertebrae except that they have no hypapophysis below the centrum.
· Centrum is well developed and acoelous.
· The transverse processes are long, flattened, highly developed and pointed backwardly.

Posterior lumbar vertebra of Rabbit (Lateral view)

4. Sacral Vertebrae

· They are 4 in number which is fused together to form a sacrum and support pelvic girdle.
· The first sacral vertebra is the longest one which has an articular surface facet attached with the ilium of the pelvic girdle its Neural spine is upright.
· The second to fourth vertebrae are similar in structure.
· Their neural spine is large and backwardly directed.
· Pre and post-zygapophyses are present as tubercules.
· Anapophyses are absent.

Sacrum of Rabbit (Lateral view)

5. Caudal vertebrae
· They are 16 in number in rabbits and progressively decreasing in size posteriorly.
· They have no transverse process.
· The last 5-6 caudal vertebrae are present in the form of the rod-like centrum.
· Intervertebral discs are present in between the central which are fibrocartilages.
· The central portion of the disc is called the nucleus pulposus. It represents the remnants of the notochord in the adult.

Caudal vertebrae of Rabbit (Dorsal view)

· It lies mid-ventrally in the thoracic wall.
· It is elongated, laterally compressed and transversely segmented.
· Sternum is composed of six rod-like bones called sternebrae which are arranged in the straight line.
· The first longest piece or bone is called presternum or manubrium.
· The remaining five bones constitute the mesosternum.
· The Sixth sternebrae is the smallest one.
· The last or the seventh sternebrae is called xiphisternum or metasternum.
· It is terminated in the large expanded plate of cartilage called xiphoid cartilage or xiphisternal cartilage.

Sternum of Rabbit with ribs

Sternum of Rabbit

· There are 12-13 pairs of thoracic ribs in Rabbit.
· They are divided into three types:
a) True ribs- First to 7th pairs of ribs [Attached with sternum]
b) False ribs- 8th to 9th pairs [Attached with last true rib]
c) Floating ribs- 10 to 12 pairs [Have no sternal part and are not attached with sternum]

· Rib is a curved rod-like bone.
· Each rib is divisible into 2 parts: the sternal part and the vertebral part.
· Vertebral part is a longer, dorsal bony part of ribs which is articulated with the thoracic vertebra by tuberculum to transverse process and by capitulum to centrum. So vertebral rib is bicephalous (having two heads).
· The sternal portion is cartilaginous which is connected below with the sternum.

A thoracic vertebra with attached rib

A thoracic rib of Rabbit

Limbs bones or Appendicular Skeletons

A. Pectoral girdle
· Pectoral girdle consists of two halves.
· Each half consists of two bones- a membranous bone called clavicle and a large replacing bone called shoulder blade or scapula-coracoids.
· Scapula – coracoids is a large, flat and triangular bone and its apex has a glenoid cavity for the attachment to the head of the humerus.
· The coracoid is fused with the scapula to form the coracoid process.
· The dorsal or vertebral edge of the scapula is made of a thin strip of cartilage called suprascapula.
· Acromian spine is present on the dorsal surface of the scapula.
· This spine continued ventrally into the free downwardly directed acromion process and a long backwardly directed metacromian process.
· Clavicle is the slightly curved rod-like bone that is articulated with the acromion process and on the other hand, it is joined with the manubrium of the sternum.

Pectoral girdle of Rabbit (Right half in outer view)

B. Pelvic girdle
· It is a W–W-shaped bone which is composed of two halves and each half is formed by the combination of 4 bones- ilium, ischium, pubis and cotyloid.
· Ilium is a blade-like bone that runs parallel to the vertebral column with an articular facet for sacral vertebra or sacroiliac joint.
· Ischium is a stout and straight bone found on the posterior-dorsal side which is thickened posteriorly to form ischial tuberosity.
· Pubis is the smaller bone found on the ventral median and the inner side of the girdle.
· Obturator foramen is present in between pubis and ischium.
· A small cotyloid bone is present on the inner side of the acetabulum between the ilium and Ischium.
· Pubis of both sides are attached to each other in the middle by pubic symphysis which is formed by fibrocartilage.
· Acetabulum is located in-between ilium and ischium on the outer side in the form of concavity to the articulation of the head of the femur.

Pelvic girdle of Rabbit (Ventral view)

Forelimb Bones

A. Humerus
· It is the bone of the upper arm. It is short but stout rod-like bone.
· Proximally, it has a large rounded head which is articulated with the glenoid cavity of the scapula.
· Close to the head, a greater tuberosity and lesser tuberosity are present at the outer and inner sides respectively.
· Biccipital groove is present in-between greater and lesser two tuberosity.
· Deltoid ridge is also present just below the head on the anterior side.
· Distally humerus bears a pulley like a trochlea for articulation with ulna. Just above this trochlea, supratrochlear foramen and olecranon fossa are present.

Humerus of Rabbit (a) Anterior view (b) Posterior view


B. Radio–ulna (Forearm)
· These are two parallel bones (radius and ulna) of forelimbs that are tightly bound together at both ends.
· Radius is smaller and shorter, and somewhat curved. So, proximally, it lies in front of the ulna.
· Ulna is longer. Its proximal end protrudes beyond the radius to form the olecranon process to articulate with the olecranon fossa of the humerus.
· Anteriorly, these two bones form the C-shaped sigmoid notch where the trochlea of the humerus is fitted.
· Posteriorly these two bones have epiphyses and are articulate with carpals.

Radius-ulna of Rabbit (Side view)

Hand Bone

A. Wrist (Carpus)
· Wrist has nine small bones called carpals.
· Carpals are arranged into two rows.
· Proximal row bears radiale, intermedium, and ulnare bones.
· Distal row contains central, trapezoid, magnum, trapezium, and unciform.

B. Metacarpals
· It has five long and slender bones of palm.
· The palm of rabbits is permanently facing downward.
· It cannot rotate its palm. So, it is called the pronate condition.
· There are five digits in each forelimb with varying numbers of phalanges.
· The number of phalanges is fourteen.
· All digits are terminated in the horny claw.
· The digital formula is 2, 3, 3, 3, 3.

Hand bones of Rabbit

Hind Limb

A. Femur (Thighbone)
· It is the longest and the strongest bone of the mammalian body.
· It has a long cylindrical shaft with expanded proximal and distal ends.
· Proximal end bears head, greater trochanter, lesser trochanter and third trochanter.
· The head is rounded, present at an inner side of the proximal end for the articulation with the acetabulum of the pelvic girdle.
· Greater trochanter lies just above the head whereas lesser trochanter is present just below the head.
· The third trochanter is present below the greater trochanter.
· Trochanters are meant for muscles attachment.

Femur of Rabbit (a) Posterior view(b) Interior view

B. Tibia-fibula (Shank)

· It is the bone of the shank of the hindlimb formed by two bones: Tibia and Fibula.
· Tibia is a large, strong and straight bone.
· Fibula is a small and slender bone that lies at the posterior side of the tibia.
· Tibia and fibula are free proximally but fused distally. Hence, its name is the tibia- fibula.
· The proximal end of the tibia bears a small sharp ridge called the cnemial crest, two oval-shaped surfaces on its upper end for articulation with the two femoral condyles.
· Distal end has an intercondylar groove at the backside for articulation with the tibia- fibula and patellar groove at the front side.

Tibia-fibula of Rabbit (Lateral view)


A. Tarsus or Ankle
· Tarsus has six tarsal bones arranged in two rows- proximal and distal rows.
· The first or proximal row has these bones- Astragalus [tibiale and intermedium], Calcaneum- has a calcaneal process and central.
· Distal row or second row: It contains mesocuneiform, ectocuneiform and cuboid.

1. Sole (Pes)
· It has four long and slender metatarsals bones.
· The thumb or hallux is absent.
· So there is no 1st metatarsal.

2. Digits
· It has twelve phalanges of toes.
· All digits have curved claws.
· The 1st digits are absent.
· The digital formula is 0, 3, 3, 3, 3.

Bones of the foot of Rabbit


· Joints are the region where movements are possible.
· It is the gap present between the two articulating bones or cartilages.
· The double-wall capsule at the point of joint is always filled by a fluid called synovial fluid which contains large polysaccharide molecules of hyaluronic acid and proteins.
· Synovial fluid is sticky and mucoid in nature.
· It is secreted by the inner synovial membrane to provide flexibility.
· In some cases, the synovial cavity is divided by fibro-cartilage into two cavities which are known as the meniscus.
· The joint lacking synovial cavity and other associated structures are called imperfect or immovable joints.

Various Types of Joints

· Broadly, Joints are of 2 types: Synovial or movable and Immovable joints.

1. Synovial joints or Movable joints

· This joint consists of two or more joints that are present in the synovial cavity and the tips of the bones are covered by hyaline cartilage.
· Their mobility is provided by synovial fluid.
· These bones are connected by ligaments.

· Types of Synovial joints:
a) Ball and socket joints

· In this joint one of the articular bones has a rounded end whereas another one has concavity to receive the round end of the first bone.
· This joint provides the greatest degree of movement between two articulating bones.
· Example: found between the glenoid cavity of pectoral girdle and acetabulum of the pelvic girdle.

b) Rotary or pivot joint
· One of the articular bone act as pivotal or fixed and another bone rotates over this pivot or fixed bone.
· For example, the joints between the atlas and the axis of reptiles, birds and mammals show this type of joint because the odontoid process acts as a pivot over which the atlas rotates to provide mobility of the head.

c) Hinge joint
· This joint allows the movement only in one direction.
· For example elbow and knee joints.

d) Sliding joint:
· In this joint two articular bones can slide or glide over each other.
· For example, the radius and ulna can glide over each other at the carpal point, zygapophysis of vertebrae are sliding over another vertebra.

e) Saddle joint

· The two articular bones display the saddle like joint in which both ball and socket are incompletely developed.
· The function of the saddle joint is somewhat like that of the ball and socket joint.
· For example joint between the metacarpal of the thumb and wrist bone.

2. Immovable joints
· They are of five types:

a. Fibrous joints or fixed joints
· In this joint, margins of the bones are fitted tightly into one another separated only by fibrous tissue.
· Example: cranium or joints of skull.

b. Cartilaginous joint:
· This joint occurs where two bony surfaces are covered by hyaline cartilage, connected by a pad of fibro-cartilage and by ligaments.
· It does not form the complete capsule around the joint and limits the degree of mobility.
· For example, Fibrocartilage disc in the vertebral column.

c. Bony joints: Example: mandibular symphysis

d. Gamphoses joints: Example: teeth of mammals attached to the sockets.

Another Classification of Joints

1. Synarthroses: In which no movement or little movements

· It is classified further into the following types:
a. Synostosis:
· In this type of joint, bones are united by bone tissue and no movement takes place.
· At an older age, this type of synarthrosis unites the skull bones which were united by dense connective tissue in children or young age.

b. Synchrondrosis:
· This is an articulation in which the bones are joined by hyaline cartilage.
· For example, The first rib unites to the sternum.

c. Syndesmosis:
· It permits a certain amount of movement.
· The bones are joined by an interosseous ligament of dense connective tissue.
· Example: pubic symphysis.

2. Diarthrosis:
· They are joints that generally unite long bones and have great mobility such as elbow and knee.
· In this joint ligament and capsule of connective tissue maintain the contact at that end of bones.
· The capsule contains the sealed articular cavity that contains synovial fluid.
· The sliding of the articular surface is covered by hyaline cartilage.
· The capsule is formed by two layers: the outer fibrous layer and the inner synovial layer.

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