Axial

I. Axial Skeleton Overview

  • The axial skeleton comprises the skeleton of the head, vertebral column, and thorax.

  • The learning objectives include being able to discuss the skeleton of the head, vertebral column, and thorax across various domestic animal species, in addition to identifying species-specific differences in the axial skeleton across these animals.

II. Skull

  • The skull can be divided into the neural part and facial part.

  • Bones of the Cranium (Neural Part):

    • The bones include the frontal bones, interparietal bones, occipital bone, parietal bones, and temporal bones, as well as the ethmoid bone and sphenoid bone.

    • Occipital bone: Forms the nuchal wall and consists of basilar, squamous, and lateral parts.

      • The nuchal crest is reduced as the nuchal line in ruminants.

      • Carnivores and horses have a well-defined median ridge called the external sagittal crest.

      • The paracondylar process is elongated in pigs, short in ruminants and horses, and bulb-shaped in carnivores.

    • Sphenoid bone: Consists of the presphenoid (rostral) and basisphenoid (caudal), each with a body and wing.

      • In horses, the basisphenoid forms the rostral border of the foramen lacerum and has three notches: the carotid, oval, and spinous notches.

      • Carnivores have an oval foramen, spinous foramen, and carotid canal, while ruminants have an oval foramen.

    • Temporal bone: Has three parts (distinct in newborns): squamous, petrosal, and tympanic.

      • In oxen and carnivores, the petrosal and tympanic parts are pyramid-shaped and firmly fused to the squamous part.

      • The mastoid process is strong and bulb-shaped in horses, smaller in other domestic animals, and serves as an attachment for the hyoid apparatus in carnivores.

      • The styloid process serves as an attachment for the hyoid apparatus in horses and ruminants but is absent in carnivores and pigs. In carnivores, the hyoid apparatus articulates with the mastoid process, while in pigs, it articulates with the nuchal process.

      • The tympanic part includes the tympanic bulla, which encloses the tympanic cavity of the middle ear and the auditory ossicles (stapes, malleus, and incus). The bulla is very large in cats.

      • In ruminants, the temporal fossa is pushed to the L side by the frontal bone.

    • Frontal bone: Consists of the frontal squama, temporal surface, orbital part, and nasal part.

      • The temporal line, which separates the frontal and temporal surfaces, is prominent in dogs, oxen, and horses, but insignificant in other species.

      • In oxen and pigs, the frontal bone forms the entire roof of the cranium.

      • The zygomatic process is a species-specific articulation.

      • Horned ruminants have paired cornual processes.

    • Interparietal bone: Present only in horses and cats.

    • Ethmoid bone: Situated deep in the walls of the orbit.

      • Partly forms the cranial and facial parts.

      • The cribriform plate is a partition between the nasal and cranial cavities with perforations for olfactory nerve bundles.

      • The perpendicular plate divides the ethmoid into two tubes (ethmoidal labyrinth).

      • The ethmoid labyrinth is composed of delicate bony scrolls called ethmoturbinates.

      • Ethmoturbinates arise from the dorsal and lateral walls and are arranged in two rows, except in horses, where there are three rows.

      • The ethmoturbinates are divided into ectoturbinates (short, more superficial, arranged in a single row, except in horses, where they are in a double row) and endoturbinates (long, deep-lying).

    • The number of endoturbinates and ectoturbinates varies among animal species:

      • Dogs have 4 endoturbinates and 6 ectoturbinates.

      • Ruminants have 4 endoturbinates and 18 ectoturbinates.

      • Horses have 6 endoturbinates and 25 ectoturbinates.

      • Pigs have 7 endoturbinates and 20 ectoturbinates.

  • Facial Part:

    • Lateral walls are formed by the lacrimal bones, zygomatic bones, and maxilla.

    • The floor of the nasal cavity/roof of the oral cavity is formed by the palatine bones, maxilla, incisive bone, and vomer.

    • The roof of the nasal cavity is formed by the frontal and nasal bones.

    • The roof or lateral walls of the pharyngeal cavity is formed by the pterygoid bones, palatine bones, sphenoid bones, and parts of the vomer.

    • Nasal bone: The rostral processes end centrally in pigs, sheep, and horses, laterally in carnivores, and have separate apices in oxen.

    • Lacrimal bone: Articulates with the frontal, zygomatic, and maxilla. In ruminants and horses, it also articulates with the nasal bone; in carnivores, it articulates with the palatine bone.

    • Zygomatic bone: Lies ventrolateral to the lacrimal bone and forms part of the bony orbit and the zygomatic arch.

      • Extends towards the frontal bone as the "frontal process," except in horses.

      • The supraorbital margin in ruminants, carnivores, and pigs is the frontal process and zygomatic process of the frontal bone.

      • In horses, the zygomatic processes of the frontal and temporal bones form the supraorbital margin.

    • Maxilla: The largest bone of the face, providing the osseous basis for the major part of the facial part and forming part of the lateral walls, nasal and oral cavities, and hard palate.

      • Parts include the body, alveolar process, palatine process, and zygomatic process.

      • Cats have 4 cheek teeth with the P4 close to the orbit, which can lead to spreading alveolar abscesses.

      • Ruminants lack an alveolar process.

      • Horses have a facial crest, pigs have a distinct facial crest that ends at the canine fossa, and ruminants have a facial tubercle.

      • The infraorbital foramen in horses is at the level of the nasoincisive notch and rostral end of the facial crest; in oxen, it is 3 cm dorsal to the 1st maxillary cheek tooth; and in dogs, it is 1 cm dorsal to the 3rd maxillary cheek tooth.

    • Incisive bone: The interalveolar margin is relatively long in horses and short in pigs and carnivores.

    • Paranasal sinuses: Air-filled cavities within some bones of the skull, including the frontal and maxillary sinuses.

      • Horses have two (rostral and caudal).

      • Pigs and ruminants have a single sinus.

  • Lateral View: Features the orbit, temporal fossa, zygomatic arch, retroarticular surface, tympanic bulla, and mastoid process.

    • Features three openings: retroarticular foramen, stylomastoid foramen, and external acoustic meatus.

  • Ventral View: Has three regions: the base of the cranium, choanal region, and hard palate.

III. Mandible

  • The mandible consists of the body and the ramus.

    • The body includes alveoli and mental foramina.

    • The ramus includes the coronoid process, condylar process, and angular process.

  • The mandible consists of two halves joined by a median mandibular symphysis.

    • In pigs and horses, the median mandibular symphysis is complete.

    • In carnivores and ruminants, it remains bipartite.

  • Mandible (body)

    • Horses and ruminants have the longest interalveolar margin/diastema between the canine and 1st cheek tooth.

    • Horses and ruminants have a single opening of the mental foramen on the lateral surface, while carnivores have 2-3 openings, and pigs have up to 5 openings.

    • Horses have a facial notch where the pulse is commonly palpated.

    • Carnivores and ruminants have a dental alveolus (canine tooth) directly caudal, while horses and pigs have some space apart.

IV. Hyoid Apparatus

  • The hyoid apparatus serves as a suspensory mechanism for the tongue and larynx.

  • It connects to the tongue and larynx and includes the basihyoid, thyrohyoid, and ceratohyoid.

  • Hyoid apparatus

    • Connects to the tongue and larynx.

    • Consists of the basihyoid, thyrohyoid, and ceratohyoid.

    • The ceratohyoid articulates caudally with the basihyoid and thyrohyoid and proximally with the epihyoid (connection to the suspensory apparatus).

    • The basihyoid is short and unpaired at the base of the tongue, with a lingual process that is long in horses and short in ruminants.

    • The thyrohyoid projects caudally from the basihyoid and is firmly fused with the thyroid cartilage of the larynx in ruminants and horses, with a movable joint.

  • Suspensory apparatus

    • Articulates with the temporal bone in a species-specific manner.

    • Consists of the tympanohyoid (proximal), stylohyoid (middle), and epihyoid (distal).

    • The tympanohyoid consists of short cartilaginous tissue in most animals, but is fibrous in carnivores.

    • The stylohyoid is laterally flattened cylinder in ruminants and horses, while the distal part remains cartilagenous in pigs and carnivores.

    • The epihyoid is fused with the stylohyoid in horses, replaced by the epihyoid ligament in pigs, and cylindrical in carnivores.

    • Ruminants and horses articulate with the styloid process of the tympanic part of the temporal bone; carnivores articulate with the mastoid process of the petrosal part of the temporal bone; and pigs articulate with the nuchal process of the squamous temporal bone.

V. Vertebral Column

  • The vertebral column includes the cervical, thoracic, lumbar, sacral, and coccygeal vertebrae.

  • Cervical Vertebrae:

    • The first cervical vertebra (C1) is the atlas, which lacks a body and has a wing and foramina (alar, transverse, lateral vertebral).

    • The second cervical vertebra (C2) is the axis, characterized by the dens and caudal articular facet.

    • Carnivores have an alar foramen and "alar notch" while ruminants have transverse foramen on C1.

    • Carnivores and pigs have a rod-like dens, while other species have a spout-like dens on C2.

    • Carnivores have an elongated spinous process that overhangs the cranial and caudal end of the body.

    • Pigs have an elongated spinous process that overhangs the caudal end of the body on C2.

    • Ruminants have a rectangular spinous process on C2.

    • The spinous process is confluent with the caudal articular process in carnivores and horses, but not in pigs and ruminants on C2.

    • The cranial vertebral notch is present in dogs and absent in other domestic animals; the lateral vertebral foramen is also present on C2.

    • The bodies of the remaining cervical vertebrae (C3-C7) become progressively shorter.

    • C3-C5 have a stout ventral crest, C6 has an enlarged, plate-like extension, and C7 has an indistinct or absent ventral crest.

    • The spinous process increases towards the thoracic vertebrae; however, only C7 possesses a spinous process in horses.

    • C3-C6 have a transverse foramen that forms the transverse canal.

    • Intervertebral foramen are formed by deeply notched vertebral arches between adjacent vertebrae.

    • The number of cervical vertebrae in birds varies with the length of the neck, with small birds having 8, chickens having 14-17, and swans having approximately 25.

    • The atlas in birds has a small ring with a depression on the ventral arch.

    • The axis in birds has a dens and short cranial articular facet.

    • The remaining cervical vertebrae in birds are uniformly cylindrical.

  • Thoracic Vertebrae:

    • The bodies of the thoracic vertebrae are short and have costal fovea for the rib head.

    • The transverse process is short and stout with a costal fovea for the rib tubercle.

    • The spinous process is very prominent and increases to the anticlinal vertebra before gradually declining.

    • The spinous process is long, large, and narrow in horses; long and wide in oxen; and long, rough, and irregular in pigs.

    • In pigs and ruminants, the spinous processes of T1-T3 increase in height and progressively become shorter up to T11 (pig) or T12/T13 (ruminants) before staying the same length caudally.

    • The spinous process is anticlinal at T10 in dogs, T12 in pigs and goats, T13 in oxen, and T16 in horses.

    • The intervertebral foramen is comparatively large and often divided by a bony bridge in ruminants.

    • Chickens have 7 thoracic vertebrae with complete ribs and a sternum.

    • T3-T5 are fused in chickens, raptors, and pigeons, forming the "notarium," and the last 1 or 2 vertebrae fuse with the lumbar, sacral, and Cd1 vertebrae to form the "synsacrum".

  • Lumbar Vertebrae:

    • The body of the lumbar vertebrae is longer and more uniform in shape, without costal facets.

    • The spinous process is approximately equal in height and inclined cranially.

    • The transverse process is flattened and projects laterally with costal processes.

    • Proximal mamillary and accessory processes are present.

    • The spinous processes are progressively longer from L1-L3/L4 in carnivores and caudally inclined in oxen.

    • Small ruminants have spinous processes oriented perpendicular to the long axis and relatively low and wide summits that are moderately thickened and axe-shaped.

    • The transverse processes are cranioventral in carnivores and pigs, with L5/L6 being the longest (other animals: L3/L4), and horizontal in ruminants and horses.

    • The last 2 lumbar vertebrae in horses articulate with S1.

    • Lumbar vertebrae are fused with the last 1 or 2 thoracic vertebrae, sacral vertebrae, and Cd1 in birds, forming the synsacrum.

  • Sacral Vertebrae:

    • These are fused, forming the sacrum.

    • The sacrum narrows from cranial to caudal.

    • The spinous processes are fused or independent.

    • The transverse processes are fused and cranially have an auricular surface (origin: S1).

    • The sacrum is quadrilateral in carnivores and triangular in other species.

    • Wings are present in carnivores, pigs, and ruminants (S1 & S2).

    • Spinous processes are independent in carnivores and horses but fused into a "median sacral crest" in ruminants; in pigs, they are replaced by an indistinct crest.

    • Sacral vertebrae are fused with the last 1 or 2 thoracic vertebrae, lumbar vertebrae, and Cd1 in birds, forming the synsacrum.

  • Caudal Vertebrae:

    • The first few caudal vertebrae resemble lumbar vertebrae, while the rest are reduced to simple rods.

    • Cranial vertebrae have paired hemal processes.

    • Hemal processes are found on Cd1-Cd8 in ruminants, Cd 5-15 in carnivores, and Cd2 in horses.

    • Ventral arches are present on Cd2-Cd3 in ruminants and Cd3-Cd8 in carnivores.

    • The spinous process is bifurcated in horses on Cd2.

    • 5 or 6 caudal vertebrae fuse, forming the pygostyle, the most caudal segment that provides support to the rectrices (birds).

  • Bony Pelvis: Firmly attached to the synsacrum and composed of R & L hip bones, lacking a ventral symphysis in ostriches and rheas.

    • The ilium is dorsal, the ischium is lateral, and the pubis is a thin rod to the ventral border of the ischium.

  • Pelvis: Acetabulum is formed by the joint ilium & ischium.

    • Pubic symphysis is present, and + rhea & ostrich is an adaption to support heavy visceral mass.

VI. Thoracic Skeleton

  • The thoracic skeleton comprises the thoracic vertebrae, ribs, and sternum.

  • Ribs:

    • Ribs have an osseous part and costal cartilage.

    • Ribs can be sternal ("true ribs"), asternal ("false ribs"), or floating (last pair in dogs and cats).

    • The shaft is cylindrical in dogs, narrow and strongly curved dorsally in horses, wide and flat with a long neck in oxen, and narrow with a distinct angle in pigs.

    • The vertebral (dorsal) part has an "uncinate process" (caudodorsally directed process) that overlaps the next rib, while the sternal (ventral) part is complete in bird ribs.

  • Sternum:

    • The sternum is a series of sternebrae joined by intersternal cartilages.

    • Carnivores have 8 sternebrae, ruminants have 7, horses have 6/7, and pigs have 6.

    • The body of the sternum is cylindrical in carnivores, wide and flat in ruminants, and has a ventral keel in horses.

    • The sternum in birds is a large, unsegmented bone with a prominent keel (carina) in very good fliers.