The Axial Skeleton | Gross Anatomy | Anjani Mishra

 

                                                                                               Written By Anjani Mishra

THE AXIAL SKELETON

The axial skeleton is the branch of skeleton which deals with the bones of the head, vertebral column, ribs and sternum.

BONES OF THE HEAD

THE SKULL BONES                  

The skull provides a means of the protection for the brain, the organs of special senses (sight, smell, hearing, equilibrium and taste), the openings for the passages of air and food, and the jaws, including the teeth for mastication. The term ‘‘cranium’’ (bones of the skull) is sometimes referred to as consisting of those bones which lodge and protect the brain devoid of the mandible and facial bones. According to the Nomina  Anatomica  Veterinaria  (NAV);1968  divides the skull in to 2 groups namely:-

 

·        Bones of the cranium &

·        Bones of the face

   # Formula to remember paired and single bones of skull

   @ SHEVOM- Single bones
   @ all the rest- paired bones

  # Formula to separate cranial and facial bones of skull

 
  @ SOFTPIE- Cranial bones
  @ all the rest- facial bones

CRANIAL BONES(paired)

Parietal Bone:-

The parietal bone is a paired structure and forms the dorsolateral wall of the cranium with the cccipital bone caudally and the frontal bone rostrally. It is composed of a parietal plane, temporal plane and a nuchal plane (in the ox). Internally the grooves and ridges correspond with the gyri and sulci of the brain. There is also an interparietal bone between the occipital bone and the parietal bone which fuses with age.

Interparietal:-

The interparietal bones are a median bone situated between the squamous part of the occipital bone and the parietal bone. The bones of either side fuse with each other and with the parietal bone.

Frontal Bone:-

The frontal bone is a paired structure joined by the interfrontal suture between the cranium and the face and enclosing the frontal sinuses. The nasal and lacrimal bones border the frontal squama section and form the zygomatic process laterally and part of the orbit dorsally. Lacrimal glands are also present near the orbit. The temporal line extends into the external sagittal crest.

Temporal Bone :-
The temporal bone is composed of squamous, petrosal and tympanic parts and forms the lateral wall of the cranial cavity. It articulates with the frontal, parietal and sphenoid bones. The squamous element joins the temporal process of the zygomatic bone to form the zygomatic arch and forms the articulating surface of the temporomandibular joint. An articular tubercle and mandibular fossa are present.

CRANIAL BONES(unpaired)

Occipital Bone:-

The occipital bone forms the nuchal wall and the foramen magnum. The pars basilaris element is the caudal base of the cranium, although rostral to foramen magnum and joined by a cartilaginous suture to basisphenoid bone. A nuchal crest is present and is easily palpable. The nuchal crest is often used as a landmark for collection of cerebrospinal fluid (CSF). There are also external occipital protuberances present which provide muscle attachment sites for the nuchal ligament. The lateral parts form the borders of foramen magnum. Occipital condyles are present which articulate with the atlas to form the atlanto-occipital joint. The paracondylar process provide muscle attachment sites for muscles of the head. The hypoglossal canal is also within this structure.

Sphenoid Bone:-

The sphenoid bone forms the base of the neurocranium and is composed of a body and wings. The bones are separated by cartilage which ossifies with age. The presphenoid is rostral and has a caudal fossa which is a hollow body with sphenoid sinuses located inside. The pterygoid processes and oval foramen are also present in this structure.

Ethmoid Bone:-

The ethmoid bone forms part of the cranial and facial parts of the skull and is located deep in the orbit. External lamina consist of the roof plate, floor plate and paired orbital plates. The ethmoid bone is separated from the cranial cavity by the cribriform plate. Numerous small foramina exist where the olfactory nerve (CN I) passes through. The perpendicular plate splits the ethmoid into two halves and the ethmoid larbyrinth protrudes from the ethmoid tubes. The tubes are composed of two rows of ethmoturbinates and air filled ethmoidal meatuses. Secondary ethmoturbinates may also be present. Ethmoturbinates are divided into endoturbinates and ectoturbinates. The first endoturbinate forms the dorsal nasal conchae and the second endoturbinate froms the middle nasal conchae. The endoturbinates form 3 nasal meatuses; the dorsal nasal meatus, the middle nasal meatus and the ventral nasal meatus.

 

FACIAL BONES(paired)

Maxilla:-

The maxilla forms most of the facial part of the skull, including the lateral walls of the face, nasal cavity, oral cavity and hard palate. It also forms the ventral nasal conchae and articulates with all of the facial bones as it is the largest bone of the face.

Premaxilla:-

The premaxilla (incisive) bones are the most rostral bone of the face and are each composed of a body, a palatine process and a nasal process. The bone is narrower and more pointed rostrally in case of ox.

Lacrimal:-

The lacrimal bone forms part of the lateral wall of the face and orbit and is situated near the medial canthus. It articulates with the frontal bone, zygomatic bone and maxilla. It also articulates with the nasal bone in ruminants and the horse and articulates with the palatine bone in carnivores. It is composed of an orbital and facial part separated by supra- and infraorbital margins. The nasolacrimal duct is present by the margin of the orbital surface. The ventral oblique muscle attaches caudal to the margin of the orbital surface. The nasal surface forms the boundaries of the maxillary and frontal sinuses.

Malar/Zygomatic:-

The molar (zygomatic) bone is in the caudolateral area of the face and forms the ventral border of the orbital cavity.

Pterygoid:-

The pterygoid bone is a paired structure bordered by the palatine and sphenoid bones. It forms the dorsal and lateral walls of the nasopharyngeal cavity. The pterygoid hamulus is formed by the pterygoid bone.

Nasal:-

The nasal bone is a paired structure and forms the roof of the nasal cavity. Dorsal nasal conchae attach to the ethmoidal crest on the internal surface. A rostral suture forms the apex and between the nasal and incisive bones is the naso-incisive notch.

Palatine:-

The palatine bone is a paired structure between the maxilla, sphenoid and pterygoid bones. It is composed of horizontal plate(forms part of the hard palate) on which nasal crest is present. The palatine sinus is present on horizontal plate.

Turbinate bones:-

These are delicate, scroll-like bones(four in number) which are attached to the lateral wall of the nasal cavity. Each bone is composed of a very thin lamina, cribriform in many places and covered on both sides with mucous membrane in the fresh state. They are arranged in two pairs, dorsal and ventral.

 

FACIAL BONES(unpaired)

Vomer:-

The vomer is unpaired and extends from the choanae of the palatine bone to the floor of the nasal cavity. It attaches to the median nuchal crest and has a septal sulcus which surrounds nasal cavity.

Mandible:-

The mandible can be divided into the body and the ramus. The body of the mandible supports the incisor teeth (rostrally) and cheek teeth (caudally). The section of the body which does not support any teeth is called the inter-alveolar margin or diastema. The mandible also contains the mandibular canal and the mental foramen. The facial notch is on the ventral surface where the parotid duct (in herbivores) and facial vessels run. The ramus extends from the caudal end of the body dorsally towards the zygomatic arch.  The condylar process articulates with the mandibular process of the skull.

Here, we see the bones of skull are flat bones, developed in membrane and the bones of the cranial base may be classed as irregular and are developed in cartilage . Only two form are permanent movable joints with other part of skull .The mandible form synovial joint with the temporal bone, and the hyoid bone is attached to the latter by bar of cartilage. The immovable joint located between most of the bones of skull are termed ‘suture’. But these suture disappear by osseous fusion on increasing age of animal. The skull present numerous foramina as mention below: - infraorbital, mental, mandibular, supraorbital, etc. through which cranial nerves and blood vessels enter and exit.

Hyoid Bone:-

·     The hyoid bone(Os hyoidiun) is situated chiefly between the vertical parts of the rami of the mandible, but its upper part extends somewhat further back.

·     It is attached to the petrous temporal bones by rods of cartilage, and supports the root of the tongue, the pharynx, and the larynx.

 It consists of a body, a lingual process, and the four pairs of cornua.

Body:

·     The body is a short transverse bar, compressed dorso-ventrally.

·     The dorsal surface is concave and smooth in its middle, and presents a convex facet or tubercle at each end for articulation with the small cornu.

·     The ventral surface is flattened and is slightly roughened for muscular attachment.

·     The anterior border carries the lingual process medially.

·     The posterior border is concave and smooth in its middle and carries the thyroid cornu on either side.

Lingual process:

·      It has a short tuberous lingual process.

·      The lingual process projects forward medially from the body, and is embedded in the root of the tongue.

Cornua:

A.    The thyroid cornua or thyroids

·     They extend backward and upward from the lateral parts of the body.

·     They are compressed laterally(except at their junction with the body), and the posterior end has a short cartilaginous prolongation which is connected with the anterior cornu of the thyroid cartilage of larynx.

B.    The small cornua or keratohyoids

·     They are short rods which are directed upward and forward from either end of the body.

·     Each of which is somewhat constricted in its middle part and has a slightly enlarged ends.

·     The ventral end has a small concave facet which articulates with the body.

·     The dorsal end articulates with the great cornu(horse), or with the middle cornu when present.

C.    The middle cornua or epihyoids

·     These are small, wedge-shaped pieces or nodules interposed between the small and great cornua.

·     They are usually transitory, and unite with the great cornua in the adult.

·     The middle cornua are almost as large as the small cornua.

D.     The great cornua or stylohyoids

·     They are the largest parts of the bone.

·     They are directed dorsally and backward, and are connected above with the base of the petrous temporal bone.

·     Each of which is a thin plate, which is slightly curved in its length, so that the lateral surface is concave and the medial surface is convex. Both surfaces are smooth.

·     The border are thin. The dorsal extremity is large and forms two angles; the articular angle is connected by a rod of cartilage with the hyoid process of the petrous temporal bone whereas the muscular angle is somewhat thickened and rough for muscular attachment.

·     The ventral extremity is small, and articulates with the small or middle cornu.

THE VERTEBRAL COLUMN

• It is the part of axial skeleton.
• It is an articulated structure of vertebra, cartilage and ligaments that gives passage to the spinal cord and its covering(meninges).
• Commonly called spine of the body.
• It comprises of five regions:

Vertebra	Region
Cervical Thoracic or Dorsal Lumbar Sacral Coccygeal	Neck Withers & back Loin Croup Tail

Definition:

Vertebral column is the fundamental part of axial skeleton consists of chain of median, unpaired, and irregular bones which extends from the occipital bone to the end of the tail.

The vertebral column is subdivided for description into 5 regions, which are named A/to the part of the body in which the vertebrae are situated. Thus the vertebrae are designated as cervical, thoracic, lumbar, sacral and coccygeal(caudal) vertebrae. The number of vertebrae in a given species is fairly constant in each region except the last, so that the vertebral formula may be expressed(for example in case of ox) as follows;

OX- C7, T₁₃ , L₆, S₅, Cy_18-20

Vertebral formula- express the total number of bones present in the vertebral column.

Vertebral formula of domestic animals and fowl:

Species	Cervical	Thoracic	Lumbar	Sacral	Coccygeal	Total(Av.)
Ox	C7	T13	L6	S5	Cy18-20	50
Horse	C7	T18	L6	S5	Cy15-21	54
Sheep/Goat	C7	T13	L6-7	S4	Cy16-18	48
Dog	C7	T13	L7	S3	Cy20-23	51
Pig	C7	T14-15	L6-7	S4	Cy20-23	53
Fowl	C14	T7	(L+S)14		Cy6	41

TYPICAL VERTEBRA:

            The vertebrae in a given region have characteristics by which they may be distinguished from those of other regions, and individual vertebra have characteristics which are more or less clearly recognizable. All typical vertebrae have a common plan of structure, which must first be understood. A vertebra consists of a body, arch and processes.

Body or Centrum:

• Mass of bone on which vertebra is built.
• It is cylindrical, solid and rod like.
• The dorsal surface of the body forms the floor of neural (spinal) canal.
• The ventral surface presents ill developed spines called infraspinatus process, for muscular attachment.
• The centrum is convex anteriorly and concave posteriorly.
• The vertebra are articulated to each other both at the centrum and oblique process.

Arch:

• The neural arch consists of lamina and the pedicle, together. (Dorsally, from either side of the body spring plates of bones called pedicles which form the wall of the neural ring. They unite above to form the roof and are called laminae).
• The arches of the opposite sides unite together to complete the neural ring.
• Series of neural rings with intervertebral membrane constitute the neural canal, which lodges the spinal cord.
• The pedicles bear notches at their cranial and caudal aspects, which form a circular opening together with preceding vertebra called intervertebral foramen, through which the spinal nerves and vessels pass.

Processes:

 

A)    Spinous processes: 

a) Supraspinous process/Neural Spine/ Dorsal spine:

·         It is a single process project dorsally form the middle of the arch, where the two laminae meet.

·         It varies greatly in form, size and direction in different vertebrae.

·         It furnishes attachment to muscles and ligaments. 

b) Infraspinous process:

• Its is on the ventral part of the centrum.
• Some vertebrae also have a ventral tubercle, ventral crest or a hemal arch. 

B) Oblique/Articular Processes:

• The vertebral segments articulate with each other, both at centrum and at oblique process.
• The oblique processes are four in numbers, two being placed anteriorly and two posteriorly.
• They bear a small articular part.
• The anterior oblique process faces above and articulates with the posterior oblique process of the preceding vertebra, which look ventrally.
• They are called oblique process due to their placement.

C) Transverse processes:

• Are two in number and project laterally from the sides of the arch or from the junction of the arch and body.
• In the cervical region the transverse processes of the 3^rd to 6^th cervical vertebrae present a cranial and caudal portion.
• The transverse processes are pierced by the transverse foramen(foramen transversarium) at their base. 

 D) Mamillary processes:

• Are found in most animals on the caudal thoracic and cranial lumbar vertebrae between the transverse and cranial articular processes or on the latter. 

 E) Accessory processes:

• When present, are situated between the transverse and caudal articular processes.

THE CERVICAL VERTEBRA

• The vertebral segments of neck (cervical) region are called cervical vertebra and are seven in number.  
• Placed serially one behind the other, being designated as first (atlas), second (axis), third, fourth, fifth, sixth and seventh cervical vertebra.
• They have long cylindrical centrum. They decrease in length from before backwards, while their width increases.
• The oblique processes are well developed and form diarthrodial joints.
• The first cervical vertebra(atlas) is completely an atypical where as the second cervical vertebra(axis) is partially an atypical or typical. Rest of the vertebrae of this segment are typical.

Atlas

·     It widely differs from the rest of vertebrae, hence, it is called an atypical vertebra.

·     In domestic animals head is suspended form atlas where as in human, atlas carries the head.

• It presents a ring and two large plates of bone, termed wing or alae.

Ring:

·     Ring is more or less rounded.

·     It has 4 surfaces: anterior, posterior, dorsal and ventral.

·    The dorsal surface presents a raised eminence called dorsal tubercle, which represents     supraspinous process of other vertebra.

·     On the ventral surface there is an obtuse eminence called ventral tubercle.

•  The anterior surface present two foramen; vertebral foramen (antero-internal) and the     foramen alare (antero-external) connected by a short furrow.

·     On the posterior surface there is a smooth articular surface called fovea dentis, which lodges odontoid process of the axis.

·    The ring anteriorly presents on either side a deep articular cavity which receive the occipital condyle of skull.

• The ring posteriorly presents two articular surface (fovea dentis and lateral articular surface) for articulation with the anterior surface odontoid process and articular surface) of axis.

Wing or Alae:

·     The wing represents transverse process of the typical vertebra.

·      It presents dorsal and ventral surfaces.

·      On the dorsal surface of the wing just behind the foramen alare (1^st) and vertebral foramen (2^nd), there are the external openings of the third and fourth foramina.

·      On the ventral surface at the junction of the ring, there is a deep fossa atlantis.

·       Foramen transversarium is absent.

 Axis

·     It is longest of the vertebrae and partly atypical.

·     On the anterior part of the body, there is a projection called odontoid process (dens), which is dorsally deeply concave and rough and ventrally articular, smooth and convex.

·     On the either side of the odontoid process, there are extensive, nearly flat articular surface, which may be considered as modified anterior oblique process. They blend with the ventral surface of the odontoid process. The entire surface thus, articulates with the fovea dentis of the atlas. 

·     The intervertebral foramen is present a little behind the anterior notch, which is circular.

·     The infraspinous process is in the form of median ridge and the supraspinous process projects a little infront over the canal.

·     Posterior oblique process placed posterolateral over the canal.

·    The foramen transversarium is either small or absent.

The third, foUrth and fifth cervical vertebra

·         These vertebrae are similar with typical vertebrae.

·         In serial placement the bodies tend to become shorter and wider form front to backwards.

·         The supraspinous process is short and tuberous centrally in the third, then gradually increases in height and length upto the seventh with forward inclination.

·         Ventrally, projects a well defined infraspionus process with increseas in length and angle of descend.

·         The oblique processes are slightly convex anteriorly and concave posteriorly, the size of which increases with the receding number.

·         The oblique processes are slightly convex anteriorly and concave posteriorly, the size of which increases with the receding number.

·         The transervse processes are divided into upper tubercular portion (project at right angles) and lower plate like portion directed outwards, downwards and forwards.

·         At the base of tubercular transverse process, the foramen transversarium is present. Series of these foramina continue to form canalis transversarious.

The centrum is convex anteriorly and concave and deep posteriorly.

THE sixth cervical vertebra

·     It is a typical vertebra.

·    The supraspinous process is well developed, while the infraspinous process is absent.

·    The oblique process is larger.

·   The transverse process is divided and the lower part is modified to form ventral branch of transverse process and laterally it forms lateral branch of transverse process, and is pierced at the base by the foramen transversarium, which is very large (largest in the series).

THE seventh cervical vertebra

·   It is a typical vertebra.
·     It bears the characters of both cervical and thoracic segments.
·     The supraspinous process is highest in the series.
·     Infraspinous process is represented by two tubercles.
·     The foramen transversarium is absent.
·     The transverse process is undivided.
·   The neural ring is very large.

·   Two semilunar demi facet are present on either side of posterior concave articular surface, which              together with head of first thoracic segment completes the articular cavities for the head of first pair        of ribs.

THE THORACIC OR DORSAL VERTEBRAE

·     There are 13 thoracic vertebrae in this region.

·     The vertebrae of this region are characterized by the presence of articular surface on the bodies for the heads of the ribs and on the transverse process for the tubercles of the ribs; and by the excessive development of supraspinous process.

·    The supraspinous process is well developed and highest.

·    The infraspinous process is in the form of thin ridge.

·    The oblique process is small. The anterior oblique process is slightly convex and placed on the dorsal part, directed upward. The posterior oblique process are slightly concave and placed at the base of dorsal spine, face downwards.

·    The transverse process are short, thick and single.

·    Each transverse process bears a tubercular facet on its ventral aspect for articulation with the tubercle of rib. They bear a thick rounded mamillary process on the dorsal aspect, which may be absent or ill developed in last few segments.

·    The bodies are short, distinctly constricted in the middle and are expanded at the   extremities.

·     On the either side at either end of the articular extremities of the centrum, there is a   concave articular surface, termed as costal facet. Each is a demi-facet as it articulates   with half the part of the head of rib.

·     The vertebral notches are shallower and smaller but anterior notches are deeper.

·     The arch is perforated by an additional intervertebral foramen on either side, in the posterior segments.

·     Spine of the first is pointed backwards. This backwards inclination increase up to the   tenth and then decrease.

·   The summits of the seventh to tenth are distinctly bifid.

     

The first thoracic vertebra

  ·      It shares the characters of both the thoracic and cervical vertebrae.

 ·     The costal and tubular facets are the largest in the series so also the transverse process

 ·     The supraspinous process suddenly rises in height

 ·   The anterior oblique process resemble those of cervical region.   

The LAst thoracic vertebra

·   The body is oval and bears no posterior pair of costal facet.

·  The supraspionus process is vertical of slightly incline forward.

The lumbar vertebrae

·   There are six lumbar vertebra and form the skeleton of loins.

·    Excessive development of transverse process is a special character.

·    Each transverse process is a long plate of bone, the transverse processes are   separated by intertransverse space. The length of first transverse process is the   shortest. It gradually increases up to 4^th & 5^th. The 6^th is slightly shorter than the 5^th.

·    The supraspinous process is broad and flattened.

·    The oblique process is well developed. The anterior oblique process is strongly   curved, facing inwards, while the posterior ones are correspondingly convex.

·    The mammillary process are fused with the anterior oblique processes , which are   thick and tuberous.

·     The bodies of lumbar vertebra are constricted in the middle, expanded at the   extremities and resemble as thoracic vertebrae.

·     The neural canal is uniform up to 3^rd and increase in length and height up to the last.

·     The posterior notches are larger than the anterior ones.

      ·  The intervertebral foramen frequently doubles in the anterior few segments.

THE first lumbar vertebra

·    Body is small and cylindrical.

·    Articulates anteriorly with the last thoracic segment.

·  Space between the two anterior processes is the shortest in the series, so also the intertransverse space.

THE sixth lumbar vertebra

·    Body is compressed form above downwards.

·    Neural ring is very wide.

·    Oblique processes are placed wide apart.

·  Supraspinous process is smallest in series.

THE Sacrum

The sacrum consists of five sacral bones fused together to form a single bone. It forms the region of croup.

Direction: it is horizontal in direction.

Location: Located between lumbar vertebrae anteriorly and coccygeal vertebrae posteriorly.

Relation : it articulates with the ilium, anteriorly with last lumber and posteriorly with the first coccygeal vertebra.

Shape: it is aeroplane shape. 

Composition: it present 2 surfaces, 2 borders, a base and an apex.

Surface:

Dorsal surface:

·        Presents centrally five sacral spines, which are fused together to form the medial sacral crest.

·        On other side of the base of the median sacral crest are the lateral sacral crest, which are the fusion of the oblique processes.

·        There are 4 pairs of dorsal sacral foramina, of which the 1^st & 2^nd pairs are located internal to lateral sacral crest, and the remaining are external to the lateral sacral crest.

·        Neural rings are triangular, anterior larger and posterior smaller.

 

Ventral Surface:

·        It forms the roof of the pelvic cavity and is concave from side to side and from before backwards.

·        It presents 4 pairs of ventral sacral foramina, and 4 transverse lines and one central median longitudinal furrow (sulcus vasculosus).

Borders:

The two lateral borders are thin, sharp, concave and irregular.

Base:

·        It is the anterior extremity of the first sacral vertebra.

·        It consists of body and two alae or wings.

·        The body is located centrally and is concave.

·        The wings are anteriorly concave and articulate with ilium by auricular facet.

Apex:

·        Apex is the posterior end of 5^th sacral vertebra and is slightly wider than the 4^th sacral vertebra.

·        It has centrally a small triangular neural ring.

·        On either side of the centrum of the last segment are backward projections, which represent the transverse process of the last sacral vertebra.

Comparison with:

A) Sacrum of Horse: (S₅)

·        Supraspinous process is distinct.

·        Four dorsal sacral foramina are on either side of the base of spines.

·        The lateral sacral crest is suppressed.

·        Lines transversae are less distinct.

·        The ventral sacral foramina are smaller than that of ox.

·        The auricular surface is elongated, oval and large.

B) Sacrum of Dog: (S₃)

·        Median sacral crests are in the form of two tubercles.

·        Lateral sacral crests are two in number.

·        The wings are high and auricular surfaces are directed outwards.

·        The ventral foramina are two in number.

C) Sacrum of Pig: (S₄)

·        Supraspinous process is ill developed.

·        Body is less curved.

·        Cranial articular processes are well developed.

D)  Sacrum of Fowl: (L+S₁₄)

·        Lumbar and sacral vertebrae are fused to form a single synsacrum.

·    This bone remains articulated with the ilia along their lateral borders.

THE COCCYGEAL VERTEBRA

·        The coccygeal or tail region consists of 15-20 segments.

·        In the first three of four, all the characteristics features of a true vertebra are found.

·        The first coccygeal vertebra may be fused with sacrum.

·        The first five have complete neural arches, supraspinous processes and large transverse processes. Anterior oblique processes are very distinct, but don't form ant articulation.

·        The first 12 present two small processes on the ventral surface of the bodies, the haemal processes, which pass the middle coccygeal artery.

·        Oblique processes disappear from the 7^th or 8^th.

Comparison of different vertebrae with:

A) Horse:

            C₇ T₁₈ L₆ S₅ Cy_15-21

·        Body of cervical vertebra is longer.

·        Wing of the atlas are pierced posteriorly by foramen transversarium.

·        Mamillary processes are in few caudal segments of thoracic vertebrae which are more developed.

·        Coccygeal vertebrae are shorter and ill developed.

 

B) Dog:

            C₇ T₁₃ L₇ S₃ Cy_20-23

·        Foramen transversarium is present in the atlas at its posterior aspect.

·        In lumbar vertebrae, the spinous processes are comparatively short, broad ventrally and narrow dorsally.

·        Coccygeal vertebrae are better developed.

 

C) Pig:

            C₇ T_14-15 L_6-7 S₄ Cy_20-23

·        Cervical vertebrae are short and bodies are wide.

·        Thoracic vertebrae are comparatively long.

·        Spinous processes of cervical vertebrae are short but large in axis.

·        First few coccygeal vertebrae present well-developed articular processes. 

D)  Fowl:

            C₁₄ T₇ L+S₁₄ Cy₆

·        In general, the vertebral column is rigid & stiff due to fusion of some vertebral   segments.

·        Atlas is in the form of ring, presents only one articular facet for single occipital condyle.

·        Cervical vertebrae have styloid projections from transverse processes.

·     Last few caudal vertebrae fuse to form a pointed bony projection, known as pygostyle.

THE RibS

   It presents a shaft and 2 extremities.

Shaft:

It is elongated and curved more at the upper part, while it is twisted and turned inwards at the lower part.

It presents 2 surfaces and 2 borders.

Surfaces:

·        The lateral surface is convex and is market by a wide groove on its anterior part.

·        The medial surface is concave, smooth and is covered by pleura.

 

Borders :

·   Anterior border is thick and concave.

• Posterior border is convex and presents costal groove.

 

Extremities:

Proximal extremity:

·        Consists of head, neck and tubercle.

·        Head (capitulum) is rounded and has a hemispherical articular surface, which is divided into two convex facets, anterior & posterior by a groove (sulcus capituli). It articulates with the costal facet placed on the body of thoracic vertebra.

·        Neck (collum costae) is a long constriction below the head and separates the head from tubercle.

·        The tubercle (tuberculum costae) bears a concave facet for articulation with tubercular facet of the thoracic vertebrae.

 

Distal/Sternal Extremity

• It is either articular or is prolonged by a costal cartilage.

Characters of Ribs in Serial placement:

·        The first rib is shortest, thickest and last curved. It body widens greatly towards the distal end. The costal groove is absent.

·        The 8^th, 9^th & 10^th ribs are generally the longest and the widest.

·        The head and tubercle decrease in size from the first to last ribs.

·        The curvature of the ribs increases from the first to last.

·        The last rib is slender and curved. The head and the tubercle of this rib are smallest and fuse with each other and hence neck is absent.

·        The distal extremity is thin and prolonged by the costal cartilage.

 

THE COSTAL CARTILAGES

·        It is the prolongation of rib, which helps to complete the lateral walls of the thorax.

·        Each is a cylindrical piece of cartilage, slightly compressed from sides to sides.

·        It is rounded and smooth on its borders and surfaces.

Comparison with:

A) Ribs of Horse:

·        18 pairs of ribs, eight sternal and ten asternal.

·        11^th is longest and 6^th is the widest rib.

·        Costal groove is distinct on the 4^th to 8^th ribs and absent on the first.

B) Ribs of Dog:

·        13 pairs of ribs, nine sternal and four asternal.

·        The first eight or nine increase in width at their distal extremity.

·        The last three of four ribs articulate with anterior costal facets, the posterior being rudimentary.

·        First rib is very long and highly curved.

C) Ribs of Pig:

·        14-15 pairs of ribs, 7 sternal and 7-8 asternal.

·        The first rib has prismatic body.

·        The last rib is usually small and floating

D)  Ribs of Fowl:

·        7 pairs of ribs.

·        Each rib has a dorsal 7 ventral segment.

·        First two ribs don’t extend up to sternum.

• Each of the dorsal segments of 2^nd-6^th ribs processes a caudal extension, called uncinate process, which supports the thoracic cage in a better way of overlapping the succiding rib.

The sternum(Breast bone)

·        It is centrally placed, segmented, osteo-cartilagenous structure, forming the floor of the thoracic cavity.

·        It is held in suspension by the costal cartilage.

·        In the adult animals, it consists of seven segments (sternebrae), which are elongated from before backwards.

·        Sternum is compressed laterally in the front, and dorso-ventrally behind.

·        It is oblique direction pointing downwards and backwards.

It presents 2 surfaces, 2 borders and 2 extremities.

Surface:

Dorsal surface

·        It is flat and rough, widens from before backwards and becomes much constricted behind the last pair of costal facets.

Ventral surfaces :

·        It is strongly convex anteriorly and is flat and depressed behind. It is market by a faint median longitudinal ridge, which divides the surface into two equal halves.

·        Ventral crest is absent.

Borders:

2 lateral borders

Lateral borders:

·        Divide the dorsal and ventral surface. Each border separates anteriorly into dorsal and ventral divisions to enclose 7 sternal facets.

Extremities

Anterior extremity

·        It is cranial end of the 1^st strnebra and forms the presternum or manubrium sterni.

·        It is compressed laterally and is placed at an angle in front of the 2^nd sternebra.

·        It bears a facet for articulation with the first costal cartilage on either side.

 

Posterior extremity:

·        It is prolonged by the xiphoid cartilage. It is concave above and convex below.

Comparison with

Sternum of horse:

·        Fusion of seven sternebrae.

·        Boat shaped, compressed laterally towards the cranial end and dorso-ventrally at the caudal end.

·        Lateral surfaces are convex.

·        Ventrally it presents a prominent crest.

·        Xiphoid cartilage is flat and rounded.

 

Sternum of dog:

·        Formed by 8 sternal segments, which generally don’t fuse with each other. Many fuse at very old age.

·        First sternebra is longest.

·        Cranial end is blunt and the xiphoid cartilage is narrow.

 

Sternum of Pig:

·        Six sternebrae has fused to form this bone

·        Bone is dorso-ventrally compressed.

·        Xiphoid process is long and xiphoid cartilage is small.

 

Sternum of Fowl:

·        It is large irregular plate of bone composed of cranial mass.

·        Cranial mass has:

o   Cranial projection : rostrum

o   Caudal extension: metasterum and lateral process.

o   Dorsal surface is concave.

o   Ventrally presents a plate like bone, which is extended up to the metasternum known as sternal crest or keel or crania.

·        The rostrum presents tow facets for coracoids.

·        The lateral processes are two in number.

·     The cranial end is flat and the caudal one is divided into a dorsal and a ventral part.

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